I have been using my Linn Intek as a pre in an active tri-amped system I'm running to great effect. Whilst enjoying some John Martyn the other night the sound cut out, there was a crackle and pop from inside the amp and I could see something catch fire. As I dove across the room to hit the power the sparks went out and the sound came back briefly before I turned it off.
Upon further investigation I found that one of the output transistors had failed (and taken the other with it). This puzzled me as I hadn't even been using that part of the amp. Then when I did some schematic hunting I found out that I was supposed to separate the power amp from the pre amp internally before using the pre out / power in combined sockets on the back. So this made a little bit of sense as I was running quite a hot signal out of the back of the pre amp into the +4dB input on my crossover and the power amp would probably have been running into clip most of the time. Although my initial reaction would be that as the amp wasn't under load this wouldn't affect it but I suppose the proof is in the pudding.
I have now disconnected the power amp board from the power supply and am quite happily running the amp as a pre amp again (if anything sounds better). But I would like to be able to get the power amp working again in case I want to use it again, I very much liked the sound of it.
Apologies for the lengthy pre amble, HERE are my queries!
I have sourced some replacement output transistors and intend to replace both channels for consistencies sake. Now I find that one of the emitter resistors was taken out when the transistor caught fire. I know it is 220R but I need to know what sort of power handling I should look for in a replacement, they don't look very substantial compared to the big ceramic emitter resistors I'm used to. Here's a photo of said burnt resistor next to the culprit transistor (it's was a TO3P case) for size reference:
Should I replace all of the emitter resistors with whatever replacement I find?
Also, I have read a lot about people replacing the driver transistor at the same time as the output transistor. I have a rough understanding of what the driver transistor does but can somebody explain to me how to locate it? Is it a wise move to replace this at the same time?
Here's an annotated schematic (sorry about the quality). Ringed red is the burnt out transistor, ringed blue is the transistor that is now open, ringed green is the burnt resistor.
https://www.dropbox.com/s/ymlwzd5jps712yh/Annotated Linn Intek Power Stage.pdf?dl=0
Upon further investigation I found that one of the output transistors had failed (and taken the other with it). This puzzled me as I hadn't even been using that part of the amp. Then when I did some schematic hunting I found out that I was supposed to separate the power amp from the pre amp internally before using the pre out / power in combined sockets on the back. So this made a little bit of sense as I was running quite a hot signal out of the back of the pre amp into the +4dB input on my crossover and the power amp would probably have been running into clip most of the time. Although my initial reaction would be that as the amp wasn't under load this wouldn't affect it but I suppose the proof is in the pudding.
I have now disconnected the power amp board from the power supply and am quite happily running the amp as a pre amp again (if anything sounds better). But I would like to be able to get the power amp working again in case I want to use it again, I very much liked the sound of it.
Apologies for the lengthy pre amble, HERE are my queries!
I have sourced some replacement output transistors and intend to replace both channels for consistencies sake. Now I find that one of the emitter resistors was taken out when the transistor caught fire. I know it is 220R but I need to know what sort of power handling I should look for in a replacement, they don't look very substantial compared to the big ceramic emitter resistors I'm used to. Here's a photo of said burnt resistor next to the culprit transistor (it's was a TO3P case) for size reference:
An externally hosted image should be here but it was not working when we last tested it.
Should I replace all of the emitter resistors with whatever replacement I find?
Also, I have read a lot about people replacing the driver transistor at the same time as the output transistor. I have a rough understanding of what the driver transistor does but can somebody explain to me how to locate it? Is it a wise move to replace this at the same time?
Here's an annotated schematic (sorry about the quality). Ringed red is the burnt out transistor, ringed blue is the transistor that is now open, ringed green is the burnt resistor.
https://www.dropbox.com/s/ymlwzd5jps712yh/Annotated Linn Intek Power Stage.pdf?dl=0
For whatever reason the image isn't working. Here's the link
https://www.dropbox.com/s/7qxkhzh3b32zrr7/IMG_1737.JPG?dl=0
https://www.dropbox.com/s/7qxkhzh3b32zrr7/IMG_1737.JPG?dl=0
Don't use a 3rd party image host if you can help it - attach any suitably scaled jpg, gif etc. to the text of your post. It's at least reliable but more importantly, it remains with your post rather than vanishing when your subscription expires - that's infuriating for people reading old threads, looking for help.
That certainly was some failure, though I don't see how leaving the power amplifier connected internally to the preamp is going to affect it if the load is disconnected - perhaps instability? 😕
The resistor is probably a half-watt metal film type, by size comparison with the T03P transistor. You could use either 1, 0.6 or 0.5W MF types for the drivers of most transistor single pair output stages. They are not as common as the ubiquitous quarter-watt MF types but major distributors like Farnell, CPC, RS, Rapid will have them. Don't try to buy small numbers of parts on the net/Ebay. You don't know what you will get and I have even seen production rejects passed off as good parts.
