kenwood TK-40 Output problem - need help with circuit debug

[rievax_60]

Re: kenwood TK-40 Output problem - need help with circuit debug

Check R8 and R11 also. Dont turn it on until the idle current setting circuit is fixed.

[jimkarl]

Re: kenwood TK-40 Output problem - need help with circuit debug

Idle current setting circuit? do you mean the D2/D4 Diode that broke?

[jimkarl]

Re: kenwood TK-40 Output problem - need help with circuit debug

Also you said "check R8" you mean for accurate value, or to note voltage? Just want to be clear.

[rievax_60]

Re: kenwood TK-40 Output problem - need help with circuit debug

Idle current setting circuit? do you mean the D2/D4 Diode that broke?

D4 and D5 and associated components are for controlling the idle current through the output transistors.. Check R8 and R11 with your meter's resistance measurement range.

[rievax_60]

Re: kenwood TK-40 Output problem - need help with circuit debug

As Budm mentioned, the output transistors can be destroyed if a fault develops or a mistake is made in this idle current setting circuit. The 500 ohm pots should also be set to their minimum resistance initially to be safe.

[jimkarl]

Re: kenwood TK-40 Output problem - need help with circuit debug

OK - Replaced Q1 in the power supply sections and D4 on the Main Amp board - both broken due to my clumsiness. Did a complete re-measure all over and have the attached results. What I find interesting is the consistent 0volts across Left channel Q2e/Q4c and Right channel q7e/Q6c respectively. (Also Q9b&e which they all tie to) Are Q2/Q7 dead and emitters fried? Too low base voltage to turn them on? Something wrong in the Q9 circuit portion that they both tie to?
I can pull them and test but figured I'd ask first if something is obviously the cause. Given the similarity in how both circuits look now, there would seem to be a unifying common cause.

[rievax_60]

Re: kenwood TK-40 Output problem - need help with circuit debug

The readings show that the Q3 BE is forward biased by about 30V. This is an impossible state for a good transistor. Look for this type of condition and check the transistor. Then check other transistors also.
Transistors that have high reverse BE voltage applied, are likely to be damaged also. It is not supposed to exceed 5V for most bipolar transistors.

[jimkarl]

Re: kenwood TK-40 Output problem - need help with circuit debug

Pulled Q3/Q5 and they both register as dead on my BK transistor tester. Checked Q2 but looked good so I put it back. Then checked Q4/Q6 Transistor tester and it registered it (strangely on two different b/c/e combinations) so not 100% sure they were good or not. These are PNP. Diode test with a multimeter shows only .174 on a couple combinations in one direction, which seems quite low? Also showed leaky on the transistor tester.

Did a quick goggle search for 2sc538A replacement and shows BC107/A157. A157 is much cheaper on Ebay but also a different package format and may be more challenging to get on the board. But price is an important factor given the low value of this receiver.

2SB89A PNP shows AC128/2n2706. Ebay shows a lot of hits on a MP13B & MP40 which they state is "similar" but I don't know. The AC128 2n2706 regulars are a bit too pricey for me to bother with for this unit, I think.

Is there an web site bible of all transistor replacements out there?

Also, anything else to check on the board as to the source cuase of the 38 volts feeding back into the circuit and destroying these transistors? I'd hate to buy these replacements then just have it do the same thing again!

Also - I never have been able to find a replacement for the TH2 thermistor part (SDT-35). With that right channel one out of circuit that's not contributing to the problem, is it? I know the safety measure is gone without it, but the immediate concern is getting the general amp back up and functioning. Not planning to drive it hard until a replacement is found)

[budm]

Re: kenwood TK-40 Output problem - need help with circuit debug

Be careful on the pin out of the original Japanese transistor and the substitute Transistors.

[rievax_60]

Re: kenwood TK-40 Output problem - need help with circuit debug

Going by the specs for the 2SC538A, BC556 and BC546 should be adequate replacements for both drivers.
The 2SB89A is a germanium transistor, very rare now. The 0.174V diode test reading would be normal for this transistor. I would replace it with a BC556.
What are the output transistors? Are they ok?
We will figure out what to do with the idle current setting circuit later.
I think the 38V is just a result of the faulty transistors.
Are R17 and R18(4.7ohm) actually fitted?

[jimkarl]

Re: kenwood TK-40 Output problem - need help with circuit debug

I'm not seeing BC556 readily available on ebay in NPN format, so I guess I'll go with the 546 for those. For the germanium replacement I'll get BC546 PNP as you suggested. (though now I think they may be ok but I'm going to replace anyway since they are already out.)

R17/R18 are not fitted. Only 4 of the six listed .47 resistors are present. Those two that are missing are just a direct line back to the power supply main cap. I'm sure it was just another slight schematic variation. There seem to be several...

The Output trans are 2SD92 and checked ok last time I tested them.

[rievax_60]

Re: kenwood TK-40 Output problem - need help with circuit debug

The BC556 is PNP and the BC546 is NPN.
Another option is MPSA05 and MPSA55. They have a higher current rating.

[jimkarl]

Re: kenwood TK-40 Output problem - need help with circuit debug

The BC556 is PNP and the BC546 is NPN.
Another option is MPSA05 and MPSA55. They have a higher current rating.

Correct - I backwards got them.

[rievax_60]

Re: kenwood TK-40 Output problem - need help with circuit debug

Keep in mind that the pin out will be different.

[jimkarl]

Re: kenwood TK-40 Output problem - need help with circuit debug

Got it. Already printed out the datasheet with the new pin outs.