ADA B200s: Channel A issues

[Iperfungus]

Hi guys!

Ok, here we go with some questions about Channel A of my B200s.

First of all, I've to say that volume pots look good for both A and B channels: not noisy or scratchy and very smooth.

Channel A had a couple of strange behaviors in the past days: the first time I powered the amp up it had very low volume, much lower than Channel B at same settings...and it was low also at 10.
I powered the unit off, disconnected cables, connected again, turned on: issue was gone.

Some days later Channel A was totally muted.
Same stuff as above: issue was gone.

Since then, Channel A has always worked fine...meaning that I always hear sound from it now.

But...

Speaking with the guy I bought the B200s from, I discovered that pots were probably scratchy in the past...and he made them "repaired" in some way.
I don't know if pots have been cleaned with some contacts spray or changed with others.
They are not scratchy or noisy for sure now.

By the way, I discovered there's a slight difference between channel A and channel B...they do not sound 100% the same at same level.
Channel B sounds more thick, present, with better bass and high freqs...while Channel A sounds a little "weaker".
I've to investigate this point better...

I didn't open the unit yet...just used a bit of contacts spray inside Channel A IN and OUT...but nothing changed so far.
Also tested different cables...and run Channel A and B in mono to see if difference is still audible and it is.
Looks like pots work in slightly different ways...

Do you have any idea, guys?

[MarshallJMP]

Could be a pot/jack issue, best to replace them. You won't believe what people sometimes spray in pots and jacks. I've seen pots that were eaten away inside from that good "contact spray" they always use.

[Iperfungus]

Could be a pot/jack issue, best to replace them. You won't believe what people sometimes spray in pots and jacks. I've seen pots that were eaten away inside from that good "contact spray" they always use.

Ok, I had a look inside it yesterday.
What I found:

1) at least Channel A pot has been changed...maybe also Channel B pot, but I'm not sure: they look different, Channel A pot has "A 10K" marked on its back, while Channel B pot has the value written in top front...the trees have different lenght...and, when I measured them, I've got A = 9,87K B = 8,57K ...  I've seen that there are a couple of caps on the little board, so the resulting filters could be the reason why channels sounds slightly different?

2) both Input jack have broken plastic thread...  I've found a way to fix them, but I don't like this situation...

3) when I insert jack, I see that tip touches a kind of little metal/solder point on OUTPUT small board (both connectors) and it doesn't on INPUT board (both connectors), like if jack goes more inside in OUTPUT connectors...I don't know if this could have any effect, now I can hear sound from both channels...

4) ON/OFF power switch has been changed and it works...but I found a little mess inside to fix it to the chassis...they used something like silicon to make it blocked from the inside... 

5) I powered the unit on without the bottom cover (removed to look inside) and the buzz/hum transformer's noise I can hear when it's turned on was lower than usual a lot...I've tightened all screws (some were loose) and when I closed the unit, put it back in the case and turned it on, the buzz/hum noise was there again at usual (low) level...is this normal? May it depend on power amp position (upside down or not)? It's not a strong noise at all...but just to be sure...

6) on the back there's a small paper sticker that says 220V and it looks like being applied over original value (117V I assume)...like if this were a USA model and tranny has been changed...the 60 Hz value is still indicated near the sticker...
Looks like this one but with a black plastic stopper insted of that white socket...

http://smg.photobucket.com/user/mrdan666/media/ADA%20poweramp/LGIM0061.jpg.html
http://smg.photobucket.com/user/mrdan666/media/ADA%20poweramp/LGIM0063.jpg.html

When I closed the unit and connected everything back, this time channel B was 100% muted...ahahahahahah!!! 
I've got it back to work turning it some times from min to max and back...then I opened the unit again and used a little amount of contacts cleaner inside them and to clean IN and OUT jack connectors.
Everything started working fine again, aside the slight channels sound difference.

So, basically I should change IN and OUT connectors for both channels and maybe also volume pots and ON/OFF switch change would be a good idea.
The problem is: can I find spare parts somewhere? Or can you suggest any compatible replacement?

What's the correct impedance value for volume pots?

Thanks guys! 

[MarshallJMP]

Can you take some pics of the jacks?

Pots are A10k

[Iperfungus]

Can you take some pics of the jacks?

Pots are A10k

Yes, I can take pictures...but I've to remove jacks very carefully...otherwise I'm not sure I can put them back in place...I will try...

What about tranny (see points 5 and 6 of previous post)?

[MarshallJMP]

So it's actually a mechanical hum?

[Iperfungus]

So it's actually a mechanical hum?

Yep...it looks like...

I will collect some pictures from outside and inside...

[rnolan]

Mike has some experience with changing the output jacks.  They have to be switching jacks (to cut the channel when nothing plugged in) and also plastic/insulated from the case.
From what you've said, new jacks (input and output) and new pots should (?) sort out the slight channel difference.  If the transformer proves to be a problem, you can get a toroidal to fit. Mounts with one centre bolt from the top. We did this with a US model B200s so it worked here (240v)  .

[Iperfungus]

Mike has some experience with changing the output jacks.  They have to be switching jacks (to cut the channel when nothing plugged in) and also plastic/insulated from the case.
From what you've said, new jacks (input and output) and new pots should (?) sort out the slight channel difference.  If the transformer proves to be a problem, you can get a toroidal to fit. Mounts with one centre bolt from the top. We did this with a US model B200s so it worked here (240v)  .

Good to know, Rich! 

Where I can get a 220V/50Hz tranny that fits my B200s?

Do you mean these jack connectors?

http://www.banzaimusic.com/Switchcraft-SC-NL-112AX.html

About pots...A = linear?
Would this fit? Or shaft is too long?

http://www.banzaimusic.com/Marshall-Pot-10k-lin-16mm.html

[MarshallJMP]

These should work too

http://www.banzaimusic.com/Alpha-16mm-split-shaft-10k-log.html

As for the jacks I need to see a pic of them.

[Iperfungus]

These should work too

http://www.banzaimusic.com/Alpha-16mm-split-shaft-10k-log.html

As for the jacks I need to see a pic of them.

Ups...cheaper a lot! I can buy many of them and select the best after measure... 

Should they be LOG or LINEAR? A = linear or log?

I will post pics of jack connectors later.

Any suggestion for a new tranny, eventually?

Many thanks!!!

[MarshallJMP]

Look at the link ,it says log   ;D  A=log, B=lin C=rev log.

Why a new tranny?

[Iperfungus]

Look at the link ,it says log   ;D  A=log, B=lin C=rev log.

 

Why a new tranny?

Well...I'm not sure if it's original or it has been changed...My B200s has a sticker reporting 220V that looks added over the original writing (I can still read original 60Hz writing and the back looks more like a USA unit)...I will post some pics to make it more clear...
Then there's that low buzz/hum noise when I turn it on and I'm not sure if it's normal...even if the power amp works fine, aside reported minor issues with jacks and pots.

rnolan suggested a new tranny, even if we're not sure it would be actually necessary.

[MarshallJMP]

Take a pic of the tranny as well.

[Iperfungus]

Take a pic of the tranny as well.

I will attempt to record the buzz/hum too...  :banana-jazz-smiley-emoticon: