Hello Guys,
My Apt Holman preamplifier has lost left channel phono input. So I'll have to open it to solve this problem.
In the mean time I've read a lot about this device and compiled some information in order to repair it and improve it a bit if I can.
I therefore have a lot of questions and i would be delighted and thankful to get answer from here.
1) the FET input transistors of the phono preamp are selected for low noise. how do you do that ? What kind of equipment you need to do this ?
2) As the fault is in the phono stage, I've searched for availability of parts. The 2N5484 seems to have vanished and it seems there is an SMD replacement available (MMBF5484). The 2N5458 seems to be available and one of the usual suspects (Farnell) suggest J109 as a replacement, but Mouser seems to have the correct original one. What do you think of that ?
3) on one thread, someone suggest to replace the power regulators which seems to age bad. Should I use the same reference or replace them with better parts ?
4) The original Op Amp are (as far as i can tell) fine. Should I leave them be or replace them ? And if i replace them, is it clever to put sockets for them ?
And last but not least, the power supply. In France we have switched to 240V for mains. The transformer is made for 220 V. Should I find a replacement to have the voltage correct (my mains is at 237V and sometimes above 240 as I'm near the substation) ? If I should change the transformer, what should I look for ?
As you see, a lot of questions, and I thank you to have read them all !
Thank you in advance for your answers !
My Apt Holman preamplifier has lost left channel phono input. So I'll have to open it to solve this problem.
In the mean time I've read a lot about this device and compiled some information in order to repair it and improve it a bit if I can.
I therefore have a lot of questions and i would be delighted and thankful to get answer from here.
1) the FET input transistors of the phono preamp are selected for low noise. how do you do that ? What kind of equipment you need to do this ?
2) As the fault is in the phono stage, I've searched for availability of parts. The 2N5484 seems to have vanished and it seems there is an SMD replacement available (MMBF5484). The 2N5458 seems to be available and one of the usual suspects (Farnell) suggest J109 as a replacement, but Mouser seems to have the correct original one. What do you think of that ?
3) on one thread, someone suggest to replace the power regulators which seems to age bad. Should I use the same reference or replace them with better parts ?
4) The original Op Amp are (as far as i can tell) fine. Should I leave them be or replace them ? And if i replace them, is it clever to put sockets for them ?
And last but not least, the power supply. In France we have switched to 240V for mains. The transformer is made for 220 V. Should I find a replacement to have the voltage correct (my mains is at 237V and sometimes above 240 as I'm near the substation) ? If I should change the transformer, what should I look for ?
As you see, a lot of questions, and I thank you to have read them all !
Thank you in advance for your answers !
As a UK resident you do know the "British company " Farnell ( or its derivative names ) was bought over by US giant --AVNET --don't you ?
Farnell has a big base in China (among others abroad) .
This was sad news to me as the original company was well know in the UK and I have bought equipment from them in the past.
Mouser ( yes I have bought from them direct via the USA complete with Homeland Security technology check certificate )-
owned by TTI.Inc which itself is owned by ---yes Berkshire Hathaway -well know to me an American Holding company - Warren Buffet ring any bells ?
Run on purely profit maximization so is one better than the other in all honesty ?
Farnell has a big base in China (among others abroad) .
This was sad news to me as the original company was well know in the UK and I have bought equipment from them in the past.
Mouser ( yes I have bought from them direct via the USA complete with Homeland Security technology check certificate )-
owned by TTI.Inc which itself is owned by ---yes Berkshire Hathaway -well know to me an American Holding company - Warren Buffet ring any bells ?
Run on purely profit maximization so is one better than the other in all honesty ?
1) Ears.
Measure the current AC through the primary winding of the transformer.
Rate the heat. Don't worry Be Happy..
Measure the current AC through the primary winding of the transformer.
Rate the heat. Don't worry Be Happy..
