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

[solderdude]

Yes those things are more than sufficient.
Do not go cheaper than this one (German website I suspect you know this)
The ones with 200V AC as lowest setting are not recommended (the ones costing 10 Euros)
These are good enough for DC and resistance measurements only.

 

[Kaktus_Kontrafaktus]

I looked around for any mentions of buzzing etc. from an AM-8500 (or AM-7040 which is very nearly identical, see attached service manuals) but couldn't find anything. What I could find is people singing its praises (favourably comparing it to amps that sell for a lot more than the Sherwoods these days) and calling it a sleeper.

Read this reminded me, that I noticed some intermittent channel drop out on the left channel when using the "A" speaker terminals when I first got the AM-8500 (I just switched to the "B" terminals and forgot about it):

Most common issue on these are intermittent channel drop out, usually caused by dirty speaker select switches and the switch for scource direct or variable input,
a good deox usually takes care of this issue, knock on wood both mine have performed like a champ for the ro 10 years ive owned them.

https://audiokarma.org/forums/index.php?threads/sherwood-am-7040.632370/#post-8381177
Another thread: https://www.audiokarma.org/forums/i...ness-strikes-thrice-sherwoods-am-7040.134855/

I picked out a few power amps from well-known brand that the "Stereoplay" put in the same "class" as the AM-8500 & AM-7040 back in the 90ies:
(from their 'eternal' ranking, see attachment)
Denon POA-3000 (1980)
Denon POA-3000Z (1985)
Denon POA-4400A (pair) (1989)
Denon POA-6600 (pair) (1987)
Kenwood L-08 M (pair) (1981)
Kenwood L-1000 M (1990)
Luxman M-02 (1984)
Luxman M-03 (1988)
Marantz MA 500 (pair) (1993)
Marantz SM 11 (1985)
Marantz SM 80 (1991)
Onkyo M-200 (1985)
Onkyo M-5060 R (1983)
Onkyo M-5890 (aka M-588) (1992)
Pioneer M-73 (1991)
Rotel RB-956 BX (1993)
Rotel RB-970 BX (1994)
Rotel RB-990 BX (1993)
Rotel RHB-05 (1994)
Sony TA-N 55 ES (1990)
Sony TA-N 901 (1984)
Yamaha M-60 (1984)
Yamaha M-65 (1986)
Yamaha MX-1000 (1989)
Yamaha MX-70 (1989)

Most of these sell for a good deal more than the Sherwood these days and they probably all come with their own potential issues,
so I'm thinking I might up my budget for the AM-8500 repair/service if the buzzing can be fixed without breaking the bank.

What other components would make sense to check/replace to improve/restore perfomance or preempt likely future defects?


Also, here's a look at the front board:

 

[Majestyk]

Yeah, this definitely doesn't look factory spec. I used to have a Sherwood Newcastle AM-9080 and it was a lot cleaner than this. It was super quiet too.

 

[Kaktus_Kontrafaktus]

Yeah, this definitely doesn't look factory spec. I used to have a Sherwood Newcastle AM-9080 and it was a lot cleaner than this. It was super quiet too.

Maybe but I think it might be, I can find very few pictures of the insides.
EDIT: To clarify, I thought you meant the appearance of the internals rather than the performance.

Where the buzzing is concerned, I very much doubt that even the most corrupt Hifi magazine would put an Amp with such an obvious flaw in the same category as the Yamaha MX-1000...

Here's mine:

Here's the only other photo of the same version that I could find (same components in the ratsnest above the power caps!):

And here's the only other photo that I could find, this one shows the 110v/220v switchable version (note the 4 fuses on the rear panel):

The AM-7040 should be mostly identical, I'll go look for some pictures...

 

[Kaktus_Kontrafaktus]

Some pictures of the AM-7040's insides.

Source no. 1:


Source no. 2:


Source no. 3:


Source no. 4 (switchable version with 4 fuses):



The AP-7040/AM-7040 also came branded as Inkel (the name of the South Korean OEM that actually made the Sherwood amps):

https://www.canuckaudiomart.com/details/648973788-inkel__mosfet__power__amplifier__huge_power_/

The AM-7040 may have been the first amp of this type & price range Inkel ever made.

 

[Kaktus_Kontrafaktus]

My new DMM should be here tomorrow and after some asking around, I have found a local repair company that has a good reputation both for expertise and for fair pricing.

28€ for diagnosis and a quote and 49€/h seems alright.

 

[Kaktus_Kontrafaktus]

When you have a multimeter you must measure the DC voltage between the red wire and black wire.
Should be around +27V.
Between the blue and black wire there should be around -27V.

The voltages are +26.9V and -27.2 and the two resistors both measure ~3.3Ω(±0.1).
I measured the 3rd resistor on the board (R503) and got ~1.06kΩ (should be 2.2Ω), could this be (part of) the issue?

 

[solderdude]

R503 is supposed to be 2.2kΩ and is a so-called 'bleeder' designed to drain C511 the moment the amp is switched off.

The voltages +/- 27V seem fine.
What you can do is measure them again but in the ACV setting.
In the unlikely event one or both of the smoothing caps (the small ones on the board) could be defective there can be a substantial (as in 5V or so) AC be measured there.

 

[Kaktus_Kontrafaktus]

Did you measure R503 with the resistor removed from the board ?

No, it's still on the board.

What you can do, when the device is powered down is see if you still measure some DC voltage across that resistor.
When this is the case the resistance measurement is wrong.

