Amplifier fault
 

My power amplifier has just stopped working. It's a Rose English made
70W stereo amplifier, about 20 years old. Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but no noise at
the speakers. Now nothing - no mechanical or electrical noise, and the
power LED doesn't come on. Also, when I disconnected the speaker cables,
one had sheared off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter? A likely cause?


--
Cheers, Rob

Amplifier fault
 

"RJH" wrote in message
...
My power amplifier has just stopped working. It's a Rose
English made 70W stereo amplifier, about 20 years old.
Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but
no noise at the speakers. Now nothing - no mechanical or
electrical noise, and the power LED doesn't come on. Also,
when I disconnected the speaker cables, one had sheared
off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse
for continuity. All fine.

Any other diagnostics I could do, given limited
understanding of such things and a basic multimeter? A
likely cause?




Look to see if you can see any relays - usually two - in the
speaker output area. If you have shorted the L/S output,
transistors (even power FETs) will destroy themselves many
times more quickly than any fuse. This could result in the
output sitting at one rail rather than earth (dual supply.)
The protection circuit that operates the relay to connect
the amp to the speaker with thus be inhibited to protect the
speakers - the LED may be switched by this protection
circuit.

Equally if one of the output transistors has gone short
circuit then it could have blown the bridge rectifier on the
supply circuits although this probably would more likely
blow a fuse first.

Have a look at the residual d.c. on the speaker terminals
(assuming you have now repaired the faulty one) - expect to
see up to about 50-100mV or so. If there is no residual on
either channel then look for d.c. on the fuses - make sure
you don't try to measure the ones carrying mains!

If you feel at all unsure DON'T fiddle, get it to someone
who knows what they are doing. Most people don't know that
Richers have a service dept that does repairs.


--
Woody

harrogate three at ntlworld dot com

Amplifier fault
 

"RJH"

My power amplifier has just stopped working. It's a Rose English made 70W
stereo amplifier, about 20 years old. Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but no noise at the
speakers. Now nothing - no mechanical or electrical noise, and the power
LED doesn't come on. Also, when I disconnected the speaker cables, one had
sheared off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter?


** Check for continuity through the AC plug active and neutral pins -
resistance reading should be about 10 ohms.

Check the fuse in the plug too.


..... Phil

Amplifier fault
 

"Woody"


Look to see if you can see any relays - usually two - in the speaker
output area. If you have shorted the L/S output, transistors (even power
FETs) will destroy themselves many times more quickly than any fuse.


** They fail internally short circuit and then fuses will blow.

However, the OP simply has no power at all to the circuitry and no blown
fuses.

One analyses this sort of scenario with an ohm meter and a pair of mk 1
eyeballs.



..... Phil

Amplifier fault
 

I'd not touch it myself even if I could see as without the circuit diagram
everything is a bit of a guess.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"Woody" wrote in message
...
"RJH" wrote in message
...
My power amplifier has just stopped working. It's a Rose English made 70W
stereo amplifier, about 20 years old. Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but no noise at the
speakers. Now nothing - no mechanical or electrical noise, and the power
LED doesn't come on. Also, when I disconnected the speaker cables, one
had sheared off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter? A likely cause?




Look to see if you can see any relays - usually two - in the speaker
output area. If you have shorted the L/S output, transistors (even power
FETs) will destroy themselves many times more quickly than any fuse. This
could result in the output sitting at one rail rather than earth (dual
supply.) The protection circuit that operates the relay to connect the amp
to the speaker with thus be inhibited to protect the speakers - the LED
may be switched by this protection circuit.

Equally if one of the output transistors has gone short circuit then it
could have blown the bridge rectifier on the supply circuits although this
probably would more likely blow a fuse first.

Have a look at the residual d.c. on the speaker terminals (assuming you
have now repaired the faulty one) - expect to see up to about 50-100mV or
so. If there is no residual on either channel then look for d.c. on the
fuses - make sure you don't try to measure the ones carrying mains!

If you feel at all unsure DON'T fiddle, get it to someone who knows what
they are doing. Most people don't know that Richers have a service dept
that does repairs.