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I would buy a bunch of FETs and put in a socket. That will allow you to pick the best one by measuring noise level with a resistor as the input, the volume set to maximum and a true RMS meter. I suspect using a 47K resistor will actually be too high a value, but you can compare the good channel with the bad one. You might also try a 100 ohm resistor.
I always add sockets when replacing ICs.
To handle the line voltage I would use a buck transformer. Get a 240 to 36 volt transformer with an output current of one amp or so. As the preamp only draws under an amp that current should be fine. Now connect the transformer's 36 volt secondary in series with the AC power line. Measure the voltage. It should be either 276 or 224 volts. If it is 276 volts reverse the buck transformer's primary connections.
If you want to be fussy use a split secondary and use one winding in series with each AC line side.
Best way to trouble shoot the preamp is to inject signal and follow the signal path with an oscilloscope. Since you probably don't have one you can use earphones. Be sure to place a capacitor in series with the earphone. Even .1 microfarad will do as all you want to do is listen for any signal not full range. The capacitor is needed to prevent DC from damaging anything.
In my Apt-Holman preamp I had one of the opamp buffers fail. Quick fix was to just jump it out of the circuit path. I would not change any of the original ICs unless they failed.
As to replacing the regulators, just check to be sure any machine screws attaching the heatsink are tight. The internet suggestion to replace them is highly suspect as is any internet advice! 😉
I always add sockets when replacing ICs.
To handle the line voltage I would use a buck transformer. Get a 240 to 36 volt transformer with an output current of one amp or so. As the preamp only draws under an amp that current should be fine. Now connect the transformer's 36 volt secondary in series with the AC power line. Measure the voltage. It should be either 276 or 224 volts. If it is 276 volts reverse the buck transformer's primary connections.
If you want to be fussy use a split secondary and use one winding in series with each AC line side.
Best way to trouble shoot the preamp is to inject signal and follow the signal path with an oscilloscope. Since you probably don't have one you can use earphones. Be sure to place a capacitor in series with the earphone. Even .1 microfarad will do as all you want to do is listen for any signal not full range. The capacitor is needed to prevent DC from damaging anything.
In my Apt-Holman preamp I had one of the opamp buffers fail. Quick fix was to just jump it out of the circuit path. I would not change any of the original ICs unless they failed.
As to replacing the regulators, just check to be sure any machine screws attaching the heatsink are tight. The internet suggestion to replace them is highly suspect as is any internet advice! 😉
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Thank you all for the answers.
Actually, as I sometimes do old tube radio repair, I've a HF generator a Tek 2235 scope and a variac. So all I miss is the true RMS voltmeter but I own a Chinese millivoltmeter of unknown accuracy and value....
For now, I gather ideas and list of parts to buy because next Sunday I leave for a couple of weeks so I won't touch it until my return.
The idea of sockets for the FET is a good one but i need some reference for correct parts.
Buck transformer is a good idea but I know a transformer maker of very high and good quality (if not cheap) So I can ask him to wind a correct one for me... Will it be a good idea to go toroid ?
I won't touch any of the op amps unless they have failed. i do not like tinkering with a good design with no flaws. I'm not an audiophile. I do not listen to the equipment, i listen to the music. (This preamp was my elderly brother second preamp. He was big into audio equipment and had a huge HiFi equipment. So I know what is audiophile ;-) )
Actually, as I sometimes do old tube radio repair, I've a HF generator a Tek 2235 scope and a variac. So all I miss is the true RMS voltmeter but I own a Chinese millivoltmeter of unknown accuracy and value....
For now, I gather ideas and list of parts to buy because next Sunday I leave for a couple of weeks so I won't touch it until my return.
The idea of sockets for the FET is a good one but i need some reference for correct parts.
Buck transformer is a good idea but I know a transformer maker of very high and good quality (if not cheap) So I can ask him to wind a correct one for me... Will it be a good idea to go toroid ?
