25-05-2011, 05:08 PM
Sorry to hear of your probs Rob.
A few pointers if you haven’t already done some or all of these:
Firstly, make sure neutral is connected to chassis and keep one hand behind your back or in your pocket when poking around to measure voltages. I hope you've replaced C22 – the mains hum suppressor cap found on the top of the chassis behind the dropper with an X2 rated cap or there is likely to be a loud bang at some stage. (if not just disconnect it for the time being).
Now test the HT: Place your meter + prod on the reservoir capacitor, C20, (or the screen grid, pin 5, of the UL41 output valve), with the – prod to the chassis to measure the HT and switch on. If you don’t like poking around with prods, use croc clips instead before you switch the set on. The HT should rise slowly as the rectifier valve warms up, and if it settles at a low value, switch off. There are three likely causes of that - a weak rectifier UY41, a faulty output valve UL41 (which can damage the output transformer) or a faulty reservoir capacitor (C20). If the voltage is low, move the probe to the grid of the UL41 (pin 6) and switch on again - if it rises above 1V switch off immediately - you need a new UL41, if it hums loudly it is likely to be C20, otherwise it is probably the UY41.
If the HT is about right (+/-10%) then this means the set is drawing about the right current and it is reasonably safe to leave switched on to look around for other faults. ). Low HT may stop the local oscillator from functioning, with the dead set result as there will be no signals presented to the IF/AF stages even if those stages are working. Start by checking the voltages as follows to see if they’re within about 10% plus or minus of the spec :
V1:
Pin2: 98V
Pin3: 47V
Pin 5: 47V
Pin 7: 0.8V
V2
Pin 2: 74V
Pin 3: 47V
Pins 3,4, & 7: 1.0V
V3
Pin 2: 74V
Pin 7: 1.3V
V4
Pin 2: 190V
Pin 5: 98V
Pin 3 & 7: 5V
V5
Pin 2: 222v AC
Pin 7: 205V DC
1. Clean all the valve pins and wavechange switch with Servisol. Don't use sandpaper on the pins and don't wipe the printing on the valves as it will come off!
Personally, when fault tracing on any set, I like to use a signal tracer and injector to divide the RF/IF stages from the audio stage as follows: Inject a 1 kHz or so signal at the wiper of the volume control, and if you hear a loud tone from the speaker, that means the audio stage is working. If you then apply a probe from a signal tracer (basically, a small amplifier with a detector diode) at the wiper of the volume control, if the IF RF stages are working, and you tune around the band, you should hear stations loud and clear. No signals, then there’s a fault in the IF/RF stages. If you apply the tracer probe to pin 6 of V2 and there are signals, that means the mixer/oscillator stages and the first IF stage is working, are working, but if you get no signals at pin 5 of V2, the second IF stage isn’t working. Try on each waveband, and if you get stations on only one band, then you know that the frequency changer and IF stages are working, but there’s a problem with the band ‘switchery’ for the non-working waveband – an open circuit somewhere, such as a detached wire on a coil, or a switch wafer not making contact. That can be checked out by continuity tests with the set switched off and unplugged.
Assuming that you don’t have a signal injector, you can still do a test of the audio stages. Try scratching the centre pin of the volume control, which should cause a crackle or hum in the speaker, or connecting a lead to pin 3 of the UBC41, (V3) and stroke the other end to chassis, which should give a massive crackle from the speaker, which will clear the output stage.
5) Then work backwards through the stages:
a) Touch the wire to the grid of the UF41 (V3) and you should hear at least loud clicks/hiss.
b) Similarly the mixer grid of the UCH42 (V1)
c) Measure the oscillator grid of the UCH42 which should read -ve if the oscillator is working.
Changing waveband should produce a crackle as the switch is turned, even if the oscillator isn't working.
I hope these notes, based on my own experiences, help rather than confuse!
Best of luck!
David
A few pointers if you haven’t already done some or all of these:
Firstly, make sure neutral is connected to chassis and keep one hand behind your back or in your pocket when poking around to measure voltages. I hope you've replaced C22 – the mains hum suppressor cap found on the top of the chassis behind the dropper with an X2 rated cap or there is likely to be a loud bang at some stage. (if not just disconnect it for the time being).
Now test the HT: Place your meter + prod on the reservoir capacitor, C20, (or the screen grid, pin 5, of the UL41 output valve), with the – prod to the chassis to measure the HT and switch on. If you don’t like poking around with prods, use croc clips instead before you switch the set on. The HT should rise slowly as the rectifier valve warms up, and if it settles at a low value, switch off. There are three likely causes of that - a weak rectifier UY41, a faulty output valve UL41 (which can damage the output transformer) or a faulty reservoir capacitor (C20). If the voltage is low, move the probe to the grid of the UL41 (pin 6) and switch on again - if it rises above 1V switch off immediately - you need a new UL41, if it hums loudly it is likely to be C20, otherwise it is probably the UY41.
If the HT is about right (+/-10%) then this means the set is drawing about the right current and it is reasonably safe to leave switched on to look around for other faults. ). Low HT may stop the local oscillator from functioning, with the dead set result as there will be no signals presented to the IF/AF stages even if those stages are working. Start by checking the voltages as follows to see if they’re within about 10% plus or minus of the spec :
V1:
Pin2: 98V
Pin3: 47V
Pin 5: 47V
Pin 7: 0.8V
V2
Pin 2: 74V
Pin 3: 47V
Pins 3,4, & 7: 1.0V
V3
Pin 2: 74V
Pin 7: 1.3V
V4
Pin 2: 190V
Pin 5: 98V
Pin 3 & 7: 5V
V5
Pin 2: 222v AC
Pin 7: 205V DC
1. Clean all the valve pins and wavechange switch with Servisol. Don't use sandpaper on the pins and don't wipe the printing on the valves as it will come off!
Personally, when fault tracing on any set, I like to use a signal tracer and injector to divide the RF/IF stages from the audio stage as follows: Inject a 1 kHz or so signal at the wiper of the volume control, and if you hear a loud tone from the speaker, that means the audio stage is working. If you then apply a probe from a signal tracer (basically, a small amplifier with a detector diode) at the wiper of the volume control, if the IF RF stages are working, and you tune around the band, you should hear stations loud and clear. No signals, then there’s a fault in the IF/RF stages. If you apply the tracer probe to pin 6 of V2 and there are signals, that means the mixer/oscillator stages and the first IF stage is working, are working, but if you get no signals at pin 5 of V2, the second IF stage isn’t working. Try on each waveband, and if you get stations on only one band, then you know that the frequency changer and IF stages are working, but there’s a problem with the band ‘switchery’ for the non-working waveband – an open circuit somewhere, such as a detached wire on a coil, or a switch wafer not making contact. That can be checked out by continuity tests with the set switched off and unplugged.
Assuming that you don’t have a signal injector, you can still do a test of the audio stages. Try scratching the centre pin of the volume control, which should cause a crackle or hum in the speaker, or connecting a lead to pin 3 of the UBC41, (V3) and stroke the other end to chassis, which should give a massive crackle from the speaker, which will clear the output stage.
5) Then work backwards through the stages:
a) Touch the wire to the grid of the UF41 (V3) and you should hear at least loud clicks/hiss.
b) Similarly the mixer grid of the UCH42 (V1)
c) Measure the oscillator grid of the UCH42 which should read -ve if the oscillator is working.
Changing waveband should produce a crackle as the switch is turned, even if the oscillator isn't working.
I hope these notes, based on my own experiences, help rather than confuse!
Best of luck!
David
