AIWA AD-F770 playing weird

[HWTest]

My AIWA AD-F770 is playing weird, the sound is only from one channel and very low volume and distorted.
When I hit pause a then play, it plays very loud from the other channel and the sound fades to zero.
Recording is OK - tested on another deck.
I've made a sample of the sound:
http://rosa.mywia.cz/Sample.wav
http://rosa.mywia.cz/Sample.mp3
I think, that also the crack sound, when I hit pause is not normal ...

Has anybody an idea what could be wrong?

[tapetech]

I would clean the record/play mode switches.

Also check the soldering of the play head wires.

Check the soldering of the output RCA jacks.

[HWTest]

Thanks for the suggestions, I'll try them tomorrow.
I will also try to swap the whole transport with my AD-F660, they should be identical.

[Nakdoc]

I think your pinch roller is either dirty or hardened, causing the tape to curl away from the head. Make an open cassette by removing the top case in the pressure pad area so you can see the tape pass over the heads. Use a crappy tape of course.

[scan80269]

Another possibility is faulty muting circuitry. In Pause mode, mute is in effect, but going back to Play, the mute is released. There may be bad muting transistors that turn back on after a few seconds of play and start shunting the signals again.

Listening to the sound sample, the problem sounds to me more like an electronic one than a mechanical one.

[HWTest]

I have swapped the whole transport from a perfectly working AD-F660 and the result is the same. So I can rule out the whole transport incl. heads, switches etc.
I have also tried the Headphones output and there is the same signal, so I think that rules out the RCA jacks.
I have made again a sample, this time playing a 3k tone.
http://rosa.mywia.cz/Freq.wav
http://rosa.mywia.cz/Freq.mp3
I think scan80269 is right, the problem is somewhere in the electronic ...

[scan80269]

It can also be bad capacitors in the playback signal path.

Troubleshooting this will require an oscilloscope to trace the signal from the PB head all the way to the line output, to locate the degradation point.

I have three AD-F990s, and they all had mechanism issues: the capstan belts melted into black goo. One deck also failed to auto-cal any inserted tape until I put in a fix.

[HWTest]

Bad caps sound reasonable, I'll try to trace it. I hope the signal from the heads is not too weak for my scope.
But I'll come to this later. Now I'm using the opportunity, that the transport is out and cleaning and lubing it.
Melted belts are a "feature" of this line of AIWAs. I have changed belts in all five of my AIWAs.
You are a lucky guy with your 990s *reelspin*
I'm looking for a nice, reasonable priced 990, to complete my collection (660,770,990), for some time but still without luck ...

[tapetech]

Inside the unit there are several small vertical circuit boards. Two are Dolby NR cards. It is common for the solder joints that connect the small board to the mother board to get cracked. From the bottom of the mother board, resolder all joints to these small boards.

Also check for cracked solder joints on the power supply regulator transistors (rear middle of mother board) and the RCA in/out joints. They may not cause your current problems, but they get cracked often.

[HWTest]

I already resoldered the joints of the power supply regulator transistors.
I'll check vertical circuit boards.

[HWTest]

I have traced the signal from the PB heads and it seem the problem is somewhere on the "Dolby PCB".
Will continue tracing on the "Dolby PCB" tomorrow ...

[tapetech]

Have you resoldered the Dolby PCB (where it connects to the mother board) as I suggested in my previous post? That is a common defect. If so, and if the unit is still defective, then check/replace the high-value caps on that same board.

[HWTest]

Yes I did. Will do, but later, now I have to first fix our dishwasher :-(

[eclectiktronik]

you now need a hairdryer and some freezer spray to narrow it down. This sounds like an IC problem to me. If it were a cap, the problem should go away as it warms up....

[HWTest]

I have both but you are scaring me. If a Dolby IC is shot ... :-(

[HWTest]

The dishwasher is fixed but I had only little time for the AIWA.
The Dolby PCB is different from the service manual, there is one additional small IC on the PCB and the layout is a little bit different :-(
I have also again traced the signal from the PB heads with an oscilloscope and I have noticed one thing - the signal after the muting transistor in one channel is considerable lower.
Maybe the muting transistor is bad? It makes sense for me, considering the strange behavior after unpausing.
Does anybody know a replacement for the 2SC945L-PQ transistor?
The original is not available.

[HWTest]

Removed both mute transistors and the transient effect after unpausing is gone.
But the signal in one channel is still about the half of the other channel - there is still something wrong before the Dobly PCB.
I'm thinking about swapping the Rec and PB Dolby PCBs because the Rec Dolby PCB seems to be OK - recording is fine.
I was also trying with a hairdryer and a freezer spray but no luck.