I won't touch any of the op amps unless they have failed. i do not like tinkering with a good design with no flaws. I'm not an audiophile. I do not listen to the equipment, i listen to the music. (This preamp was my elderly brother second preamp. He was big into audio equipment and had a huge HiFi equipment. So I know what is audiophile ;-) )
Left channel works thru Aux inputs? Are you sure it's not the Input Select switch (others have reported this at fault)?
If you read the schematic, it says "2N5484 OR 2N5458".
Are you sure the FET has failed? The 2SC1345E transistors are known to go bad with age. It's far more likely that one of these is faulty than the input FET. I had to replace 2 of these in my Apt line section.
Before randomly replacing parts, to isolate the fault, you should at least check voltages at the test points indicated in the manual.
Measure the output voltages. If w/in spec, not much to gain by replacing.
If both channels work fine thru Aux input, then opamps are good (phono stage has no opamps).
Yes.
NOT needed. You should read EVERY paragraph of the manual:
"2.3 POWER REQUIREMENTS
2.3.1 Input line voltage is 120 Vac nominal, 240 Vac with internal wiring change, 50-60Hz.
The range of normal operation for the 120 Vac connection is 95-135 Vac (190-270 Vac connected for 240 Vac operation)."
I don't see any reason for a preamp to use a toroidal transformer other than to reduce magnetic flux leakage into the rest of the circuitry. If that is not an issue right now, then changing to one might introduce other problems such as the wider bandwidth introducing noise.
Since you have test gear, check the DC voltage before the regulators. High line AC line voltage might show a problem here but I suspect it will be fine as is.
In the U.S. the 220 volt AC line is often 240 volts but may go as high as 264 volts. So before replacing things checking it out may save you effort.
As to what FETs to try, you might want to find a surface mount to lead adapter to try the surface mount ones along with the others you mentioned
Since you have test gear, check the DC voltage before the regulators. High line AC line voltage might show a problem here but I suspect it will be fine as is.
In the U.S. the 220 volt AC line is often 240 volts but may go as high as 264 volts. So before replacing things checking it out may save you effort.
As to what FETs to try, you might want to find a surface mount to lead adapter to try the surface mount ones along with the others you mentioned
Hello Sean,
For now, i've not opened the preamp. i gather a list of what to do and try to figure out the amount of work involved and the ease or ability to find replacement parts. I've to buy parts online as most component selling shops are now gone in France, so I try to devise a list to place only one order. Order which will be placed when the fault is found. (early august, if I can work on it when I get back home).
If I had to replace the 2SC1345E, should I replace the other channel also or wait for it to fail ?
As per the mains voltage, I've read somewhere that you have to contact the factory for 240V operation, so I assumed the transformer was made for only 220V, but the device says 240V mains on mine ! And Google pointed me to people having had problems with the original transformer...
So one thing not to test/change.
i will of course check both transformer voltages and rectified voltages and regulated voltages. I plan to change the filtration caps also (I had to change one cap in the phono section which had leaked), so this time I'll be more thorough ...
Thanks a lot for your advice and help !
Only the phono left channel is silent. I routinely use the tuner, and Aux1& 2 without problem. i've switched the RCA connectors from right to left and the phono cart and cables are OK. (left channel plays when plugged on right input). I've also tried phono2 input and, again, left channel is silent. So i guess this rules input selector out ?
For now, i've not opened the preamp. i gather a list of what to do and try to figure out the amount of work involved and the ease or ability to find replacement parts. I've to buy parts online as most component selling shops are now gone in France, so I try to devise a list to place only one order. Order which will be placed when the fault is found. (early august, if I can work on it when I get back home).
If I had to replace the 2SC1345E, should I replace the other channel also or wait for it to fail ?
As per the mains voltage, I've read somewhere that you have to contact the factory for 240V operation, so I assumed the transformer was made for only 220V, but the device says 240V mains on mine ! And Google pointed me to people having had problems with the original transformer...
