Arfa Daily wrote:
wrote in message
ups.com...
I've got an Integra DTR-5 (nearly identical to the Onkyo TX-DS 575)
thats in circuit protection mode (immediately goes into standby as soon
as the the power switch is pressed on, and opens the main power relay).

The most likely scenario is that it is detecting a DC offset on one of the
channels, but if you are new to faultfinding, you will probably struggle to
get to the bottom of the problem with any certainty that you can confidently
switch back on, without finishing up with another pile of dead components.
The reason that you read infinity on the speaker lives, is because these are
connected to the output stages via the output relay(s), which will be
remaining open, if the unit is in protect.

Thanks for the concern. It's already a dead pile of parts, so I might
as well learn something new. Saw the relays. That explains that...
The receiver has separate front channel and center/surround channel amp
transistor boards. I think it's been narrowed down to the surround
board - when I unplug the surround board's rail power, the circuit
protection does not kick in. I tried to catch the offset voltages on
the surround board before the protection kicks in, but the DMM doesn't
register fast enough.

To check for DC offset, you need to measure at the output transistors'
midpoint, which will be at the junction of the two low value emitter
resistors (assuming that the amp uses conventional output devices, and not
an STK hybrid). At idle, you should have zero volts, or very very close to
zero, on this point. If you find any offset on any channel, then it's likely
that one or other or both of the output transistors has failed, but that is
unlikely to be the end of the story.

Thanks - I see that (.22 ohm middle tapped resistors after the amp
stages, but before the coils). I've got to get external voltage to the
surround board rails somehow. The rails are 45V, but would driving them
at 12V (two 12V batteries, one for each rail, to give 24V delta)
provide enough voltage for a diagnosis?

With a DC coupled amp, the problem can
be coming from virtually anywhere right back to the front end, and other
transistors besides the outputs, will likely be faulty. If it does use an
STK hybrid - and I seem to recall that Onkyos don't - then any DC offset
will normally be cured, without any other issues, by simply replacing it. If
you don't detect any offset, then the problem is with the protect circuit
itself (not unusual), but sometimes, these monitor many parameters, and can
be an absolute bitch to troubleshoot.

This receiver has the C5198 and A1941 transistors attached to the heat
sinks (no STK's that I can see). It's a 5 channel + sub pre-out, along
with separate two channel B speakers (quasi-multi room). The surround
output circuit is fed by a LC4966 chip, which is fed by an M62447SP
chip (that feeds the front and center amp circuits directly as well).
Data sheets show the LC4966 as a quad bilateral switch and the M62447SP
as a 6 channele electric volume chip. Hopefully it's something on the
transistor board. I can almost follow analog circuits, and get a
concept of whats going on... I'd love to power each individual channel,
but there is one feed to the entire surround board, and the rest are
PCB traces and hard jumpers soldered in place (which I suppose I could
unsolder, if thats what I'm supposed to do).

If you have a full-rail offset on all channels - say +45v - then look for a
missing negative rail or vice versa.

Thanks - will check.

I don't mean to put you off, but honestly, if you are new to faultfinding,
this kind of project is not something I would recommend to a beginner. Most
of us who are involved in repairing this kind of gear daily, bypass the
power switching, and use a variac to bring up the supply slowly, so that we
can measure what's going on (wrong !!) before all those shiny new
transistors fail again ... Good luck with it.

I appreciate the advice, and love a challenge. I wish I had a Variac (I
see many flavors on Ebay, for $10=$100+) or knew of someone locally
(Farmington, CT) that had one, but I'll have to figure out another way
to bring up the supply voltages gradually. It's been a good learning
experience so far, and if this works, I've got a bookshelf system the
lost its right channel after it was dropped that would be nice to use
in the bedroom, and another one for the basement exercise room. I think
every point you mentioned generated about 2 hours of googling,
schematics, and testing, but I'm more comfortable already, and feel
like I'm starting to get the concepts. Thanks again!

Andris