CT-F9191 Humming

[pustelniakr]

I have a CT-F9191 that is humming. It is significantly more in the right channel than the left, registering on the VU meter at -35db on the right meter. All of this is when I put it in play mode while there is no tape in it (properly actuated, so the heads come up and the rt hub spins as long as I hold the tape sensor actuated). If I put a blank tape in, the hum drops so that it can only be heard at loud volume settings, and barely tickles the meters (either one).

Normally I would suspect the REC/PB switches being dirty, but I have just gone completely through this unit (Deoxit et al), and all switch contacts register in the milli-ohms (measured) when closed/active.

It is bizarre that the hum reduces when a tape is actually playing. Have resisted the spool and capstan motors, to no effect, so its not mechanical. I have also let go of the tape sensor so both motors stop. It's not the motors. Also, the unit sounds excellent otherwise. The music is not fuzzy. It is clear and full, with no fizzing, popping, or excessive hissing.

I have actuated the REC switch and the noise disappears while in rec mode, but returns when REC is released, but ramps in, taking about 1 sec to get back to its stable level.

At the moment, I am suspecting a bad 'lytic cap. I don't like shotgun repairs, so I want to find the bad part before replacing. So I'm not going to replace caps till the problem disappears. I'm hoping that this unit has something common to the breed so I can fix her more quickly. I am confident I could find the problem, but would rather take a shortcut to the diagnosis if possible.

Thanks in advance...
Rich P

 

[markthefixer]

Hi Rich.... This sounds like a good one!!! What type of test equipment do you have or can borrow? Sounds like you have at least a dmm and ideally can you borrow a scope? You said you De-oxited it, how long ago and how much did you excercise the rec/pb switch/relay ??? That is probably not it, but is suspicous...because the ramping and disappearance when switching back and forth sounds like a high impedance charging/discharging a cap slowly.... but the hum appearing over time as opposed to disappearing is a little counter-intuitive to me... but hey... like I said, this is a good one. Especially when it is confined to ONE channel.

You did the measurements of the contacts on both of those 2 Loooooooong switches?? wow....... (that implies a GOOD dmm)

Also does switching the Dolby in/out or switching tape eq settings make a difference???

Lets see, you Did win a service manual for the 9191 on ebay about a month ago?? right???

Here's a test.. that shouldn't hurt anything.... IF DONE RIGHT!!! the head leads come off of the heads and loop around to the back of the transport, then go to a small pc board ( connector assembly rwx-084, 1+2 are r/p right head, 3 is shield for 1 and 2, 4 is shield for 5 and 6,5+6 are r/p left head, 7+8 are erase head on THAT little board)

Boiling down and analyzing the input circuits:
input of this amplifier is literally switched between the mic input during record and the tape head during playback... the switch could influence things... which is why deoxit is so effective here...
The input is capacitor coupled so it won't change any dc biases...

DON"T PUT THE DECK INTO RECORD IF DOING THIS...

IF YOU DONT HAVE A MANUAL TO POSITIVELY IDENTIFY THESE WIRES, DONT DO IT!!!!!!
short the head wires 1 to 2 with one clip and 5 to 6 with another clip...

Then try playback and see if there is any hum then.
Any induced noise in the cables will be squashed....

If you want to check dc resistance readings on the head cables, the explanation of what you will see will get a little complicated with respect to measuring ohms to ground....