So one thing not to test/change.
i will of course check both transformer voltages and rectified voltages and regulated voltages. I plan to change the filtration caps also (I had to change one cap in the phono section which had leaked), so this time I'll be more thorough ...
Thanks a lot for your advice and help !
Probably, but not necessarily. Check for continuity across the switch phono contacts. If no continuity, try cleaning/deoxidizing the switch contacts. Cheapest & easiest solution.
I don't replace unless it's suspect. In my case, Q115 had failed short, putting -16v on the opamp. I replaced its pair Q114 as well, just in case it had been overstressed. The 2SC1345E is NLA obsolete. KSC1845 is the recommended replacement.
That is what's written on the schematic, but see para. 9.4 in the manual: "240Vac connection".
The transformer is dual-primary, can be wired for either 120 or 240v. It's a simple change, though it can be a bit tight to get at with a soldering iron.
Earlier you had said "For now, i've not opened the preamp", but you have been inside & worked on it before?
There's only 4 electrolytics in the phono section:
2 are used for +/- power regulation
1 is output coupling cap
1 is RIAA ground reference
1 is output coupling cap
1 is RIAA ground reference
Which one did you replace? and Left channel by any chance?
Not necessarily.
The switch as a whole may work fine but an unused pair of contacts, the phono ones, can easily be covered in grime , oxidation, you name it. A very thin layer of course, but a problem at such low voltage levels.
Other contacts self-clean because of normal use friction.
A product such as Deoxit can work miracles.
Try to avoid pulling parts for "testing" unless really non working, messing with parts introduces more problems.
Sean,
This preamp had a fault some time ago.
You helped me sort it out. See here :
Apt Holman preamp problem
And yes, it was also the left channel.
At that time I did not touch anything else, not having the parts to change the capacitors or anything else.
As per today's fault, no, I have not opened the preamplifier so I can't tell what the fault is ;-)
JMFahey, The phono input has plenty of use as I listen a lot to my records. And I switch often from phono to tuner so I hope the mechanical movement cleans the grime on the contacts. But I'll do some cleaning in the repair procedure. Thank you.
This preamp had a fault some time ago.
You helped me sort it out. See here :
Apt Holman preamp problem
And yes, it was also the left channel.
At that time I did not touch anything else, not having the parts to change the capacitors or anything else.
As per today's fault, no, I have not opened the preamplifier so I can't tell what the fault is ;-)
JMFahey, The phono input has plenty of use as I listen a lot to my records. And I switch often from phono to tuner so I hope the mechanical movement cleans the grime on the contacts. But I'll do some cleaning in the repair procedure. Thank you.
I've seen black oxidation on switch contacts that required mechanical scraping to remove.
Just exercising the switch, even with cleaner, may not do much.
Just exercising the switch, even with cleaner, may not do much.
Dada Electronics in the UK sells upgrade kits for Quad equipment. They replace the old TL071-TL072 opamps with OPA604 or LME49710 or OPA2604 or LME49720 opamps. This doesn't help the phono preamp circuit, but I thought you might be interested in an opamp upgrade.
Georges,
just finished rebuilding my recent APTquisition. One channel was out but it was due to a wiring problem someone else did when doing a mod. Put everything back to "stock" to start.
Did find two bad electrolytics - one coupling in the mode circuit. Found it by doing frequency sweeps and noting that the results were far from flat. May not have noticed it or spent a lot of time troubleshooting by ear. When I removed the bad caps, you could see some electrolyte seeping out the bottom which wasn't noticeable when looking at the cap in place. Based on this, I'd suggest you replace ALL the caps especially in the signal path. Since I had enough newer dual op amps and the time, I replaced them also.
The unit sounds great and in phono has NO hum, just noise when the volume is turned all the way up. Regulators seem fine - but replaced the filter caps also. The RIAA accuracy (via an inverse network) was as good as I've seen.
No switch or mute relay problems - but this was a later unit - PC board says "Rev 7"
Love the functionality! Imagine the mode control would work really great with mono records.