No DC voltage.

Also (a simple check) is to measure resistance in both directions.
When in both directions it measures the same (1.06k) you can be pretty sure it is defective.

Same result in both directions.

The voltages +/- 27V seem fine.
What you can do is measure them again but in the ACV setting.

Nothing on ACV, I got +27.2V & -27.5V on DC this time.

In the unlikely event one or both of the smoothing caps (the small ones on the board) could be defective there can be a substantial (as in 5V or so) AC be measured there.

Well, fuck.
I had a hard time getting good contact with the probes. I got AC voltages on C506 a few times (only for a moment, I couldn't get a stable reading), got careless while trying to get better contact and crossed the probes.

I got a pop from the speakers (not extremely loud, about normal listening volume) and now the outputs are dead (as are the LEDs on the front panel but the transformers are still buzzing).

The two fuses on the REG board are blown (I'm assuming they shouldn't measure as ~12MΩ...), the other fuses (2 on the rear panel, 4 above the 'rats nest') are still fine.

 

[solderdude]

I edited my initial post:

R503 is supposed to be 2.2kΩ and is a so-called 'bleeder' designed to drain C511 the moment the amp is switched off.

It is not audio related.

When the probes don't make good contact on a DC voltage in the AC position you will get short readings indicating a voltage.

Just replace them, probably you shorted something by accident with the probes.
They are 0.5A (500mA) ? check if there is an F, S or T on them as well.
Chances are no harm was done (except to the fuses)

 

[Kaktus_Kontrafaktus]

Just replace them, probably you shorted something by accident with the probes.
They are 0.5A (500mA) ? check if there is an F, S or T on them as well.
Chances are no harm was done (except to the fuses)

5x20mm T 500mA L250V, cheap and easy to find. I'll stop by Conrad tomorrow.

Since I have nothing else productive to do now... I dug up an old copy of Stereoplay to find some measurements for the AM-8500:
(and two other power amps from the same issue for comparison)

AM-8500 | AVM Evolution A3 | Rotel RB956AX (5ch)


AM-8500:

AVM Evolution A3:

Rotel RB956AX:

The corresponding Preamps, while I'm at it:
AVP-8500 | AVM Evolution V3 | Rotel RSP960AX

 

[solderdude]

It would appear as though the AM8500 has some hum in the signal at -90dBV = 31uV and harmonics are also visible making the hum more audible (the Rotel too b.t.w.)
This is 100dB below 1W (8 Ohm)
( I could be misinterpreting the plot regarding the actual levels)

 

[Kaktus_Kontrafaktus]

Am I missing something? The plots are labeled "1kHz, 25W, 4Ω", so where's the 1kHz peak and it's harmonics?

 

[Kaktus_Kontrafaktus]

Right, the new fuses are in and the amp is back to it's buzzing old self.

 

[solderdude]

Am I missing something? The plots are labeled "1kHz, 25W, 4Ω", so where's the 1kHz peak and it's harmonics?

The all left peak is to be ignored.
The first peak on the left is 1kHz (2kHz per division)
The harmonics are on the right of the first pole (2nd = 2kHz, 3rd = 3kHz usw)
The 'bump' just right of the downward slope is the 50Hz signal.

 

[solderdude]

Right, the new fuses are in and the amp is back to it's buzzing old self.

The measurements so far are indicative that the amp functions properly and the common part (the power supply for the input amp and DC servo) seems to be O.K.
As John already mentioned because both channels hum the same it is unlikely both separate power supplies have the same issues.
Certainly not because there are 2 smoothing caps in parallel on each rail.

Your meter may not be suited but measure the output voltage on the speaker terminals with the smallest ACV setting (hopefully 200mV otherwise 2V).

 

[Kaktus_Kontrafaktus]

Your meter may not be suited but measure the output voltage on the speaker terminals with the smallest ACV setting (hopefully 200mV otherwise 2V).

It's autoranging (the lowest range is 2V). I always get the same reading (~0.015V) even when I shut off the terminals with the switches on the front panel.

 

[Kaktus_Kontrafaktus]

Right, I think I've found a decent (qualified and fair on pricing) repair shop.

1. Are there any simple measurements left that I can do to narrow down the cause of the buzzing (to save on repair cost)?

2. What components would make sense to have replaced while it's in the shop?

If I can get rid of the buzzing, I'll probably keep the AM-8500 for a long time. I might get rid of it if I switch to active speakers but I'm quite happy with my Quadral Aurum 770 and a significant upgrade isn't in the cards budget wise.

 

[solderdude]

1: None come to mind, The most obvious one (the common pre-amp power supply) does not seem to be the culprit.
Mechanical buzzing from the amp itself will be impossible or very hard to remove anyway.
Judging from the plots the amp has some hum from itself and have no idea if this is worse in your particular amp and why that would be.
As your speakers are quite insensitive (needs a powerful amp) I would not expect the hum shown in the plots to be clearly audible.

2: Perhaps the speaker output relays ?

 

[solderdude]

Right, I think I've found a decent (qualified and fair on pricing) repair shop.

1. Are there any simple measurements left that I can do to narrow down the cause of the buzzing (to save on repair cost)?

2. What components would make sense to have replaced while it's in the shop?

If I can get rid of the buzzing, I'll probably keep the AM-8500 for a long time. I might get rid of it if I switch to active speakers but I'm quite happy with my Quadral Aurum 770 and a significant upgrade isn't in the cards budget wise.

Did you resolve the issue ?
Just curious.