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

[Mark Hennessy]

Excellent progress, Trevor

Those 5 pin connectors are notorious for playing up. Contact cleaner and a squeeze often does the trick, but many times the soldering needs to be reflowed, even if it looks OK. It's a shame they didn't use a different connector with more pins, as then they could have used two earths - one for signal and one for power.

I'm not sure I have any spare AM IFTs for that vintage of set - I haven't scrapped any as yet. To widen the net a little, the RP35 Herald and RP36 Helmsman uses the same IF PCB. What I don't know off the top of my head is if you'd need to find a germanium set - the manuals I have here don't list part numbers for the IFTs in the supplements that cover the silicon versions of the IF PCB.

[Murphyv310]

               
Hi.
Thanks Mark for your reply.
I've taken some pictures of the work so far. As you'll see the "lid" was off the IF can on receipt so may have been part of the issue as to the damage to the ferrite and connections. Actually the diode can be checked without dismantling the can and they are a reliable component.
I'm sure after another can is fitted as we have good signal levels to the base of the last transistor, all the transistors are AF117's now and they have all had a "just in-case flash"
Next instalment on Friday  

[Bonky]

Trevor,

I'm full of admiration; so quick; so detailed.

In some ways I'm glad that the issue is' "multi-factorial'' and that I'm not to blame for most of it (although perhaps the can is mainly mine - but perhaps not the ferrite core).

To help me (and I hope others), please could you explain how you arrived at your diagnoses?

eg:

1. Where did you put the probe of your 'scope. Did you inject a signal...if so..what was it?
2. How did you know the various transistors were dud? etc.

Very many thanks,

Richard.

[Murphyv310]

Hi Richard.
OK well here we go........ As the set had already been worked on and the detailed voltage readings you gave I thought I'd firstly see if I could trace out the mixer/oscillator. Using the scope I put the probe on the collector on the small board with one transistor T1. there was no results, i should have seen a sine waveform on MW & LW. So a little prod around with the meter I saw the voltage jump when on the base and at this time a low waveform was seen on the scope as i left it connected. Moving from the base further up the print the voltage was different, I then ad a look with the magnifying glass and could see the crack in the print.
Now remember what I said originally..... use ears, eyes, and nose. I actually think this was your original fault as it was away from the "land" where the base connection of the AF117 is soldered to. I then was happy with that area and fitted a known good AF117 at that point to the component side.
Moving on to the IF amp board I removed T2 and fitted another AF117 as what we call a blanket job as I have plenty of "repaired" AF117's so I could then eliminate the transistor and any discrepancies there. Signal was at its collector but nothing on T3 collector, I whipped out the AF114 you had fitted and found a dead short between collector and screen, in other words a tin whisker problem. I'll repair the AF114 though and it will be fine for another 30-40 years. Then I hit problems with the last stage, I'd already seen the can off the IF transformer but wanted to make sure I got results before delving into it.
All the tracing was done with the scope, it is my favourite piece of test equipment and in a case like yours is the best to use. A signal generator would have been useless as you'd not get any further back from the damaged IF transformer. At least I know the set will work once the transformer is fixed. A signal tracer may have been of some help too but in this case I'm sure all will agree in my method.

[Murphyv310]

I did a little more before I went out and am just about to leave for a night away.
I found the second IF transformer had a short to the screening can. This was due to the 270of cap lead shorting a bit of tape on the inside of the can cured that ill. I also noted the cores were sloppy as the rubber strips were missing. To be fair I think that these faults were on the set prior to Richard obtaining it. A difficult repair for a novice, multiple faults can be very confusing.

[Murphyv310]

Hi.
Now back home so doing some more later on.

[Mark Hennessy]

Posts about the oscilloscope and probes moved to a new thread: http://golbornevintageradio.co.uk/forum/...p?tid=6607

[Murphyv310]

   

Hi.
Look what I found. No idea where its from or how long I have had it but its a perfect donor board for the IF cans I need

[Bonky]

Wonderful!! Very many thanks!

BW

Richard.

[Mark Hennessy]

That's a bit of luck