Kenwood CS-4025 Spoiled HighVoltage circuit (power supply)

[Colorito]

Hello,

This is my first topic in this forum, I will apreciate help with that, thanks in advance.

I bought a broken 20Mhz Kenwood CS-4025 Oscilloscope and I have set myself the challenge of fixing it!!

The guy who had it broked it plugin a bare wire to the wall 220Vac plug, i suppose he connected the ground to the hot wire.

When I powered it for first time, the green led is on but nothing in the screen. Searching the failure with the service manual i found that the high voltage for the CRT is dead.

For accessing the power supply pcb I needed to take out the CRT, the test points that i write are without the CRT connected.

TP1: ok
TP2: ok
TP3: ok
TP4: ok
TP5: ok
TP6: KO
TP7: KO
TP8: KO (only noise, no 1800V)
TP9: ok
TP10: seems ok, but the waveform seem to bo slighly different
TP11: seems KO (having a square waveform with minimum voltage of -50V)
TP12: KO (0V)
TP13: ok
TP14: ok (but is 15-20V less in voltage)

I attach the service manual:
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Kenwood-CS-4025-HU-Service-Manual.pdf
page 17: High voltage checking procedure
page 28: Power supply schematics and TPs (test points).

Any suggestions to continue?

Many many thanks!!!!!

[coromonadalix]

The psu seems an self oscillating one with no special ic,  check the U2 and Q8 Q9, do you have the +10 and +8 volts  ?, if the 1800v is dead, this line is the feedback for U2 ?? and the filament voltage will be affected too.



Here's a thread covering the hybrid U1 ic (kma01)  you'll have to register ...  there's a lot of talk about this hybrid, many voltages are done with it, +8 -8v, +140v, +5v,     U1 seems okay if your tp1 thru tp5 are ok.

https://www.vintage-radio.net/forum/showthread.php?t=89122

https://www.diyaudio.com/forums/parts/199275-kenwood-hybrid-ic-kma01.html

Here's the recreated KMA01 hybrid and infos

[Colorito]

Hello Coromanadalix, and thanks for your repply and taking time to me!! ;-)
I agree you with you that U1 seems to be ok
As I dont fully undertand the oscillating node I attach a captures and details about what U2 sees and gives. (both are the same signals but with diferent time scale)

U2 pin1 (A output) has over +7.45Vdc (corresponds to CH1 AC-coupled on DO capture)
U2 pin2 (A- input) has over 2.6 Vdc (corresponds to CH2 AC-coupled on DO capture)
U2 pin3 (A+ input) has 0V, logical as is pointed to groud over R30
As U2 has a feedback capacitor I understand that I works as some kind of integrator…but the capture seems to show that is not working properly.
Seems like U2 is dead, do you think I’m correct?
Do you advise me to change Q8 and Q9 too? I don't have any idea off what is most probably damaged in that case of blown up (ground conected to hot line)

Many Thanks again for your time and sharing your knowledge!!!
T

[coromonadalix]

U2 is not costing a fortune,  you can test the transistors q8 q9 outside the circuit,  the surrounding capacitors maybe weak, do a proper check if you can.

Dont change parts thinking it may solve all problems...  take your time

[Colorito]

Thanks, I will do it as you are suggesting. I supposed Testing the junctions with a multimeter is enough in that case?
I will update the thread ASAP.
Thanks ;-)
T

[Armadillo]

U2 is dead. Aside from that, C20 must be checked. Anyway, time to change. I think both play the part of the 50Khz Oscillation. U2 for regulation of the -1800Voltage, C20 for the 50Khertz. But U2 must be working to talk about C20.

For the 2 transistors, you need to measure the BCE voltages of the 2 to confirm. The manual says all referenced to -8v but I think as long as you are consistent with the measurement with reference to ground, should be easy to understand.

+8V is there. double confirm -8V is there and +10V is available.

[coromonadalix]

You van check trnsistors in ohms or diode tests

https://www.wikihow.com/Test-a-Transistor