Hacker Hunter RP38A buzzing

[D o i v e]

Listening to PM yesterday evening on my faithful old Hunter, I noticed that the bass was starting to buzz through on people's voices, and especially on music clips. Tuning to other stations didn't help, but moving the bass adjustment even slightly upwards (set at lowest at present) caused the output to change to a loud BRRRRRRRRRRRRRR noise. Switch off and on, back to normal for a few seconds, then back to BRRRRRRRRRRRR again. Very unpleasant, seemingly independent of volume control as well.

Left it over night, came down this morning and operated all switches and pots through range, treble up down, bass up down, select LW MW VHF, twiddle tuner, volume full and off again. Powered set up, switched on to be greeted by perfect sound at all volumes and all stations. That was 10am this morning, and still going fine 5 hours later.

So is it just a case of getting a bit of contact cleaner in round the set? It's been perfect for the last six years, great little set, so I don't want to go taking it apart looking for a problem that doesn't exist.

[Darren-UK]

Going through it with switchcleaner would be a good move regardless, but I suspect the problem might have been some sort of external, perhaps low frequency, interference.

If it returns try moving the radio directionally (and the aerial if on FM) and move the radio around the house. That will soon tell you if it's external or if the radio is at fault.

[jjl]

It sounds as though the audio amp could have been motorboating i.e. oscillating at low frequency. This can be caused by failing decoupling capacitors or even a low battery.

John

[Darren-UK]

I thought of motorboating but it's unlikely given that the OP says the radio has been working fine for several hours today. Mind you, a failing battery can cause motorboating on FM but will continue to function well on AM for a long time at a reasonable volume level.

[ukcol]

Low battery occured to me as already mentioned by John (jjl).

Colin M

[mhennessy]

Whenever I've had this problem, it's been an intermittent connection at the 5-way plug on the amplifier PCB. Give it a wiggle and see if it provokes the connection.

Cheers,

Mark

[D o i v e]

Thanks for your thoughts gents. The battery can't be the source, as I run it on a mains adaptor at all times. I will give the amplifier PCB connector a closer look to see what's what. The old girl ran from 10am to 7pm today without fault, so I will keep an eye on it. Long way from retirement yet!

[Hybrid tellies]

I have nearly the same fault on my Hacker Hunter RP38A. With the volume set at reasonable level and the bass is turned up to more than a half way it starts buzzing loudly and continues even with the volume control turned to minimum. The only way to stop it is by switching it off and backing the bass off before turning the set back on.
I have checked the capacitors on the output stage using an Atlas LCR tester and all appears to be ok. I am sure this motor boating problem is in the output stage and suspect one of the caps has gone microphonous. I have also cleaned the contacts and am running the set off 18v from my workshop supply.

[Herald1360]

Originally Posted by Hybrid tellies

I am sure this motor boating problem is in the output stage and suspect one of the caps has gone microphonous.

Is that a lower valency state than microphonic?

[Hybrid tellies]

Apologies not feeling too good at the moment I meant microphonic.

[mhennessy]

Hi Simon,

I've only seen that exact fault on the Sovereign III (and derivatives). There was a fix for it, which is detailed in the service manual - it involves running an extra wire from the chassis earth tag to the earth trace on the tone control PCB.

Of course, the RP38A is different, but in light of the above, it's definitely worth checking and double-checking the earthing arrangements, as tags can go loose, and solder joints go bad...

However, the smart money is always on the 5-pin amplifier connector. You've probably already checked it, but it might be worth giving the earth contact another clean and squeeze, just in case.

Regarding the capacitors in the amplifier, they are generally pretty well behaved. It's hard to see how microphony could be the cause. As already mentioned, it might be worth checking the decouplers though - even though you've checked them with your LCR meter, that didn't measure ESR. Just putting a known-good capacitor in parallel is good enough for a quick check

Good luck,

Mark

[Herald1360]

Originally Posted by Hybrid tellies

Apologies not feeling to good at the moment I meant microphonic.

No need to apologise- microphonous sounds rather good as a new word!

Get well soon.....

[Hybrid tellies]

All sorted the problem was a hairline crack on the audio output panel underneath the 5 pin plug and socket SK 1 off the treble pin. This is now repaired plus a good clean of the plug and sockets using a small screw driver and an alcohol soaked cotton bud.it is now buzz free with loads of bass and treble. Just a bit of tiding up to do then will put a full write up for success stories.
Thanks for the helpful suggestions very much appreciated.

[Meat Pye]

Thanks for telling us how it was cured - so annoying when people simple say "fixed it"! Hairline cracks are a common occurrence on any electrical device, along with poor soldering (Roberts!) or poor contacts on switches and connectors.

[Nickthedentist]

Well done Simon.

This is a known fault on other Hackers too, presumably because pressure from the plug being inserted flexes the board, causing cracks.

Nick.

[Hybrid tellies]

Thank you for your kind words. Hacker repairs are a bit jinxed for me. I have got three Sovereign II's and Sovereign 111 which have all developed strange faults, sadly on one all the IF coils on the AM tuner became loose. My Sovereign 111 also has that buzzing fault which I will get to take a look at especially with that information from Mark.
I have nearly completed the Hunter RP38A and will put a write up on success stories.

[Nickthedentist]

I've been working on my first RP38A tonight.

The symptoms were the same as Doive and Simon report, i.e. motorboating. It was mainly dependent on the setting of the bass control (worst at minimum or maximum) and occurred with the volume control at any setting.

The cause was the plug/socket between the tuner and amp sections, but in my case, was caused by the female receptacles on the amp PCB being cracked leading to a very poor grip on the plug's pins.

The cure was to remove the female receptacles from the PCB, abrade the outside surface, then poke a thin piece of very thin tinned copper wire through the pin that's soldered to the PCB, and wrap the rest of it around the receptacle to make tight coil, finally applying solder to bond the coil to the frail receptacle. The repaired parts were then inserted into the PCB, the plug inserted to aid correct alignment, and the receptacles soldered back to the PCB. Finally, the inside of the receptacles were cleaned with Servisol Super 10 on a dental "TePe" brush.

The repair has been completely successful so far with no hint of instability, but I have another set with an even worse problem which seems to have been caused by battery acid. I think the socket will have to be bypassed here.

Nick.

[Hybrid tellies]

Nice one Nick, it seems that this 5 pin connection on these Hackers is becoming a bit of a weak spot. But at least we are now becoming more aware of the strange faults that this plug and socket link can throw up.