Kenwood 700C wont turn on.

V

vuntruong

Active Member

• Mar 5, 2015

• #1

So recently, I acquired a Kenwood 700C. It is in good condition, and was working great for about 3 weeks or so. I mainly listen to it via its headphones amplifier. One day, I turn the amp on, and I hear nada, Nothing. I could not hear the usual CLiCk! from the soft start relay. The amp simply refuse to turn on. So i decided it was time to reccapped the amp, and change out the Omron relay.

So replace the old relay with Omron LY2 24VDC relay,
Change all the electrolytic caps with new ones,
change all the 2sc1451 transistors to 2sc3902T

I turn the amp, on, and still no click,
I measured the voltage of the power supply, for pin #33 to be -50V, and pin # 19 to be -25 volts.

I know that Vrk1 (on the PSU board adjust the +-50 volts, Anyone knows what Vrk2 ( one the PSU board ) Adjust?

 

V

vuntruong

Active Member

• Mar 5, 2015

• #2

Actually, I figured Vrk1 adjust the -50 volts PSU and Vrk2 adjust the +50 volts PSU. So, I got everything on the PSU adjusted to +- 50.5 volts, and +-25.6 volts. Still, the 700C wont click on.

 

gslikker

Super Member

• Mar 5, 2015

• #3

What about the 23 volts unregulated supply for the relay board and its fuse?

 

V

vuntruong

Active Member

• Mar 5, 2015

• #4

23 volts unregulated power supply? i am not sure what you are referring to. Measurements of pin 9 gave a reading of 7.11 volts, Pin 12 ( -7.11 volts), and pin 34 ( 26.9 Volts )

 

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EchoWars

Hiding in Honduras

Sponsor

• Mar 5, 2015

• #5

9 and 12 should be close to +/-5V, but that's neither here nor there, and those 5V supplies are used only in the headphone amp.

Both relays (the big Omron LY2 and the small reed relay) are powered from the voltage generated by the 20Vac winding and the little Dk6 bridge rectifier. Your voltage at pin #34 is in the ballpark. So the focus shifts to the relay block. Make sure you see that same voltage at pin #5 on the relay block. If so, then you may have a couple of bad diodes (Dq1 & Dq2) or a couple of weak/dead relay driver transistors (Qq1 & Qq2).

 

V

vuntruong

Active Member

• Mar 6, 2015

• #6

So, I checked the Voltage of pin Number #5, and I got a reading of 27 Volts. So I guess, I need to replace Dq1, Dq2. and/or 2sc1416 and 2SD220.

 

gslikker

Super Member

• Mar 6, 2015

• #7

It could be, but since the board is small and the components are easily accessible, it is quite easy to make a list of the voltages you measure compared to ground at each connector pin and maybe the answer will be there.

In case you do not have the components at home and have to buy them, it would be a disappointment if the problem would be something else.

I guess you have the schematics? you can download them from hifiengine.

 

V

vuntruong

Active Member

• Mar 6, 2015

• #8

Yes, trouble shooting this preamplifier has been a little bumpy. I Thought replacing the LY2 relay would do the trick, but I was wrong. :sigh: So now the task have turn into a full restorations. :yes: Since the relay board have only 2 diodes, and 2 transistors, I figured it is probably best to replace them ( or put the order in for them). If one of the diodes or transistors failed, the other one is probably not too far behind.
So now, I need to source some diodes, transistors, and possibly put the order in for some other rectifier diodes on the PSU board while I am at it.

Diode V06B
Qq1 2sc1416 Gr
Qq2 2SD220

Diode V03C
YZ-140
W04M
Cz-245

 

V

vuntruong

Active Member

• Mar 10, 2015

• #9

So i checked the Diodes on the relay board, they seems to be ok. I've also measured pin # 8 = 0.00 volts, pin 9 = 20 volts, pin 10 = 26.7 volts and pin 11 =26.7 Volts. I am thinking I probably need to replace 2sc1416 (Gr) and possibly 2SD220. Any suggestions?
I was thinking i can replace 2sc1416 with Fairchild KSC185YTA, and replace the Sanken 2SD220 with ON Semi 2SC6043. Do you think these transistors will work for the relay board?

 

EchoWars

Hiding in Honduras

Sponsor

• Mar 10, 2015

• #10

KSC1845Y is NLA. I'd use a KSC1845FTA. It'll do fine. For the bigger trannie, the KSC2690AYS (TO-126 style) works great. I'd also replace the diodes, simply because. A simple meter test might catch 70% of failures if you're lucky. Pop some new 1N1004's in there. And replace that 47µf cap as well.

As always, download the data sheets if you're not totally sure of which pin on the new transistor is which.

 

V

vuntruong

Active Member

• Mar 10, 2015

• #11

Thanks Echo. Very much appreciated.

 

V

vuntruong

Active Member

• Mar 13, 2015

• #12

So yesterday, I got the parts i needed to trouble shoot the relay board for the kenwood 700C preamplifier. I replace the 2SD220 (Qq2) with KSC2690AYS (TO-126), and now the OMRON LY2 (24VDC) relay is clicking on like it should. However, I am still not getting any signal output, I can hear anything :worried: I proceeded to replace the 2SC1416 GR (Qq1) with KSC1845FTA, and replace the Dd1&Dd2 with UF1004T diodes.
I turn on the preamp, a few seconds later the big relay click, but the preamp still has no output. It seems like the reed relay is still not engaging
I took some measurements, for the relay board. Pin#5 = 25.2 volts, pin #9 = 19.5 volts, pin#10 = 0.478 volts and Pin # 11= 25.9 volts.

 

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V

vuntruong

Active Member

• Mar 25, 2015

• #13

Just an update, I got the preamp up and running nicely now. thanks again EchoWars for all the insights

 

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mech986

AK Subscriber

Subscriber

• Mar 25, 2015

• #14

Congrats on the repair! and thanks to EW for pointing the way.