Hacker RP 25 -Motorboating now now AM and high quiescent current

[boater sam]

CCC strikes again.

Change the mixer back to Ge, I have found that you may get away with Si IF transistors, but the mixer is more critical on the biasing.

Fault find wins out over component swapping every time.

[Murphyv310]

I concur with Sam.
Even using Silicon in IF stages can be hit or miss. Often they knock alignment out or introduce instability. Germanium equivalents are still out there, perhaps more expensive than in the 60's but if you take inflation into account they are still reasonably priced especially if you buy the Russian ones.

[Mark Hennessy]

I've rarely tried swapping Ge for Si either. There's no point, and there's enough to conspire against you at the best of times without adding new variables. Occasionally I'll try it on a set that is already working - purely as an experiment - and would agree that oscillators are fussy, but you might be OK with IF amplifiers.

My success rate with zapping AF11x transistors is practically 100%. No returns either. If I do it for a customer, I give a 5 year guarantee. But that's for Hackers, where the transistors are easy to get at. For a Roberts R707, I'd fit AF12x types.

In case the OP hasn't seen it, my article on radio transistors is here: http://www.markhennessy.co.uk/articles/v...istors.htm

[Bonky]

Ignorant trier here again...

Re:Hacker Sovereign 2. (RP25). I have got rid of the motor-boating. It works fine on FM but not on AM. I have now replaced T1 to T4 on boards 502 and 501 with more authentic ones (AF178s). But still no sound. Most of the electrolytic caps have been replaced.

I have a sig gen and a Vellemann signal tracer (now working properly after early issues). I can get sounds from some components on both boards, I was advised to start at the detector which I think is on board 502 and under the 'can' of L8. I have tried to unsolder this item with no success. Imagining where the diode pins may be on the pcb, I do get a sound from one 'end' and not the other. I don't know if this is correct(?)

I am at a loss to know how to move on. I know I need the discipline to follow the circuit diagram properly and not just stick a probe in at random, but I wondered if anyone could give me some advice...

(I live in Sheffield but periodically visit Golborne and Bath. If any kind soul would like to peruse the 'giblets'I would gladly pay).

I hope it's acceptable to copy this on two other websites (Golborne and HackerGroups).

Yours,

Richard

[Crackle]

Hi Richard
Inject your modulated IF signal into the collector of T4 and listen for the signal on the speaker.
If that works turn down the output from the signal generator so you can just hear it.
Connect the signal generator to the collector of T3 and listen for output.
Turn down the generator again so you can just hear the tone.
Then connect the signal generator to the collector of T2, turn down the generator output.
Repeat again on T1 collector if you get a signal in the output, then suspect T1.
If the signal stopped along the way then suspect the transistor immediately upstream (towards the audio amp) to where the signal stopped.

If I am talking complete gibberish then will someone please tell me.

Mike

[Bonky]

That's really useful! Many thanks.
BW

[Bonky]

Hi all. Once again, many, many thanks for you all on this site and VintageRadios for all the help.

Update:
I am no further forward.

1"David Potter:
One question.
Is the AM oscillator actually oscillating?
One simple method is to place the radio under test, near to another working radio, tuned to a medium to weak station up the high end of the band, 1.2 meg and above. Tune the test radio lower down the band to see if you can pick up the oscillator of the test radio on the working radio, (you will soon know if you find it).
You can practise with two working radios first. Tune slowly as it will be very narrow."

I couldn't find any signs of the pick-up from the oscillator in a working radio.

2. Voltages on the collectors of Tr1 to 4
There are no voltages (!) except on TR1 (On another board) where it reads 1.27V

3.The HT line on the 502 board should read 9V. It reads 2.23V

I thus thought that the switches must be at fault, but shorting the pins on the pcb made no difference.

I haven't yet tested the 'dropping' resistor R13 but if this were faulty then why do I get a voltage on the collector of TR1 (via R3)?

I can't see how board 502 gets its +ve supply from.

A tell-tale sign may well be that the whole set is slow to warm-up ( perhaps 30 secs or so) - even on FM (which works well)

Thanks,

Richard.

[Murphyv310]

Hi.
Something is dragging the supply down. Double check the transistors are PNP and fitted the right way round. Check for solder splashes or bridges causing a short circuit. Also make sure electrolytic capacitors are in the right way round. Do these checks before delving in, man made faults are the most common especially if faults are on going.

[Terry]

 Voltages on the collectors of Tr1 to 4
There are no voltages (!) except on TR1 (On another board) where it reads 1.27V

I think that would be a very good reason that it isn't working - don't you?
There are two options: either there is no supply to that part of the set or there is a dead short across the supply rail.

The HT line on the 502 board should read 9V. It reads 2.23V

This suggests that the fault is a short but as I don't have circuit to hand I can't comment in any more detail.

[Murphyv310]

Hi.
We were always taught three simple things to use. Ears, Eyes & Nose.
So in your case, no sound on AM. Then look for bridges, on tracks, incorrectly fitted transistors, burned resistors, wrong wiring, smell and see if you can detect burning.
This is the fundamental way of what we call first line checks. Once satisfied then progress deeper.