The picture of the main power supply caps (the big red ones) answers the Voltage question: they are rated at 50V, so the supply is probably + and - 45V, or less. Either way, 100V types will be perfect for signal capacitors, 50V for supply caps.
The transformer pic shows what looks like a 1990 date code on the toroid. 20 years old! THe electrolytics wil be VERY stale by now.
I agree that one should not 'slaughter' the film caps, but the electrolytics will be sounding terrible.
The big red reservoir caps in the picture must be replaced with quality parts. They look like 10000uF 50V. Use a known-good sounding type. The minimum is the Kemet-BHC ALS series. These have a fine track-record in audio equipment.
Also, looking at that bridge rectifier (ordinary PN type) something should be done to prevent rectifier noise getting through. Building a schottky bridge out of MBR1060 diodes is easy enough, and you can then compare before and after!
The transformer pic shows what looks like a 1990 date code on the toroid. 20 years old! THe electrolytics wil be VERY stale by now.
I agree that one should not 'slaughter' the film caps, but the electrolytics will be sounding terrible.
The big red reservoir caps in the picture must be replaced with quality parts. They look like 10000uF 50V. Use a known-good sounding type. The minimum is the Kemet-BHC ALS series. These have a fine track-record in audio equipment.
Also, looking at that bridge rectifier (ordinary PN type) something should be done to prevent rectifier noise getting through. Building a schottky bridge out of MBR1060 diodes is easy enough, and you can then compare before and after!
Shotgunning a product like this changing many parts trying to find the bad one is really bad practice. The process of replacing parts can add more failure points. It was (is) clearly a very premium preamp from a very premium manufacturer. The Pass caps alone were very expensive (tin foil caps that were some of the first to use the design).
Its really important to find the source of the problem- described as a level difference combined with a rolloff on the low channel. Time with a generator and an oscilloscope would be indicated but lacing that do the detailed diagnosis with available resources.First reverse input channels and output channels to confirm its the preamp. Then check all line input pairs to see if the balance problem is in all of them. Thyen check line in to tape out to see if there is a balance issue there. Does the balance problem change with level?
With a meter check the supplies. The main supply caps are 50V. Good practice ois to riun them at 25V or so. The regulated supplies on the amp circuits should be around 15-20V. if you can figure out where those supplies are you will be able to check them on each channel.
I really doubt that there are electrolytic caps in the audio chain. There may be some in bias networks or possibly in a servo but using a $20+ Pass cap would be pointless if there are electrolytic coupling caps. The caps are very high grade and I suspect changing them will only change the color of the sound a little. My guess is that a transistor has failed or a connection is open.
While new caps may work better I have quite a few older instruments where the original caps perform as they did when new. Checking the ESR would be the only way to confirm the caps have degraded. I have even compared 30 year old Spectral preamps with old and new caps and not noticed a significant difference but for some it fees better to replace them (at the risk of new reliability problems).
Its really important to find the source of the problem- described as a level difference combined with a rolloff on the low channel. Time with a generator and an oscilloscope would be indicated but lacing that do the detailed diagnosis with available resources.First reverse input channels and output channels to confirm its the preamp. Then check all line input pairs to see if the balance problem is in all of them. Thyen check line in to tape out to see if there is a balance issue there. Does the balance problem change with level?
With a meter check the supplies. The main supply caps are 50V. Good practice ois to riun them at 25V or so. The regulated supplies on the amp circuits should be around 15-20V. if you can figure out where those supplies are you will be able to check them on each channel.
I really doubt that there are electrolytic caps in the audio chain. There may be some in bias networks or possibly in a servo but using a $20+ Pass cap would be pointless if there are electrolytic coupling caps. The caps are very high grade and I suspect changing them will only change the color of the sound a little. My guess is that a transistor has failed or a connection is open.
While new caps may work better I have quite a few older instruments where the original caps perform as they did when new. Checking the ESR would be the only way to confirm the caps have degraded. I have even compared 30 year old Spectral preamps with old and new caps and not noticed a significant difference but for some it fees better to replace them (at the risk of new reliability problems).
That's usually true, but where electrolytics in amplifiers are concerned - change 'em wholesale. Once you have tried this on a few old amps, even at 10 years, you will know what I mean.Shotgunning a product like this changing many parts trying to find the bad one is really bad practice.
I really doubt that there are electrolytic caps in the audio chain.
The local decoupling caps in the supply to each stage. They will be electrolytics, and are in the signal path (where does the signal current come from)?
While new caps may work better I have quite a few older instruments where the original caps perform as they did when new. Checking the ESR would be the only way to confirm the caps have degraded. I have even compared 30 year old Spectral preamps with old and new caps and not noticed a significant difference but for some it fees better to replace them (at the risk of new reliability problems).
