Hacker Sovereign II RP25A Dead on all Wavebands

[OldTechFan96]

Hello all!

I have a Hacker Sovereign II that is dead on all wavebands. When the volume is turned up to the max a hiss can be heard. Sometimes a strong station can be faintly heard on both MW and VHF.

The chassis has been cleaned and the switches and potentiometers have been sprayed with Servisol Super 10. The two PP9 batteries that I'm using are in good order.

Using my signal generator (Heathkit IG-102) I injected an audio frequency into the middle tag of the volume potentiometer and the signal came through loud and clear. I also was able to play music through the phono input of the radio without a problem.

I decided to focus on the AM section of the radio first.

Thinking that the fault must lie before the audio stages I injected a 465KHz RF signal into the collectors and bases of T1 to T4 of the AM section of the radio, including the detector diode. I could not hear a thing.

I measured the voltages on the amp board and the AM section with my DMM and they all seemed too high or too low when compared to the specification given in the Hacker service manual. I will measure them again sometime.

I think that the radio may have been used with some form of external power supply as it came with plastic thing by TRC where you place the battery connectors. Also, a small section of the rear case has been removed to facilitate a power supply lead.

I am very much a beginner when it comes to radio fault finding and would like to use this radio as something to learn/practice on along with getting better at using my sig gen and DMM.

Where should I look next?

Thank you

[paulsherwin]

I would normally suggest a transistor fault, but this radio effectively uses different circuitry for AM and FM operation so failure of both would require multiple transistors to fail.

However, you should familiarise yourself with the possible transistor issues which may affect this set. Search the forum for 'lockfit' and 'af117'.

The fact that you don't hear anything when injecting a 465kHz modulated signal at the detector diode is very puzzling.

[mhennessy]

I'd start with the gram input socket, as these are notorious for getting poor contacts. If you're lucky, a bit of switchcleaner dribbled in the top (plus some gentle agitation with a toothpick or similar) will fix it. If not, you'll need to replace the socket or simply bridge the contacts together.

Unless they've all been sold by now, authentic NOS replacements are available from Partridge Electronics.

[julie_m]

If the audio stages work fine when you inject a signal downstream of the detector diode, but you get nothing injecting a modulated signal upstream of the detector diode, then that would suggest the diode itself. Is it hidden inside the final IF can, or is it a separate component? It will probably be a small glass tube, about 5mm. long by 3mm. diameter, with a red marking on one end.

EDIT: Beaten to it. And shame on me for not thinking of the obvious first

[paulsherwin]

The symptoms do point to a relatively simple switching fault. You could simply short out the contacts if they're bad and you don't need the aux input, as Mark says.

A bad detector diode would only affect AM reception.

[julie_m]

True, but there could be a separate fault in the FM section. So I pounced on the idea of the detector diode at fault, ignoring the contacts of the input jack and wavechange switches. It's that whole System 1 / System 2 thing again .....

Now, maybe if I'd had the actual set sitting here in front of me, I might have thought to eliminate any switch contacts before condemning the diode. Especially if it was inside the IF can, because I'm sure I'd want to be really sure it was faulty before going digging for it. And that pause might just give long enough for System 2 to kick in -- It is my considered assessment that there is a greater probability that a mechanical contact is to blame, than that there is an unrelated fault in the FM section even given that the detector diode is faulty. So who knows?

Always think of the obvious, should be obvious; but one person's "obvious" can be another person's "WTH?" .....

[OldTechFan96]

Thanks for the above advice!

This radio use Lockfit transistors which seem to be going bad in some sets. I've dealt with AF11X transistors in a Roberts R700. I replaced them with a modern transistor that was suggested by Mr. Sherwin and it worked well.

The detector diode is within the IF can, L8.

I'll have a good look at the switches and sockets to see if they are a problem.

I've included two photographs of the plastic thing that came with the radio.

[OldTechFan96]

I bridged the contacts of the gram input and the radio came back to life! Only on VHF.

I tested TR1 to TR4 yesterday and found that TR4 (BF195C) was bad. Today I replaced it with an BC337 and MW/LW came back to life.

Now I'll attempt some of the adjustments and alignment mentioned in the service manual.

It is a bit of a shame that the case of the radio lets it down. The grille is battered in places, the dial plate has two splits in it, the on/vol knob has been reglued, the aerial looks to be from a Vega Selena 215 and parts of the case have warped so they don't quit fit together. This is not everything, though!

Thanks for all of the help! I'll probably have questions about the adjustments but I may make another post for that.

[paulsherwin]

The BC337 isn't an ideal transistor to replace a BF195, though if it works then it works. Ideally I would choose a general purpose transistor optimised for lower current operation, such as a BC548 or 2N3904, or even better a VHF transistor like an MPSH10 or 2SC2999. These circuits are a lot less fussy than is often assumed though, especially the AM stages, and it's amazing what you can get away with.

I would caution against realigning unless you have a real reason for doing so. It rarely results in improved performance if the factory alignment hasn't been disturbed by the phantom twiddler.

[mhennessy]

OK - that's good news

From your description of the cosmetics, it sounds like it'll be an ideal shed or gardening radio. There's plenty out there, so sooner or later a better one will appear. In the meantime, enjoy the Hacker sound quality!

The object in post #7 is part of a TRC Adaptavolt (spelling?) mains adaptor. I've got one somewhere. It insulates the PP9 connectors once they've been connected to the output of the adaptor, so preventing them coming into contact with anything metal inside the set.

[mhennessy]

Pictures of the Adaptavolt that came with a Hunter many years back. Not the safest thing...

[OldTechFan96]

I think I'll order a MPSH10 transistor to replace the BF195. This might explain why MW/LW sounds a bit quiet.

[paulsherwin]

No, don't order single transistors at random on the offchance it'll make a difference. If your AM performance really is poor then it may just need an RF realignment. This is basically just a matter of moving the ferrite rod coils about until the signal peaks.

However, it makes sense to stock up on versatile transistor types from Chinese suppliers as they will last you a lifetime. You can buy 50 2N3904s or 2N3906s for 99p including postage, and 50 2SC2999s for under £2. MPSH10s seem to be a bit more expensive at the moment, but you just have to keep an eye on eBay and order when the price is right.

[OldTechFan96]

Originally Posted by paulsherwin

No, don't order single transistors at random on the offchance it'll make a difference. If your AM performance really is poor then it may just need an RF realignment. This is basically just a matter of moving the ferrite rod coils about until the signal peaks..

Duly noted. Both the AM and FM performance could be better so an alignment is the next thing that I'll do.

Thanks for the information on the Adaptavolt, mhennessy. That explains the piece cut-out of the rear of the radio. If anybody wants the plastic piece they are welcome to have it.

Despite the cosmetic faults of the radio it actually looks pretty good once assembled!

[crackle]

I have to agree Mark, that power adaptor dose not look the safest of items to plug into the mains. It looks like there is a fuse in there but what stops the transformer and all rattling around in the case and shorting on the mains tags.
I think I would destroy that so it cant be used.
Mike