Pioneer SX-780 need help signal tracing. Powers on no sound. Darlington packs are good.

After deoxit bath. AUX now produces sound and flipping the switches doesn't put it in protection mode. Volume pot still very static.
In Stereo left channel no sound. In Mono both channels have sound.
I'm going to give it another deoxit bath.
Stay tuned.

 

Myrlynski you talked about taking out the Darlington packs and putting in 1k resistors to bypass the STK'S and perform further testing. In the schematic you highlighted what appears to be 4 resistors. Am I understanding that right.
I would connect the resistors to the four pins you highlighted and tie them in to what???

 

They are for DC troubleshooting. The resistors just go in place of the STKs, they don't tie in anyplace else. One goes from pin 0 to pin 8, another one goes from pin 1 to pin 3 on the SX-780 circuit board for each STK location. What they do is complete the DC current feedback path so that the amp stabilizes. Of course there is no current gain because the STK is missing, therefor no audio to go to speakers (which should not be connected). But you can see if there is another problem in the voltage gain part of the amp, or something else causing a major DC offset problem. The DC levels are not exactly the same as if there were STKs in place, but they are fairly close, and since there is no STK a bad one won't be messing up the amps DC balance. Also the Power Supplies will not be overloaded by a bad STK either, so PS troubleshooting is easier because you can power up without the DBT. You also do not risk frying an STK while troubleshooting 'other' problems. You do have to put the STK in to get audio output, and to set final DC offset and (with the STK replacement module) set bias. Hope that helped and is not too much information, sometimes I ramble on . . .

 

If both channels have output in MONO mode, both output amplifiers are operating. The investigation should be upstream, where the signal is split or blended ahead of the MONO switch. The prime suspect is the stereo/mono switch, and tape monitor switches ahead of that.

 

Tape 1,2 and FM tuner all produce sound out of left and right channel in stereo.
AUX still wont produce sound in left channel

I took out the stereo/mono switch out and cleaned it internally. I used 2000 grit sandpaper, eraser, and Fader lube, this did not fix the problem. Should I continue cleaning out the rest of the switches starting with tape monitor 1 and 2. (Before and after pics below.)


Sometimes when using the function switch I hear a loud pop and the receiver goes in protection for a short period. I'm going to take apart the function switch for inspection and cleaning.

 

If you want the "easy" way out, acquire some jumpers with miniature alligator clips or grabber hooks, then jumper the switches at different points in the circuit.

For example, for the AUX switch L and R channels, jumper from Pins 52 (L) and 48 (R) on the board edge to R207 (L) and R208 (R). That eliminates all the questionable switches and their connections and solder joints. Plug in your source and test the operation.

 

Would I need two jumpers per channel. One jumper on one side or resistor to pin, and another jumper on the other side of resistor to pin?

 

Is that question is directed at the resistor substitution I suggested? I just removed the bad STK completely and tack soldered the resistors to the PCB where the STK pins would be soldered if it was present.

Watthour is talking about bypassing noisy/dirty switches with clip on jumpers to temporarily troubleshoot noise and/or signal loss. A different problem than bad STK units.

 

I was referring to what Watthour was talking about. I want to try what he said, then I will know if there is a problem with the switches or not and I can save time buy not having to take apart and clean every switch one at at time. (Note once I get the unit up and running to proper specs I can always go back and clean each switch independently. I just don't know exactly how to hook up the jumpers. For example if I was doing channel A would I connect one jumper to pin 52 and the other end to R207, then another jumper to pin 52 and the other end to the other side of R207. So I would need two jumpers per channel?

If afer this test I can't isolate the problem to the switches I will try merlynski's method. Thank you

 

FWIW I do not believe substituting resistors for STKs will help troubleshoot/solve noise or signal loss upstream from the power amp section of the circuitry. That process is useful for analyzing bad STKs and STK related power supply problems. I believe you are past that step since you have audio out of both channels in Mono.
Follow Watthour's instructions to diagnose the noise and signal loss in the switches. Dirty/corroded switches can be a major problem with these old systems, sometimes disassemble and clean is the only way to resolve bad switches without replacing them.

 

One jumper per channel. Just pick an end of either resistor, or go to the ungrounded ends of the volume pots (but it's a little more congested there).

 

Using this jumper method I get sound from both channels in Stereo. Does this mean the problem is not in the controls or at least not in the AUX selector.
I also have a Scope and signal generator that I'm trying to learn to use.

 

That means the loss of signal is somewhere between the points you jumpered. The most likely place is the selector switches, S1, S2, S3, and S4. It's also possible the connections to the switches (circuit traces, solder joints, etc.) are unreliable. What it does prove is that the audio amp sections are just fine.

If you want to isolate it even further, try jumpering switches one at a time. If you jumper a questionable switch and the sound returns, you've found the culprit.

 

I found the problem it was the function switch (AES 108). After opening it up for inspection I noticed the small little tabs, there are six of them, are all bent up. It looks like most of the tabs got bent and jammed up in there. I spent a long time trying to rebend the tabs and get the switch operational and now I have signal on Left and Right stereo in Aux. But now on FM I am only getting signal in one channel. Also a loud pop after rotating the function switch. It seems those little tabs are still jamming up in there. Is there a place to order those tabs? I did order a replacement switch (AES 107) that should be the same as the AES 108. Once I receive that I will install it and that should fix that issue.

Now on to the next problem. The FM radio gets horrible reception, I can only get one station and that's with an antenna. Also the AM radio gets no reception.

 

good for you taking the switch all apart.
real basket case ...
FM:
, disconnect antenna, tune to a noisy spot, is the center tune meter in the dead center? if not you need a discriminator zero adj to start with.
signal meter defect full scale with a very strong signal?
test gear, inject RF and scope it out, check thd, alignment?

 

When the audio switch is repaired, we can walk you through a few tuner front-end checks in addition to the suggestion by rcs.

 

I got the switch installed and I have sound out of the left and right channels on all sources.
Now I get more FM stations but the stereo light wont turn on. I checked the LED and it is good so it might be something to do with the multiplex decoder.
I only get one station on AM and it is very faint.

Disconnect antenna, tune to a noisy spot, is the center meter dead center? Yes
Signal meter detect full scale with strong signal? I don't know what this means.
Test gear inject RF and scope it out, check thd, alignment? I do have a heathkit SG-8 RF signal generator but it has some weird connections https://www.ebay.com/itm/Custom-Sig...G-102-BK-E-200D-Eico-324-Others-/332421775922
Im looking into making some leads I think the connector is called a microphone connection.

 

Since the audio problem resolved, this should now transition to the RF section.

Given that the other parts of the unit evidently had dirt/oxidation problems, it might be good to review this: http://audiokarma.org/forums/index.php?threads/how-to-clean-a-tuner-analog.144163/

 

Looks like the old Motorola "PL259" type connector, from the pic in the link, they are common, adapters or cables are available.
the sx has 75 ohm and is a bare wire connection.

with an FM antenna connected (folded dipole or better) tune across the dial, do you get any signal strength deflection on the meter? what number? if you get no deflection you have to check that the the ckts are getting power then have to work from the RF front end thru the IF to the detector PA chip.

 

Are you suggesting to start another thread dedicated to the RF. Should I start a new thread in the Tuner section or in Pioneer?
I followed the directions on how to clean a tuner but still cant get any stations in Stereo and still have no AM radio the needle doesn't even move for AM