Hello!
I'm brand new to this forum and somewhat green when it comes to reading schematics, which I have for the amplifier I'm trying to fix/modify.
My problem is with my Technics su-v7 stereo amplifier. I've used this amp for a few years now after finding it next to a dumpster. At the time, it was fully functioning with the exception of the switches for mono/stereo, viatone/straight dc, low filter, high filter, and loudness. These switches were dirty and crackling a bit when I found it, but after switching them on and off a bunch they cleaned up well enough to not notice any loss of fidelity.
Now they are either dirty again, or the switches are wearing out enough to not provide a good connection.
I never use the amplifier with any of these options on (straight dc operation with no tone or filter in stereo mode), but now the switches are causing a bad connection even in the off position, especially the loudness switch. If I wiggle the switches while they are in the off position I can eventually achieve clean sound, but it's pretty irritating having to do this every couple of uses.
My question is: would it be simple enough to just bypass all of these switches by soldering the pcb so that all of these options are permanently in the off position, or would it be better to try some kind of cleaning solution, and if so, what? I would like to just get rid of these options all together as they are useless to me and only causing problems.
Obviously I'm being cheap (actually, completely free), but I'd like to keep it that way if possible.
I love this amp and would be happy with it for a long time to come if I can fix this issue.
Thanks very much in advance for advice!
I'm brand new to this forum and somewhat green when it comes to reading schematics, which I have for the amplifier I'm trying to fix/modify.
My problem is with my Technics su-v7 stereo amplifier. I've used this amp for a few years now after finding it next to a dumpster. At the time, it was fully functioning with the exception of the switches for mono/stereo, viatone/straight dc, low filter, high filter, and loudness. These switches were dirty and crackling a bit when I found it, but after switching them on and off a bunch they cleaned up well enough to not notice any loss of fidelity.
Now they are either dirty again, or the switches are wearing out enough to not provide a good connection.
I never use the amplifier with any of these options on (straight dc operation with no tone or filter in stereo mode), but now the switches are causing a bad connection even in the off position, especially the loudness switch. If I wiggle the switches while they are in the off position I can eventually achieve clean sound, but it's pretty irritating having to do this every couple of uses.
My question is: would it be simple enough to just bypass all of these switches by soldering the pcb so that all of these options are permanently in the off position, or would it be better to try some kind of cleaning solution, and if so, what? I would like to just get rid of these options all together as they are useless to me and only causing problems.
Obviously I'm being cheap (actually, completely free), but I'd like to keep it that way if possible.
I love this amp and would be happy with it for a long time to come if I can fix this issue.
Thanks very much in advance for advice!
Hi and welcome to diyAudio 🙂
I think that bypassing the switches if you never use them is the most sensible approach. Just identify which pins get switched to which (input and output) when in the position you use it and jumper only those. Some tinned copper wire should probably do the trick.
Tony.
I think that bypassing the switches if you never use them is the most sensible approach. Just identify which pins get switched to which (input and output) when in the position you use it and jumper only those. Some tinned copper wire should probably do the trick.
Tony.
Thanks for the quick response Tony!
I attached a pic of the schematic zoomed in on the loudness switch in hopes that you or someone else can make sense of it. I can provide more pics if this one doesn't provide enough info.
With the amplifier casing removed top and bottom, I can see exactly where the switch is soldered to the pcb, but the switch itself is encased in plastic and I can't see which connection is being made (if any) on the pcb to put the loudness in the off position. My concern with leaving the switch soldered to the board and bypassing it is the possibility that while it's in the off position, the switch is making a necessary connection. Also, I worry that if I simply bridge the points on the board to make the loudness off, and if while in the off position a connection is still being made by the switch, it would cause a damaging or hazardous short if the switch were to be pressed to the on position.
Thanks again for bearing with the newbee!
I attached a pic of the schematic zoomed in on the loudness switch in hopes that you or someone else can make sense of it. I can provide more pics if this one doesn't provide enough info.
