Fidelity HF35T (Transistor version).

[smiler411]

maybe the driver transistor is shorted or you have an open resistor feeding it from the power rail, looks like there is no bias current for the transistors. That will be the small plastic one in the picture I think. I think points 3 and 4 that you measured should be around half the supply

[Marconi_MPT4]

Fidelity circuit is mostly similar to one published by Mullard in Transistor Audio And Radio Circuits (2nd Ed.)

I managed to reverse engineer the PCB from pictures posted earlier in the thread as it was fairly simple circuit, but might help with diagnosing the problem. I did briefly check it out to see if the amplifier worked.

Attached circuit....

What is the measured voltage on junction of R9(120k) / R7(330k)

Rich

[HRichard]

Originally Posted by Marconi_MPT4

What is the measured voltage on junction of R9(120k) / R7(330k)

Rich

Circa 17v

[EmleyTim]

Maybe I’m missing something but the circuit diagram is available from Paul for both this version of the HF35 (albeit with a T suffix to denote transistor rather than the earlier valve version, I imagine) ref A18411 and the later IC version ref A18412.

A18411 is only 2 pages - component values and circuit diagram, respectively.

I’ve repaired a couple in the past with no output and in both instances, it was a tin whiskers problem, hence my suggestion in post #10.

It could of course be something entirely different but, having established that the supply was ok at 27V, I’d check the output transistors next by the method of removing the common heat sink and briefly testing again. Not particularly scientific but it’s an easy check.

Regards, Tim

[Marconi_MPT4]

17V is slightly lower than expected but not enough to cause this fault.

What is the voltage across R6(2.2k)? Should be about 0.6V to 0.7V.

If within these limits it strongly points to the driver transistor (Q4) could be at fault, as suggested in post #21.

Rich

[julie_m]

@Marconi_MPT4, thanks for reverse-engineering that circuit more quickly than I could!

[HRichard]

Originally Posted by EmleyTim

I’d check the output transistors next by the method of removing the common heat sink and briefly testing again. Not particularly scientific but it’s an easy check.

Regards, Tim

Exactly the same readings.

[HRichard]

Originally Posted by Marconi_MPT4

What is the voltage across R6(2.2k)? Should be about 0.6V to 0.7V.

Rich

Measuring 4v

[smiler411]

If you have 4V across R6 then either R6 is open circuit or Q4 is faulty I would have thought

[smiler411]

Q4 will probably be the small black plastic transistor towards the outside of the PCB. I would think any PNP type would do like BC558 or similar, wouldn't think its anything special

[HRichard]

Originally Posted by smiler411

Q4 will probably be the small black plastic transistor towards the outside of the PCB. I would think any PNP type would do like BC558 or similar, wouldn't think its anything special

I've removed R6 and measured it to be in spec so I will try checking / replacing Q4 next.

[HRichard]

Q4 is dead.
It looks like an ME040x ?
Other than a BC558 (which I don't have) any other suggestions for a suitable replacement?

[Keith956]

2N2906? Any general purpose small signal silicon PNP should work.

[HRichard]

I've gone through everything I have and there's nothing suitable (all NPN) so I'm going to need to order some anyway.

[HRichard]

OK, so we have progress.
Replaced the ME0404 with a BC508B.
The amp is now working in so much as there is sound from the speaker and a hum when switched on, but, there is no volume.
I can hear sound with my ear against the speaker but nothing from 2 feet away.
Volume control is on full, tone control doesn't change anything, sound is slightly muffled.
Please help, again

[Station X]

Buzz test?

Voltage readings?

[HRichard]

If I touch the red wire the hum gets louder, but no loud buzz.
Green wire, no noise at all.
Voltages on Q1 are circa 25, 13, 13v however Q3 gives 0, 13, 13v

[Sideband]

Originally Posted by HRichard

OK, so we have progress.
Replaced the ME0404 with a BC508B.

ME0404 is PNP silicon, BC508 is NPN silicon (according to on-line data sheet)!! Wrong replacement! You need to change it for a suitable PNP like a BC558. With the correct transistor, check the lead outs as they may be different from the original (for example a lot of transistors have collector in the centre, BC558 has base in the centre) check the data sheets.

[HRichard]

Sorry, that was a typo. It is a BC558.
I couldn't find a pinout for the ME0404 though.

[HRichard]

Finally found the correct datasheet for the ME0404 and I may have fitted the BC558 upside down
Will report back later after I've eaten.