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Buzzing power amp (Sherwood AM-8500) [UPDATE: Now measuring]

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#1
Hi,
I found this amp a couple of months ago for what seemed like a very good price.
(270€ together with the AVP-8500 preamp which I immediately sold for 135€)

I've been rather happy with it but it does produce a buzzing noise both on the speaker outputs and directly from the amp.

At my typical listening distance, the buzzing from the speakers is on the edge of audibility, so it's not too pressing an issue but I'd like to get it fixed.
I'm on a rather limited budget at the moment, so I could use your in narrowing down the cause, estimating the cost of repair and deciding whether it's worth it.

The noise doesn't change with the input signal and persists when I disconnect the inputs and shut off every othe device in my flat (incl. lights, fridge, etc.), connecting the amp to another power outlet at the opposite end of the flat doesn't change anything either.
The noise in the speaker outputs seems identical on both channels and what's coming from the amp itself sound rather similar, too.

Mic in front of the tweeter:
freq.jpg wavform.jpg


Mic in front of the midrange driver (150-2600hz):
midrange_freq.jpg midrange_wavform.jpg

Mic on top of the amp (right above the transformers):
amp_freq.jpg amp_wavform.jpg


Pictures of the amp (please excuse the potato quality):

AM-8500_01.jpg AM-8500_02.jpg AM-8500_03.jpg AM-8500_04.jpg AM-8500_05.jpg AM-8500_06.jpg AM-8500_07.jpg AM-8500_08.jpg AM-8500_09.jpg AM-8500_10.jpg

Specs:
AM-8500_spec.jpg
(since it's not listed, weight is ~26kg/56lb)
Edit: Production date is October 28 1993


best regards
Jan
 

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tmtomh

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#2
That's power supply noise - 50Hz and multiples thereof. You're in the EU and 50Hz is the AC frequency.

The most likely issue is a grounding problem. Others here are more qualified than I am to advise you on the easiest ways to deal with a grounding/ground-loop issue.

The only other thing I can think of is that the amp's power transformer is humming because of DC in the AC line (aka dirty AC from your local grid). In that case a DC blocker will do the trick.
 
OP
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Thread Starter #3
That's power supply noise - 50Hz and multiples thereof. You're in the EU and 50Hz is the AC frequency.
Well, obviously. The question is how is it getting into the signal path.

The most likely issue is a grounding problem. Others here are more qualified than I am to advise you on the easiest ways to deal with a grounding/ground-loop issue.
It can't be a ground loop if there's nothing to loop with, neither disconnecting all other devices from the phase the amp is on nor disconnecting the amp from ground changes a thing.
One thing that could cause this sort of thing, as far as I could gather, are broken ground connections inside the amp.
I would have to largely disassemble the amp to even get a look at all of them, though.

The only other thing I can think of is that the amp's power transformer is humming because of DC in the AC line (aka dirty AC from your local grid). In that case a DC blocker will do the trick.
That way a lot of obvious voodoo bullshit lies; and - as far as I can tell - very little that's convincingly not bullshit...

I guess I could move the Amp to a friends place (>100km away) to see if the noise persists.
 

Wombat

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#6
That power supply area looks to be coated in dust/cigarette smoke/ cooking fumes. There may be a leakage path in there. A clean will tell.
 

RayDunzl

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#7
Amazing how the driver boards vertical profiles nest into each other to maximize the space for the heat sinks.

1574316587714.png
 
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Wombat

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#8
Lots of old electrolytic capacitors. Look at the ones in the power supply for distorted cases or leaks.
 

Wombat

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#9
The rat's-nest wiring, centre-rear, doesn't look 'factory'. Who knows? Small cap bypassing big electro audiophile mod? Pics, of B and G models don't show those polyester capacitors.

AM-8500_09.jpg
 
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OP
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Thread Starter #10
How to win friends ……………………………… , etc.

I'll risk a reply:

Schematic, here: https://www.hifiengine.com/manual_library/sherwood/am-8500.shtml
Thanks, that's much more helpful.

Wait for the pros to weigh in. I'm not one. I will guess that the filter capacitors are bad on the main power supply board A. There are 8 caps on that board.
That power supply area looks to be coated in dust/cigarette smoke/ cooking fumes. There may be a leakage path in there. A clean will tell.
I'll take a closer look and post some pics shortly.
 
OP
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Thread Starter #11
The rat's-nest wiring, centre-rear, doesn't look 'factory'. Who knows?

View attachment 39671
Here's a closer look:
IMG_20191121_073441.jpg


Transformers:
IMG_20191121_073512.jpg IMG_20191121_073520.jpg IMG_20191121_073528.jpg IMG_20191121_073544.jpg

Caps:
c00.jpg c01.jpg c02.jpg c03.jpg c04.jpg c05.jpg c06.jpg c07.jpg c08.jpg c11.jpg c12.jpg c13.jpg


As far as the crammed mess of cables goes and whether that's original, keep in mind that Sherwood (actually the South Korean OEM Inkel, then owned by Etronics) didn't have much experience with "high end" stuff.

They made one Stereo Pre/Amp combo (AP-7040 + AM-7040, the amp is near identical to the AM-8500), this combo (the AVP-8500 generates center and rear channels from from the stereo inputs and has integrated amps for those 3 channels) and a 5-Channel Pre/Amp combo (AVD-9080r + AM9080).
Edit: There was a 4th one, apparently: P-965/A-965 (7 channel)

It's hard to to find good information one these, so I'm wondering how much repair/service cost would make sense...
 
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Wombat

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#12
Also lots of 'spade' connectors at the power supply that could be checked for oxidation.
 

RayDunzl

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#13
Whose bright idea was it to put plastic covers on the top of caps?
 

Wombat

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#14
Those polys are on the diode bridges. I don't think that is a good application for polys but my memory of such things is out of practice.
 

Wombat

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#15
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OP
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Thread Starter #16
A lot can be discovered carefully using an insulated prodder to tap, push and poke with.

I use one of these or similar: https://www.ebay.com.au/itm/8-Diffe...503683?hash=item4b6e441883:g:j9oAAOSwtYRdxB~2

Some say a plastic knitting needle is OK. Maybe for up to 120V, maybe not.
Many components aren't accessible without removing the boards, I don't really feel qualified to do that (and get it back together again).

Edit:
Any idea what this yellowish mass is? (I've carefully cut through it with a razor blade)
yellow.jpg c00.jpg

Is that just glue to keep those two caps in place or might there be some damage?
 
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Wombat

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#17
Try the power supply. That is reasonably accessible. Keep the probe in one hand and the other hand in your trouser pocket.

IF IN DOUBT DON'T.

As for sending it out for repair I will let the bench-savvy guys comment.
 
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Thread Starter #18
Also lots of 'spade' connectors at the power supply that could be checked for oxidation.
Like these?
spade.jpg


I pulled off three of those and found them perfectly clean, the one on the bottom right is being stubborn and I'm afraid I'll rip the connector off the cable while trying to get it loose. The cable runs all over the amp are very tight (the cable on that cable takes a sharp turn and disappears somewhere downwards), some of the connectors are under strain already (like the two on the reg board between the transformers).
 

solderdude

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#20
yes, just glue.

You need to check the DC power supply voltages with an oscilloscope.
Also the derived voltages (need schematic) need checking.

Likely candidates: big capacitors and regulator circuits. Maybe the bridge rectifiers ?
 
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