Bush AC34 fault

[brymac]

A restoration of this Bush went well, but I have been left with a residual problem which is that there is an intermittant fault with the volume. The radio will sometimes operate properly and then the volume will dip for about 10 minutes and then come back again, or it will be at low volume at switch on and then perk up. I have tried:

tapping the valves and chassis
cleaning the pins
replacing all the wax caps and the low voltage electolytic (smoothing can appears OK)

I have not (yet) tried replacing the valves

Any ideas?? The HT does not dip with the fault!!

[Darren-UK]

Hi Brymac,

From what you describe it seems the UL41 and UY41 are functioning properly. Faulty UL41s will often fire up as normal and then cause volume to fade after 10 - 20 minutes. Maybe a fault on the F/C or a screening fault somewhere.

Could possibly be a bad connection/soldered joint or, maybe more likely poor connection(s) on the frame aerial. You don't mention the switches; is the volume control and wavechanger ok ? contacts good and clean ?

Finally, is the volume change accompanied by any pops or crackles ? if not, it would likely rule out any electro- mechanical fault and suggest an aerial/screening/signal fault.

[brymac]

Thanks - yes, I've dismantled and cleaned the volume and tone controls and cleaned the wavechange switch. This fault was apparent when I collected the radio after buying it - as I listened, the volume suddenly increased to the normal level after about 5 minutes - nothing I've done since has fixed the fault!! I'm thinking about trying a replacement UL41 though and will look at the frame aerial - although I don't think it would be intermittant. The set does seem to settle once it's been running for some time though......

[brymac]

sorry - should have said no pops or crackles!

[ukcol]

There is one simple test you could try that would indicate which “half” of the set the fault is in.

Put the wave-change switch in the gram position and connect an audio source to the pickup socket, (One channel of the output from a CD player, tuner or cassette deck would be suitable). Now run your set from this signal source. If the symptom still occurs the fault is in the audio stages and your attention should be focused on the circuits around V3 (UBC41) and V4 (UL84). If the fault does not occur then the fault is in the radio receiver section before the volume control, V1 (UCH42), V2 (UF41) and the detector section of V3.

Another thought, if the volume change is not abrupt, that is it fades down and fades up; you may have a fault in the valve heater chain.

Regards

[Darren-UK]

Hmmm....strange indeed. Won't do any harm to try another UL41 or one of it's substitutes such as N142 or 10P14. I've found these two substitutes to be more reliable, for some reason.

Following your last posting, temperature comes to mind. What caps have you installed ? are they nice new audio grade polyprops ?

Certain Bush AM/FM sets which retain old capacitors are famous for wandering off-station on VHF a couple of times during the first few minutes after switch. My VHF70 does this and I know it's the old caps which are at fault - one day I'll get round to sorting it out

Of course, this is not relevant to your AC34 but I am wondering if temperature is the cause of the problem with your volume. I use freezer spray to trace such faults applied, of course, with the set disconnected from the mains during application and with care not to get spray onto hot valves

Is the o/p transformer ok ? any problem with the loudspeaker and the leads thereto ?

[brymac]

Thanks - the volume change is abrupt, not gradual and the fault occurs with a feed into the gram socket. I've tried taking the output from the ext L/S feed to another L/S, so it's not the speaker or output transformer - I think? New caps are Samhwa 630VDC polypropylene film from Pete Roberts. The only ones I've not yet replaced are the tone caps and the one across the L/S.

[quantum]

How about voltage checks? Try checking the voltages at the anode and cathode of the UBC41 and the cathode of the UL41 and see what happens when the volume goes up and down.

[brymac]

It's just done it again! On at 7.10pm - perfect volume and then dropping down (not so you can't hear it, but enough to notice) 10 minutes later.

I've checked the valve voltages - they appear OK, but the probelm is getting the wretched thing to exhibit the fault while I'm looking at the voltages!!

I'll have another stab at it tomorrow, picking up some of the advice above - any more most welcome!!

[ukcol]

Originally Posted by brymac

The only ones I've not yet replaced are the tone caps and the one across the L/S.

Replace C29 (0.05uF), C30 (0.001uF) and C31 (0.01uF). There should not be a capacitor across the LS unless you mean C30 which is across the output transformer primary. BTW circuit ref numbers are from Trader sheet 1147.

Regards

[brymac]

Thanks - those are the only caps I've not replaced. I'll do that tomorrow!! Yes - I meant C30 and I know it's across the output t/f primary. And yes - I have the trader sheet.....

Thank you all for an excellent collection of responses - as I've got two more radios lined up ( a Marconi 219 - already underway - and 2 Bush PB55s from which I intend to get one decent one!!) , I shall return!!!

But - I'm going to crack this problem first and will post the final solution once I get there!!!

Thanks again!! Bryan

[Tim]

Hello Byan
Time for my two penneth!
It could be something as simple as a bad contact in a valve holder.
The pins of those rimlock valves seem to tarnish.
I recently found broken socket contacts in an AC11 (same chassis) presumably caused by someone forcing in a valve.. As you say the fault also happens on gram, I would check the rectifier,output and UBC41 sockets and valve pins in particular.
If a broken contact is found there are a few spares in the rectifier socket!
Good luck, I know how annoying these kind of faults can be.
Tim

[mjizycky]

Hi Bryan,

I'd go with Tim's suggestion about the " Rimlock " valveholders - they can be a real pain in some sets (like the Murphy A362). I'd be inclined to look at the voltages around the output stage, and in particular the screen-grid connection to the output valve. Don't know why, something's nagging me about that... oh, and tone-correction capacitors on the output transformer. I had a Pye P75 with an intermittent capacitor that went near-s/c and that drastically reduced the volume, as well it might!

[brymac]

OK - well, it's nearly gone out of the window!! All 3 tone control caps replaced, together with the only wax cap left in the set. All valve pins cleaned. Frame aerial connections checked.

It's still doing it!!

I'm awaiting the arrival of some valves (from Germany) and will then substitute them in turn.

Meanwhile, I'm going to crack on with the Marconi!!

Bryan

[ChristianFletcher]

Just a thought could it be a problem with the output transformer, may be a shorting turn when the set is warm. Have you looked at the anode current of the output valve.

Regards Chris

[brymac]

I've only checked the anode voltage on the output valve, not the current - but bear in mind that the volume comes back up again after a while, so it's unlikely to be an output transformer fault - and I've tried it with an external speaker and the problem is still there!!

[brymac]

OK - for those of you who have been losing sleep over this one (!!) - once I replaced the UCH42 frequency changer, it was fine!! Please see my new thread for my latest project! Thanks to all of you who helped with this one!!

[ukcol]

Originally Posted by brymac

....and the fault occurs with a feed into the gram socket.

Originally Posted by brymac

....once I replaced the UCH42 frequency changer, it was fine!!

I'm a bit puzzled by the above quotes; the first suggests that the fault was in the audio amplifier stages, the second that it wasn't.

[humorist2751]

Hi all.

Maybe I'm being hopelessly naive, but might not this occur if the UCH42 had a dodgy heater? If it went intermittently high resistance, the performance of all valves would suffer, including those in the audio section.

Just a thought.

[ukcol]

Originally Posted by ukcol

Another thought, if the volume change is not abrupt, that is it fades down and fades up; you may have a fault in the valve heater chain.

Originally Posted by brymac

Thanks - the volume change is abrupt, not gradual and the fault occurs with a feed into the gram socket.

As you can see we'd covered that possibility David.

I'm still puzzled.