Right Channel Problems B&K ST202Plus

[allenzachary]

So this is weird...

During the fabulously delightful weekend I spent at Carlman's at the beginning of May, we descended to the sanctity of his listening room only to find his McIntosh amp had a strangely diminished right channel. It wasn't distorted or raged sounding, just weak and thin.  The left channel had deep, powerful bass and a terrifically balanced sound as I've come to expect from his Piegas (Sidebar:  anyone with some cash needs to snag these up fast; they are a steal at $3K), but the right side had little bass and reduced volume.

When I got home I turned on my system and noticed some oddness in the way it sounded.  I turned it off until I could focus on it.  Today, after a few troubleshooting tests, I've determined the right channel of the amp is diminished, much the same as Carlman's system sounded. Mids are clear and undistorted, but not as loud as the left channel.  Bass is present, but almost as an afterthought.

I've owned the amp 20+ years without an issue.  It's so strange that the problem happens the same week that Carl's took the same nosedive.

Any ideas as to how to fix it?

[richidoo]

If Carl hadn't already told me about his amp then I would suggest you clean out your right ear. 

Hmm...     Mercury is not retrograde until June 17.

[bpape]

I was thinking the same thing as I read that.....

Bryan

[allenzachary]

I wish it was that simple.  The final test of the troubleshoot was to swap speaker wires at the amp, the "bad" sound jumped across to the left speaker, proving that the only wax that is in the way are the stacks of records overflowing from every closet.

Seriously, I know some Nervosan can explain what is happening inside my amp.  Right?

[_Scotty_]

Try checking to see if you have blown fuse on the channel that is missing the bass.
Scotty

[richidoo]

Preliminary diagnosis of Carl's amp looks like damaged input stage (opamp) from a source fault, will be replaced with much tastier chip. Could yours have a similar issue? Any lightning? Any loud pops or erratic signal from the source feeding the amp?

Does yours have the optional balanced inputs?

[allenzachary]

I leave the amp on 24/7 so any thing could have taken it out.  Then again, I've had the amp since 1990 without incident...I guess it was due.

The inputs are only RCA connectors.

It makes sense that the problem is in the opamp section.  I've heard blown outputs and they don't sound like this.  

I haven't ventured inside the amp yet.  There isn't much to see though.  

There are two sets of fuses, but I believe both are on the output side and if any were blown the channel would be out, rather than diminished.  

The weekend is here.  I'll take a lot inside to see if there is anything obviously wrong.

[_Scotty_]

If the speaker fuse is by-passed by a cap then you would still hear some highs but the bass would be missing.
Scotty

[allenzachary]

Cracked the top and checked both sets of fuses, which are good.  *sigh*

Nothing obvious...no swollen caps, burn marks etc.

Next?

[richidoo]

You need a tech to probe a LF signal through the amp.

I can't find the schematic online to see if it is an opamp input or discreet transistors. Can you get a picture of the top and bottom of the amp board? I would bet they are discreet transistor input stage.  Which would mean a simple and easy repair if that's what's wrong.

If you can get it to me, I know someone who can repair it well and reasonable. Allow 8 weeks turnaround because he's busy working on a few other AN members problems.   Maybe you have a good tech down your way?
Rich

[BobM]

Unfortunately it could be anything from a bad solder join to a  blown resistor or cap to a bad op-amp to a transistor. No way of really knowing unless you go in with a meter. If sound was gone completely it might actually be easier to identify.

Sorry

[Carlman]

Really? This is nuts. Sorry to hear it, Allen.
Fyi, mine wasn't failing reliably. Sometimes it would be fine and then sometimes it wouldn't. A loose solder joint somewhere? Idk but it's in the shop. All 125+ lbs of it. Oy!

I took the opportunity to have some other upgrades/fixes done too. My preamp will have new caps so I'm interested in how that sounds.

Allen's amp isn't super heavy. So don't rule out shipping. Plus it'd be a great excuse to come back to NC and hear it working.

-C

[AcidJazz]

In my CJ Sonograph 250, I get reduced volume when an internal fuse blow on the respective channel. Check internal fuse visually and with a VOM (meter).