Vintage Synth Repair

I have a Fostex Model 600 power amp.
A while ago one of the channels started to quietly pop and click.
I pulled the 1/4inch jack (from my mixer) in and out and it cleared up (the plug socket is not switched) so I thought it was a dirty contact.
The problem returned and was fixed several times.

Now the pops and clicks are intermittent and the replugging has no effect.

It doesn't seem thermal because if it is left alone with just the quiescent current flowing though it, it can start noisy then it can be quiet for and hour and the pops just start up again.
The DC measurements are close to the cct diagram.

Before I start replacing all the caps, should I be looking at some other components?

Thanks

Royce

Royce
 
I would start by replacing the socket, just in case there is a bad contact
between the jack and the socket.  This would automatically take care of any
broken solder joints on the socket connections.  Should this not cure the
fault completely then start replacing caps.
 
Regards
Brian G3OYU
www.g3oyu.co.uk
 
  _____  

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of rpcfender
Sent: Sunday, 02 August 2009 03:44
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] Amp clicks
 
  
I have a Fostex Model 600 power amp.
A while ago one of the channels started to quietly pop and click.
I pulled the 1/4inch jack (from my mixer) in and out and it cleared up (the
plug socket is not switched) so I thought it was a dirty contact.
The problem returned and was fixed several times.

Now the pops and clicks are intermittent and the replugging has no effect.

It doesn't seem thermal because if it is left alone with just the quiescent
current flowing though it, it can start noisy then it can be quiet for and
hour and the pops just start up again.
The DC measurements are close to the cct diagram.

Before I start replacing all the caps, should I be looking at some other
components?

Thanks

Royce

I would argue against replacing the caps as the stages of the amp are  
all DC coupled withe exception
of C601, the 1 uf coupling cap at tyhe input staGE. My money( we are  
all guessing here) is on the input differential
stage as the culprit, specifically TR601,that's the 5 legged dual  
transistor. 2SC2259?
if you are just determined to replace caps, try c601 first.

On Aug 1, 2009, at 10:44 PM, rpcfender wrote:

> I have a Fostex Model 600 power amp.
> A while ago one of the channels started to quietly pop and click.
> I pulled the 1/4inch jack (from my mixer) in and out and it cleared  
> up (the plug socket is not switched) so I thought it was a dirty  
> contact.
> The problem returned and was fixed several times.
>
> Now the pops and clicks are intermittent and the replugging has no  
> effect.
>
> It doesn't seem thermal because if it is left alone with just the  
> quiescent current flowing though it, it can start noisy then it can  
> be quiet for and hour and the pops just start up again.
> The DC measurements are close to the cct diagram.
>
> Before I start replacing all the caps, should I be looking at some  
> other components?
>
> Thanks
>
> Royce
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>

Thanks to everyone for the help.

I did re solder and then replace the jack but no joy there.

> I would argue against replacing the caps as the stages of the amp are  
> all DC coupled withe exception
> of C601, the 1 uf coupling cap at tyhe input staGE. My money( we are  
> all guessing here) is on the input differential
> stage as the culprit, specifically TR601,that's the 5 legged dual  
> transistor. 2SC2259?
> if you are just determined to replace caps, try c601 first.

It was the first thing I tried, but unfortunately it wasn't the problem.

I'll try and track the transistor pair down.

Thanks 
Royce

--- In vintagesynthrepair@yahoogroups.com, "rpcfender" <rpcfender@...> wrote:
>
> Thanks to everyone for the help.
> 
> I did re solder and then replace the jack but no joy there.
> 
> > I would argue against replacing the caps as the stages of the amp are  
> > all DC coupled withe exception
> > of C601, the 1 uf coupling cap at tyhe input staGE. My money( we are  
> > all guessing here) is on the input differential
> > stage as the culprit, specifically TR601,that's the 5 legged dual  
> > transistor. 2SC2259?
> > if you are just determined to replace caps, try c601 first.
> 
> It was the first thing I tried, but unfortunately it wasn't the problem.
> 
> I'll try and track the transistor pair down.
> 
> Thanks 
> Royce
>
Hey...I dont know if they ship to down undah but:

http://www.bdent.com/search/part.jsp?partnum=2SC2259

Hi 
Thanks for the address, but I think I have found a local place.

All the best
Royce

> >
> Hey...I dont know if they ship to down undah but:
> 
> http://www.bdent.com/search/part.jsp?partnum=2SC2259
>

Hi
The culprit turned out to be C602, a 180p ceramic (what else!) which looks like it is part of an input filter. A 10c fix.

Thanks again for all the help - no more clicks, yea!

Royce