With the amplifier casing removed top and bottom, I can see exactly where the switch is soldered to the pcb, but the switch itself is encased in plastic and I can't see which connection is being made (if any) on the pcb to put the loudness in the off position. My concern with leaving the switch soldered to the board and bypassing it is the possibility that while it's in the off position, the switch is making a necessary connection. Also, I worry that if I simply bridge the points on the board to make the loudness off, and if while in the off position a connection is still being made by the switch, it would cause a damaging or hazardous short if the switch were to be pressed to the on position.
Thanks again for bearing with the newbee!
Attachments
![technics_su-v7_[ET]5.jpg](/community/data/attachments/226/226060-5325cdd2156c120e3773b92fecacc0eb.jpg?hash=UyXN0hVsEg)
Just a hint from another SUV Technics user - yes mine did the same with the tape monitor and tone bypass switches dying all the time until I realised that the problem was silver plated contacts on the tiny open backed alternate action switches.
This is bad in urban environments, as the silver forms insulating compounds with various components of the very air we breathe. I used contact cleaner/lubricant with lots of "working" of the switch with great success but this needs a re-clean every couple of years in my coastal environment.
Hey, it's dead easy and it works, ok?
This is bad in urban environments, as the silver forms insulating compounds with various components of the very air we breathe. I used contact cleaner/lubricant with lots of "working" of the switch with great success but this needs a re-clean every couple of years in my coastal environment.
Hey, it's dead easy and it works, ok?
Thanks again guys.
I spent the $8 on some CRC contact cleaner and everything works perfectly. Thanks for the tip Ian! I didn't get the lubricating kind because a local vintage audio shop advised against it. Not sure why he recommended not using the lubricating kind though. Anyone know?
I spent the $8 on some CRC contact cleaner and everything works perfectly. Thanks for the tip Ian! I didn't get the lubricating kind because a local vintage audio shop advised against it. Not sure why he recommended not using the lubricating kind though. Anyone know?
It's a tradition to avoid lubricants that may attract dirt and produce breakdown products which are also bad news with relays and contact switching. However, at signal levels there won't be breakdown products like with arcing contacts and the dirt build up is, well, minuscule with the front panel PCB in most SUV models. It's broad brush thinking that ignores positive experiences with careful application. IOW - BS!
In, fact, ornery 'ol WD40 has been used for audio with good results too! Problem is, it's not an overpriced specialist product and it's not marked "for audio use" This makes audio guys and engineers in particular, really jumpy but a friend's SUV40 has been chugging away, sounding as good as always on just one thorough dose about 5 years ago. I don't recommend the stuff but lubricated types last a lot longer than plain cleaner, so there you go - make sure you mask the PCB and nearby parts as best you can and clean all residues from the PCB in that case. - enjoy.
In, fact, ornery 'ol WD40 has been used for audio with good results too! Problem is, it's not an overpriced specialist product and it's not marked "for audio use" This makes audio guys and engineers in particular, really jumpy but a friend's SUV40 has been chugging away, sounding as good as always on just one thorough dose about 5 years ago. I don't recommend the stuff but lubricated types last a lot longer than plain cleaner, so there you go - make sure you mask the PCB and nearby parts as best you can and clean all residues from the PCB in that case. - enjoy.
good to hear that it's working again 🙂 I use the CRC CO myself when I need it. completely residue free.
I once had a can of the crc contact cleaner with lubricant, it is horrid stuff, the guy gave you the right advice. It gets onto the circuit board, if you try and wipe it off, it takes the silkscreening with it, and generally makes a sticky mess.
Tony.
I once had a can of the crc contact cleaner with lubricant, it is horrid stuff, the guy gave you the right advice. It gets onto the circuit board, if you try and wipe it off, it takes the silkscreening with it, and generally makes a sticky mess.
Tony.
damn!
Very happy with the response and solid advice on this forum!
Maybe this should be a thread of it's own, but I'm curious about others opinions on the su-v7 or whatever model of suv you're using. It seems that enough of you have used them, so I can't help but ask.
