Hahah thanks Hans. But I would be more cautious and wait at least a few days before celebrating.
Funny enough I found a pre-restoration picture where the black cable was in the same exact position, so if the problem was caused by mechanical stress on the Wima capacitor it makes sense that the popping noise was present before the restoration as well. Also, since the 0.33uF Wima capacitor is in parallel with the green one, it makes sense that if it was the cause, it produced the same issue as in your case with the failing green cap, right?
Funny enough I found a pre-restoration picture where the black cable was in the same exact position, so if the problem was caused by mechanical stress on the Wima capacitor it makes sense that the popping noise was present before the restoration as well. Also, since the 0.33uF Wima capacitor is in parallel with the green one, it makes sense that if it was the cause, it produced the same issue as in your case with the failing green cap, right?
Attachments
Last edited:
Yes, it is a snug fit, so closing should always be done with great care.
Most likely the popping amp that I once repaired had been closed before without attention to this detail.
Hans
Most likely the popping amp that I once repaired had been closed before without attention to this detail.
Hans
Well, after one week no pops, so I closed everything back and it’s working great. It was tough to close it properly though, that cable really wanted to go and get stuck between the two capacitors.
When I first heard the pops I was desperate as I thought I would have been nearly impossible to find the cause but I was lucky enough that the problem was evident o simple inspection.
When I first heard the pops I was desperate as I thought I would have been nearly impossible to find the cause but I was lucky enough that the problem was evident o simple inspection.
Thanks you guys I really hope this is the end of this journey. All flaws with this amp are now fixed and it sounds great ��
Hello Hans, it's been a while, and have been enjoying my ML23.5 since the repair.
Last week I finally received the power supply capacitors I had ordered in September (long wait!), and installed them (a bit tricky since slightly different diameter, but everything worked out pretty well).
Well, I really wasn't expecting such a huge improvement! Apparently the old Spragues had reached their end of life, and although the amp sounded fine, there was a subtle hum. But when I installed the new capacitors (39000uF and 2200uF), which I think have excellent specs (temperature rating, life, ESR) the improvement was unbelievable, like changing power amp altogether, much improved soundstage, threedimensionality, dynamics, clarity and silence. Really a no-brainer for 300 USD and 8 months of wait.
The reason I'm writing is because I'm finding it challenging to calibrate precisely the voltages. I don't know if the capacitors need still a bit more break in (things have improved in the first few days), or if it's because of the season (hot temperature but highly variable depending on whether I turn on the AC or nor). However the Vreg change from 39,8V to 40,2V (but the 4 voltages don't change all equally) depending on ambient temperature (I got a precision meter), bias drifts about + or - 10mV from where I set it depending on ambient temperature.
In the first couple of days things were much worse, with bias ranging from 40mV to 75mV, now the variability is less wide, but it's still more than it was before I think.
Is it normal acceptable drift? Could it be because the new caps are 1 cm less in diameter and there is much improved air circulation inside, that makes the amp more susceptible to ambient temperature changes?
Thanks!
Last week I finally received the power supply capacitors I had ordered in September (long wait!), and installed them (a bit tricky since slightly different diameter, but everything worked out pretty well).
Well, I really wasn't expecting such a huge improvement! Apparently the old Spragues had reached their end of life, and although the amp sounded fine, there was a subtle hum. But when I installed the new capacitors (39000uF and 2200uF), which I think have excellent specs (temperature rating, life, ESR) the improvement was unbelievable, like changing power amp altogether, much improved soundstage, threedimensionality, dynamics, clarity and silence. Really a no-brainer for 300 USD and 8 months of wait.
The reason I'm writing is because I'm finding it challenging to calibrate precisely the voltages. I don't know if the capacitors need still a bit more break in (things have improved in the first few days), or if it's because of the season (hot temperature but highly variable depending on whether I turn on the AC or nor). However the Vreg change from 39,8V to 40,2V (but the 4 voltages don't change all equally) depending on ambient temperature (I got a precision meter), bias drifts about + or - 10mV from where I set it depending on ambient temperature.
In the first couple of days things were much worse, with bias ranging from 40mV to 75mV, now the variability is less wide, but it's still more than it was before I think.
Is it normal acceptable drift? Could it be because the new caps are 1 cm less in diameter and there is much improved air circulation inside, that makes the amp more susceptible to ambient temperature changes?
Thanks!
Hi Stefano,
It’s too late now to respond, I’ll try to come back on your posting tomorrow.
Hans
Stefano,
When power caps are causing a hum, it means that their capacity dropped and the regulated power supply receives a voltage in the dip that it can no longer process correctly.
But this is only for the 2200uF caps.
So replacing them was a good idea.
Another reason for sound improvement can be because many connections have to be taken apart and reconnected again, breaking an oxidation film that might have developed over the years.
when the large 39000uF caps are showing the kind of expected ripple, I don't think they will have much effect on sound perception, but you never know.
