Bush DAC90A - Very Low Volume

[DonaldStott]

Originally Posted by Station X

I'm beginning to wonder about the AF amplifier function of V3. What are the voltages like on that valve?

V3 UBC41

Heater voltage (Pins 1-8): 12.82V AC

Pin 2: 58.5V

Pin 5: 0.2V

Pin 7: 0.6V

Originally Posted by Tim

What’s the anode voltage on the UBC41?
Is it’s 150K load resistor OK/ wired to the correct point?
Resistors in this position very often go high/open.
There is an IF filtering cap there somewhere( a few thousand pF ISTR). Is that s/c or leaky, or replaced with an incorrect value?

Anode voltage on the UBC41 is 58.5V as noted above - a bit low as the spec is 74V.

R7 (100kΩ) is the V3 Grid Stabiliser at this location and is connected to Pin 3 - this has been replaced and is reading 100.6kΩ

The IF Filtering Cap C12 (100pF) must be one of the most inaccessible components in this set! Impossible to get a reading for this and C16 (50pF) without taking these mica caps out of circuit - if I'm doing that I may as well replace them! Only value I could get with my ESR meter for C12 was 461kΩ

[Station X]

Originally Posted by DonaldStott

[Anode voltage on the UBC41 is 58.5V as noted above - a bit low as the spec is 74V

Check that R9 150k, the anode resistor on pin 2, hasn't drifted high in value.

[mark2collection]

Has C17 been replaced? And by what value? Original was 0.003uF, I think.

It's a tone corrector, mounted chassis to tag board, next to the volume control (I believe the earth connection for the cap uses the O/P TX mounting bolt(?)). It's not uncommon for these to be hiding under the tag board itself ... sneaky. If this is resistive, it could keep the set quiet.

Mark

[ms660]

The manufactures voltages/current around V3 don't make sense anyway so far as I can make out.

Lawrence.

[DonaldStott]

Originally Posted by Station X

Check that R9 150k, the anode resistor on pin 2, hasn't drifted high in value.

R9 has already been replaced and currently measures 149.5kΩ.

Originally Posted by mark2collection

Has C17 been replaced? And by what value? Original was 0.003uF, I think.

C17 has already been replaced and currently measures 0.003μF (630V cap). This new cap is connected to the O/P TX mounting bolt as you mention and I will go back and ensure that this tag is properly coupled to the chassis. I normally do the kalee20 mod for this cap, fitting it directly on the UBC41, but not on this occasion.

Originally Posted by ms660

The manufactures voltages/current around V3 don't make sense anyway so far as I can make out.

Thanks Lawrence - do my measurements make any sense for V3?


***

As I noted in my Post #1 above " ... I replaced all the old paper caps one at a time along with any resistors which were 20% over tolerance" - good though to re-check everything, just in case!

In my Post #21 above I also noted that I was finding it almost impossible to get a reading for C12 (100pF) and C16 (50pF) without taking these mica caps out of circuit. What I'd like to know before proceeding is whether or not faults with either (or both) of these mica caps could account for the very low volume I have with this set?

[Station X]

C12 is wired across the volume control so its value cannot be checked in circuit. The fact that you're measuring 461kohms (ie the resistance of the volume control) across it shows that it's not short-circuit, but it could be breaking down in service.

With these impossible faults all you can do is:-

Check the wiring accounting for every wire and component on every tag.

Check that new components have the right value, it's easy to misplace a decimal point where capacitors are concerned.

Check all components to make sure they're not faulty.

Clutching at straws does the output transformer's primary measure correctly in circuit? If there's a short across it it will make little difference to the voltage readings, but the amplifier won't work.

[G6Tanuki]

Originally Posted by Station X

Clutching at straws does the output transformer's primary measure correctly in circuit? If there's a short across it it will make little difference to the voltage readings, but the amplifier won't work.

A few shorted-turns on the primary - while not really easy to detect with a simple resistance-measurement - will absorb a good chunk of the audio-signal.

[I've had this with modulation transformers in some 'sixties mobile-radio gear: winding resistance values were within 5% of the specified values but the thing never managed more than about 30% of the intended output....]

[DonaldStott]

Originally Posted by G6Tanuki

A few shorted-turns on the primary - while not really easy to detect with a simple resistance-measurement - will absorb a good chunk of the audio-signal.

[I've had this with modulation transformers in some 'sixties mobile-radio gear: winding resistance values were within 5% of the specified values but the thing never managed more than about 30% of the intended output....]

Interesting - thanks

The output transformer's Primary measures 702Ω in circuit and 16.2V across the two tags?

How would I go about checking for shorted-turns?

[Station X]

Given that the O/P transformer was rewound by Ed Dinning I very much doubt it has shorted turns.

Is C20 the correct value? 0.01uF or 10nF, 500V or greater.

[G6Tanuki]

Testing for shorted-turns on an output-transformer is not going to be easy with typicl testgear; it's the sort of part where a service-manual is likely to say "Test by substitution!", which is OK if you happen to have a spare. In practice almost any small-valve-radio output transformer will be a 'near enough' for a test-by-substitution.

