Bush AC 11

[Pepelephew]

hello All,
looking for some help please in restoring a Bush AC 11. I started with a dead set, not even scale lamps. I started by replacing the old wax caps and any out of tolerance resistors I found. I now have set lighting up with scale lamps but only have a low hum from the speaker. Not a peep from any wavelength.
Valves are lighting up whereas they were not beforehand. Any pointers please for the next best step to trace faults.
thanks
Bill

[McMurdo]

I would check the obvious like dirty valve holders and pins, (wiggle them); dirty or open volume control, that sort of thing. The output stage is probably ok if there's any hum at all but you can do a hum test on the gram input if you like.

After that I'd get the diagram out and do a bit of probing for valve voltages. Confident fault-finders will inject hum or signals at different points, but it's only for folks competent to work on a live set.

A good, rough & ready Les Lawry Johns tip was to operate the wavechange switch. Loud clicks will show the IF-stages forward are passing signals, and the fault's probably in the FC stage .

The lack of valves and dial lamps wouldn't have been a capacitor issue, more likely a sticky mains switch etc which you've cured or disturbed while tidying up the out-of-tolerance parts.

There's probably a sticky up there somewhere for basic, general fault finding to minimise any needless repetition!

[Station X]

You'll find some helpful info here:-

https://www.vintage-radio.com/repair...ion/index.html

[Pepelephew]

Thanks Kevin,Graham, I will follow up and let you know how I got on
Bill

[Pepelephew]

Hi lads,
I have cleaned up volume switch and checked for loose valve fittings etc. On the wave change I don't hear anything from the speaker when changing wavelength, only the click of the switch itself.
Below are voltages and they don't look great.
UY 41 Pin 2 235 AC
Pin 7 276
UL 41 Pin 2 1.1
Pin 3 3.5
Pin 5 68.7
Pin 7 0.8
UBC 41 Pin 2 44.2
Pin 7 0.8
UF 41 Pin 2 49.3
Pin 3 0.7
Pin 4 0.7
Pin 5 41.0
Pin 7 0.8
UCH 42 Pin 2 68.0
Pin 3 1.7
Pin 5 50.8
Pin 7 0.7

thanks again
Bill

[ms660]

The anode voltage on the UL41 is virtually non existent, hopefully it might be the audio output transformers plug and socket connections/contacts, if not then it could be the primary winding of that transformer that's gone high resistance or open circuit.

Whatever, don't apply power until you find the cause of the missing anode voltage as doing so might cause the valves screen grid to exceed it's maximum power dissipation rating, the screen grid seems to be pulling about 16mA according to your voltage measurements

Lawrence.

[Cobaltblue]

Th UL41 pin 2 should be almost full HT I would check the primary of the output transformer for an open circuit cheers Mike T

Crossed with Lawrence

[Pepelephew]

Hi Lads,
I will do some checks and revert
thanks
Bill

[Pepelephew]

Hi Lads,
Following comment from Lawrence.....
"hopefully it might be the audio output transformers plug and socket "
I found bad connection in the plug, re-checked voltages below, looking a bit better but still some issues around UCH 42. I tried 2 x UCH 42 and got the same readings on both.
UY 41 Pin 2 240ac
Pin 7 250
UL 41 Pin 2 230
Pin 3 7
Pin 5 111.2
Pin 7 7.1
UBC 41 Pin 2 77.2
Pin 7 1.2
UF 41 Pin 2 76.3
Pin 3 1.4
Pin 4 1.4
Pin 5 61.1
Pin 7 1.4
UCH 42 Pin 2 110.0
Pin 3 0.1
Pin 5 81.0
Pin 7 1.3
Thanks Bill

[ms660]

The voltage on the anode of the V1's triode is missing, that supply comes from the screen grids feed resistor (R2, 15k) via the oscillator coil(s) and switch contacts, the screen grids voltage is present (81 volts) so in theory there's a break between the screen grids and the anode of the triode, check for the voltage on both sides of the oscillators primary coil and both contacts of the corresponding wave change switch that's been selected.
Is that voltage missing on all three wavebands?

Lawrence.

[Pepelephew]

Hi Lawrence,
Voltage is missing on all wavebands, not sure of procedure to check oscillators, can you advise on this please
thanks
Bill

[ms660]

Just trace the circuit back from pin 5 to pin 3 of V1 until the voltage disappears.

Use the schematic as an aid.

Lawrence.

[Pepelephew]

Thanks Lawrence, will do

[TrevorG3VLF]

If the oscillator is working, then the grid voltage will be negative. It may help to measure through a high value resistance in case the meter damps the resonance.

What pin is the oscillator grid of the UCH42?

[Boater Sam]

Triode grid is pin 4

[TrevorG3VLF]

So, the voltage has not been measured.

[ms660]

The osc anode voltage has been reported as 0.1, but then again that might be a case of RF up the cheapo DMM syndrome, a capacitor from probe to chassis would reveal if it is.

Lawrence.

[Pepelephew]

Pin 4 V1 measures 1.07

[Pepelephew]

Had a look back at some photos I had taken before starting work on this set and found additional resistor and cap, seems to be between pin 3 V1 and R2, see photo. This was removed but I missed it again when working through set.
Is this an earlier repair?should it be replaced
thanks Bill

[ms660]

Looks as though that extra resistor was connected between HT and either pin 3 (osc anode) or chassis, if you still have the resistor what colour are the coloured bands on it and what resistance does it measure.

Lawrence.