Pioneer A-8 hot transformer and humming.

QSilver

Super Member
I got one of these a few months ago but only just got into using it this week but I've noticed that the transformer does hum noticeably and gets quite hot. Even when idling. Is this anything to worry about? The hum is from the transformer itself rather than the amp.
 
I took off the top and put an ammeter in series with the main T4A fuse on the power supply board and powered on reading Amps AC. After the initial inrush, it settled down to 2.8mA and after a warm up time of 10 minutes it had risen to 3.2mA. Does this sound normal? 3.2mA doesn't sound like much...

I then put the ammeter in series with the other fuse, a 1.5A fuse. I haven't found my service manual for it, I will try to dig it out tomorrow.... but this fuse had 9.72VAC and VDC across it and measured 3.3mA. My volt readings were taken with my black lead on the chassis.

I put my hand on the top of the transformer and I can feel the hum and the transformer seems to warm up around the edges. Don't know if this helps... the amp was not under any speaker load when I took these readings.
 
I just moved it back into my system for now and when I went to power on, I got a lot of crackling from the plug and not much life from the amp. I powered it back down and took the plug apart. Found that the wires were a bit of a mess - the neutral just slipped out of its post and ends caps of the fuse fell off.... I restripped the wire and rewired the plug, replacing the fuse too and I'll keep an eye on it but I think the transformer might be a bit cooler now.... not sure...
 
It's still humming quietly you can just feel the hum on the case. Maybe I could measure the case temperature? Its still only warm but I think it is still getting warmer... much slower than it was and doesn't seem to be making the cabinet its in as hot either...
 
Can you suggest a device?

I think its getting as hot as it was now. Mostly focused around the transformer and the left channel power amp area from the top of the case.
 
could also be a few things like loose laminations . poor insulation .
once we know the power draw for the primary it should help .
 
Yes, I was wondering about loose laminations... I'll buy one of these and hook it up to it...

Transformer humming could be due to mains DC offset.
I don't think so as I have just moved house and its behaivior hasn't changed in the new house... though, I do have a large mains conditioner for my computer (2200VA) I could plug the amp into that and see if it helps with the hum?
 
Yes, I was wondering about loose laminations... I'll buy one of these and hook it up to it...

I don't think so as I have just moved house and its behaivior hasn't changed in the new house... though, I do have a large mains conditioner for my computer (2200VA) I could plug the amp into that and see if it helps with the hum?

You could try that. Anyways, transformer humming due to mains DC presence sounds more like growling, in a way very different to mechanincal vibration.

As a side note, transformer heat build up should be very light under no load. If you notice that it's heating up real high then the transformer might already be damaged, or poorly designed/made in the first place.
 
As a side note, transformer heat build up should be very light under no load. If you notice that it's heating up real high then the transformer might already be damaged, or poorly designed/made in the first place.

The amp isn't really working hard - the indicators for 0.01W don't even light up but the transformer is very warm... It's a Bando transformer which I thought were quite a good make?
 
The amp isn't really working hard - the indicators for 0.01W don't even light up but the transformer is very warm... It's a Bando transformer which I thought were quite a good make?

Bando transformers can fail.

Does your A-8 have a mains voltage selector? Looks like there are units with and without this feature.

Also, can you take a power supply voltage reading? That would be on C1 & C2 (12000/63) with the amp idling.
 
Does your A-8 have a mains voltage selector? Looks like there are units with and without this feature.

Also, can you take a power supply voltage reading? That would be on C1 & C2 (12000/63) with the amp idling.

I get +54VDC and -54VDC when the amp first switches on and then over ten minutes, the voltage creeps down to +53VDC and -53VDC

My A-8 has the multivoltage selector I think you can change it between 240V and 220V. It is set correctly.
 
My Pioneer SX-8 used to hum when on as well and I never figured out why. I chalked it up to loose laminations. It was also a Bando tf, I wonder if it was the same PN.
 
It's fine for a transformer to hum a little due to loose laminations (IF IF you're willing to accept the noise). What's not heathy at all and sympthom of malfunction is excesive heat. If you feel heat build up is in excess of warm then there is an issue with it.


I get +54VDC and -54VDC when the amp first switches on and then over ten minutes, the voltage creeps down to +53VDC and -53VDC

My A-8 has the multivoltage selector I think you can change it between 240V and 220V. It is set correctly.


+/-53V is OK.

As follow up, you might need to carry out a voltage reading at or near full rated output. Should not drop below 47V as per specs. I suggest doing so with a couple of dummy resistors and with a test tone (1khz would be fine)

http://onlinetonegenerator.com/
 
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