Yes, there is often a small glow depending on the total of all idling currents in all circuits. This varies a lot between designs, features etc. I see there was indeed a problem with DC offset on one (or both?) channel and I suspect there are some small caps around IC1 needing replacement.
Regarding the change in operation now, I suspect the amplifier may not have been used for some time and there are 2 likely problems with the electrolytic caps, not just the main power supply smoothing caps. First, they may have required "reforming" which can be done over 12-24 hours with a small current of < a mA. This can also be done by the circuit they are used in, but not as effectively. Second, they are probably useless by now, having dried out, if they are the original caps.
It seems the amplifier can be restored so why not recap it with nice, new parts in the power supply and power amplifiers at least. Good that you have some progress here
Regarding the change in operation now, I suspect the amplifier may not have been used for some time and there are 2 likely problems with the electrolytic caps, not just the main power supply smoothing caps. First, they may have required "reforming" which can be done over 12-24 hours with a small current of < a mA. This can also be done by the circuit they are used in, but not as effectively. Second, they are probably useless by now, having dried out, if they are the original caps.
It seems the amplifier can be restored so why not recap it with nice, new parts in the power supply and power amplifiers at least. Good that you have some progress here
I was actually suspecting you were going to suggest a total recap but this has been done already. I got the amp with one channel blown out so I fixed that by replacing the output transistors and drivers. Once that was working I did a full recap with Elna silmic II and Nichicon Gold tune caps.
Hmm....so you have done a ful recap and the DC offsets should settle fast if the speakers are connected. I guess you could check the capacitor date codes but it may be waste of time if you can reduce this error by leaving the amplifier on for say, 24 hours and rechecking then. I never believed the nonsense that some folk post about breaking-in new capacitors but perhaps this an example where it proves necessary. 3V with about 6R speaker coil load is a hefty 0.5A current though - a bad sign if that voltage continues to take a significant time to fall.
Is that the same in either channel or are you just saying that the unloaded channel takes a lot more than 3 seconds to settle at 3mV? That would be expected but I can't see any reason for the DC offsets to be the same 3V in either channel to begin with.
Anyway, it would seem that the end is in sight!
Is that the same in either channel or are you just saying that the unloaded channel takes a lot more than 3 seconds to settle at 3mV? That would be expected but I can't see any reason for the DC offsets to be the same 3V in either channel to begin with.
Anyway, it would seem that the end is in sight!
I will try the breaking in process and will let you now.
In the mean time, some more measurements : with only one speaker on the active system the DC goes up to 4v and settles at 3mV after 2 or 3 seconds.
with both speaker on the active system DC gets up to 5v (R and left approximately the same) and doesn't seem to lower at all).
Without any load I have a DC reading of 5v as well but only for less than a second before stabilising at 3mV. This is the same with both speaker systems.
I should have checked this before, could have saved us quite some time but I was sure the problem came from the speaker switches at first.
In the mean time, some more measurements : with only one speaker on the active system the DC goes up to 4v and settles at 3mV after 2 or 3 seconds.
with both speaker on the active system DC gets up to 5v (R and left approximately the same) and doesn't seem to lower at all).
Without any load I have a DC reading of 5v as well but only for less than a second before stabilising at 3mV. This is the same with both speaker systems.
I should have checked this before, could have saved us quite some time but I was sure the problem came from the speaker switches at first.
It may not be relevant but the BP electrolytic caps around ICI may not be just cheap bipolars. I have made the mistake of fitting these types when the original parts were actually NP or non-polarised electrolytics and the difference in sound was audible and not good. BP types are cheap and often falsely marked and sold as NP so this matter can be confusing.
Interesting that the amplifiers won't settle when loaded with the speakers. It would seem that the DC servo is fighting itself or the tone control circuit in the NFB loop! I need some help myself to understand how that should operate reliably!
Interesting that the amplifiers won't settle when loaded with the speakers. It would seem that the DC servo is fighting itself or the tone control circuit in the NFB loop! I need some help myself to understand how that should operate reliably!
The 10µF, 25V has been replaced with Nichicon Muse es series but I couldn't find the 4.7µF 16V so these are basic Nichicon MP series. Would it be better to replace these by a 4.7µF, 25V ES series ?
Is there anything obvious in those two that could be wrong and that I can easily check before we get some help with this ?
Is there anything obvious in those two that could be wrong and that I can easily check before we get some help with this ?
I've been trying keep track of this thread and so far nobody has mentioned the speaker relay. At first turn on, speakers or no speakers, the relay should NOT energize until the amp has settled down, ie the DC servo should have taken over the offset. It seems everything is "instant on" right now. Could it be that the servo can't keep up without the time delay?
Craig
Craig
Fuse resistors are usually blue in color and are installed up off the PCB.
Without going back and re-reading everything, have you tried it without the bulb tester? The bulb tester may be upsetting the voltages enough to cause a problem. I've been at this (electronics) since 1980 and have yet to use or need a bulb tester. My Variac with current meter suits me just fine.
Craig
Without going back and re-reading everything, have you tried it without the bulb tester? The bulb tester may be upsetting the voltages enough to cause a problem. I've been at this (electronics) since 1980 and have yet to use or need a bulb tester. My Variac with current meter suits me just fine.
Craig
The ones I was Talking about are blue/grey and indeed lifted up from the board, they don't seem to be off spec though, what kind of problems could hey be causing ?
Without the bulb tester I still have about 5v dc offset on both channels for about 2 to 3 seconds before dropping to 3mV. I don't want to risk connecting speakers without the bulb tester.
Without the bulb tester I still have about 5v dc offset on both channels for about 2 to 3 seconds before dropping to 3mV. I don't want to risk connecting speakers without the bulb tester.
There is no voltage ref. for Q10 so the first guess of capacitance multiplier would be closer to the mark.
I just got a message from someone that has this amp and he says this actually is perfectly normal on the ones that don't have the protection circuit, it's just recommended to deactivate speakers before turning on the amp. I measured the dc offset on power up from a Sansui A-60 and that gives me exactly the same result. Without the bulb tester and speakers on that results in a pop when I turn on the amp, nothing worrying it would seem.
Thank you for all your help even if it seems that this wasn't necessary, sorry for that
Thank you for all your help even if it seems that this wasn't necessary, sorry for that
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