Tascam 38 - record level wavering +/- 3dB

[mtconnol]

Howdy ya'll!

I'm working on bringing a Tascam 38 back from the grave (defined here as sitting in someone's closet for a number of years.) So far I'm in it for a new capstan belt, an MRL tape, and some fresh RMGI SM911 tape. I went through a whole calibration / align procedure but am still stuck on this problem which looks to be a PCB-level failure.

Current problem is that, fed a nice 0VU (-10dBV) signal from my DAW and recording with output set to Repro, many channels are wavering +/- dB in an intermittent, jerky fashion. Furthermore, the distortion component is varying in time with output level.

At first I was afraid this was a mechanical tape problem, but I have pretty much ruled that out at this point.

Then, the revelation: on testpoint TP1 on the affected channel cards, I can observe my input signal mixed with the 150 kHz bias signal. On good channels, this is rock-solid, and looks like the summation of the two signals. On affected channels, the amplitude of this tone+bias signal wavers up and down in time with what I hear on repro (offset by the head latency.) Meanwhile, TP2 and TP3 are rock-solid 150khz waveforms on all channel cards.

So there's some failure on these channel cards that makes the contents of TP1 (which I believe is pretty close to what is placed on the sync head) waver wildly in amplitude. And here's the kicker: the failure can be induced by flexing the offending card up and down in the frame. I have tried to narrow it down to a particular region of the board or component (all the passives look fine and there don't appear to be any cold solder joints anywhere) but am pretty stuck.

Anyone seen this failure before, or just have some general suggestions for my next move?

Thanks so much!

Michael

 

[RRuskin]

What happens when you play back the recorded tone from the sync head?

 

[mtconnol]

PLayback from sync

When I play the tape back off of sync (or repro), the waver pattern is there. That is, it is happening on teh way to tape. By contrast, when I play the MRL tape, all 8 channels are dead solid (since it was recorded with a good stable signal.)

 

[RRuskin]

When I play the tape back off of sync (or repro), the waver pattern is there. That is, it is happening on teh way to tape. By contrast, when I play the MRL tape, all 8 channels are dead solid (since it was recorded with a good stable signal.)

Then it's probably not your heads. Pull the cards and clean the contacts on the bad channels. Exercise all the trim pots, too. In fact, do all the dumb little things 1st before changing circuit board components.