Charles
just finished rebuilding my recent APTquisition. One channel was out but it was due to a wiring problem someone else did when doing a mod. Put everything back to "stock" to start.
Did find two bad electrolytics - one coupling in the mode circuit. Found it by doing frequency sweeps and noting that the results were far from flat. May not have noticed it or spent a lot of time troubleshooting by ear. When I removed the bad caps, you could see some electrolyte seeping out the bottom which wasn't noticeable when looking at the cap in place. Based on this, I'd suggest you replace ALL the caps especially in the signal path. Since I had enough newer dual op amps and the time, I replaced them also.
The unit sounds great and in phono has NO hum, just noise when the volume is turned all the way up. Regulators seem fine - but replaced the filter caps also. The RIAA accuracy (via an inverse network) was as good as I've seen.
No switch or mute relay problems - but this was a later unit - PC board says "Rev 7"
Love the functionality! Imagine the mode control would work really great with mono records.
Charles
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OK, I recall it now. I didn't partake in that discussion, as I didn't see it until after you had it all fixed in post #17, where you had replaced C13 & C17.
So are you saying it has been working fine for 8 months, then the left channel suddenly quit?
BTW, one of the voltage charts in the Apt manual has incorrect information:
Yes. I enjoyed it since. Then, one morning, in a middle of a record, the left phono channel went out.
As I had put a ferrite bead (*) around the phono cables to kill the huge FM radio station polluting my hearing in pianissimo, I thought the weight had made one RCA disconnect. So I checked them. No joy. I swapped Right for Left and still sound on right channel. i put the RCA on Phono 2 input. Still right channel only.
So I suspect a fault in the phono preamp ;-)
*: see here : Apt Holman preamp : hearing an FM station on phono input
As I had put a ferrite bead (*) around the phono cables to kill the huge FM radio station polluting my hearing in pianissimo, I thought the weight had made one RCA disconnect. So I checked them. No joy. I swapped Right for Left and still sound on right channel. i put the RCA on Phono 2 input. Still right channel only.
So I suspect a fault in the phono preamp ;-)
*: see here : Apt Holman preamp : hearing an FM station on phono input
As you have two channels the normal troubleshooting technique is to take voltage and signal readings from the same point on both. The stage at which they differ is where you look for the problem. Not quite as simple as it sounds due the the way circuits compound.
OK. I thank you all for all your advices and lead. I will be gone until the 7 of August, so the Apt will stay put on it's shelf for now.
But, I'll report back when i have opened it and share my findings. And this time, I'll change the electrolytic I did not change last time i was in.
Have a nice summer and enjoy your day !
But, I'll report back when i have opened it and share my findings. And this time, I'll change the electrolytic I did not change last time i was in.
Have a nice summer and enjoy your day !
IMHO, that symptom points to a 2SC1345 transistor failure. Similar to my failure: one day it worked fine, next day one channel was dead from power-on.
simon7000s advice is excellent. Compare voltages to right channel, particularly at BCE of each 2SC1345.
This Spice sim drawing may help. You won't get these exact voltages, because they depend on regulated supply differential and specific device model parameters (had to use KSC1845 models since there are none for 2SC1345). Use the voltages measured from right channel.
Point is, the 3 BCE voltages should be different. If any 2 are exactly the same, that transistor has failed.
Nice sounding pre-amp I bought one of these an month ago to go along with my hafler dh-220 (with infinity Qb speakers) and recapped it with nichicon FG sourced from Hifi Collective. There were a few electrolytic caps with very high ESR. So did the lot of them, took about 3 hours in total, and cost me under 30 euro. Also converted mine from 120v to 240V simple enough to do as I did not want to run an converter.
I usually use KSC1845, A992, 2SA1015, 2SC1815 in most repairs and never had any issue with either of them.
I usually use KSC1845, A992, 2SA1015, 2SC1815 in most repairs and never had any issue with either of them.
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