Thread: Bush A.C.91
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Old 23rd Jun 2017, 2:27 pm   #164
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
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Default Re: Bush A.C.91

If I get stuck like this, I print off the circuit, then as I check each connection one at a time, I go over the circuit with a highlighter pen so I know where I've checked. In a set which is working correctly, the lamp limiter should glow dimly, but with a 60 Watt bulb, enough to see the glow. As was pointed out by Tony in post #150, when the set is switched off, in no circumstances should the lamp glow - however dimly.

At some point it would be useful to measure the HT current being drawn by the set. You can conveniently do this by putting your multi-meter on say the 200V DV range, applying the test prods to each end of R18 - the 10K Ohm HT load resistor - and reading off the voltage dropped across R18. Then applying Ohms law, divide that Voltage by 10,000 (Ohms) and it will give you the current in Amps (or a fraction of an Amp in this instance).

Hopefully, with careful scrutiny you'll find a misplaced wire here or there and after putting the right, the lamp limiter will stop drawing excess current. At that point, even if the set still isn't working, you'll be able to check the voltages of the screens and anodes of the valves and compare them with the maker's data to give you some pointers, but of course, with the lamp limiter in circuit, the voltages will read maybe 10 - 20% low, but that wouldn't stop the set from working.

As far as you can, it's best to do as much testing with the set switched off and unplugged. You can measure the continuity of wire connections, coils, and resistors, and as you go along, can write the results down, and can highlight the circuit diagram to show what you've checked.

In a dead set, having checked voltages etc, I always use a signal injector/tracer to try to discover which stage(s) is faulty. For example, if a signal is injected at the volume control and you get a loud tone from the speaker, that shows that the audio stage is working, if only after a fashion. If you don't have a signal injector/signal generator, an alternative approach is to scratch the centre tag of the volume control with a screwdriver and you should here loud scratching noises from the speaker.

To check the RF/IF/Detector stages, if you apply a signal tracer to the centre tag of the volume control and can tune stations in, that means those stages are working even if the . If you get signals on one band but not on another, or if you get signals on AM but not on FM (in a set which has both modes), that helps to narrow down where to look for the fault.

As to the mains switch being in the neutral lead rather than line, it was quite commonplace on AC/DC sets. I recall someone suggesting that the technical reason for this was to prevent arcing of the switch contacts when the set was used on DC, but whether that was so, I've no idea and it's irrelevant to us with AC only, so I invariably change it over and put the switch in the line rather than neutral as I instinctly dislike the idea of part of the set being live when switched off.

Good luck in your endeavours Alistair.
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