When I found this amplifier I was using a Technics sa500 receiver which I believe is a B class amp. The su-v7 is a "new class A" amplifier, which is basically an AB amplifier, right?
Anyway, the difference in sound quality was astoundingly apparent from the get go, even with the dirty connections.
Are all B class amps harsh, or is the sa500 just a bad sounding B class amplifier?
The su-v7 when compared to the sa500 has a much, much smoother and more controlled sound than the sa500, and I believe this is an observation that anyone could pick up on, not just audio nuts with a "trained" ear.
I'm a bit confused by this because most things I read about "upgrading" your sound system have said that the amplifier has the least effect on over all sound quality, putting speakers first, room second, and source third.
Upon finding this amp next to the dumpster, I definitely did not expect it to be such an obvious and audible upgrade to the sa500 I had been using for so many years.
Now that I've seen first hand what a difference the amplifier can make in sound quality, I wonder how this amplifier sounds compared to other types of amplifiers.
Very happy with the response and solid advice on this forum!
Maybe this should be a thread of it's own, but I'm curious about others opinions on the su-v7 or whatever model of suv you're using. It seems that enough of you have used them, so I can't help but ask.
When I found this amplifier I was using a Technics sa500 receiver which I believe is a B class amp. The su-v7 is a "new class A" amplifier, which is basically an AB amplifier, right?
Anyway, the difference in sound quality was astoundingly apparent from the get go, even with the dirty connections.
Are all B class amps harsh, or is the sa500 just a bad sounding B class amplifier?
The su-v7 when compared to the sa500 has a much, much smoother and more controlled sound than the sa500, and I believe this is an observation that anyone could pick up on, not just audio nuts with a "trained" ear.
I'm a bit confused by this because most things I read about "upgrading" your sound system have said that the amplifier has the least effect on over all sound quality, putting speakers first, room second, and source third.
Upon finding this amp next to the dumpster, I definitely did not expect it to be such an obvious and audible upgrade to the sa500 I had been using for so many years.
Now that I've seen first hand what a difference the amplifier can make in sound quality, I wonder how this amplifier sounds compared to other types of amplifiers.
Comments on the model/family won't be out of place.
AFAIK, the SUV series is a long running family developed from the SEA or New Class A series (Sandman's class S) from around 1980. The principle of these all differ in the details but broadly speaking, there is a tiny class A amplifier attempting to drive the load in a bridge sensing circuit which allows it to be buffered by a large, class B amp. which supplies the actual muscle yet almost has the subtle crossoverless amplification of class A, as it emulates that source.
Many 'pooh-pooh' the scheme but it definitely sounds better than your average class AB amplifier in smooth detail and imaging IMO. It runs dead cool most of the time and otherwise has a long, trouble free life. Yes, I like even my later, budget SUV50.
That is from a series labelled "Super Class AAA+ , whatever that meant.
'Very clever designs and awful to adjust and stabilise. The design details are not in the schematics. These are buried in the proprietary chips or hybrid power modules in the Technics SUV models, so I can't say what the actual details are but there have been threads here which you can search from the top of the page.
AFAIK, the SUV series is a long running family developed from the SEA or New Class A series (Sandman's class S) from around 1980. The principle of these all differ in the details but broadly speaking, there is a tiny class A amplifier attempting to drive the load in a bridge sensing circuit which allows it to be buffered by a large, class B amp. which supplies the actual muscle yet almost has the subtle crossoverless amplification of class A, as it emulates that source.
Many 'pooh-pooh' the scheme but it definitely sounds better than your average class AB amplifier in smooth detail and imaging IMO. It runs dead cool most of the time and otherwise has a long, trouble free life. Yes, I like even my later, budget SUV50.
That is from a series labelled "Super Class AAA+ , whatever that meant.