Unless you are concerned about aesthetics, you don't need to replace parts unless they show signs of stress.
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The 220 ohms will never normally see anything over 0.8 volts or so (they are across the B-E junction of the outputs) and so a 0.125 or 0.25 watt is fine.
You need to check the 0.47 ??? ohms that are in the collector circuit of each output.
Edit...the drivers are the two pairs of three paralleled devices. I would advise you power this up with a bulb tester after repair to save any disasters and blowing of devices.
You need to check the 0.47 ??? ohms that are in the collector circuit of each output.
Edit...the drivers are the two pairs of three paralleled devices. I would advise you power this up with a bulb tester after repair to save any disasters and blowing of devices.
Attachments
Thanks for posting the relevant part of the schematic, Mooly. I despair of ever seeing schematics for so many UK models that are no longer of much commercial interest.
That model alone would would have kept Zetex busy making all those little E-line transistors, though. What an odd approach to manufacturing economy, if that's what they were really aiming for.
That model alone would would have kept Zetex busy making all those little E-line transistors, though. What an odd approach to manufacturing economy, if that's what they were really aiming for.
Perhaps last week's application might have something to do with it. I was running a pair of ESL57's off the amp and they are notorious for blowing up amps with their high capacitance level. I'm so thankful that this didn't happen when they were plugged in! 😎
After finishing a long struggle with my Intek: Yes those are 220 ohm 0.6W metal film resistors. When any of the outputs fail, they usually take some - or all - drivers with them, + the emitter resistors. Check the small ZTX drivers (6/channel) clipped to the small U-shaped heatsink, also ALL emitter resistors, sometimes they fail without any visible mark. A warning about the ZTX series transistors: they are so called low saturation voltage types, which means they open at a lower E-B voltage, than common (BC, BD, 2SA, 2SC series)driver transistors. This means, that you can substitute them with any decent 100V 1A driver transistor, but then you will have to replace both NPN and PNP side devices, AND readjust idle current, which is not so easy, since it is done in the Intek with two fixed resistors plus a transistor (Q208, R209,R210 on the left channel). ((After my Intek killed a few woofers on various occasions, I finally added a TA7317 based protection circuit to it, that works for shorted outputs, DC on the outputs, +overload protection, excellent little circuit))
Good luck!
Good luck!
Thanks for all the help guys!
Neat idea dragonweed, you must like your Intek as well then! 😛 I was planning on getting myself a pair of Rogers to hang off it when I finally get fed up with my 3 way active system but after this I was a bit apprehensive. Hopefully I can recover mine and if it fails again set it up with a little protection circuit.
Neat idea dragonweed, you must like your Intek as well then! 😛 I was planning on getting myself a pair of Rogers to hang off it when I finally get fed up with my 3 way active system but after this I was a bit apprehensive. Hopefully I can recover mine and if it fails again set it up with a little protection circuit.
While I am at it--- Repairing and Intek can become a quite nightmarish experience, even for someone who does this for about 30+ years. Check the thermistor related circuit too, that can cause random muting sometimes, also see that the main transfo AC connecting 3 pole plugs are OK, because they can become loose, or even have burnt contact prongs- I replaced mine with similar sized PC solderable 3 pin screw terminals. Last but not least the 2 sided PC board's numerous interconnecting points can have bad, intermittent contacts, so check those too with a continuity checker on your multimeter. Other than that, after some substantial mods I like the sound very much, it is clean now, without the "Linn blur", have good authority, and much wider frequency response than the original......
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Your comparison hifi must be a lot better than mine, I thought it sounded great before anything happened to it! 😀 Thanks for all the advice.
I am not saying that it doesn't sound good in its original state, but after the mods, it is a LOT better. If you are interested I can tell what I did to mine.....🙂
That's great, I would be very interested to hear about some of your mods (my electronics skills are not very extensive but I'm keen to learn more)!
I have just checked the drivers and they all appear to be operating normally with a hFE of around 240 for the PNP's and around 190 for the NPN's, also the emitter resistors seem ok. Is there anything else I need to look out for before going ahead with the repair? I don't want to go and blow another pair of power transistors straight away, they aren't cheap!
I have just checked the drivers and they all appear to be operating normally with a hFE of around 240 for the PNP's and around 190 for the NPN's, also the emitter resistors seem ok. Is there anything else I need to look out for before going ahead with the repair? I don't want to go and blow another pair of power transistors straight away, they aren't cheap!
Well, there are a few more things you can do when you start up your amp, first check the idle current (all this without load on the speaker terminals), on the emitter resistors, it should read 55-60 mV between the two outer legs, the DC on the outputs should not be more than 10-20 mV. Let it run like this for a half an hour, then recheck, if they don't change much you are likely OK to try it with load. (If you have a scope, see if there is no HF oscillation on the terminals).
