Thread: Bush A.C.91
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Old 24th Jun 2017, 6:38 pm   #175
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

Yes, near enough 14mA, which is not excessive, so should not cause the lamp limiter to glow brightly, so if it is, that's puzzling. (The heater chain current is 200mA, total voltage dropped in the chain is 72.6V AC plus 3.5V for the dial lamp in series with the heaters - 76V in all). A defective C29 could cause excess current to be drawn. 135V dropped across R18 accords with the maker's data, which states 245V on the anode of V4, and 115 on the screen, so 135V drop across R18 in spot on.

I'd be inclined to check the voltages on the anodes, screens and cathodes of the other valves and compare them with the data on page 6 to see if anything stands out. Also, the cathode currents are listed, but if the voltages on the cathodes are fairly close to the data, then so should be the current. (Dividing the cathode resistor into the voltage gives the current. So for example, the cathode voltage of V4 is listed as 4V, across R17, 150R, so 4/150 = .027A, (27mA) - the data shows this as 30mA, which is near enough.

If any of the measured voltages differ greatly from the data, that may help to pinpoint a problem. So much work has been done on the set, both before and after it came into your hands, that I'd be inclined to check any disturbed wiring/components and compare with the circuit - as I said earlier, by using a highlighter pen to show what you've checked.

If you hear no sound from the set when scratching the slider tag of the volume control as I mentioned earlier, then there's a problem in the audio stage. If you had a signal tracer - just a small amplifier - eg an amplified computer speaker or whatever would do as a makeshift tracer, and could apply that to the slider of the volume control and could tune in stations, that would mean that the frequency changer, detector and IF stages are all working, so that would be a big step forward.

Good luck in your endeavours Alistair - you've persevered with the set over so many months that you deserve a lucky break! Don't get despondent - think of it as a learning exercise - when you've sorted this set, anything else will be a doddle!
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