Replacing them after so many years is a good idea anyhow.
New caps have to break in, which can be felt because of a higher temperature in the beginning, dropping after a few days.
But what I don't get is the Vreg that should be ca. 84 Volt instead of the 40 Volt that you mention.
That Vreg has some temp dependency is true, it is based on relatively simple circuitry around non temp compensated zeners.
But the drift in bias seems absolutely not normal and is very atypical after changing power caps.
Is this true for both channels ?
Hans
When power caps are causing a hum, it means that their capacity dropped and the regulated power supply receives a voltage in the dip that it can no longer process correctly.
But this is only for the 2200uF caps.
So replacing them was a good idea.
Another reason for sound improvement can be because many connections have to be taken apart and reconnected again, breaking an oxidation film that might have developed over the years.
when the large 39000uF caps are showing the kind of expected ripple, I don't think they will have much effect on sound perception, but you never know.
Replacing them after so many years is a good idea anyhow.
New caps have to break in, which can be felt because of a higher temperature in the beginning, dropping after a few days.
But what I don't get is the Vreg that should be ca. 84 Volt instead of the 40 Volt that you mention.
That Vreg has some temp dependency is true, it is based on relatively simple circuitry around non temp compensated zeners.
But the drift in bias seems absolutely not normal and is very atypical after changing power caps.
Is this true for both channels ?
Hans
Hello Hans,
Thanks for the reply.
I’m sorry I mistyped the Vcc Reg voltages. They actually range between +-83.8V to +- 84.2V more or less.
Yes the amp used to run a lot hotter in the first couple of days, I feel this has improved in the last couple of days. Also the drift in voltages is much more limited now. Bias still drifts a bit (both channels), I would say +-5mV at least. All measurements were made with the amp fully warmed up (at least 24 hours). However I suspect one of the reasons for the bias drifting so much might have to do with the fact that I normally kip the lid closed, but for measurements I have to open it, and it takes a while to connect the leads so the temperature might decrease in the meanwhile, especially if I have the AC turned on
Will try and see if it gets better in the next days.
I measured the old caps for capacitance and they measure all 8 above specs, but the ripple was almost twice as normal when I measured during the repair in October, although I didn’t go ahead and measure the current draw on each rail. I would like to get an ESR meter and see if that shows high readings on the old caps, but the amp now is better in every possible way, dynamics and soundstage most apparent.
p.s. as I expected the recap didn’t fix the transformers mechanical buzzing. I think I will have to measure current draw, and then maybe try an external DC blocker. If nothing works I guess it’s the toroidal fault, right?
Thanks for the reply.
I’m sorry I mistyped the Vcc Reg voltages. They actually range between +-83.8V to +- 84.2V more or less.
Yes the amp used to run a lot hotter in the first couple of days, I feel this has improved in the last couple of days. Also the drift in voltages is much more limited now. Bias still drifts a bit (both channels), I would say +-5mV at least. All measurements were made with the amp fully warmed up (at least 24 hours). However I suspect one of the reasons for the bias drifting so much might have to do with the fact that I normally kip the lid closed, but for measurements I have to open it, and it takes a while to connect the leads so the temperature might decrease in the meanwhile, especially if I have the AC turned on
Will try and see if it gets better in the next days.
I measured the old caps for capacitance and they measure all 8 above specs, but the ripple was almost twice as normal when I measured during the repair in October, although I didn’t go ahead and measure the current draw on each rail. I would like to get an ESR meter and see if that shows high readings on the old caps, but the amp now is better in every possible way, dynamics and soundstage most apparent.
p.s. as I expected the recap didn’t fix the transformers mechanical buzzing. I think I will have to measure current draw, and then maybe try an external DC blocker. If nothing works I guess it’s the toroidal fault, right?
Anyway, right now I have the amp closed, but relatively soon I’ll have to open it again to mount the clamps for the big capacitors (since they are smaller and I didn’t want to drill new holes on the bottom of the chassis I’m having custom made clamps made by a professional, with longer feet to reach the existing holes). When I’ll reopen the amp, I will make sure to check the current draws of each rail to rule out that possible cause of transformer buzzing.
Out the top of my head, the amp should draw between 160 and 200Watt from mains.
When you bias setting is correct, but your amp draws more power, it probably means your transformer is not o.k. and is humming because of that.