Some basic mods (the most "audible" ones): Replace the main rectifier diodes with ultrafast diodes -I used BYW 29-200 , but UF5404, or MUR 420 are good choices too. If you can't (or don't want to) replace the main filter caps (I did to 12.000 uF 56V ELNA Audio Blue types) then bypass them with 2.2-3.3uF 100V flat case film capacitors. Remove C206, C306, they simply short the output at high frequencies, replace C207 and 307 with 0.1uF film caps, also replace R217, and 317 with 4.7 ohm 2W metal oxide resistors.
To improve the effective bandwidth of the power stage (this does a lot to clarity and transient response) replace C204 and 304 (220uF) with 470-1000uF 16V low ESR high frequency type elctrolytics. Finally replace C201 and 301 (2.2nF) with 470pF styroflex, or silver-mica types.
The last thing is, to replace the venerable op-amp (NE 5532) with anything your heart desires -I used LM4562, definitely cleaner, faster than the original... (OPA 2604, OPA 2134 are OK too, but stay away from anything faster than those!)
Good luck with your amp,
Cheers: Andrew
Some basic mods (the most "audible" ones): Replace the main rectifier diodes with ultrafast diodes -I used BYW 29-200 , but UF5404, or MUR 420 are good choices too. If you can't (or don't want to) replace the main filter caps (I did to 12.000 uF 56V ELNA Audio Blue types) then bypass them with 2.2-3.3uF 100V flat case film capacitors. Remove C206, C306, they simply short the output at high frequencies, replace C207 and 307 with 0.1uF film caps, also replace R217, and 317 with 4.7 ohm 2W metal oxide resistors.
To improve the effective bandwidth of the power stage (this does a lot to clarity and transient response) replace C204 and 304 (220uF) with 470-1000uF 16V low ESR high frequency type elctrolytics. Finally replace C201 and 301 (2.2nF) with 470pF styroflex, or silver-mica types.
The last thing is, to replace the venerable op-amp (NE 5532) with anything your heart desires -I used LM4562, definitely cleaner, faster than the original... (OPA 2604, OPA 2134 are OK too, but stay away from anything faster than those!)
Good luck with your amp,
Cheers: Andrew
Very interesting. I've changed filter caps in a NAD before to very great effect actually, and I've never tried changing OP amps before, other than in my DAC and that made a huge difference. Where did you get the design for your protection circuit from?
It bugs me that there is no way to alter the bias without swapping fixed resistors. Have you considered a mod to put in a pot for easy adjustment? I don't have a scope but I do have a pair of car speakers that have seen high DC too many times so should be ok! 🙂
Thanks for all the advice, sounds like a lot of fun to be had.
Josh
It bugs me that there is no way to alter the bias without swapping fixed resistors. Have you considered a mod to put in a pot for easy adjustment? I don't have a scope but I do have a pair of car speakers that have seen high DC too many times so should be ok! 🙂
Thanks for all the advice, sounds like a lot of fun to be had.
Josh
I would recommend using a bulb tester to power the amp up initially. If you apply a short circuit across C and E of the vbe multiplier (Q288 ??? diagram is blurry, the one with the bias resistors) then that will force the quiescent current to zero. The amp should operate normally apart from slightly higher distortion. If all OK then remove the short and check the bias. The bulb should be very dim. If its bright you have a fault. Don't turn up the volume to high with a bulb fitted, the circuit could latch unpredictably.
You could replace replace the lower 200 ohm with a 470 ohm preset to give manual adjustment. Always begin with the resistor on highest eg 470 ohms to give minimum initial current.
And use a bulb tester at first but always do final checks and adjustments on full mains.
You could replace replace the lower 200 ohm with a 470 ohm preset to give manual adjustment. Always begin with the resistor on highest eg 470 ohms to give minimum initial current.
And use a bulb tester at first but always do final checks and adjustments on full mains.
It is Q208 and 308 respectively. Yes I forgot to mention that I've replaced the two lower 200 ohm resistors with a 500 ohm multiturn cermet trimmer.... If you google TA 7317 (or UPC 1237) you'll find dozens of applications for them in different amps.
Be careful with the bulb tester method on this amp, due to the thermal protection circuitry it can produce some strange behaviour sometimes, especially when pre and power stage supply voltages get far from their normal operational values (the power stage can latch, or worse: switch the negative PS voltage to the output- both variants happened to mine more than once....)
Be careful with the bulb tester method on this amp, due to the thermal protection circuitry it can produce some strange behaviour sometimes, especially when pre and power stage supply voltages get far from their normal operational values (the power stage can latch, or worse: switch the negative PS voltage to the output- both variants happened to mine more than once....)
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