Bush DAC90A - Very Low Volume

[DonaldStott]

Originally Posted by astral highway

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.

Thanks for your ongoing interest and support.

C20 has been replaced with a 1kV 0.01μF ceramic capacitor but I can easily remove that and change if required.

I do like a systematic approach rather than randomly changing components in the naive hope that the problem will somehow be resolved.

I'm pressing the PAUSE button on this Thread - as I said earlier "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!"

[Station X]

That capacitor should be fine. I normally use polypropylene capacitors myself, so I'll leave it to others to comment on the suitability of a ceramic capacitor here.

I too favour a systematic approach to fault finding, but it seems to have failed in this case so we're reduced to checking the value of components and speculation.

If I were in your situation I'd be getting my AF signal generator out to check which stage lacks gain.

[DonaldStott]

Originally Posted by Station X

That capacitor should be fine. I normally use polypropylene capacitors myself, so I'll leave it to others to comment on the suitability of a ceramic capacitor here.

I too favour a systematic approach to fault finding, but it seems to have failed in this case so we're reduced to checking the value of components and speculation.

If I were in your situation I'd be getting my AF signal generator out to check which stage lacks gain.

Thanks Graham as always.

From reading numerous Threads and other online material I was led to believe that a 1kV ceramic cap was suitable in this location?

I have a working restored Heathkit signal generator (AF and RF) but I don't yet have an isolation transformer to deploy this safely with an AC/DC set like the DAC90A.

[David G4EBT]

Originally Posted by DonaldStott

I have a working restored Heathkit signal generator (AF and RF) but I don't yet have an isolation transformer to deploy this safely with an AC/DC set like the DAC90A.

You don't need to use the audio output of the Heathkit Signal Generator - the K7000 audio injector will do fine, and as it's battery powered, there are no issues in connecting the ground lead to the DAC90A chassis and using the injector probe. You can also use your K7000 tracer. You've already ascertained that at the slider of the volume control you can
hear signals loud and clear from the RF/IF stages.

Your K7000 tracer is just a small amplifier and in effect, is a substitute for the triode section of V3 and for V4, so you have therefore ascertained that those stages are not working as they should (or are working, but only after a fashion). By using you your tracer, if you apply it to various points as I've suggested on the updated AF section of the circuit attached, you should be able to find the culprit.

EG, you know the signals are present at the VC slider, so are they still there after C14? If so, are they present after R3 at pin 3 of V3? If so, are they present at C18 (where - if V3 triode is amplifying, you should have to turn down the volume of your K7000). Are the signals present at pin 6 of V4? If so, as they're not present at the speaker, that points to V4, R14, C20, T1 and the speaker.

Alternatively, if you inject an AF tone at pin 6 of V4, do you hear it at good volume in the speaker? If you, the audio stage is working OK. If so, if you inject a tone at pin 3 of V3, (which provides and extra stage of audio amplification), if V3 is working correctly, the volume of the tone should be louder.

One amplification stage or the other, or both are clearly not working, so to ascertain why, that would be my approach, but I'm sure that if I'm on the wrong track, someone will apply a light touch on the tiller, steer me back onto an even keel and suggest a better approach.

Hope that might help a bit.

[Ian - G4JQT]

Looks like the fault has been narrowed down to the AF stage. But if it turns out not to be in there, I had a quiet DAC90A that took ages to fix. Turned out it was a metal spindle had parted from a ferrite core in one of the IFs. The spindle turned but only occasionally engaged with the core. So it seemed to be adjusting, but wasn't peaking!

Disassembly, spots of super glue, reassembly. Job done.

[DonaldStott]

***Bush DAC90A - Very low volume. Problem Resolved***

I seem to remember saying "I'm pressing the PAUSE button on this Thread.." but Post #44 from David G4EBT inspired me to make one final push. He came to my rescue by clearly setting out what my systematic approach should be on this occasion and that my Velleman K7000 would suffice. What I needed to do was follow the signal back and find the point where it disappears - so obvious! I followed David's instructions carefully and everything checked out ok until I got to C18, the audio coupling cap which was the first one I changed - at this point the signal went very quiet! A bit of fiddling and prodding proved that one connection was 'dodgy' and this joint was de-soldered, cleaned and tinned. A dab of flux and some new solder made a bright new joint and everything was right with the world again - DAC90A volume restored!

So why had I missed this during my numerous checks - I can only assume in the very act of testing that my test clips had mechanically held the joint closed so that C18 measured correctly - just my guess (or my excuse)?

