Hacker Autocrat RP33

[Hammonds]

Good afternoon to all

I'm still a Newbie.

I have a Hacker Autocrat, which I'm looking at for a friend. He believes the RF and IF are ok but that the problem lies with the audio board. I've detached the audio board and connected it to my stabilised power supply. It's drawing 240mA at 5 volts!
I've removed the AC127s and AC128s and checked them using my trusty Fluke. They all read about 0.178V in the forward direction and OL (overload) in the reverse direction.
I've replaced R11, which was 870 ohms, C8 which was leaky and C3, which had developed a short.

It's still drawing the same current regardless of the settings on RV1 and RV2. T4 gets quite warm.

I'm at a loss to know what to try next.

Any help would be gratefully appreciated.

Dave

[tiv48]

As it is a direct coupled amp I would check the other 3 transistors .
these are a pain to fault find as a problem in any stage of the amp can have knock on effects .
so I would check TR 1/2/3 preferably by substitution .
Rob T

[AlanBeckett]

Hmm. I learned by bitter experience about 47 years ago that the sensible thing to do is change all of them.
If any old ones test good save them for the next job.

Alan

[Refugee]

I have done amps like this.
What I have to know is what kind of direct coupled amplifier it is first and then check witch way the bases of the output stage are being driven. Then I can work out the faulty driver transistor needs to be replaced.
If you are not sure then change all of them and save any that pass a bench test.

[tiv48]

have you read joes battle with one?
see http://golbornevintageradio.co.uk/forum/...r+Autocrat

[Joe]

Don't scare the poor so-and-so, Rob!

- Joe

[Hammonds]

Many thanks to all who replied.

I bit the bullet and removed all the transistors from the audio board (even the BC108!).
With all transistors removed I cleaned the PCB where the transistors were, with solder wick. Found some more leaky caps and replaced them for good measure.
Got some matched pairs of AC127/9 from vintage-parts. I had a spare BC108 anyway.
Fitted all the transistors and connected to the power supply and BINGO, quiescent current down below 10mA.
Adjusted the pots to give 8.85 volts and 4.0mA.
Fitted the audio board back into the set and reconnected.
It now works like a dream on both wavebands.

I just need to check those pesky germanium transistors to see which one was the culprit.

Thanks again for a great forum and all your help

Regards

Dave

[Joe]

Good to hear.

- Joe

[boiss]

Before you get their third leg in a twist, It might have been one of those leaky caps..
I got a few old germanium audio transistors but have never had a prob with them, which can't be said for their RF brothers.
Mike