--
Woody

harrogate three at ntlworld dot com

Amplifier fault
 

"Brian Gaff"

I'd not touch it myself even if I could see as without the circuit diagram
everything is a bit of a guess.


** It's amazing how much better one can do things, solve problems etc with
the benefit of eyesight.

Guessing and imagining is all any blind person can ever do.




.... Phil

Amplifier fault
 

On 18/03/2014 23:32, Phil Allison wrote:
"RJH"

My power amplifier has just stopped working. It's a Rose English made 70W
stereo amplifier, about 20 years old. Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but no noise at the
speakers. Now nothing - no mechanical or electrical noise, and the power
LED doesn't come on. Also, when I disconnected the speaker cables, one had
sheared off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter?


** Check for continuity through the AC plug active and neutral pins -
resistance reading should be about 10 ohms.

Check the fuse in the plug too.


Thanks - no continuity across one of the pins.

I didn't see a fuse holder built into the 3 pin mains socket at the back
of the amp. One of these:

http://www.maplin.co.uk/p/1a-time-de...-10-pack-gl59p

has blown. I'll get some tomorrow and give it a go.


--
Cheers, Rob

Amplifier fault
 

On 18/03/2014 20:26, Woody wrote:
"RJH" wrote in message
...
My power amplifier has just stopped working. It's a Rose
English made 70W stereo amplifier, about 20 years old.
Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but
no noise at the speakers. Now nothing - no mechanical or
electrical noise, and the power LED doesn't come on. Also,
when I disconnected the speaker cables, one had sheared
off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse
for continuity. All fine.

Any other diagnostics I could do, given limited
understanding of such things and a basic multimeter? A
likely cause?




Look to see if you can see any relays - usually two - in the
speaker output area. If you have shorted the L/S output,
transistors (even power FETs) will destroy themselves many
times more quickly than any fuse. This could result in the
output sitting at one rail rather than earth (dual supply.)
The protection circuit that operates the relay to connect
the amp to the speaker with thus be inhibited to protect the
speakers - the LED may be switched by this protection
circuit.

Equally if one of the output transistors has gone short
circuit then it could have blown the bridge rectifier on the
supply circuits although this probably would more likely
blow a fuse first.

Have a look at the residual d.c. on the speaker terminals
(assuming you have now repaired the faulty one) - expect to
see up to about 50-100mV or so. If there is no residual on
either channel then look for d.c. on the fuses - make sure
you don't try to measure the ones carrying mains!

If you feel at all unsure DON'T fiddle, get it to someone
who knows what they are doing. Most people don't know that
Richers have a service dept that does repairs.


Thanks for that - I was a bit dippy and didn't notice the fuse holder in
the IEC socket. Never thought to look . . . never noticed one before.

--
Cheers, Rob

Amplifier fault
 

On 18/03/2014 23:32, Phil Allison wrote:
"RJH"

My power amplifier has just stopped working. It's a Rose English made 70W
stereo amplifier, about 20 years old. Here it is with the top off:

http://flic.kr/p/mbRV1M

When I switched it on there was a noticeable 'phutt', but no noise at the
speakers. Now nothing - no mechanical or electrical noise, and the power
LED doesn't come on. Also, when I disconnected the speaker cables, one had
sheared off at the banana plug. So a short is a possibility.

I've checked the 4 internal fuses and the mains 5A fuse for continuity.
All fine.

Any other diagnostics I could do, given limited understanding of such
things and a basic multimeter?


** Check for continuity through the AC plug active and neutral pins -
resistance reading should be about 10 ohms.


Out of interest, what's the significance and proper application of that
test? I tried it on an amp that I know to work (a NAD 3020) and it reads
20 ohms. On a CD player and DAB tuner - both 100 ohms.


--
Cheers, Rob

Amplifier fault
 

On 19/03/2014 08:41, Brian Gaff wrote:
I'd not touch it myself even if I could see as without the circuit diagram
everything is a bit of a guess.

Brian


Well, yes.

But if a safe and educated guess, with the help from this NG for
example, gets me in the ballpark, I can't see what harm's done. From
there, I can decide whether to brave fixing it myself, having it fixed,
or taking it to the tip for example.

--
Cheers, Rob