Oh I misread your post. I read the output is AC coupled but you wrote input. Indeed it makes no sense. Probably some low amplitude inaudible hiss that alters DC readings?
I will measure XLR output from the preamp anyway.
I will measure XLR output from the preamp anyway.
No, input was correct.
Both inputs from your 23.5 are AC coupled, so it is strange that the DC output of the inverting amp changes, although it is an insignificant amount.
Hans
Both inputs from your 23.5 are AC coupled, so it is strange that the DC output of the inverting amp changes, although it is an insignificant amount.
Hans
Ok got it, it was mostly a curiosity. Plus, with the amp still open (no air circulation), and having to switch the amp off each time I connect or disconnect the preamp, it is also possible I'm measuring with the amp not properly warm up and I get different readings.
Rechecked carufully and now the offset seems to be ok. It was probably just a warm up issue then. Sorry for bothering you.
An out of topic question, I just recapped the entire preamp (ML28) eith Panasonic FCs, with excellent sonic results except for when I replaced the main filter caps (four pole Sprague 88D 6800uF) with Mundorf Mlytic. it still sounds fantastic but the bass is gone. Are regular two pole caps not an adequate replacement or are just the Mundorf that are bad in your opinion? I can always put back the original Spragues but I would also like to try other caps like Nichicon KG. Do you have any ideas?
Thanks
An out of topic question, I just recapped the entire preamp (ML28) eith Panasonic FCs, with excellent sonic results except for when I replaced the main filter caps (four pole Sprague 88D 6800uF) with Mundorf Mlytic. it still sounds fantastic but the bass is gone. Are regular two pole caps not an adequate replacement or are just the Mundorf that are bad in your opinion? I can always put back the original Spragues but I would also like to try other caps like Nichicon KG. Do you have any ideas?
Thanks
Sorry I can't help you with this.
I don't believe that much in those fancy boutique audio caps that are quite expensive.
All I know is that those Spraques are serious caps, so if the aren't broken, just leave them in.
Hans
I don't believe that much in those fancy boutique audio caps that are quite expensive.
All I know is that those Spraques are serious caps, so if the aren't broken, just leave them in.
Hans
I think I agree with you. I don’t have much experience so I’m experimenting. I came to the same conclusion. However putting the Panasonic in place of some generic rubycon that someone used to replace the original in the phono and line PCBs made a huge improvement overall.
Hi Hans, the 23.5 worked great in the past couple of weeks, but yesterday I heard a pop from the left loudspeaker a few minutes after switching it on. Then it worked well and stable. But today in the first few minutes after switching on (amp not playing) I heard three pops that were much louder and I therefore switched it off not to damage my speaker.
I hope it could be the main filter caps occasionally discharging, since they are the only electrolytically I didn’t replace yet, but if it’s some cold joint or failing transistor I don’t think I would have the time and energy to undergo a repair attemp.
As a further detail, from this channel I recently starte to hear very quiet pops after prolonged use, before I underwent the restoration.
During restoration I inspected all solder joints and those that looked bad i resoldered. I also inspected all the joints I did when replacing components.
Additionally, as you know, I replaced all the transistors in the inverting input section with original NOS transistors. These looked original to me but is they were fake and failed soon I couldn’t know.
P.s. it’s not the preamp since I tried inverting the interconnects and the pops are always on the left channel
I hope it could be the main filter caps occasionally discharging, since they are the only electrolytically I didn’t replace yet, but if it’s some cold joint or failing transistor I don’t think I would have the time and energy to undergo a repair attemp.
As a further detail, from this channel I recently starte to hear very quiet pops after prolonged use, before I underwent the restoration.
During restoration I inspected all solder joints and those that looked bad i resoldered. I also inspected all the joints I did when replacing components.
Additionally, as you know, I replaced all the transistors in the inverting input section with original NOS transistors. These looked original to me but is they were fake and failed soon I couldn’t know.
