Sony STR-6065 Receiver Left Channel Issues

[Draal1000]

Hi folks,
I have a Sony STR-6065 receiver that is in great shape except for one major problem: no audio from left channels. Both Main and Remote lefts are not working. When I turn on the receiver, I get an initial "pop" from the left channel speaker, then the right channel gradually comes on as you'd expect from a receiver.

There's 1 fuse and that's not related to the output of either channel. If that was blown, the unit wouldn't even turn on, so I know that's not the issue.

What I've done so far:
1. Checked all connections to speakers and channels
2. Swapped speakers/channels with no change in problem
3. Used Deoxit on all switches and pots.
4. Purchased the service manual and I feel the problem may be traced to the power amplification board. (This is just an assumption and reason why I'm asking for help).

So far, I haven't seen signs of bulging caps. What recommendation do any of you have? This is a real nice looking receiver with decent a decent following and I'd like her to rock again.

[Draal1000]

Oops. I bet this topic is in the wrong section. Sorry, first post here...

[Mooly]

Nope... it's in the right section

We'd have to see the circuit but most faults like this are caused by a power amp problem... either failed output transistors or a failed "STKxxxx" module if it uses those.

You need to check the DC offset at the L and R amplifier outputs (on the board before any protection relays) as a first step.

[Draal1000]

Thanks for the tips Mooly ! I'll be studying that area more closely soon. Here's a segment of the power amp circuit :

[Mooly]

Check that the DC voltage on those two 0.5 ohm 5 watt resistors is zero on the faulty channel as measured from ground. Can't make the numbers out... they go to Q901 and 902

It should be essentially zero on each end of each resistor.

[Draal1000]

Just checked the 0.5 resistors (R738, R739) this morning. Getting a 0.00 reading on both.

[Mooly]

OK... so with the amp on you have zero volts DC on those 0.5 ohms. I'm surprised tbh, but that's a good result. In a way that means 95% that the problem is elsewhere in the amp... strange

I don't know what test gear you have... a scope and generator would help but I'll assume you haven't access to either.

You need to confirm that the six supplies to the faulty channels power amp are correct. Be very careful measuring, one slip will cause a lot of damage.

That's +50volts DC on Q902 and -50 volts DC on Q901.

And confirm the auxilliary supplies of +54 and +55 and - 54 and -55 volts DC from the regulator transistors Q801 and 802. Measure on the collector and emitter of each. These voltages will only be approximate and could be different by 10% or so... we are just confirming they are there.

Make sure these supplies arrive at the power amp as there are some safety resistors along the way.

[Draal1000]

I don't have access to the gear you mentioned but I did check the transistors on the board. Q705's collector is showing 23.6v when it should read 0.8. My initial testing of the other transistors on that board shows numbers that are similar to the service manual's. Nothing as off the charts as Q705.

I will re-read your latest recommendations while I take a break.

[Mooly]

Just very quickly... and its not easy to read the circuit tbh... bit blurry

23 volts on Q705 collector ? Check the Base/Emitter volt drop across Q 708 (the driver) to the "top" output of the output pair. Is it open circuit ? Is the 5.6 ohm OK
Base/emitter going open circuit is a typical failure mode...

All the base emitter volt drops should not exceed around 0.8 volts max.

[Draal1000]

Yeah, that upload is a bit blurry. Let's see if this one is better. My 11 month old son just woke up, so I'll do as you suggest once I get some free time later today .

Your help has given me greater confidence in finding the problem.