A huge thanks to everyone for their patience and perseverance throughout this Thread and for all the helpful suggestions. I was open to anything and everything as I just wasn't getting anywhere or seeing where the problem lay?

Hope that no one is underwhelmed by the solution which was really rather simple involving a 'dodgy' joint!

[Norman Raeburn]

Well done, it’s a steep learning curve at times. Sometimes we need a push in the right direction and with Davids excellent advice you have solved it. Onwards and upwards to the next one, Norman

[David G4EBT]

Well done in sorting it!

It underlines the worth of methodically using the signal injection/tracing approach to fault-finding, (whether on valve or transistor radios).

[Station X]

Alleluia!!

Glad it's fixed.

[G6Tanuki]

Rejoice!

And don't beat yourself up or be embarrassed about how a seemingly 'obvious' fault has held you up; anyone who's worked in the real-world will have plenty of similar stories to tell!

At least it's now working properly, so you can listen to it as you relax with a glass of something suitable....

[DonaldStott]

Thanks for the kind and supportive words - much appreciated.

Now that we have resolved the Very Low Volume problem it's time to move on and consider how we might improve the reception which is a bit noisy?

[Al (astral highway)]

Originally Posted by DonaldStott

Now that we have resolved the Very Low Volume problem it's time to move on and consider how we might improve the reception which is a bit noisy?

Congrats, Donald! I feel happy for you and that your persistence paid off.

Noisy reception is pretty much a given on MW and LW these days. There are quite a few existing threads on mains-borne noise and what, if anything, can be done about it. Maybe have a little search first and that will be a good starting point to anchor any new suggestions!

[DonaldStott]

Unfortunately this DAC90A is still giving me some grief, as if to echo the title of this Thread, as I am still hearing audio even with the volume control turned right down. I have checked both sides of the pot with my DMM and I am indeed reading 0.00Ω at both extremes. Excessive hum is not an issue with this set.

I've done the Pin 4 Mod on the UL41 and have replaced the faulty on/off switch volume control with a new Bowood Electronics 470K log switched pot, replaced the Output Transformer with a rewound job from Ed Dinning, replaced the usual waxy paper capacitors and replaced lots of crumbling wiring around the Dropper before powering up.

I understand from other Threads that UL41 internal leakage is the usual suspect in such instances but for testing purposes this has been substituted with a valve from a known working set and with a couple of NOS valves as well?

[snowman_al]

On a 90a there is always some audio with the volume turned fully down.
Very little, but still some... R6

[Graham G3ZVT]

I mentioned my DAC90A back in Post #2.
It has the original volume control, and like your set it does not turn down completely, sound can faintly be heard at the lowest setting. I consider this an advantage, making it less likely that the set will be left switched on unattended.

Whether they "all do that" or is a symptom of a fault, I'm not sure, but looking at the way AVC appears to be applied, my guess is it's normal.

Edit.
X with Snowman.

[Station X]

This had me checking out my own DAC90A which has been fitted with a new volume control pot. There is indeed residual sound with the control at the point of opening the OFF/ON switch.

Not something I've knowingly noticed before and it doesn't bother me.

[DonaldStott]

Erratum - I should have said:-

"I have checked both sides of the pot with my DMM and I am indeed reading 0.00Ω - 430kΩ and 430kΩ - 0.00Ω at both extremes"

Would an additional resistor across the pot help?

@Graham - it only bothers me as the audio volume is quite loud and becomes far too loud very quickly?

[Station X]

Is the replacement pot a log law one?

If unsure set the knob at the halfway point and check that the readings from the wiper to each outer tag are nowhere near equal. If they are nearly equal it's a linear pot which is unsuitable.

[ms660]

Measure the resistance between the wiper and chassis with the volume control turned to minimum (but not so's the switch operates)

Lawrence.

[G6Tanuki]

There still being some 'background volume' even when turned to minimum is quite a common thing with some designs of radio: it's often caused by stray capacitive coupling within the valve-envelope between the detector- or AGC-diodes and the grid or anode of the first audio-amplifier.

The high-level RF from the diode anodes couples across to the grid and anode of the first-audio stage where any non-linearity will cause 'phantom detection' and noticeable non-turn-downable background audio.

(Some valves [the B7G 6AT6 and its ilk] have badly designed electrode assemblies and radios that use them suffer from this problem a lot).

See https://www.radiomuseum.org/r/bush_dac90a_dac_90_a.html for a suggested 'fix'.