EDIT: I hadn't noticed that the transformer has already been rewound by a trustworthy source!

[ms660]

External signal in at V4 g1 (via a capacitor) then take it from there.

Lawrence.

[David G4EBT]

To state the obvious, given that you have tuneable signals with your signal tracer at the slider of the volume control, that exonerates the RF and IF stages. And given that you can hear signals from the speaker, but at much reduced volume, that indicates that the detector stage is working. That points to the IF amp (V3 triode section) and the AF amp (V4).

That's the area marked in red on the part circuit below.

As to Voltages, of all the DAC90As that have passed through my hands, I've never had one yet in which, in all respects, the voltages compare with the spec., but that said the datasheet was based on voltage being take with the radio on the MW with so signal tuned in, using an AVO 7 (1,000 OPV) on the 1,000V range for HT measurements, and for low voltages (cathodes etc), on the 10V range. Using a digital multi-meter with maybe 20 M Ohm input impedance, the voltages will differ to an extent, but not perhaps by much.

Some variations will arise due to different mains voltages. For example, locally, the mains voltage is 237V, so If I set the dropper at 250V to be kinder to the valves, the voltages will be lower than if I set it on the 230V tapping.

For what it's worth I've attached the table of voltages on my working DAC90A (and what was until a few days ago), the non-working one. The fault which caused the malfunction in that set was an IFT capacitor and that had no bearing on the voltages. Both sets are now working really well, except that the mains switch on the 'working' one doesn't now switch off so I'm going to have to fit a new 470k Pot.

If you're satisfied by substitution that V3 & V4 are sound, I can only suggest you focus on the area of the circuit I've marked in red.

Hope that might help a bit.

Good luck with it.

[Al (astral highway)]

Hi Donald,

More to offer moral support , more than anything else! So you have essentially rebuilt the whole receiver, and still no joy. That must be very frustrating and I hope you can solve this...

Wild card:

did you ever check C20? I've known failure of the capacitor serving this role.

Watching with interest!

[Station X]

Originally Posted by David G4EBT

That points to the IF amp (V3 triode section) and the AF amp (V4).

Is not V3 an AF amplifier?

[DonaldStott]

Thanks to everyone for giving up their time to contribute to my DAC90A problems - as usual I've learned lots of new stuff along the way which will be catalogued and stored until the next time. Unfortunately we don't yet have a successful outcome and I have a few more things to try but I am more convinced than ever that the problems, whatever they may be, are probably of my own making!

I fully agree with David's comments in Post #32 above about DAC90A voltages i.e. "I've never had one yet in which, in all respects, the voltages compare with the spec." Nevertheless he has helpfully narrowed down any further investigations to the area outlined in RED in his attached circuit diagram - thanks David. It will be a question of working my way through every component, every wire and every connection shown - I thought I'd already done that but there must be something I've missed!

Originally Posted by Station X

Is not V3 an AF amplifier?

I checked the UBC41 details at the Valve Museum and it does say "Mullard describe the UBC41 as a high gain triode for use as an AF voltage amplifier, combined with twin diodes, for DC/AC mains operation." Another technical sheet I found thanks to Google states "The diode system is intended for detection and A.G.C. leaving the triode system for A.F. amplification". Notwithstanding my navel-gazing, David G4EBT has clarified that in relation to V3, my focus should be on the triode section.

I think I'll put this DAC90A back on the shelf for a few weeks until I've got the stamina to tackle all those checks!

[David G4EBT]

Originally Posted by Station X

Is not V3 an AF amplifier?

Yes indeed - double diode triode, detector, AVC, AF amp.

thanks for reading the post Graham - my mistake - well spotted.

[Herald1360]

Confused by calling the output stage the AF Amp (which it is of course) and looking to call the previous stage something else maybe?

Could call them AF Voltage Amp and AF Power Amp I suppose....

[G.Castle]

I read through this little lot thoroughly, I think. I noted the speaker had been repaired, has it actually been tested?

[DonaldStott]

Originally Posted by G.Castle

I read through this little lot thoroughly, I think. I noted the speaker had been repaired, has it actually been tested?

You deserve a medal for persevering through this Thread!

Speaker has been repaired and tested with a known working DAC90A but thanks for the suggestion.

[Al (astral highway)]

Originally Posted by Herald1360

Confused by calling the output stage the AF Amp (which it is of course) and looking to call the previous stage something else maybe?

Could call them AF Voltage Amp and AF Power Amp I suppose....

‘First audio amplifier and loudspeaker valve’? Saw that idea in an Eddystone user group I think.

Donald, now that you’ve checked the speaker, what about C20? The reason I mention it is that in some fault conditions, EHT can develop on the primary side, making this leaky. This was linked to the first ever problem I encountered in a valve radio.

That would be one candidate for your symptoms. May as well systematically work backwards from one side of the speaker into that red zone.