Hacker Hunter RP38A fault

[Vintage_Man2012]

Hello everyone,

I have a Hacker Hunter RP38A set that slowly loses its volume over a period of about 20-30 minutes.
I initially thought I had cured the problem by re-flowing a joint on C4 but alas the problem still persists.

I’ve had a poke around of the transistor voltages on the A209 amplifier and T1 has no voltage on its emitter. Can anyone advise what is dragging down the voltage on this transistor?
T2 and T3 have 13.5V and 16V respectively which I believe is too high according to the service data I have.

Paul

[cathoderay57]

Hi Paul, not sure what the voltages you measured are referenced to? If measured with respect to chassis it would be more helpful to measure collector-emitter, collector-base, and base-emitter voltages to enable a more direct comparison with the service data. The biasing of the transistors on the amplifier board are interdependent on account of direct coupling so if one is out it can affect the others. Start with Tr1 and work forwards. On mine almost all of the resistors were worse than 20% tolerance (some much worse) and so, if one of them is marginal, after running 20 minutes or so the thermal effects could drift it beyond the tipping point such that one or more transistors gets slowly biased off. The BC148 lockfit transistors have a notorious reputation (see elsewhere on this Forum) but more often owing to noise rather than gradual fade-out. Cheers, Jerry

[vinrads]

Hi Paul try isolating the output transistors from the heat sink, with a piece of mica, Mick.

[paulsherwin]

I would swap out the BC148 as it's easy to do in these sets. The BC548 is an identical part but all the common Si NPN types will work well - BC182, 2N3904 etc. Even if this isn't causing the fault, replacement while the set is on the bench is good preventative maintenance.

You will need to check the quiescent current.

[Vintage_Man2012]

Hello everyone and thanks for your replies.

The voltages are measured with respect to chassis, I have attached the voltage readings on T1, T2 and T4.

I should also point out that T1 and T2 lockfits were previously replaced with BC108 TO-18 package.

[paulsherwin]

OK, in that case they're unlikely to be bad.

[agardiner]

Does your set have those small resistors which have like a sort of end cap on them? Sorry, not sure what type they are called.

I have had a few of these sets where one or two resistors in the audio amp have developed hairline cracks where the end cap meets the main body. You can usually find them by poking around with a plastic stick, or gently grabbing them with a pair of pliers.

[cathoderay57]

You should also check the 2 preset pots on the amplifier board since dirty wipers will cause biasing problems. Jerry

[cathoderay57]

The voltage measurements that you made in Post #5 compare to the service sheet for the RP38A as follows:
T1 T2 T4
Vce 14.13 (9.69) 1.70 (7.85) 3.20 (9.20)
Vbe 0.600 (0.525) 0.600 (0.625) 0.700 (0.625)

To me this indicates that T1 isn't passing much current - has R4 gone high?
Also it would appear T2 maybe isn't passing much current either - check R6, RV3 and R8. Try those for a starter for ten. Jerry

[Vintage_Man2012]

The set does have some strange looking resistors, they almost like small mylar capacitors but the resistance band is printed on the end of them.

I've pulled R4 and it does appear slightly high, 550 ohm, I expect there are possibly others too but I have yet to check them.
I had a problem with the presets in another Hacker radio I repaired and ended up replacing them so I do have some spare 50k and 10k presets -

I will check those others tonight and report back!

Paul