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CT-F 9191 hum on both channels in play

brainbanger

brainbanger

Member
Hi guys,
it has been quite a while since I posted something here. Now I am stuck fixing the hum in a CT-F 9191.

The hum occurs when pressing the play button and is equal on both channels (meters show around -20db). This can be reproduced even without a cassette (relevant switches engaged). It is NOT present with the record button pressed.

I have successfully de-oxided all switches, pots and especially the long switches on the REC/PB board. The deck runs fine besides the hum. The power supply caps have all been replaced, currents are in spec and stable.
I have checked all ground wiring on every board, no issues or broken/missing wires.
All relevant screws in components/boards are in place.

I guess it must be a grounding issue, but do not know what else to check/look for. DMM and scope are available. Looking for clues and hope for some comments.

Greetings from Germany,
Andreas
 
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You didn't leave much open to suggest, you covered it all rather thoroughly.

Play shows hum at -20, and the only thing connected to the deck is the ac power line?

Did you verify by ohms testing the deoxiting, those long switches are rather resistant to deoxiting.

It almost seems that an analog signal ground is open somewhere,
and that it is seeking out a ground, catching a power supply or lamp current return along the ground path.
Maybe a lamp return is open.

You make no mention of the history of the unit, did it come to you like that?

Trace the AC CURRENTS across the grounds from the schematic - I call them the "hum zones".
 
brainbanger said:
No clues at all?? Please, somebody to the rescue :⁠-⁠). Without a hint I might have to sell it for parts :⁠-⁠(
Click to expand...
Can I add another hint? Yes
Before selling it for parts it should be cured of the hum (Technically).
When I have hum issues the first thing I look at is the PS. And what did I do?
There is a simple center tapped two diode rectifier arrangement used in the power supply.
Maybe looking into that, after checking Marks more experienced response.
 
Thanks for jumping in guys!

@ Mark,
The unit is only on AC, nothing else attached.
I will have to revisit the two switches again, although it is tough not to break any wires while working on this board... Did not do any ohm checks yet, so will do this first.
I bought this thing years ago and you already did the clutch for me - works like charm! Besides the drive somebody was in the circuits before. I need to check at least the PS for replacement parts used and will test the 'hum zones'.

@zebulon1,
will check the rectifier circuit while reworking the PS on my own, thanks for the hint!

I will report back next week.
 
Pay close attention to the meter analog ground and the lamp current return path "ground".

The lamps run on AC.

The meter ground goes back to pin 110, and picks up ground at the headphone buffer and meter driver circuits.
Right after the output level control.
I'm not sure electrically how far away those grounds are from the heads playback amps ground.
Think of a head signal like a phono or mic level signal. Very low and vulnerable.
 
Update as promised:
- PS is fully working and delivers correct voltages
- no issues found in "hum zones"
- no issues with meter analog ground


Hum occurs in PLAY, but not when PAUSE is engaged. So in PLAY/PAUSE I pushed the head assembly with my finger against the cassette and it played fine without any hum or distortion. Looks like the issue is caused by the pinch solenoid or its circuit.

I will have a look into page 11 of the SM in order to check the involved components. Stay tuned!
 
You could also have a cold joint/wire hanging by a thread on one of the grounding connections on the heads themselves. Tiny little buggers.
 
I guess this will require some more time to resolve :(.
Just discovered that someone in there before replaced Q901 & Q902 (2SC789) on RWX-083 with MJE15032 ?!? I checked the data sheets and decided to replace them with 512-KSD526Y.
Now I can't hear a "hum" via headphones, but on full volume there is a hiss on both channels that triggers the meters to -20dB. And it is already there after switching the unit on, even without pressing any key... Hope to trace this down with the scope.
 
Don't know how, but the hiss is gone... Maybe a loose wire or a cracked joint?
What's left is a loud hum when switches "Memory Stop/Play", "Bias" and ""EQ" are "ON". If "Memory Stop/Play" is "OFF", the hum is gone. Need to deep dive the switch circuit and its connectors.
NB: switches are de-oxited ;)
 
I tracked the issue down on the switch assy board. When I disconnect the indicator lamp for MEMORY STOP/PLAY everything works perfect, no hum no hiss. Now my problem is, that the switch assy board in the unit is not an RWS-031 as per schematic, but an RWS-042... The circuit layout is different and I will need to inspect the board by hand and eyes in order to understand the root cause.
EDIT: downloaded an "Additional Service Manual" from Manualslib with the correct schematics, stay tuned!
 
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Had the same Problem and fixed it.
It might come back but the 2-long switch under the metal plate and were the middle sol is and when it actives it pushes the 2 sw's for play and record and they are dirty.
I just pulled my CT-F9191 out and have not used it for decades but it worked and than the started humming than after moving it again it worked and came back again I'm guessing that every time I moved it the switch made better contact.
short way to do it, is to remove the sol assembly and spray/clean the 2-switches (you will see to white levers that can be pushed in and out) w Deoxit or remove the board and do it.
also clean w DeoxIT Fader the output pot.

Later all the pots and switch will need to be cleaned, most of the pioneer stuff go thought all the switches.

Good luck
 
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