2275 right amp problem

[Perkinsman]

Hi all,
I’m working on the 2275 & have a problem. I replaced the 3 caps on the right amp board, cleaned the old heat sink compound off the large transistors & replaced those plastic gaskets with new plastic & compound. I
also gently lifted those two driver transistors, cleaned & put new compound on them. When I powered on, I got very low volume & it was flubby from that channel. (It was working before.)Sound like something u may have come across?

 

[catrafter]

My Mantra 'If it was working before you messed with it and it doesn't work now, check what you did.'
Oops.

Tom

 

[Perkinsman]

Agreed Tom! I’m concerned that I may have shorted one of the two smaller transistors I lifted or the large power transistors I removed & replaces. Is there a way to test them in place or do they need to be removed? I’m getting sound but it’s low volume & garbled.

 

[catrafter]

An 'Octopus' if you have an o'scope with X-y capabilities or Huntron tracker will allow you to test semiconductors in circuit with ease. Otherwise, you may have to remove them to use the diode test method out of circuit.

Tom

 

[Perkinsman]

I'm still troubleshooting my right channel FM volume imbalance problem. With the assistance of Runningdog, we were able to determine that the right amp and preamp were operating correctly. The right channel operates loud and clear on every section, ie, Tape, Aux, Phono, AM with the exception of FM. On FM, the right channel is clear but the volume is 50% of the left channel . I only worked on the P200, FM IF board so I'm pretty sure the problem is there. Today I tested all the voltages at every transistor on the board and they all operate within the voltage spec's of the schematic. The caps are all new. Are there any other components on that board that would cause the volume imbalance?

 

[catrafter]

The 2275 IF amp contains the FM discriminator (also called a demodulator). I wonder if it is possible to mis-tune the discriminator in such a way that the right channel is weak.
Read all about the FM stereo process here.

Tom

 

[Perkinsman]

I went back & resoldered each cap I replaced on the FM IF board & now I have right channel (at full volume) but lost the left channel! I looked at the discriminator circuit but would it cause this issue?

 

[catrafter]

I doubt that the loss of the left channel is due to your work on the IF board. I would look at the P300 board down stream of the decoder IC H321(HA1156) Pin 4. This would include the low pass filter (L301, 303 et al). If you get sound on both channels when the Dolby FM button is pressed, I would suspect the problem is in the buffer amp H304,305 (and it's associated circuitry) or the wire at J311.
Please excuse my crude drawing.
Tom

 

[Perkinsman]

Tom, I don't get both channels with the Dolby button pressed in, but do with the R mono button pushed in, but I think that's just the right amp playing thru both speakers. When i started the recap initially, I had both R&L channels but lost the R after working on the R amp and P200 FM IF boards. There were a couple lifted trace/solder/broke wire problems on the FM IF board and after correcting them, it regained the R channel. The next day, i turned it on and no R channel again. I hadn't worked on the P300 FM MPX board which is why I've been focusing on the P200. With that said, after your last message, I started measuring and pin 4 was 9.5vdc, the schematic reads .5v. That seems significant. H306 & H307 collectors were 20.5vdc, 35% over spec. I started poking around with a chopstick over your outlined path and tapped on the big cap in the middle of H304,305,306 & 307 area. It seemed loose so I wiggled it and the right channel volume jumped significantly, which I believe is the correct volume since it never really produced good volume...but still no L channel. It's pretty obvious that P300 needs to be rebuilt but I'm confused as to why the problem seemed to correct after working on P200, only to return.

 

[rBuckner]

A possibility is the poking around is moving a fractured solder joint, pinched wire or cracked trace somewhere else making it look like where you're messing around is where the issue is but it's elsewhere. It IS very frustrating. The FM IF board in and of itself is unlikely, but not impossible, to be the source. If it is I'd love to see what it was!

I would suggest this test. Put a clip lead on J311 and with the unit playing touch the free end to J313. If the volume is the same then on both channels, which is very likely, the issue is on or before the P300 MPX board. Next I'd do the same style test, applying the jumper with the power off. Connect the jumper from the junction of R315 and C323 (about in the middle of Tom's blue line marking) to the similar spot on the other channel, R326 and C324. If that brings the two levels the same the issue is before the two transistor output buffers and I'd be checking for a very low ohms reading on the two coils before that thinking one may be open circuit. I would short across any other points which would short out the two IC pins, it may blow it up. It may be toast anyway!

 

[runningdog]

Put a clip lead on J311 and with the unit playing touch the free end to J313. If the volume is the same then on both channels, which is very likely, the issue is on or before the P300 MPX board.

Thank you Randy and Tom. The OP and I worked on this 2275 today. Test done, the result is as you expected. Volume matches on both sides.

Connect the jumper from the junction of R315 and C323...to the similar spot on the other channel, R326 and C324.

I think you meant R325? These two locations are the same as J318 and J320. I think they go to the Dolby circuit, before the buffer amplifier? With the jumper is place across J318 and J320, and the oscilloscope leads at J311 and J313, the volume level did not come up.

To summarize, with the 'scope leads directly on J311 and J313, one side is amplified nicely, the other side not. Jumpering them together makes the volume match.
With the 'scope leads directly on J318 and J320, one side is much weaker than the other.

I think this means the buffer amp is working, and the issue is further upstream on the MPX board, or back on the IF board? We keep returning to the IF board because we have found and fixed soldering issues on it. There could be another one. The MPX board has not been touched, but that does not mean it might not have failed. I understand what you mean about wiggling one place and getting a result may still mean the issue is not quite where you are wiggling. We did not reproduce the volume jumping around today despite wiggling the MPX board components mercilessly.

Here's a portion of the schematic, redrawing courtesy of Tim Cartier:

 

[rBuckner]

It's playing hard to get!! Keep cranking on it.