'Very clever designs and awful to adjust and stabilise. The design details are not in the schematics. These are buried in the proprietary chips or hybrid power modules in the Technics SUV models, so I can't say what the actual details are but there have been threads here which you can search from the top of the page.
I have another issue with this amplifier that I didn't mention initially because I thought it was related to the dirty switches and pots, but after cleaning everything and powering up the amp in a quiet environment, I can still hear a hum (more like a buzz) that was always present at about 60hz coming from the amplifier itself, and after the relay switch kicks in, the noise is sent to the speakers. Adjusting the volume does not effect the volume of the noise getting to the speaker, or the volume of the noise within the amplifier. Upon closer examination, I noticed a component on the speaker selection circuit that looks like a plastic rectangular box marked with "5W 220ohm J, 10 362" that looks a bit melted and blackened around the contact points with it's pcb.
With my limited understanding of circuitry within amplifiers, I'd assume that this problem is with the power supply since the sound is being made by the amplifier even with no speaker connected, and also that the sound is coming from the side of the casing with the power supply. dur?
How do I fix this???
With my limited understanding of circuitry within amplifiers, I'd assume that this problem is with the power supply since the sound is being made by the amplifier even with no speaker connected, and also that the sound is coming from the side of the casing with the power supply. dur?
How do I fix this???
Wecome back to old amp corner 🙂
I'd be a bit concerned about a hum before the speakers are connected. I assume it's the power transformer but perhaps an electrolytic is really bad. See if you can pin-point what is buzzing mechanically (don't risk shock!), I have used a screwdriver as a kind of stethoscope, like a motor mechanic, for this in the past but it's a little "left -field" for some folks.
Otherwise, any amp. over 20 yrs old is due for a re-cap. It would be best to test each electrolytic from the main power ones down to the decoupling and AC coupling ones using a DMM/ capacitance meter range that is greater than the largest electro. At say 10,000 uF, old and basic meters can't do it, so maybe you'll have to just routinely replace everything, stopping and checking after each pair or similar-use parts is changed. Like, first the big main power supply electros and so on. in each channel, checking one after the other channel so you always have a known working channel for reference.
Yeah, it's slow and very expensive that way but this is not an easy problem if both channels go down for some reason.
The fried resistor is a concern. Is it in fact one of a pair and does either still get warm in use, driving soeakers? It may only be the headphone dropping resistor that may have been shorted in the past but it could also be part of the bridge circuit so I will search for a schematic meantime. Check the speaker switch contacts for damage there too.
Do try to source industry standard parts like Panasonic or local product as long as sizes and lead spacings will fit. Caps have shrunk over the last decades and there really should be no problem getting parts at Mouser, Digi-key, Newark or any local parts supplier worth being called that. Apex Jr. also has a surplus store here in the vendors forum. At the least, change just the main electros, most problems stop there or in other warm locations.
Keep at it and take care with electrolytics that have been powered up to rail voltage. They often stay that way until you make a short path to ground and get the jolt!
Sorry if you already know this stuff, but it's good to care for those who don't 🙂
I'd be a bit concerned about a hum before the speakers are connected. I assume it's the power transformer but perhaps an electrolytic is really bad. See if you can pin-point what is buzzing mechanically (don't risk shock!), I have used a screwdriver as a kind of stethoscope, like a motor mechanic, for this in the past but it's a little "left -field" for some folks.
Otherwise, any amp. over 20 yrs old is due for a re-cap. It would be best to test each electrolytic from the main power ones down to the decoupling and AC coupling ones using a DMM/ capacitance meter range that is greater than the largest electro. At say 10,000 uF, old and basic meters can't do it, so maybe you'll have to just routinely replace everything, stopping and checking after each pair or similar-use parts is changed. Like, first the big main power supply electros and so on. in each channel, checking one after the other channel so you always have a known working channel for reference.
Yeah, it's slow and very expensive that way but this is not an easy problem if both channels go down for some reason.
