15-05-2011, 09:51 PM
Thanks for the replies which I've only just noticed.
It was after changing the transistors that the interference effect was noted. Given the amount of time I've fiddled about with it in the past I can't believe that it was doing it before.
The audio stages are fine as I've been using it as an amplified speaker for months. I finally set about changing the AF115s partly out of interest and also because something was guzzling current - 85mA quiescent.
As for proving a fault before tackling it there didn't seem anything else left to try; it seemed no less logical to change transistors which have a bad reputation than to rip out Hunt's capacitors on sight. Getting to the module was much easier than I'd expected but I wouldn't have touched it had I realised how tightly packed the inside was; there was no chance of getting at the screening leads to cut them and the PCB tracks were so delicate that it seemed to make more sense to hit it hard and hopefully sort it in one go than constantly keep dis-assembling and re-assembling it.
I've got a set of schematics for those Mullard modules but as the one in the radio hasn't got a model number on it's a bit difficult knowing which it is as none of the pin-outs seem to match.
There's nothing really lost, apart from a few bob on the BF450s, as it still works as an amplified speaker and the quiescent current is down by about half. It's not in great cosmetic condition either so isn't a disaster to our national heritage.
My current plan is to remove the chassis and put an LM386 based amplifier in the case using the original speaker. All connections and controls will be on a new top panel so this modification could be reversed. This would certainly be more use to me as I've got perfectly good 3-band radios standing spare but a battery operated active speaker is something I don't have.
I'm inclined to regard the chassis as a source of spares either for my own use or anybody else's. Besides whatever it is that's stopping it from receiving anything the aerial is broken, the handle tatty and the main switch bust - I've got 'round that by utilising a pair of spare contacts on the Tone control. That said my natural bloody-mindedness will mean I'll probably carry on fiddling and either kill or cure it.
As for the Ekco, Jamie, that's dead on VHF and the chassis looks horribly congested and unfortunately confidence is waning instead of growing. That said fears about the 'high' voltages involved seem to have left me and I'm now no more nervous in that regard than with any mains powered-equipment.
I've had some success as I've already prototyped the amplifier alluded to above and I did fix my mate's Vidor. I tried to buy a DAC90A earlier thinking that if I couldn't fix that I really should hang my soldering iron up but didn't manage it - it was the old story of it going for a price I'd pay but not when the postage is added on. This was probably for the best.
Sorry for rambling.
- Joe
It was after changing the transistors that the interference effect was noted. Given the amount of time I've fiddled about with it in the past I can't believe that it was doing it before.
The audio stages are fine as I've been using it as an amplified speaker for months. I finally set about changing the AF115s partly out of interest and also because something was guzzling current - 85mA quiescent.
As for proving a fault before tackling it there didn't seem anything else left to try; it seemed no less logical to change transistors which have a bad reputation than to rip out Hunt's capacitors on sight. Getting to the module was much easier than I'd expected but I wouldn't have touched it had I realised how tightly packed the inside was; there was no chance of getting at the screening leads to cut them and the PCB tracks were so delicate that it seemed to make more sense to hit it hard and hopefully sort it in one go than constantly keep dis-assembling and re-assembling it.
I've got a set of schematics for those Mullard modules but as the one in the radio hasn't got a model number on it's a bit difficult knowing which it is as none of the pin-outs seem to match.
There's nothing really lost, apart from a few bob on the BF450s, as it still works as an amplified speaker and the quiescent current is down by about half. It's not in great cosmetic condition either so isn't a disaster to our national heritage.
My current plan is to remove the chassis and put an LM386 based amplifier in the case using the original speaker. All connections and controls will be on a new top panel so this modification could be reversed. This would certainly be more use to me as I've got perfectly good 3-band radios standing spare but a battery operated active speaker is something I don't have.
I'm inclined to regard the chassis as a source of spares either for my own use or anybody else's. Besides whatever it is that's stopping it from receiving anything the aerial is broken, the handle tatty and the main switch bust - I've got 'round that by utilising a pair of spare contacts on the Tone control. That said my natural bloody-mindedness will mean I'll probably carry on fiddling and either kill or cure it.
As for the Ekco, Jamie, that's dead on VHF and the chassis looks horribly congested and unfortunately confidence is waning instead of growing. That said fears about the 'high' voltages involved seem to have left me and I'm now no more nervous in that regard than with any mains powered-equipment.
I've had some success as I've already prototyped the amplifier alluded to above and I did fix my mate's Vidor. I tried to buy a DAC90A earlier thinking that if I couldn't fix that I really should hang my soldering iron up but didn't manage it - it was the old story of it going for a price I'd pay but not when the postage is added on. This was probably for the best.
Sorry for rambling.
- Joe
