Help, please!

My much-loved 30 year old A60 has recently developed a fault, but having
no electronics experience, I have no idea how to diagnose it. If anyone
can at least give me a clue, and an idea of how easy it might be to
repair, I would be very grateful.

The symptoms are that the left channel, after the amp has warmed up
distorts very badly. This is only when using the loudspeakers - the
headphones output sounds fine.
The symptoms are the same from any source, using either the switched or
non-switched terminals - I've also swapped speakers to confirm that the
left speaker has not developed the fault.

Thanks in advance for any advice - I really don't want to dump this amp...

Ade.

--
Adrian Ball
http://www.hp5.co.uk/
Do not use spam-trap - please use:-
ade at hp5 . co . uk

On Sun, 06 Jul 2008 13:08:28 -0500, Adrian Ball wrote:

Help, please!

My much-loved 30 year old A60 has recently developed a fault, but having
no electronics experience, I have no idea how to diagnose it. If anyone
can at least give me a clue, and an idea of how easy it might be to
repair, I would be very grateful.

The symptoms are that the left channel, after the amp has warmed up
distorts very badly. This is only when using the loudspeakers - the
headphones output sounds fine.
The symptoms are the same from any source, using either the switched or
non-switched terminals - I've also swapped speakers to confirm that the
left speaker has not developed the fault.

Thanks in advance for any advice - I really don't want to dump this
amp...



There is a service manual for this amp he
http://www.retrohifi.co.uk/ar_cambrige.html

I realise that it might not mean much to you, but it could be invaluable
for someone trying to diagnose the problem. It may be worth your while to
print it out then take it to a local electronics "bod" to have a look at.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net
Filtering everything posted from googlegroups to kill spam.

On Sun, 06 Jul 2008 18:30:16 +0000, mick wrote:

On Sun, 06 Jul 2008 13:08:28 -0500, Adrian Ball wrote:

Help, please!

My much-loved 30 year old A60 has recently developed a fault, but
having no electronics experience, I have no idea how to diagnose it.
If anyone can at least give me a clue, and an idea of how easy it might
be to repair, I would be very grateful.

The symptoms are that the left channel, after the amp has warmed up
distorts very badly. This is only when using the loudspeakers - the
headphones output sounds fine.
The symptoms are the same from any source, using either the switched or
non-switched terminals - I've also swapped speakers to confirm that the
left speaker has not developed the fault.

Thanks in advance for any advice - I really don't want to dump this
amp...



There is a service manual for this amp he
http://www.retrohifi.co.uk/ar_cambrige.html

I realise that it might not mean much to you, but it could be invaluable
for someone trying to diagnose the problem. It may be worth your while
to print it out then take it to a local electronics "bod" to have a look
at.

Thanks for taking the time, it's much appreciated! I had found that site
earlier tonight, and thought "this might prove useful to someone that
knows what they are doing" - just got to find the local electronics bod
now...

Cheers,
Ade.

--
Adrian Ball
http://www.hp5.co.uk/
Do not use spam-trap - please use:-
ade at hp5 . co . uk

"Adrian Ball" wrote in message
...
On Sun, 06 Jul 2008 18:30:16 +0000, mick wrote:

On Sun, 06 Jul 2008 13:08:28 -0500, Adrian Ball wrote:

Help, please!

My much-loved 30 year old A60 has recently developed a fault, but
having no electronics experience, I have no idea how to diagnose it.
If anyone can at least give me a clue, and an idea of how easy it
might
be to repair, I would be very grateful.

The symptoms are that the left channel, after the amp has warmed up
distorts very badly. This is only when using the loudspeakers - the
headphones output sounds fine.
The symptoms are the same from any source, using either the switched
or
non-switched terminals - I've also swapped speakers to confirm that
the
left speaker has not developed the fault.

Thanks in advance for any advice - I really don't want to dump this
amp...



There is a service manual for this amp he
http://www.retrohifi.co.uk/ar_cambrige.html

I realise that it might not mean much to you, but it could be
invaluable
for someone trying to diagnose the problem. It may be worth your
while
to print it out then take it to a local electronics "bod" to have a
look
at.

Thanks for taking the time, it's much appreciated! I had found that
site
earlier tonight, and thought "this might prove useful to someone that
knows what they are doing" - just got to find the local electronics
bod
now...

Cheers,
Ade.