The fried resistor is a concern. Is it in fact one of a pair and does either still get warm in use, driving soeakers? It may only be the headphone dropping resistor that may have been shorted in the past but it could also be part of the bridge circuit so I will search for a schematic meantime. Check the speaker switch contacts for damage there too.
Do try to source industry standard parts like Panasonic or local product as long as sizes and lead spacings will fit. Caps have shrunk over the last decades and there really should be no problem getting parts at Mouser, Digi-key, Newark or any local parts supplier worth being called that. Apex Jr. also has a surplus store here in the vendors forum. At the least, change just the main electros, most problems stop there or in other warm locations.
Keep at it and take care with electrolytics that have been powered up to rail voltage. They often stay that way until you make a short path to ground and get the jolt!
Sorry if you already know this stuff, but it's good to care for those who don't 🙂
I tried your screwdriver stethoscope trick (slick, by the way) and the hum is definitely coming from the power transformer. Are these easy enough to find for this amp? Also, would it need to be the exact same power transformer, or would a close match be good enough? The hum is even in volume between the right and left channels, just thought I'd drop that bit of info in case it matters.
I live close to a surplus store called Ax-man, and they always have huge bins of caps, resistors, switches, pots, transformers, etc. for cheap (I think they were mostly scrapped from electronic equipment, not new, so maybe not to be trusted). If I end up having to spend more than 40-50 bucks on this amp, I might just put that $ into a decent kit and build my own. As is, the amplifier is totally usable since the hum doesn't increase with the volume of the amplifier, and at even a quiet listening volume, it's barely even detectable, it's just not something I'd like to stick with much longer if the hum isn't fixed.
The resistor that looks burnt doesn't seem to be for just one channel on the amp, as I can't find a matching one anywhere else in the amplifier. Also, it does warm up quickly during use, enough to make the casing right above it a bit toasty, but not too hot to touch.
Thanks very much for the help with this, and yes, I'm new enough to this kind of work to appreciate any "beginner" tips along the way. Getting shocked and/or breaking stuff really sucks!
I live close to a surplus store called Ax-man, and they always have huge bins of caps, resistors, switches, pots, transformers, etc. for cheap (I think they were mostly scrapped from electronic equipment, not new, so maybe not to be trusted). If I end up having to spend more than 40-50 bucks on this amp, I might just put that $ into a decent kit and build my own. As is, the amplifier is totally usable since the hum doesn't increase with the volume of the amplifier, and at even a quiet listening volume, it's barely even detectable, it's just not something I'd like to stick with much longer if the hum isn't fixed.
The resistor that looks burnt doesn't seem to be for just one channel on the amp, as I can't find a matching one anywhere else in the amplifier. Also, it does warm up quickly during use, enough to make the casing right above it a bit toasty, but not too hot to touch.
Thanks very much for the help with this, and yes, I'm new enough to this kind of work to appreciate any "beginner" tips along the way. Getting shocked and/or breaking stuff really sucks!
Useful feedback for others there too. Thanks, Macrofilkins. 🙂
The buzzing transformer indicates either heavy or unbalanced load or loose laminations. windings etc. Some cheap stuff is like that from new, though unlikely in this classy old 120W beast, These SUVs all seem to have small power transformers and work on the limit of sag under load. Some even have switched windings to cut voltage on 4 ohm loads. The quality is good though. and it may be just the electros failing.
If preloved, test electro value is within say, -10, +40% of marked value before accepting. Then swap out the main electros because hum is on the power rails for sure. You could see it with a 'scope or even measure the ripple unloaded across the electros. with a DMM set to AC. Just check the replacement's pin spacing and overall diameter for fit as well as ratings as before.
'Sounds like a worry with that 5W resistor, though and I don't have the SUV7 schematic, Sorry, perhaps others will comment and I think Sakis (Greece) may have a copy. At $15 US, I'm not quite so curious, somehow. 😀
The buzzing transformer indicates either heavy or unbalanced load or loose laminations. windings etc. Some cheap stuff is like that from new, though unlikely in this classy old 120W beast, These SUVs all seem to have small power transformers and work on the limit of sag under load. Some even have switched windings to cut voltage on 4 ohm loads. The quality is good though. and it may be just the electros failing.
