3-pin Trimmer Potentiometer

jdjohn

I have an Onkyo TA-2056 cassette deck on the bench, with an issue regarding recording levels, which is tied to a known problem with the trimmer pots...they have not aged well. I tried disassembling and 'repairing' the trim pots, but no luck, and have actually broken at least one of them at this point. I found (what I hope are) suitable replacement pots, but need to make sure the measurement points are correct.

In this screenshot from the deck's schematics, which pins on R239 should have the variability for 0-10K ohms? (You can ignore the circled 'Accu Bias Adj' for this question.)



The readings I get on the replacement trimmer pot for pins 1 and 2 go from 0-10K (turning knob from left to right). For pins 2 and 3, I get 10K-0 ohms. Pins 1 and 3 give a constant 10K ohms regardless of turning knob. I believe pin 1 is the chassis ground wire, and it has a clear insulator. Pins 2 and 3 have the pink and white wires, respectively, which I believe are the + and - signal wires.

The 10K-0 ohms variability between 2 and 3 seem correct for the trimming (since it means max resistance [low output] when turned left, and no resistance [max output] when turned right), but I'm not sure if the schematic supports this configuration. I could always move the pins (and wires) on the PCB, if needed.

Are 3-pin trimmer pot configurations standardized, so I needn't worry about it?

Thanks for any insights!

SeleniumFalcon

I found the schematic and printed out the complete circuit and with a magnifier and some different colored pens traced the circuit. R239 pins 1 and 2 are part of an adjustment for IC NJM4558DX which is an integrated circuit that contains an equalization amplifier. Pin 1 is for EQ "preset 3" and pin 2 is for "preset 2" and pin 3 is for "preset 1". Pin 1 connects to resistor and then to circuit ground. Pin2 connects to a resistor and then to the IC connection #2 (input). Pin 3 connects directly to the IC connection #1 (output).
I would think pin 1 and pin 2 of the pot should be the 0 to 10k legs.

jdjohn

Thank you, Ken! Thorough, as always!

SeleniumFalcon

Good, I'm glad the information was helpful.

jdjohn

Thanks again, Ken! I was able to find and install the three replacement trimmer pots I needed, and recording now works! Here's a pic of the deck in action.

Not the best pic, but I was trying to shield light from a glare. This Onkyo TA-2056 is a pretty nice little cassette deck - 3 heads, Dolby C, and a surprisingly wide frequency response, IMO.

I had to fix two broken capstan motor mounts, replace two belts, and replace the three recording-level trimmer pots as discussed in this thread. I'm glad I stuck with it, though, since it sounds really nice.

As a public service announcement (PSA), I found the trimmer pots at TME Electronic Components (https://www.tme.com/us/en-us/), which is located in Poland (of all places). I placed two different orders from them, and both orders arrived within three days via FedEx. I highly recommend them, and they might even have some hard-to-find parts not available on our side of the pond.

SeleniumFalcon

Great looking deck! Working on a cassette deck is a little like building a ship in a bottle, a lot happening in a small place.
There are some surprisingly good sounding pre-recorded cassettes available. I wish I could find some of the early Advent Process CR/70 tapes.