Hans
Ok, I measured the current draw from the mains and I think it's too much high: 1,2A, with 220V. Bias had been just reset correctly...by the way after leaving the amp on for 24 hours I came back home today and it felt too hot. The bias had gone to over 90mV on the right channel, 75 mV on the left. I'm not sure it is normal for capacitor break in. I feel like I should give up on this amp and enjoy it as it is, it sounds very good to me, but there must still be something wrong with it.
p.s. a couple more things, both transformers hum equally loud. After installing the new caps I heard three not too loud pops on the right channel, spread within an hour, then nothing for days and the amp is working fine aside for the bias drift. Yesterday I mounted back the transformer metal cover, put the lid back on, turned the amp on and I heard two more pops on the right channel, then nothing...this time I cannot see any cables stuck between the PCBs, the smaller caps leave plenty of space to close everyting, but replacing the cover of the transformers back in could have put some stress on the cables/connections the pass through the notch on the metal cover...just thinking
p.s. a couple more things, both transformers hum equally loud. After installing the new caps I heard three not too loud pops on the right channel, spread within an hour, then nothing for days and the amp is working fine aside for the bias drift. Yesterday I mounted back the transformer metal cover, put the lid back on, turned the amp on and I heard two more pops on the right channel, then nothing...this time I cannot see any cables stuck between the PCBs, the smaller caps leave plenty of space to close everyting, but replacing the cover of the transformers back in could have put some stress on the cables/connections the pass through the notch on the metal cover...just thinking
Last edited:
Some units are just too mysterious to finish repair. This unit sounds like a candidate. I have one unit that works fine out of the case, works fine when I twist & push on boards, goes silent as soon as I put the cover on. Its mono, to the attic with it. Buy a nice CS800x or CS800s or something with lots of support. I've fixed several of those since burglars like to carry them off to the flea market, and churches like to trade them in for NEW active thumper speakers that might last a year. Salesmen tell such beautiful stories.
I think this might be the case. I’m really frustrated because I love the sound of this amp, especially with the new caps, and it will not be easy for me to find one I might like at least as much, without spending significantly more than I paid this (2k euros), I would say at least three times more.
I don’t understand, the bias and voltages staid pretty stable for months before I replaced the caps.
Is it possible that the new caps, with their much lower ESR, are putting too much stress on the transformers, making the amp unstable?
the transformers always used to him a little since I own the amp, but I always thought it was DC on the mains that caused that, I’m not so sure anymore. That would be a dead end, I don’t think it is worth , if at all possible, to replace the transformers.
Transformers that hum can have mechanical problems like loose core rivets. Not worth trying to fix. I don't think high ESR caps do anything good for output transistor idle current. Or transformers either. Most of my Peavey & Allen re-e-cap amps settled right down after the work. (exception that silent dog when enclosed). I had one 12 input Peavey mixer with a humming transformer, but the burglar carried it off before I worked up the nerve to take 300 knobs & 100 nuts & screws off. The PV8 mixers I replaced it with are as silent as a church.
The model amps I suggested are ~$200 here working feebly, ~$100 blown. Ebay or craigslist. $100 in ecaps, clean the heat sink, Maybe a volume pot or busted speaker connector, good as new. A new fan might extend reliabiity a bit but only if ball bearing. Lots of **** fans for sale. 100 db noise below full power gets them way quieter than the traffic noise at my house. BLown units might need $140 in blown semiconductors, but that is also a problem akin to mowing the lawn. Bar band veterans have been installed & removed hundreds of times. Lots of chances to short the speaker wiring in that service. Which blows output transistors, drivers,VI limiters, sometimes all the way back to the input op amp. Transformer models like CS800x are especially cheap because bands hate to carry a 1200 VA transformer. But the freight if remote counterbalances that. The CS800s has a well documented switcher supply that weighs 5 lb. I've fixed two-caused by overaged ecap that blew a fuse on 5v in one case.
If you can't put the amp under a couch or behind the record rack to muffle the fan, the Peavey M-2600 has no fan. I paid $140 for mine, working. Did put new e-caps in it, 30 year old rubber sealed caps are a bad bet.
As for $3000 amplifiers, I spend my big bucks on speakers, microphones, musical instruments. Was eyeing an $18000 Mossman bassoon this spring, then Germany wimped out on Ukraine. Nothing like real wood & brass for real sound.
Last edited:
Thermal instability for Bias current has absolutely nothing to do with new powe caps or better ESR.
When you want to find the cause, measure the voltage over the individual emitter resistances because I suspect that one transistor carries quite a lot more current as it neighbours and is the cause of the runaway.
Hans
I'm testing the left channel for now. I set the bias and all emitter resistors are between 8mV and 10mV. I guess I just have to leave it on and wait for the bias to drift and then check which transistor is drawing more current right? The last time about 24 hours passed (without playing, no input, speakers connected) before I noticed the amp got hot and bias reached 90-100mV.
Between 8 and 10mV means between 36 and 45mA for a 0.22 emitter resistor.
When having 5 compl. pairs (that's what you have, true ?) this would mean (5 x 45mA) * (2x0.1R) = 45mV instead of the 90 to 100mV that you are measuring.
So leave it on and see what happens.
Hans