If preloved, test electro value is within say, -10, +40% of marked value before accepting. Then swap out the main electros because hum is on the power rails for sure. You could see it with a 'scope or even measure the ripple unloaded across the electros. with a DMM set to AC. Just check the replacement's pin spacing and overall diameter for fit as well as ratings as before.
'Sounds like a worry with that 5W resistor, though and I don't have the SUV7 schematic, Sorry, perhaps others will comment and I think Sakis (Greece) may have a copy. At $15 US, I'm not quite so curious, somehow. 😀
At this point, I think I'm more concerned with the burnt resistor. I have the schematic for the amplifier, but unfortunately, it's file size is too large for upload on this site. I'll try cropping it down with photoshop to a small enough size tomorrow when I have time to disassemble the amp and find the resistor in question on the schematic.
Also, I feel that I should mention that the hum from the power transformer is quiet enough to not be heard in an almost completely silent room from more than 5 ft. away from the system. Is this just normal? Have I been picking at a non-issue this whole time, or is the hum really something to be concerned with?
Again, Ian, thanks very much for your help with this. If only I could buy you a beer!
Also, I feel that I should mention that the hum from the power transformer is quiet enough to not be heard in an almost completely silent room from more than 5 ft. away from the system. Is this just normal? Have I been picking at a non-issue this whole time, or is the hum really something to be concerned with?
Again, Ian, thanks very much for your help with this. If only I could buy you a beer!
Let's put the resistor on hold for the moment but do replace the main caps. Don't be surprised then, that the hum vanishes too.
If there is only the single 220 ohm 5W resistor, it has to do with an ancillary circuit, like maybe relay power to shut the outputs down in a fault condition and delay the turn-on to avoid that thump when you power up many transistor amps.
You can check this out, assuming you have a DMM or even an old multimeter. Connect the -ve lead to earth or chassis somewhere it cant short if it comes unclipped etc. Then test the positive probe with a suitable range like 100V DC on both ends of the resistor. If, when you then power up and hear a click after a short delay and your meter reading dives at the same time, that is what it does. Look on the schematic for the speaker relay circuit and find the power to that and go from there.
You could even simply test across the resistor too but that may be awkward and lead to accidents so I hesitate to suggest juggling 2 probes and a switch at once with confidence but you may be fine with that.
Of course, it could be another function but this is most likely on an '86 model like this. Good hunting! 🙂
If there is only the single 220 ohm 5W resistor, it has to do with an ancillary circuit, like maybe relay power to shut the outputs down in a fault condition and delay the turn-on to avoid that thump when you power up many transistor amps.
You can check this out, assuming you have a DMM or even an old multimeter. Connect the -ve lead to earth or chassis somewhere it cant short if it comes unclipped etc. Then test the positive probe with a suitable range like 100V DC on both ends of the resistor. If, when you then power up and hear a click after a short delay and your meter reading dives at the same time, that is what it does. Look on the schematic for the speaker relay circuit and find the power to that and go from there.
You could even simply test across the resistor too but that may be awkward and lead to accidents so I hesitate to suggest juggling 2 probes and a switch at once with confidence but you may be fine with that.
Of course, it could be another function but this is most likely on an '86 model like this. Good hunting! 🙂
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R651 220Ohm 5W in series with two 14V 50mA lamps. Voltage on lamps stabilised by three Zener diodes. Serves the purpose of illuminating the sign "New Class A" on front paned below the volume knob. Easily replacable with two 12V LEDs in series to reduce power consumption. Reason of burnt resistor can be improper bulbs, for example. Nothing to worry about, absolute no impact on sound, IMHO 🙂
Great info! the "new class A" panel under the volume nob recently stopped working! I thought maybe it was just a dead led and wasn't worth mentioning.
