Kenwood KX-56CW Duel Cassette Deck Repair

[dustfilledhobo]

Hello!

I'm trying to fix my Kenwood KX-56CW dual cassette deck (seen in the picture below).



Everything functions as it should except that the motor for tape deck B plays back extremely fast (tape deck A functions properly). I've located the two trim pots on the board that control the speed of deck B and they were only able to bring the speed down a little bit. I was able to find a schematic online (attached) with suggested voltages, so I measured the board while playing a tape in deck B (as per the instructions on the schematic). Some voltages were close and others were substantially different. Here's what I measured and a screenshot of the relevant portion of the schematic.



Q31 (2SC1740S) C=70mV, B=20mV, E=0V
Q32 (2SC1740S) C=8.3V, B=9V, E=11V
Q33 (2SC1740S) C = 0V, B=14V, E=0V
Q34 (2SD863) C=14V, B=14V, E=14V

Based on these incorrect voltages, can you tell me where to start looking? Do I need to replace the transistors? Resistors? Something else?

Another thing to note (though I don't think this would only affect tape deck B) is that the following two electrolytic capacitors are leaking pretty badly. I've ordered replacements, but don't have them in my possession yet to swap them out.

C70 1000uF 25V
C71 1000uF 35V

Thanks in advance for any help you can provide.

Dusty

[stj]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

look at the back of the motors, a lot have speed regulators built-in with a hole for the ajustment.

[dustfilledhobo]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

look at the back of the motors, a lot have speed regulators built-in with a hole for the ajustment.

Unfortunately the motors don't have speed adjusters on the back.

Thank you for the help though!

[R_J]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

I think these voltages

Q33 (2SC1740S) C = 0V, B=14V, E=0V

are just measured wrong, C&B reversed. Q31 base is too low and not being turned on. To test, you can short Q31 E-C it should play at normal speed. If It does, then check the base circuit to find out why it's not being tuned on

[petehall347]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

see if you figure why q31 base is low . meaning q31 is off it needs to be on .

[petehall347]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

might be a problem with the speed switch
then again other deck works ok .

[dustfilledhobo]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

I think these voltages are just measured wrong, C&B reversed. Q31 base is too low and not being turned on. To test, you can short Q31 E-C it should play at normal speed. If It does, then check the base circuit to find out why it's not being tuned on

Thank you for your help! You're right, I mixed up the B & C on all of my measurements! Also, I just realized that Q32 is a different type of transistor. Here are the correct voltages/parts list:

Q31 (2SC1740S) B=70mV, C=20mV, E=0V
Q32 (2SA933S) B=8V, C=9V, E=11V
Q33 (2SC1740S) B=0V, C=14V, E=0V
Q34 (2SD863) B=14V, C=14V, E=14V

I shorted E-C on Q31, but it didn't slow the motor down. Does this mean the transistor is bad?

I shorter E-C on Q32 and it slowed the motor down, but it is still faster than it should be.

Any other ideas?

[dustfilledhobo]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

might be a problem with the speed switch
then again other deck works ok .

Thank you! I used a continuity tester to test the switch and the switch itself seems to be working properly. I'll follow the circuit from the switch and see if I can find anything that looks off.

[R_J]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

With the correct voltages, Q31 has .7v on it's base so it is already tuned on so shorting it e~c will have no effect.
It could be that Q32 is bad as it should be turned on and have the same voltage on E & C. If it slows down but is still fast, adjust VR4 (you may also need to adjust VR3 to get the correct speed). Also check the value of R124, R125 VR3 and VR4 if any are open this will cause high speed.

The speed is controlled by the resistance across CN4 pins 11&12. in high speed, Q32 is open so the speed is controlled by R125 and VR3. In NORMAL speed Q32 shorts connecting R124 and VR4 across R125/VR3

[dustfilledhobo]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

With the correct voltages, Q31 has .7v on it's base so it is already tuned on so shorting it e~c will have no effect.
It could be that Q32 is bad as it should be turned on and have the same voltage on E & C. If it slows down but is still fast, adjust VR4 (you may also need to adjust VR3 to get the correct speed). Also check the value of R124, R125 VR3 and VR4 if any are open this will cause high speed.

The speed is controlled by the resistance across CN4 pins 11&12. in high speed, Q32 is open so the speed is controlled by R125 and VR3. In NORMAL speed Q32 shorts connecting R124 and VR4 across R125/VR3

When I measure the resistance of R124, VR3, and VR4 with the power on and the motor running, it measures correctly, but when I measure R125 it measures as "overload" which I'm assuming is an open circuit. This fell in line with what you thought could be the problem, so I removed R125 from the circuit. Since R125 was measuring as open I turned the circuit on again and expected it to run as it was before, but the motor was running even faster than before. So I measured R125 while it was out of the circuit and it measured correctly. I then re-soldered R125 into the circuit and measured it again (still measured open). This time I noticed that when I measured R125 the motor slowed down substantially. R125 is still measuring as open, but while my probes are touching both leads on R125 the motor slows down to almost the correct speed.

Any ideas on why R125 measures open in the circuit, but correctly out of the circuit?

Any ideas on why the motor slows down when I measure R125?

I think I need to buy some transistors, will any general purpose PNP resistor do? Or do I need to buy the same exact 2SA933S transistor for Q32? If a variety of PNP transistors will work, which one(s) do you recommend?

Thank you again for taking the time to help me with this!

[dustfilledhobo]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

So, I replaced Q32 with an exact replacement (2SA933S) and nothing changed.

Since R125 was acting funny I shorted it out and now I can tune the motor to the correct speed, but the sound has a warble to it. Is this because of the shorting or should I investigate the belts? Is it dangerous to short out this resistor?

Without the short the motor spins so fast that it's hard to tell if there's a warble. Also, the cassettes play fine (correct speed and no warble in cassette deck A, so I know the issue isn't the cassette itself).

Thanks!

[R_J]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

When I measure the resistance of R124, VR3, and VR4 with the power on and the motor running

You DO NOT CHECK RESISTANCE with POWER ON!

[dustfilledhobo]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

You DO NOT CHECK RESISTANCE with POWER ON!

Thanks, that explains it.

Any ideas on why shorting R125 gave me the desired results?

Anywhere else I can look for what might be causing the issues with the motor spinning too fast?

[petehall347]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

replace r125 and the pot if it measures wrong

[R_J]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

Thanks, that explains it.

Any ideas on why shorting R125 gave me the desired results?

Anywhere else I can look for what might be causing the issues with the motor spinning too fast?

If you short R125 that places the pot itself across the motors speed control, This pot, depending on the adjustment position could be near zero ohms or near 4.7kΩ or somewhere in between. I suspect the motor's internal speed control circuit might be bad. This could also explain the warble sound

[dustfilledhobo]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

If you short R125 that places the pot itself across the motors speed control, This pot, depending on the adjustment position could be near zero ohms or near 4.7kΩ or somewhere in between. I suspect the motor's internal speed control circuit might be bad. This could also explain the warble sound

Thanks for your reply! I'll probably have to shelve this for now, but I really appreciate your help and patience with walking me through this process.

[budwich]

Re: Kenwood KX-56CW Duel Cassette Deck Repair

have you actually measured the resistance outputs of the pot to see if it is changing with the adjustment? On an "long unused" turntable that I had, the speed adjustment could not bring the platter into speed but after multiple back and forth rotations of the adjustment pot, the "oxide" wore off and the speed was readily controllable again.