P.s. it’s not the preamp since I tried inverting the interconnects and the pops are always on the left channel
Hi Stefan,
Sorry to hear you are still having trouble.
What you experience can be anything that can die, transistors, caps or whatever.
The times I had such problems, it turned out to be Wima polyester caps in the Vreg part on the top PCB causing these pops.
The problem though is that it is very difficult to find when popping happens only now and then.
I can set my scope on triggering on a pulse and storing one event. That way I can find my way back to find the cause.
Without it will be difficult.
What about the TO3 transistor that measured so strange, was it indeed your multimeter or is the part not o.k. after all ?
But to start with, you could detach the output of the inverting amp to make sure the pop is not coming from this part, but you better use a cheap LS.
this can be done by removing one side of the 604R//100K resistors and connecting these two to gnd, see image.
When this doesn't stop the popping it excludes this part from the list.
I can probably give you some other tests, but try this one first.
Hans
Sorry to hear you are still having trouble.
What you experience can be anything that can die, transistors, caps or whatever.
The times I had such problems, it turned out to be Wima polyester caps in the Vreg part on the top PCB causing these pops.
The problem though is that it is very difficult to find when popping happens only now and then.
I can set my scope on triggering on a pulse and storing one event. That way I can find my way back to find the cause.
Without it will be difficult.
What about the TO3 transistor that measured so strange, was it indeed your multimeter or is the part not o.k. after all ?
But to start with, you could detach the output of the inverting amp to make sure the pop is not coming from this part, but you better use a cheap LS.
this can be done by removing one side of the 604R//100K resistors and connecting these two to gnd, see image.
When this doesn't stop the popping it excludes this part from the list.
I can probably give you some other tests, but try this one first.
Hans
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Thanks for the support Hans. I will try the test in the next days, unfortunately I’m this period I’m very busy at work and have limited time. Also the pops are so sporadic that it will take time for each test to be sure I’m not missing the pop. First I will reopen the amp and connect it to a cheap speaker to see better the frequency and timing of the events. It seems to me it happens during the first 10-20 minutes after switch on from cold but when warmed up I didn’t hear anything so far.
The TO3 measured well and stable with the new meter so I guessed it was ok and left it there, but if it acts strange so rarely I could have missed it anyways. Unfortunately I have an analog scope so I cannot store events. Today I tried to look at the scope for a while after switch on with no speaker connected. I tried tapping the boards and a few components but noticed nothing. I then tapped the heat sinks on the positive side and saw the line on the scope rise way out of scale and then set back, but i saw no spikes and it didn’t repeat again by keeping tapping. So it could have been a coincidence.
The TO3 measured well and stable with the new meter so I guessed it was ok and left it there, but if it acts strange so rarely I could have missed it anyways. Unfortunately I have an analog scope so I cannot store events. Today I tried to look at the scope for a while after switch on with no speaker connected. I tried tapping the boards and a few components but noticed nothing. I then tapped the heat sinks on the positive side and saw the line on the scope rise way out of scale and then set back, but i saw no spikes and it didn’t repeat again by keeping tapping. So it could have been a coincidence.
Ok, good news. I’m definitely able to trigger the pops by tapping on the heat sinks of the left channel during warm up, but only for a few times then it no longer pops. However it could still be anything since the amp is closed and everything is connected so the vibrations for the tapping could be spread to both boards. Will try to better isolate the area. Tapping components with a spider is not enough to trigger the pops. With no tapping I didn’t hear any pops for hours.
