Weird problems with a receiver - STK-040 power pack

[ChopperCharles]

Got it and the left channel was out. Left it on for a bit, and the left channel came in super distorted. Left it on for a few hours, and when I went back to it the left channel was fine! Turned it off and came back 4 hours later, and the left channel is dead again. I'm hoping it's the STK-040, since I have a spare and I can easily swap parts... what do you guys think?

Unfortunately I have no test equipment. I did hook the tape output of the receiver into another receiver, and both channels exist there, so it's not a dirty switch. Likewise, when the internal integrated cassette deck is put into record mode, the VU meters show level on both channels. (Did I mention this is a Panasonic RA-6500 integrated all-in-one with a built-in tape deck? Low-fi for sure, but I want it working for my bedroom, so I can play tapes in there without having an arseload of equipment)

Thoughts? Is there an easy way to test the STK-040? I'm going to fix the cassette deck first and then start swapping parts.

Charles.

 

[orange]

Try the pots and switches before you go nuts operating on Mr. Sonic. If the amp pack were bad my guess is that it would either be completely dead on one channel or worse. It either works or it doesn't. You have something going out of spec or something, not the amp.

 

[stuwee]

Hi Charles, congrats on getting an all-in-one, these are surprisingly nice sounding units, their weak point is the super cheap speakers they came with...a nice set from craigslist should have you jammin' when you get the left channel sorted, good luck and post a pic of the bad boy when you can!!

 

[ChopperCharles]

I don't have the original speakers, I'm driving a pair of ADC-303AX speakers with it now. Or, I guess to be more clear, I'm driving one speaker.

I've already cleaned up all the pots, that's not the problem. Problem exists in headphones as well. I'll trace and see if I can't find where the signal enters the power pack. If I can swap channels and the problem moves to the other channel, it's not the power pack.

Charles.

 

[ChopperCharles]

Well I bridged pins 1 and 16 together with some tiny alligator clips, and got output from the left channel. So the power pack isn't bad. Somewhere the signal is interrupted. I have a feeling it's going to be a bear to troubleshoot this :/

Charles.

 

[Northwinds]

One stinking cold solder joint somewhere probably

 

[ChopperCharles]

I'm potentially looking at TR503 (the tone control preamp) on the main amp board (P3). Going by the block diagram, TR501 (AF preamp) is bypassed when using the external tape input. Since the problem exists whether or not I'm using aux or (external) tape input, TR501 cannot be the culprit. From the tape monitor switch the signal goes directly to the tone control amp and TR503. I'm going to try swapping transistors (503 and 504) to see if the problem changes channels. If not, well then I'll be stumped. Thoughts?

Service manual attached!
View attachment panasonic_ra-6500.pdf

Charles.

 

[ChopperCharles]

Okay, so further investigation:

Swapping TR503 and TR504 had no effect.

Upon further inspection, I'm getting signal at the input to the STK-040 amplifier for both channels.

If I turn the balance pot all the way to the left, and then bridge the input from left to right, I get output on the right channel. So, that's not the problem.

So i went ahead and replaced the STK-040... and that's not the problem either. It appears from the circuit diagram that the STK-040 is symmetrical from left to right. Is this true or not? If it's supposed to be symmetrical, then there is something wrong with the circuits feeding the STK-040 power to the left channel. :/

Charles.

 

[orange]

From my memory that particular IC amp model is in a few of the lower powered sets I've seen, so hold onto it as there is a 040 II possibly I think but you will need it for something else eventually if not that Panasonic.

 

[ChopperCharles]

Well, I fixed it! After noting that the schematic was identical for both sides, I started tracing the flow of power, and I found that there was a bad trace on the board. I soldered a wire bridge and it works now! It was missing the -16v source on the left channel due to that bad trace.

So now all that's left is to troubleshoot the cassette deck. It makes chirping noises when playing with no tape, and wiggling cables changes the chirp. When playing a tape sometimes there's no sound, sometimes there's a chirping, and sometimes there's music and chirping, depending on how I wiggle the cables. So I'm guessing bad solder joint.... it's just a ton of cables going through there so it's going to be difficult to figure out which one it is. Maybe easier to just touch up all of the cable connections.

Charles.

 

[orange]

One stinking cold solder joint somewhere probably

A trace is not as good as a joint? But good call!