Troubleshooting help for Pioneer H-R99 8-track deck

[EmoCowMoo]

I have been trying to restore a Pioneer H-R99 8-track deck. Before and after replacing all the electrolytic capacitors, the motor seemed to be moving in a wavy or "wowowow" fashion. Adjusting the pots (VR1 & VR2) on the motor control board would adjust the waviness, but I could not eliminate it completely.

I then replaced the 3 transistors on the motor control board (Q1, Q2, & Q3) and there was no change in the waviness problem. I then replaced the 2 transistors (Q3 & Q4) and the thyristor (SR1) on the power supply board and no change in the waviness. I adjusted the motor control board pots and got it almost cleared, but I could still hear the waviness.

I then fast forwarded a tape (to get to a song I wanted to listen to and adjust to). While fast forwarding, smoke started coming up from the power supply board. The R15 resistor was burned completely up and the R13 resistor was darkened from cooking. I replaced both of them. Normal playback still included the waviness, but then FF again, smoke was coming up again. This time the R13 was the only one visually affected.

#1: What could be causing the resistors to burn up only when I fast forward the motor?

#2: What might be the underlying cause of not being able to adjust out the waviness while in normal playback?

Could these issues be related? I'm no expert when it comes to troubleshooting electronics.

Also, hooking the oscilloscope to the incoming power to the motor control board and also output to the motor would yield sinusoidal waves each time I would test them after adjustments. Am I supposed to see an oscillating wave at motor control board incoming power? I would assume I'm not supposed to see that going to the motor. Or maybe I am, i have no clue. I don't have a good unit to test against so I have no idea...

Also I have replaced the capstan, belt, motor, and motor pulley with the same results.

Any ideas would be greatly appreciated! I attached two screenshots of the schematics and board layout relating to the motor control circuit.

 

[EmoCowMoo]

So I replaced that Q3 transistor again and I continued to blow smoke. I replaced the thyristor again and this time no more smoke. The new thyristor was shot. At least that fixed one problem.

I still had the waviness. I replaced the whole motor control board and no difference. I then replaced the all the diodes in the bridge rectifier circuit and said eff it and replaced the remaining 3 there. I was able to tune out the oscillations for the most part. It will come and go. When the motor starts up or when the track changes, it happens again for about a minute before it goes away. The counter light oscillates at the same time when that's occurring.

Would a cap on the incoming power to the transformer help with these oscillations?

 

[JRhoBurton]

Have you replaced the belt? Lubed capstan? Are capstan bearings loose? Trying various tapes?

 

[SupraTrooper]

I have a similar problem on my H-R100. Lots of flutter and if you adjust the pots wrong it actually makes the lights sorta flash to the frequency the motor is wowing at. I replaced the big transistor on that board, tested all the resistors out of circuit, tested the small transistors, replaced all capacitors......Still not right

The (Permanent magnet brushed DC) motor in this is controlled by Pulse Width Modulation. I think the two transistors (complimentary NPN and PNP pair) make an oscillator. I guess the pots control frequency and the big transistor does the switching to drive the motor. If I can't make it run right, I might whip something up involving a 555 timer and some other crap.....

I have a schematic of this circuit somewhere I'll post later.

 

[EmoCowMoo]

So I beat my head on this issue. I'd think it was fixed, but then it would end up oscillating again. I removed the 2 pots from the motor control board and ohmed them out. There were a couple of dropouts I found in their full range. I replaced them with brand new Bourns pots, but I still had oscillations. I replaced some of the same transistors I had already replaced. Same issue, except i could not "tune" out the oscillations like I could before by adjusting the pots.

I replaced the Thyristor on the power supply board (SR1, the 02AM-1 part). I had used NTE5401 as the replacement. It then ran GREAT except it wouldn't automatically stop at the "end". It only behaved like the "Endless" switch was on. Beating my head on this issue, I discovered the leg pinout on the NTE part was different than the original part. Once I fixed that, I had oscillations, but the tape ended when it should have.

I then remembered transistors have what they call hfe values that correspond to the letters that come at the end of the transistor part #. When I was troubleshooting initially, I was swapping them out not paying attention to the letters. I had saved all them original transistors, so I put them back in and it was almost fixed. It would work and then randomly the torque would "flicker" causing some oscillations and then it'd work normal. I replaced the power transistor on the motor control board, 2SC1398 again, and it worked great!

I occasionally still get oscillations, but I think I know the issue now. If you put the original bipolar 1uf cap back on the motor control board, it works fine. But when I used my new Nichicon 1uf 50V MUSE bipolar cap, it causes oscillations. I had read somewhere that low ESR caps can cause instability in DC motor control circuits. I take it that is probably what I'm seeing now. Measuring the ESR, the new cap measures 2.7ohms. The old cap measures a whopping 28 ohms.

Ultimately, I think the issue might have been a combo of the Thyristor and that power transistor failing, but since I changed pretty much everything else in the process, it's now a matter of getting it back to "normal".

So a couple of questions before I run some more trials... could I theoretically buy a 25.3 ohm resistor to add in series with the MUSE cap and have it work like the old one? Is the ESR constant or does it change as the current/voltage changes? If it changes, this might not work since that physical resistor isn't going to fluctuate.

And lastly, does anyone know of any capacitors that might work better in this situation that have a high ESR value like the original one?