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Hi Hans, I think I'm close to find the cause. I did some further inspection of the solder joints and boards and noticed that when closing the left amp, the black Vprereg- cable got stuck between the main filter cap and one of the Wima caps of the upper board (attached picture, by the way, is it one of those you were mentioning in your post?). The Wima capacitor is consequently slightly tilted towards the left, stressing the solder joints. By tapping with a sponge on the cable and/or the Wima capacitor I was able to trigger quite a few pops. So silly to have not noticed that the cable was stuck during reassembly, but that would be a great news and fixing it would only require removing (for the third time) the upper board.
p.s. the plastic case of the Wima gor melted with the shaft of the solder iron while desoldering a nearby pin for the last time, but I'm quite sure it is fine.
p.p.s. Actually I’m not sure the Wima is fine, I have an exact replacement so I will change it just in case
p.s. the plastic case of the Wima gor melted with the shaft of the solder iron while desoldering a nearby pin for the last time, but I'm quite sure it is fine.
p.p.s. Actually I’m not sure the Wima is fine, I have an exact replacement so I will change it just in case
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I’m glad you found a possible cause, but what nightmare to have this upper board removed again and again.
You have obviously the proper tools to get this done without causing any damage to the PCB traces, that’s great.
And replacing this Wima cap is a good idea, but also closely inspect the traces around to be sure there are no hairline cracks caused by mechanical stress.
Hans
P.s. You see the little green cap next to the red Wima ?
That’s the one I had to replace so you better replace this one too, out of precaution.
You have obviously the proper tools to get this done without causing any damage to the PCB traces, that’s great.
And replacing this Wima cap is a good idea, but also closely inspect the traces around to be sure there are no hairline cracks caused by mechanical stress.
Hans
P.s. You see the little green cap next to the red Wima ?
That’s the one I had to replace so you better replace this one too, out of precaution.
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I removed the board. Honestly the joints and traces look fine under magnifying glass, was hoping to see some cracked solder joints but no. Ok thanks, I will replace the green one as well, since the tilted red Wima was touching the green one it could also be the problem was the green one actually. Thanks for the tip.
Stefano,
Sounds like you may be narrowing things down. As I suggested in my offline conversation with you, a wood chopstick is very useful. It can be used very specifically on small components, which is more difficult with something large and soft like a sponge. In cases where only the bottom of the board is available it can be used on each solder joint.
That melted corner is not likely deep enough to be a problem but it could have a cracked lead beneath the potting causing the popping so it should get replaced as well as the green one suggested by Hans.
Sounds like you may be narrowing things down. As I suggested in my offline conversation with you, a wood chopstick is very useful. It can be used very specifically on small components, which is more difficult with something large and soft like a sponge. In cases where only the bottom of the board is available it can be used on each solder joint.
That melted corner is not likely deep enough to be a problem but it could have a cracked lead beneath the potting causing the popping so it should get replaced as well as the green one suggested by Hans.
Greg, thanks again for the advices. I actually meant I used a plastic spudger not sponge, the phone automatically corrected it.
Hans, regarding the green Wima, I could replace it with a Wima PP FKP2 capacitor, You mentioned the green ones are polyester but I guess it’s the same right?
How did you diagnose the problem with the Wima capacitor? Did it fail completely? Mine measures ok for capacitance.
Hans, regarding the green Wima, I could replace it with a Wima PP FKP2 capacitor, You mentioned the green ones are polyester but I guess it’s the same right?
How did you diagnose the problem with the Wima capacitor? Did it fail completely? Mine measures ok for capacitance.
Yes, FKP2 is o.k.
Popping was as you described but at very irregular and long intermediate intervals, but then finally it collapsed into a short circuit after a week in permanent use playing a 1Khz square wave.
After replacing this cap, the popping never returned.
Hans
Interesting, so you left it playing continuously to accelerate the failure and be able to diagnose it?
Anyway, I did what I had to do, replaced both caps, check all solder joints and traces under a digital microscope (everything looked good) and reassembled the channel. Now the amp is switched on and still no pops even after tapping on the caps in that area and on the heat sinks. But I will wait a few days or a week before closing the chassis, is no pops occur (fingers crossed) I could h consider its fixed…
Anyway, I did what I had to do, replaced both caps, check all solder joints and traces under a digital microscope (everything looked good) and reassembled the channel. Now the amp is switched on and still no pops even after tapping on the caps in that area and on the heat sinks. But I will wait a few days or a week before closing the chassis, is no pops occur (fingers crossed) I could h consider its fixed…
