I've got the readings. First of all, this is the multimeter:
Now, DC readings:
A: 38.7
B: 77.2
C: 38.7
D: 77.5
AC readings (on the mV AC setting)
A - 0
B - 0
C - 0
D - 0.6
PS: The readings jumped a bit at first (mainly on AC) but I left the probe a few seconds and waited for them to stabilize. I guess that's normal, but just in case (I never measured voltage values so small).
Now, DC readings:
A: 38.7
B: 77.2
C: 38.7
D: 77.5
AC readings (on the mV AC setting)
A - 0
B - 0
C - 0
D - 0.6
PS: The readings jumped a bit at first (mainly on AC) but I left the probe a few seconds and waited for them to stabilize. I guess that's normal, but just in case (I never measured voltage values so small).
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well, with readings as that, taking in account that:
-buzz same in both channels
-can't see any electrolytic caps on boards (only with them we can expect probability of same issue developed in same time on both channel pcbs)
-amp is not evidently messed with
conclusion is that culprit lies in some part of construction common for both channels ........ and that is only PSU
bridges - any problem with them should result in much bigger issues
so - logic sez - big reservoir caps, even with so small AC reading on them
now, you got to moment when only after their replacement you can know more ....... which is well known and plenty practiced repair technique - replace and see
-buzz same in both channels
-can't see any electrolytic caps on boards (only with them we can expect probability of same issue developed in same time on both channel pcbs)
-amp is not evidently messed with
conclusion is that culprit lies in some part of construction common for both channels ........ and that is only PSU
bridges - any problem with them should result in much bigger issues
so - logic sez - big reservoir caps, even with so small AC reading on them
now, you got to moment when only after their replacement you can know more ....... which is well known and plenty practiced repair technique - replace and see
Thank you! I'm not sure exactly when I'll replace those big 4 caps, it'll depend a bit on the cost (since it's not certain that the issue lies on the caps). Since you're in Europe, where do you usually buy yours and which brand do you advise me to buy?well, with readings as that, taking in account that:
-buzz same in both channels
-can't see any electrolytic caps on boards (only with them we can expect probability of same issue developed in same time on both channel pcbs)
-amp is not evidently messed with
conclusion is that culprit lies in some part of construction common for both channels ........ and that is only PSU
bridges - any problem with them should result in much bigger issues
so - logic sez - big reservoir caps, even with so small AC reading on them
now, you got to moment when only after their replacement you can know more ....... which is well known and plenty practiced repair technique - replace and see
Not sure that'll help, unless too little is also prone to hum.
The tech that replaced the bias pots didn't know the bias specs for this model so he left it at 5mV for each channel. And the hum was still there, just the same.
One of the things that I forgot to mention and maybe I should have (totally forgot). When biasing the amp I checked both for temperature and DC on the emitter resistors. I still haven't checked with this new DMM, but each resistor (on the same channel) listed different values (some with 20%+ differences). Should all those 10 grey resistors read the same?
OK, the that dilemma is solved
as it seems from that one pic you posted - nothing else was replaced on pcbs?
regarding biasing - when you're going by temperature on heatsink, that is main criteria, and voltage across emiter resistors is irrelevant as criteria ( temp at heatsink arriving first as critical/final value)
though, as always, voltage across these resistors needs to be checked, exactly from same reasons arising question mark here - equal share of sum Iq per each transistor
considering that amp looks not butchered, I resume that it came from Factory as that .... so 20% of spread is nothing to worry about
of course , only Pa can recollect is it true or not
as it seems from that one pic you posted - nothing else was replaced on pcbs?
regarding biasing - when you're going by temperature on heatsink, that is main criteria, and voltage across emiter resistors is irrelevant as criteria ( temp at heatsink arriving first as critical/final value)
though, as always, voltage across these resistors needs to be checked, exactly from same reasons arising question mark here - equal share of sum Iq per each transistor
considering that amp looks not butchered, I resume that it came from Factory as that .... so 20% of spread is nothing to worry about
of course , only Pa can recollect is it true or not
I'm not first owner but my uncle was (AFAIK) and I don't recall him mentioning this ever being serviced so yeh, I'm pretty sure it's all original besides the bias pots. And no, my tech did not touch anything else. At least he didn't mention it.
I see servicing these can be a nice little puzzle
I see servicing these can be a nice little puzzle
Yeh, you're right. I wish I had the proper tools. My main dilemma is that the hum doesn't bother me that much, but the fact that it's there and the way it could mean that there's something wrong with the amp doesn't let me give it up. But, on the other hand, it's not enough for me to go and spend a ton of money to get someone to fix it for me when I'm usually comfortable on doing this type of stuff myself. But, again, I'm missing proper tools... oh well
Thank you! I still need to see from where Mouser ships their stuff to Europe because if I order from the US I'm looking at 23% VAT on top of total price (shipping included) + handling tax. But I'll have a look for sure
You need to measure the AC readings at the speaker output in mV AC.
The ABCD points above are at the PSU caps and will be in 100mV AC range and might not show up. I am surprised it’s only 0 mV AC as that is at the smoothing caps which is usually not that low.
For the measurements I was asking about, connect probe to your output speaker binding posts. If speakers are connected they will give it the proper load if you have some DC offset that might be some of the problem. So also measure the DC at your speaker outputs. DC offset should be under 50mV DC.
Anything more than 1mV AC at speaker outputs will be audible hum.
Your DC voltages on the caps looks ok.
Do you hear hum on speaker when using phone as source on only one channel?
This is with music off.