This is always the part that folks have trouble believing. But, if you take out a lot of old electrolytics from an amp, measure their ESR and see that it is fine, but COMPARE the sound to new caps, you will forget about measurements like this. The caps go stale-sounding. I have recapped dozens of amps for customers, are they are always pleased with the result. Closer to this forum, see what a DIYer said about my recommendation for BHC in the main reservoir position:
http://www.diyaudio.com/forums/tubes-valves/164286-replace-bypass-cap.html#post2147456
Sure, changing caps can make a major improvement in an amp, depending on the state of the caps prior. However its not a good way to troubleshoot a balance problem. Also changing a cap does several things and you can't be sure that its changing the cap or the process of resoldering the joint that made a difference. Cap technology has been improving steadily and newer caps will perform better (and last much longer) than older caps. However not all circuits will respond the same and in some cases an instability that was masked by the esr/esl of a cap will surface when a different cap with different parasitics is substituted. John Curl told me about a problem like this that lead to high levels of emotional reaction with colleagues over cap performance.
In the US, persons giving advice on the internet can be sued for negligence. I think it is highly unlikely a transistor preamp has 400 V in it, but he needs to measure it before he touches it to make sure, to follow the cap discharge procedures in the tube forum sticky thread. Even if the power supply is 100V, that can stop the heart if the skin is broken, and is outside the range of the "baby on electric train track" safety exclusion on 24 V. Re-engineering from another hemisphere is difficult, but the advice stands- caps should be rated for the power supply voltage if lower than installed, if they are available in that lower voltage. Unless a proper circuit analysis is made.I'm afraid you have rather missed the point here!
They are 400V rating NOT because that is necessary, but that is (was) all there is in that value. Very often true for small films from the age of tubes.
The OP should get a generator and 'scope and MEASURE the difference between the channels rather than the wholesale slaughter of all the caps!
As far as shotgunning the electrolytic caps, my last un-answered question was how old this unit is. If over twenty years, or even 10 years if the electrolytic caps are cheap, he can save resources by buying a scops and finding out exactly what component is bad (less the resources in the scope, of course). However, time is not cheap, and if the caps are twenty years old, another one will go bad next year and he can do it all again. If he intends to auction the item off on E-bay, fix the one and let the next ninny suffer. If he intends to keep it, and he called it a "beloved" preamp, he should replace all the slectrolytics in one shipment, two at a time for debug facilitation if there is a problem.
I do not have the schematic.
Here is the schematic.
Attachments
🙂 thanks, glen. i am in the process of building the b1 buffer to temporary replace my dr6. there's something wrong with my b1 power supply at the moment. then i'll replace the capacitors of my dr6. the dr6 psu caps looks good and they are expensive to replace, maybe i'll leave them alone.
The schematic helps a lot. The first thing I noticed were pots on the input of each line stage for offset trim. They are pots that return to ground and reduce the input resistance to "offset" the input bias current. If taken too far they will also short out the input signal causing the symptoms you described.
A few minutes with a scope should make everything clear.
A few minutes with a scope should make everything clear.
I agree with doing the big Power Supply caps last. They are comparatively expensive to the little electrolytics on the signal PCB, and would probably cause hum if they were bad, which you are not complaining about. Due to the huge ratio of volume to iffy rubber seal on 5000 uf and larger caps, maybe they don't dry out as badly as small ones. I'm not finding as much deterioration on the huge caps I have, even the ones I did replace. However, big power transformers can force a big explosion in a PS cap if it does dry out and develop a hot spot. Explosions are more likely on a power amp than a preamp, due to the weaker transformer. Maybe it would be cheaper to fuse the input of the PS caps than replace them. Good luck. I replaced my 20 year old power amp PS caps, also the preamps PS caps, for that reason, but the old huge ones measure okay, I kept them, and I use them on little experiments etc.🙂 i'll replace the capacitors of my dr6. the dr6 psu caps looks good and they are expensive to replace, maybe i'll leave them alone.
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This thread seems truncated. Was the OP hit by a bus? What was the end result?
Anyway, I have a Classe Audio Six and I was having problems with the left channel going soft and a slight hum on the right. Both problems were cured by aggressive rotation of the volume control over the full range that I normally adjust the volume -- i.e. from 9 o'clock to 1 o'clock. I did this action with the input selector on CD (where I had no player connected) and without muting (so that I could hear the scratchiness slowly fade away).
I am certain the problem will return, because I have had to perform this procedure a few times. (I have owned this unit since 1995.) The pots are sealed, so a proper cleaning would be quite an effort.
Anyway, I have a Classe Audio Six and I was having problems with the left channel going soft and a slight hum on the right. Both problems were cured by aggressive rotation of the volume control over the full range that I normally adjust the volume -- i.e. from 9 o'clock to 1 o'clock. I did this action with the input selector on CD (where I had no player connected) and without muting (so that I could hear the scratchiness slowly fade away).
I am certain the problem will return, because I have had to perform this procedure a few times. (I have owned this unit since 1995.) The pots are sealed, so a proper cleaning would be quite an effort.
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