Hacker Herald RP37A repair

[NickG0HIK]

I attended the Cumbria Steam Gathering yesterday, of course the first part of the site to visit is the jumble.
A dealer had a couple of Hackers that I just could not refuse at £5 each, I didn't even try to haggle.
One was a Helmsman, which works on all bands except SW 2 & 3, but that is another story, as it is in need of some TLC
I've started on the Herald firstly.
First of all just a simple cosmetic clean, some switch cleaner and lube the aerial.
I replaced one of the damaged battery connectors and powered it up then on the shop PSU.
Working, but distorted audio at 18v and about 200mA being drawn.
I set about setting up as per the manual, but no joy, I found that T1 BC148 faulty, so replaced this. Still the same fault, inspection of RV3 47k trimmer had the rivet missing on one of the terminals, this was replaced and could now get more sensible voltage readings around T1.
But the amp was drawing way too much current adjusting RV4 I still had around 50mA drawn quiescent at minimum setting and a lot of noise.
I went for the freezer spray, but found the can to be empty so started pushing a hot iron on to components, eventually found what appears to be the culprit, T3 OC71, I've not got one of these (Can anyone let me know the best use in it's place) I've put in a AC128 in for now. The noise has improved, but still have about 40mA draw and the RV4 still not working correctly.

So for now, I've boxed it back up as it's sort of working and will do a more though restoration in the future.

One of the tone control knobs has a broken centre, is there any wisdom as to how repair this, maybe fill with modellers clay, then push in a shaft and let it set.

[Herald1360]

I expect Mr. Hennessy will be along soon.......

Otherwise, if you fancy a bomb proof upgrade, here's what I did:

https://vintage-radio.net/forum/showthread.php?t=54342

You may well have a whiskery output transistor or leaky capacitor somewhere- you've sorted the dodgy pots. T3 does thermal compensation for the output stage- pretty much any small signal Ge PNP will do.

[NickG0HIK]

Thanks for the info, if it keeps raining today, I may have another go at the set later in the day.
I'd never have thought about the Polystyrene cap being a fault in yours, but I have had these faulty myself in the past.
I'll check that and the output transistors too.

[Radio Wrangler]

Thanks, Chris, I've got an AM herald and a Sovereign 2 to do shortly. The cosmetics are done on the Herald, I just have to make it speak again.

For the broken knob, I'd turn down a metal knob, or a brass insert and araldite it in.

David

[paulsherwin]

Originally Posted by Herald1360

T3 does thermal compensation for the output stage- pretty much any small signal Ge PNP will do.

Agreed, this isn't a critical component and any spare small signal Ge transistor should be OK. An AC128 will be as good as anything else, so you might as well leave it in place unless you're trying to conserve your AC128s.

I see you've found another bad Lockfit. Rip 'em out I say! They're little ticking time bombs

[NickG0HIK]

AC128's and BC148's are two types of transistor that I seem to have plenty of in my stores

[NickG0HIK]

Originally Posted by Radio Wrangler

For the broken knob, I'd turn down a metal knob, or a brass insert and araldite it in.

Thanks I like it, I'll see if my friendly model maker will help again.

[paulsherwin]

I wouldn't use NOS Lockfits for anything. Remember a BC108/BC548 is exactly the same as a BC148 and will drop straight in. In fact any Si NPN general purpose type is likely to be fine.

I wasn't joking about ripping them out. I see so many failed Lockfits nowadays that they have become a change on sight item. They are normally cheap and easy to replace.

[NickG0HIK]

Ok Thanks for the information, all noted.

[NickG0HIK]

Originally Posted by Radio Wrangler

For the broken knob, I'd turn down a metal knob, or a brass insert and araldite it in.

Great, I've just used the Dremmel to cut out the insert from another knob from my junk box and as you said, I Areldited it.
A nice repair that only took a few minutes to complete.

[john.north]

Hello,
40mA is still too much. You may have a leaky electrolytic.
Can the quiescent current be set correctly at the link or is it too high?
If not then the likely candidates are the output pair.
Can the midpoint be set correctly with rv 3? If not then t4 is faulty.
AC 128 is ok for t3.
John.
Ps I have two knobs if you want them.

[mhennessy]

Originally Posted by Herald1360

I expect Mr. Hennessy will be along soon.......

Fashionably late, as always

Good advice given so far. It might be worth checking T4, as that can cause strange faults.

It's possible the output devices are leaking - especially if they've been over-heated for some time. To test for this, temporarily short the two bases together. The quiescent current (via the test link) should drop to nothing in this state - if not, try replacing the output devices.

Good luck,

Mark