122 MKIII motor speed drift

[steve908]

Hi.
I have been working to repair a 122 MK III that originally had a motor speed problem. I have read many forums here and on Tapeheads about replacing the electrolytic caps on the motor PCB, so the first thing I did was tp replace the caps with tantalum polymer capacitors, which are superior in many ways to the leaky and high ESR electrolyics, and I have SMD soldering experience. I also replaced C12 and C14 with bipolar 4.7uF, because I didn't have any 3.3uF at the time, and I read on Tapeheads that increasing them to 4.7uF could quite motor noise, but that may have not been a wise choice. I will have some 3.3uF caps soon to replace them if they are the cause of this issue.

Now, regarding the original issue, after I replaced these caps, the motor/capstan worked in a seemly smooth and stable manner, but the tape played too slow. I went to adjust the tape speed with the pitch control set to OFF, and the trimmer pot maxed out at the fastest setting and the speed still wasn't fast enough! It's as if my capacitor mods lowered the entire speed range of the motor to the point where the adjustment range can't find the correct speed. However, if I set pitch control to ON and use the front panel knob + the trim pot, I can get the speed of the tape to normal. But, it is at the high end of the trim pot and panel knob with not much margin to spare...

Second part of the issue is that once I get this speed control dialed in, it seems to drift easily with temperature. If I first power on the device and wait a few minutes, the speed increases about 4%. If I just blow are on the motor PCB while it is running, it decreases speed by about 12%. I am measuring the speed by looking at the sawtooth waveform on the speed control wires, and targeting 600Hz.

I am surprised that the speed can change this easily. I would think for a professional tape deck that this would have better temperature compensation. I also don't understand why the speed adjustment range falls short after the cap mod. I wish I could find a schematic of the motor PCB, but I know the motor PCB is only singular part in the 122 service manual, without and specific design details on it.

 

[steve908]

I don't know how I missed this after weeks a research, but I just found this video on YouTube that shows the exact issue I am having.

So, perhaps it wasn't the air from my breath changing the speed - it was the humidity from my breath changing the speed!

The guy used CRC 2-26 lubricant / moisture displacer, and it fixed the issue. I never heard of the stuff, actually. I will pick some up from home depot tomorrow, and then I'll clean the whole PCB off with IPA before applying the 2-26.

I guess the electrolyte from the old leaking caps really soaks into the board and causes humidity-sensitive current leakage. I'll post an update this week when I try it out.

 

[SkywaveTDR]

Yes, It was found some time ago that the C2 being a cheap ceramic causes this to happen which is why I initially put a .001uFd Polystyrene on that cap position but later found that a .01uFd Mylar worked just as well. I do not put esoteric caps on places where electrolytics were put as these film caps may seem like an improvement but the electrolytic has a lower esr as it does it through a wet electrolyte. I use 105*C caps that are through hole. I will have nothing to do with SMD parts except to throw them away. If you want a product to fail soon just use SMD parts.
Further there is a pot at the top of the motor that can be tuned to get to speed. Only in one case did I have to change a resistor around that pot to get the right speed. The other reason this motor is not perfect is that there is no quartz reference to keep it at speed but then units like the Pioneer CTF1250 also has speed problem regardless of the quartz reference not to mention cracked heads or worn out ones.

 

[SkywaveTDR]

There is no need to use special chemicals or conformal coat if the right parts are mounted on the board. In the deck I have repaired the speed is pretty stable but I get rid of the ceramic C2.

 

[steve908]

Thanks for the reply, @SkywaveTDR. I have read many posts of yours on Tapeheads, and I know you have a lot of experience with these Tascams. I find the broken motor / bad cap issue on these 122's very interesting and challenging because it's not the typical broken belt issue like most cassette decks have. I am determined to fix this and get full spec performance out of this deck. I appreciate the input!

The first time I cleaned the PCB after removing the original SMD cans, I used isopropyl alcohol to clean the board. After seeing the speed fluctuations related to humidity, I feel as though I didn't use the correct solvent, or not enough of it, to clean out all the electrolyte. Thanks to your advice, Skywave, I started looking at C2 as the source of the low speed problem. Before I removed C2, I measured the resistance across it on the board. It attaches between Pin 1 of IC1 and ground. The resistance measured about 5Meg ohms. When I blew humid air on the board, it dropped quickly to about 1Meg ohm. This cap is the hold capacitor for the sample and hold function in the servo controller, so I wonder if this leaking current is what's offsetting and destabilizing the speed adjustment range. I measured the capacitance of C2, and it was 1nF. I am not sure if the current is leaking between PCB traces or across the cap itself, but I'll still be replacing this cap with a new one. The other three ceramic caps are 100nF.

Some of the traces in the leaky cap zone look pretty ugly and discolored. I ohm'ed them out and they still have low near-zero resistance throughout. Hopefully, I don't have to start replacing them with wires.

@skywave, what do you use to clean the electrolyte off the board? I am thinking about using IPA one more time, and then using the CRC 2-26. As a water displacer, I think it's not going to hurt. If anything, it would help to stop the corrosion. The description seems like it's meant for this issue:

"Plastic safe lubricant, penetrant and corrosion inhibitor that helps prevent electrical malfunctions caused by water penetration, humidity, condensation or corrosion. Restores resistance values and helps stop current leakage."

 

[SkywaveTDR]

The discolored foil patterns are already compromised and if not today they open then in the future- I put redundant wire across them and then it never matters if the open. The cap for C2 was initially a .001uFd 50V polystyrene but since some interaction with another Technician I have started using .01 Mylar that make less the noise these motors sometimes make. A motor like this should be silent and rotate very easy. Also some people play with the thrust adjustment just any old way. the correct way I have found is to solder on a test pin to the test point near lower edge corner and then as the motor turns watching the scope maximize a sine wave without clipping or other abnormal signs. This would be the right place. I use AMSoil on the capstan bearing and the end plate bearing is Lunriplate 105. My shop has used Denatured Alcohol for about 47 years so far- it is available at Home Depot, Ace, True Value and other places maybe even Minards. It is Ethanol that you can not drink.