DAC90a - Not working??

[Bushbaby]

Hi All,
I must be the only person who can't get a DAC90a working- shows my electronic skills!

I've connected it all up (started through the lamp limiter initially and all ok) - the dial lights work to show that the heater chain is working. However, all I get is a terrible buzzing sound in the short time that it on and I could see the speak cone going up and down rapidly.

I'll check through the connections - any suggestions would be extremely helpful at this stage.

Regards

Rob

[Aerodyne]

Either you have a VERY faulty UL41 or more likely there's no effective smoothing of the HT line. I'd cold-check the reservoir and smoothing cap (in the combined can). Failing any fault being found you'll need to work backwards from the output valve. Ground the slider of the volume control. If that stops the buzz it may be due to IF instability, in which case bridge C10 (Trader sheet number). It's the IF amp screen grid decoupler.Also try a similar cap from the HT line to chassis. You'll need a high working volts cap here for safety even though the line is unlikely to be above 200V.
Another slight possibility - the output valve grid may be floating and so picking up AC field.

I don't wish to teach egg-sucking and I apologise in advance for this, but remember this is a live chassis set. Keep one hand in your pocket, work on a live set with insulated tools such as sleeved screwdrivers and when bridging components, remove the mains plug, don't rely on the switch. I did apologise, didn't I?
-Tony

[Yorkie]

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

[Bushbaby]

Hi David,
Yes, I have replaced C22 with a 0.1 X2 cap (250v) rated. I have been taking photos a I have been going along but in my haste to finish I didn't take enough photos!!!

I shall have a go at the at the fault finding tips that you have provided - thanks

Best Regards

Rob

[Joe]

Sincere commiserations, Rob. I know that feeling all too well. At least you should have got other sets you can borrow valves from to test.

I'm copying your notes out, David, as they are well-worth keeping.

Cheers
- Joe

[Shaman]

Hi RoB, do not borrow valves from another set, if you have faulty valves it multiplies trouble, have you got a valve tester, or access to one if so test valves, but i suspect you have a cap either of wrong value, or in the wrong place, ill phone you later, see if you can get it going again. David gives good advice, go through the list slowly, i still plump for the wrong capacitor in the wrong place, Tony could well be right, i have two DAC90,s one had a hum fault, it was the smoothing cap. Rob that cap you bought from NVCS, did you re-form it??, if you fitted it to this set.....regards....Alan.

[Shaman]

How did you get on Rob, any luck ring you later....regards....Alan.

[Bushbaby]

Hi All,
I must admit I've had a busy day and haven'y followed Davids (Yorkies) good advice just yet.

I've replaced the dual valve a duff UF41 - blow me its still buzzin' !!

Regards

Rob:@

[Steve_P]

A loud buzz, not variable with the volume control and no sound. It's the smoothers. These are C19 and C21 in the Bush Manual. C20 and C21 on the Trader Sheet.

Make sure it's earthed OK - A gentle tap with a hammer helps, or get a piece of wire and ground the metal case of the capacitors - if that does not work then you'll have to change them..

Might be worth checking the resistor in between the two caps as well. But my money's on the capacitors.

Cheers,

Steve P.

[Bushbaby]

Hi All,
I'd found that I had incorrectly wired the smoothing cap (doh!). Now that this has been dealt with - no buzz, but no output either. Alan (Shaman) kindly guided me through some tests when I had to stop as I notices smoke coming from the 10k HT resistor the bottom of it has almost burnt off!!

I am now wondering if the transformer has gone kaputt!!

Regards

Rob