--
Adrian Ball
http://www.hp5.co.uk/
Do not use spam-trap - please use:-
ade at hp5 . co . uk


A&R became Arcam, so you could ask their Service Dept. Failing that (and
not many people know it) Richer Sounds also have a Service Dept and may
be able to help you.

It sounds to me like a bias fault - one of the output transistors is
getting hot and a sensor - usually a transistor fitted into the
heatsink - is sensing the rise in temp and trying to switch the output
transistor off. Net result is a seriously distorted output.

The headphone output on good quality amps often came from its own source
to keep noise and distortion levels down.

I notice you are in Wirksworth. Lowe Electronics on Chesterfield Road in
Matlock are an Amateur Radio equipment supplier and probably don't do
repairs any more but they may well know someone locally who can help you
if all else fails.


--
Woody

harrogate three at ntlworld dot com

"Woody"

It sounds to me like a bias fault - one of the output transistors is
getting hot

** Only one ?

That is not even possible with a bias problem.

and a sensor - usually a transistor fitted into the heatsink - is sensing
the rise in temp and trying to switch the output transistor off.


** Got no hope of doing that - since it only stabilises the bias current
setting.


Net result is a seriously distorted output.


** Wot a load of claptrap.




...... Phil

Phil Allison said:

** Wot a load of claptrap.

Why not demonstrate your superior knowledge
in a constructive way and help the OP with his problem?

--
Ken

"UnsteadyKen"
Phil Allison said:

** Wot a load of claptrap.

Why not demonstrate your superior knowledge
in a constructive way and help the OP with his problem?


** If he cares to send me his amp, I will fix it for a fee.

But the freight to and from Australia might be a problem ....

Anyhow, it is constructive to expose a false analysis.

All way over your thick, wooden head.





....... Phil

Phil Allison said:

All way over your thick, wooden head.

ROHL

--
Ken

Woody wrote:
It sounds to me like a bias fault - one of the output transistors is
getting hot and a sensor - usually a transistor fitted into the
heatsink - is sensing the rise in temp and trying to switch the output
transistor off. Net result is a seriously distorted output.

It's possible one of the devices in the output stage has lost contact
with the heatsink (Loose securing bolts/ clip). I've had that happen
with similar symptoms before now. It may even be the heat sensing device
in the bias setting chain being defectove or loose, which is usually
either a transistor or diode secured to the heatsink with a spring clip.

At least with the other channel apparently working correctly, there's a
comparison avaliable without using the schematic.

--
Tciao for Now!

John.

For what it's worth, I have 2 A60s and they have both displayed similar faults at some stage in their lives.
In both case it turned out to be the horrible volume pot. They wear and then a length of the resistive carbon track falls off and shorts the pot out. I assume that this then results in the output stage seeing an input level it was never intended to deal with.
As the pot's are rubbish anyway, you could do worse than replace yours, as it's probably overdue even if it doesn't cure the fault in your case it eliminates a common cause of problems on the A60.
It's not a difficult job, just a bit time consuming as you have to detach the PCB from the metal base which is pretty much a complete stripdown.
Maplin now seem to be the only people supplying them, but they are cheap, which is good as they are very out of balance at the bottom of their range.
That is fixable to some extent:

"While converting the master volume in a friend's Twin Reverb to post-pi,
here's how I balanced an Alpha 250k dual-ganged pot. You can only get to the
inside of the rear pot, unless you want to figure out a way to destroy the
crimp on the end of the shaft and glue it all back together afterwards. So,
I just worked on the back pot.

I started at the full ccw position. I turned it cw in little increments and
checked the resistance from bottom to wiper on both pots until I found a
spot where the difference was 1k. I went by 1k because that was the lowest
amount I could read on the VOM - if I were using a lower-valued pot and
reading the resistance on a lower scale, then I'd look for a smaller
difference. Anyway, if the back pot had more resistance than the front one,
I would take a pencil and put some graphite down at the outer edge of the
conductive element until they were the same. If the back pot had less
resistance than the front pot, then I would scrape at the edge of the
conductive element with a penknife or one of the test probes. It takes just
the tiniest perceptable scratch to increase the resistance 1k. I kept going
in steps all the way up to full cw. The pot is now balanced within 1k.

I was careful to make all the additions and removals outside the path of the
wiper. The conductive element was a little bit wider than the path of the
wiper all the way around.

Will the pot stay balanced over the long run? I don't know, but it's plainly
better than stock now. If wear and tear is going to unbalance it, it still
won't be any worse than a stock pot with the same wear and tear."