About replacing those main caps... are you, "Ian" talking about the two big ones next to the heat sink for the transistors, or do you mean caps closer in proximity to the main power transistor?
Clearly I'm a big fat newb, so I apologize for my lack of understanding of advice thus far.
Clearly I'm a big fat newb, so I apologize for my lack of understanding of advice thus far.
Hi
Without schematic or overlay diagram, I can't be sure what you are referring to there. It's confusing with this design since it is 2 amps in 1 and not straightforward.
The largest size and value caps will be the main, class B amplifier's power supply caps which will be closely connected to the to the bridge rectifier, following the transformer. They will be wired from + and - rails to ground and obviously have the main rail voltages, say +/- 60V applied to them. They will be high, like 10,000 uF in value, perhaps even 2 on each rail, making 4. That would be what I was looking for, check your schematic for the power supply and identify the largest caps, rectifier and supply there. If in doubt, quote what you find and we can verify that.
There will also be a third pair of transformer windings and rectifier, I expect, to supply lower voltage for the preamp, safety circuits etc. It may use lower value and smaller parts that indicate what it is and the rails there may be only +/- 15V or so.
'Good of Tornisly to come in with resistor details. As you say, no problem to deal with that, even using smaller bulbs. You may also be referring to caps of around 300-1,000 uF that are sometimes placed close to the main amplifier output transistors to ensure low resistance power for transient sound bursts. These could be in worse shape due to proximity to the heatsink but I don't think these get too warm in average home use.
The class A anp will have much smaller value capacitors but all will need replacement unless you can test their ESR (equivalent series resistance) which is a proper way to be certain whether caps are stuffed or not. These meters are not cheap or common, so we remain uncertain. Why put off the inevitable when only 4 cheaper, small caps are needed for both amps?
Well, we seem to be swimming along well now - great! I trust it will soon be sounding sweet and noiseless too.
Without schematic or overlay diagram, I can't be sure what you are referring to there. It's confusing with this design since it is 2 amps in 1 and not straightforward.
The largest size and value caps will be the main, class B amplifier's power supply caps which will be closely connected to the to the bridge rectifier, following the transformer. They will be wired from + and - rails to ground and obviously have the main rail voltages, say +/- 60V applied to them. They will be high, like 10,000 uF in value, perhaps even 2 on each rail, making 4. That would be what I was looking for, check your schematic for the power supply and identify the largest caps, rectifier and supply there. If in doubt, quote what you find and we can verify that.
There will also be a third pair of transformer windings and rectifier, I expect, to supply lower voltage for the preamp, safety circuits etc. It may use lower value and smaller parts that indicate what it is and the rails there may be only +/- 15V or so.
'Good of Tornisly to come in with resistor details. As you say, no problem to deal with that, even using smaller bulbs. You may also be referring to caps of around 300-1,000 uF that are sometimes placed close to the main amplifier output transistors to ensure low resistance power for transient sound bursts. These could be in worse shape due to proximity to the heatsink but I don't think these get too warm in average home use.
The class A anp will have much smaller value capacitors but all will need replacement unless you can test their ESR (equivalent series resistance) which is a proper way to be certain whether caps are stuffed or not. These meters are not cheap or common, so we remain uncertain. Why put off the inevitable when only 4 cheaper, small caps are needed for both amps?
Well, we seem to be swimming along well now - great! I trust it will soon be sounding sweet and noiseless too.
C408/C409 15 000 x 50V - I believe they are only two of a kind. I do not have complete service manual, but, based on schematics, they are next to heatsink. Regarding the hum of transformer - just try the bolts it is attached to chassis with. I've seen a lot of vintage amps with almost every bolt loose. Dunno, they use them in trams, maybe?
And, oh, well, looking at schematics, it is not dual mono amp. One transformer, one set of filter caps, both channels share the same power supply for preamp as well.
And, oh, well, looking at schematics, it is not dual mono amp. One transformer, one set of filter caps, both channels share the same power supply for preamp as well.
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