Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Denon PMA-770 problems

Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed & checked,
all good. Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed
for parts.




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Default Denon PMA-770 problems

Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed & checked,
all good.


Hmmm... that doesn't sound like a fried-output-transistor sort of
behavior.

Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed
for parts.


Given the age and the symptoms, I'd suspect the usual few culprits:

- Dried-out capacitors in the signal path (although one web page I
see on this amp suggests that it's a direct-coupled design with
no such 'lytics).

- Bad switches, potentiometers, or relays - either dirty, or with
worn contacts.

- Blown fuse in the supply to the output or driver transistors.

The amp seems to use some sort of active servo and biasing circuit...
if this has failed you might not be getting any signal drive into the
output transistors.

Seems to me it'd be worth the usual "inject a signal at the left
input, follow it through the circuit, and see where it goes away"
diagnostic technique. Would be a shame to bin it, if it's repairable.

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Default Denon PMA-770 problems

On 1/12/2012 20:32, Dave Platt wrote:
Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed& checked,
all good.


Hmmm... that doesn't sound like a fried-output-transistor sort of
behavior.

Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed
for parts.


Given the age and the symptoms, I'd suspect the usual few culprits:

- Dried-out capacitors in the signal path (although one web page I
see on this amp suggests that it's a direct-coupled design with
no such 'lytics).

- Bad switches, potentiometers, or relays - either dirty, or with
worn contacts.

- Blown fuse in the supply to the output or driver transistors.

The amp seems to use some sort of active servo and biasing circuit...
if this has failed you might not be getting any signal drive into the
output transistors.

Seems to me it'd be worth the usual "inject a signal at the left
input, follow it through the circuit, and see where it goes away"
diagnostic technique. Would be a shame to bin it, if it's repairable.


I did find a manual with adjustments, & schematics online. Unfortunately
my electronics level is not good enough to read a schematic & figure out
where the problem could be. I am good at R&R parts, I do have a decent
Protek digital VMM. I'll try cleaning & checking the balance & volume
pot. I didn't see any bulging caps, & all fuses that I could find were
good. Then feed the inputs & try to trace where it goes bad. Thank you.


--
Steve Walker
(remove brain when replying)
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Default Denon PMA-770 problems

On 2012-01-13 03:56:23 +0100, Steve Walker said:

On 1/12/2012 20:32, Dave Platt wrote:
Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed& checked,
all good.


Hmmm... that doesn't sound like a fried-output-transistor sort of
behavior.

Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed
for parts.


Given the age and the symptoms, I'd suspect the usual few culprits:

- Dried-out capacitors in the signal path (although one web page I
see on this amp suggests that it's a direct-coupled design with
no such 'lytics).

- Bad switches, potentiometers, or relays - either dirty, or with
worn contacts.

- Blown fuse in the supply to the output or driver transistors.

The amp seems to use some sort of active servo and biasing circuit...
if this has failed you might not be getting any signal drive into the
output transistors.

Seems to me it'd be worth the usual "inject a signal at the left
input, follow it through the circuit, and see where it goes away"
diagnostic technique. Would be a shame to bin it, if it's repairable.


I did find a manual with adjustments, & schematics online.
Unfortunately my electronics level is not good enough to read a
schematic & figure out where the problem could be. I am good at R&R
parts, I do have a decent Protek digital VMM. I'll try cleaning &
checking the balance & volume pot. I didn't see any bulging caps, & all
fuses that I could find were good. Then feed the inputs & try to trace
where it goes bad. Thank you.


the bad caps are not always bulky ...
you need an esr meter to check them really...
see my web site below for lots of schematics diy.
regards,

--
----------
Kripton

the ESR Repository @ http://kripton2035.free.fr/esr-repository.html
the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html

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Default Denon PMA-770 problems

Steve Walker wrote in message
...
Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed & checked,
all good. Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed
for parts.




--
Steve Walker
(remove brain when replying)



Check the tracking of vol and balance pots , compacted grease under a wiper
or bent wiper




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Default Denon PMA-770 problems

On Jan 13, 3:39*am, "N_Cook" wrote:
Steve Walker wrote in message

...

Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed & checked,
all good. Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed
for parts.


--
Steve Walker
(remove brain when replying)


Check the tracking of vol and balance pots , compacted grease under a wiper
or bent wiper


Since the left and right channels are identical, use the working
channel to diagnose the failing one. Check for the simple stuff
first. Check the signal at the tape out jacks. This should verify
that the signal is getting through the source switch. I would verify
that the +/- 15 supplies are there for the left side. If you have a
signal generator, feed the same signal into botht he left and right
and trace both sides. There is an IC inline that may be defective.
Check the pins on that IC for signal (either the generator signal or
just a CD.)

Update this with any progress you have made.

Dan
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Default Denon PMA-770 problems

"Steve Walker" wrote in message
...
Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed & checked, all
good. Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed for
parts.




--
Steve Walker
(remove brain when replying)



Check the emitter resistors and that the speaker relay is working
correctly - it could have bad or damaged contacts, even just on the one
channel.

For that matter, check for any other possibly burned resistors. Could be
loss of voltage to the driver transistors on that channel.

Mark Z.

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Default Denon PMA-770 problems

On Jan 13, 8:04*am, "Mark Zacharias"
wrote:
"Steve Walker" wrote in message

...

Right side works, left side not, unless volume is set way high, then
horrible staticy low volume. All output transistors removed & checked, all
good. Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed for
parts.


--
Steve Walker
(remove brain when replying)


Check the emitter resistors and that the speaker relay is working
correctly - it could have bad or damaged contacts, even just on the one
channel.

For that matter, check for any other possibly burned resistors. Could be
loss of voltage to the driver transistors on that channel.

Mark Z.

It could be an FET in an interstage portion of the channel that's
breaking down. The older amplifiers that used FET's sometimes suffered
from this. We used to touch them with a soldering iron and that would
temporarily restore operation. Or perhaps an oxidized switch contact.
My 1974 Sansui receiver does this and I have to operate the tone cut
switch to get it working again. Disassembly is a nightmare and I've
been putting it off.
It might be something very simple but with all due respect this may be
beyond your abilities to find it. I sincerely wish you luck however if
as you say you cannot read a schematic then I would hold little hope
of your success. Lenny
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Default Denon PMA-770 problems

An earlier poster said what I should have said. As the right channel is
working, you have a perfect reference. Find out what's different between the
channels, and you should be close to fixing it.

Of course, it helps to have a schematic, so you know where you're poking
your probes.


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