Noisy Otari MX5050 BII-2 Tape deck

[KilgoreCemetery]

Good news / bad news time. Turns out this does not actually affect both channels equally.

I was checking voltages at switches SW302 and SW402 (SEL-REP Channels 1 & 2 respectively) and found that pin 1 of 302 had zero voltage. Pin 1 of 402 had -18V. Short version is, I pulled both switches, disassembled them, and cleaned them manually. They both had a blackish-green goo inside that I'm assuming was some sort of dialectic grease.

It looked old and tarnished, so I removed as much as I could, sprayed the insides with some Deoxit, reassembled, and now the right channel is clear as day. Left channel still has loud static.

It's super late here, so I'll pick it up in the morning, but I think we can rule out the power section now.

 

[kellys_eye]

I have not yet tried disconnecting the signal path to isolate the problem

basic principle of fault location......

but I had previously pulled, and tested the JFETs Q106 - Q111 using the working Otari.

theses FETs switch in the relevant gain resistance to control the amplifier. If the wrong FET is being switched or the gain resistor has gone 'high' (and multi-megohm resistors are prone to) then the gain could be waaaaay too high and introduce noise.

it does, in fact, have a cylinder-type transformer.

??? pics?

 

[KilgoreCemetery]

Do you have to disconnect the signal path to trace the noise? I know that the problem went as far back as the preamp chip. If the problem is ahead of that, and you don't disrupt the signal, will it still show up at the preamp or before it?

I'm assuming it's a transformer because of the schematic labeling. All the coils are "Lxxx", but this one is T501. It has TF 41002A printed on it.

 

[KilgoreCemetery]

I had a Eico 322 Signal Generator given to me a while back. It wasn't working, but I took the time to open it up and found some bad tubes in it last night. Swapped in some good ones and it seems to be working ok. This may not seem relevant, but this morning I went to work testing the Otari again.

theses FETs switch in the relevant gain resistance to control the amplifier. If the wrong FET is being switched or the gain resistor has gone 'high' (and multi-megohm resistors are prone to) then the gain could be waaaaay too high and introduce noise.

I probed the IC501 Preamp chip input pins (3 & 5) and found that the left, noisy, side was waaay louder than the right. There was also some bleed-over from the right channel. Not sure if the bleed-over is because of the proximity of the signal probe though.

I went ahead and tested the highest value resistors I could find in that section, R196/R296, which are between the FETs and the Preamp chip. They're supposed to be 1.5M. Left channel measured 1.67M with a leg lifted. Right channel measured 1.51M. For the sake of brevity, I simply swapped them to see if the 1.67M was too high, but the noise is still in the same, left, channel

 

[KilgoreCemetery]

Still testing everything I can in and around IC501. Introducing signal at the preamp side of Q204, Q205, R223 all produce sound at the headphones. These points on the 100 series, left side, produce zero sound. I've tried swapping the Q104 and Q105 with Q204 and Q205 with no change. I lifted one leg of R123 and it tested within tolerance

 

[KilgoreCemetery]

I finally found it! The trace on the negative leg of C113 was messed up. I went to pull the cap to test it and the trace came right off. I jumpered the cap over to Q105 and the noise is gone. I can now hear my tape!

Thanks for your help @kellys_eye. I still have more reading to do, but I've learned a lot from this project.

 

[kellys_eye]

I'm assuming it's a transformer

indeed it is - small audio transformer, often used for impedance matching/balanced inputs and/or small oscillator circuits.

Glad to hear you found the fault - although it may not have been immediately apparent, it may have also fallen under the same fault-finding technique as 'eye's, ear's and nose' as first-line discovery techniques!

But all fault-finding takes into account the breakdown of circuits to their individual component sections and then the individual parts-within.

Obvious sources (finger-faults) are part1, the eye's, ear's, nose is part2, PSU is part3 etc.

I might draught a flowchart for fault-finding various (common) items...... unless anyone else has something they can link/post (separate thread maybe) to start with?

I'll be waiting for the flowchart that has the two final choices as 'bin it' or 'blame someone else'...!