Bush DAC90A - Very Low Volume

[DonaldStott]

I've been inspired by David's (David G4EBT) tenacity with his DAC90A Front End Woes to look again at my own DAC90A which I eventually got working, after a fashion, but which has sat on a shelf over the Christmas and New Year period! I glowered at it most days in the naive hope that somehow it would be cowed into submission - alas, no joy.

I have restored a number of DAC90As and DAC10s so I am somewhat familiar with the circuit layout and most of the common faults although I am no expert by any means!

What I have done to get it working was to replace 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 coupling capacitors and replaced lots of crumbling wiring around the Dropper before powering up. So far, so good and when it was switched on it sprang into life albeit with very low volume with the knob turned to max?

I replaced all the old paper caps one at a time along with any resistors which were 20% over tolerance and made the Pin 4 modifications to the UL41 but none of this work had any impact on the volume level?

I also replaced all valves, one at a time, from a known working set - even with all five valves substituted the volume remains very low?

Armed with the Bush Radio Service Instructions, Bush Trader Service Sheet (1161) and the excellent 51-page illustrated DAC90A Download from Yannis Karalis I've now tested the continuity of almost every wire I can find, measured the values of all components I can access and checked the voltages of all the valve pins against the specification but I can't find anything that jumps out!

Hopefully it's something obvious I've missed and that some Forum member will have encountered similar issues with very low volume on a DAC90A or similar?

[Graham G3ZVT]

Do you have a known good UL41 to try? I guess not, otherwise you would have said.

I had the same low volume on my DAC90A when I first acquired it. Really low, I could just about hear the stations if I put my ear to the speaker. The voltages were in the correct ball-park. I couldn't raise a second mortgage for a NOS UL41 so I took my chances on a "tested" one. I don't know how the seller tested it, but the heater was open circuit so I got a refund.

In the meantime I had left the set running with the original valve and noticed that slowly but surely the volume was increasing, and has got to the point where it is now quite acceptable.

[Sideband]

Do you have any means of checking the IF amplifier....i.e a signal generator? Since you have changed everything likely to cause a problem there isn't much left. Possible it is suffering from IF transformer problems similar to David's saga. It should be fairly easy to find out which one but it's MUCH easier with a signal generator. There are two possible problems:- either one of the tuning capacitors inside either transformer is O/C and thus grossly affecting the IF tuning or one of the ferrite cores are broken with a similar effect. The second problem can usually be proved fairly easily by gently turning the chassis upside down and listening for any clicks from the transformers, try it a couple of times and see. If this is the case, the offending transformer can be removed and opened and hopefully the core can be glued back together. If successful it will need to be peaked up afterwards.


Easy enough to prove the output stage UL41 but make sure the chassis is at mains neutral first! Turn the volume control to full and touch the centre contact of the volume control with your finger. There should be a loud buzz.

Come back with your findings.

[ChristianFletcher]

It could be anything. The DAC 10 I did the other week was suffering from low volume it responded to alignment but in the process I found the primary of the first IF wouldn’t peak. I didn’t fancy opening the IF can as you probably know I Have know idea what I am doing. I suspected an open circuit capacitor across the 1st IF and replaced it with an external 100pf capacitor. The set burst into life and received station all across the MW band. So definitely worth check for an alignment issue. Before I added the capacitor I did find that waggling the pins on the first IF made an intermittent repair. I would be interested to know from anyone that’s had these cans apart for future reference. Thanks regards Chris

[DonaldStott]

Thanks to all for the quick responses - always happier when I have options to try!

@rambo1152 - I have now tried substituting a UL41 from a known working set and two NOS UL41s I have in my stash (no second mortgage involved, thankfully). No change to the very low volume.

@Sideband - although I have a Heathkit signal generator I haven't yet acquired an isolating transformer to undertake any of this work safely! I had a problem with broken ferrite cores in an earlier Bush AC91 restoration (!) and learned then about turning the set upside down to listen for clicks - no such issues with this set.

I've checked the values in ohms of all the coils (if that helps) and I am getting: -

L1 - Spec: 1.5 Measured: 2.5
L2 - Spec: 1.0 Measured: 2.1

L3 - Spec: 3.25 Measured: 3.5 (1.08mH)
L4 - Spec: 3.25 Measured: 3.3 (0.17mH)

L5 - Spec: 12.5 Measured: 15.9
L6 - Spec: 12.5 Measured: 12.5
L7 - Spec: 12.5 Measured: 14.0
L8 - Spec: 12.5 Measured: 12.3

Strangely when I turn the volume control to full (chassis is at mains neutral) and touch the centre contact of the volume control with my finger I'm not getting the usual loud buzz? Tends to imply that there is something amiss with the output stage but it seems unlikely that all three of the UL41 subs I have tried, one from another working set and two NOS, are all faulty?

Although the HT is slightly low across the whole set the voltage measurements for the V4 - UL41 pins are as follows: -

Pin 2 (Anode) - Spec: 190 Measured: 168
Pin 5 (Screen) - Spec: 98 Measured: 84
Pin3&7 (Cathode) - Spec: 5 Measured 4.5

@ChristianFletcher - I don't think there is much wrong with the IFTs but as always I am willing to entertain the possibility that the problems may lie here?

[Techman]

When I did my City & Guilds at college way back when, the first thing we were taught when fault finding was the "half split method (or rule)", and that is to divide the circuit in half, which you now have done. Is the fault an AF fault, or is it an IF/RF fault? It looks from your 'buzz test' that it's an AF fault - unless you've got a very 'un-buzzy' touch! Once you've narrowed it down to the half of the circuit that the fault is in, you divide in half again. The one problem that you have now is that the radio circuit isn't really divided in half, the AF circuit being more like a third or even a quarter of the entire circuit, so not a lot more scope to subdivide down any more, which actually in this case simplifies things. So the thing to do now is take voltage readings around the output valve, although I'm just wondering if you've made an error fitting the rewound output transformer, so worth checking your work there too. Whatever, you should soon track the fault down.

Edit: Silly me, you've already taken readings!

Anode a bit low may not be anything, but may be a small clue.

[ms660]

Originally Posted by DonaldStott

Strangely when I turn the volume control to full (chassis is at mains neutral) and touch the centre contact of the volume control with my finger I'm not getting the usual loud buzz? Tends to imply that there is something amiss with the output stage but it seems unlikely that all three of the UL41 subs I have tried, one from another working set and two NOS, are all faulty?

Bit of a clue there then, have you double checked all your work?

Lawrence.

[Techman]

To save adding 'edit' additions to the last post and risk them being missed, it may be worth you checking that the replacement pot is wired correctly, also check its actual value, regardless of what's written on it.

[ChristianFletcher]

I always use an isolation transformer when working on live sets but you dont need make a physical connection with the signal generator to do an alignment etc. You can use a loop of wire or even a wire drooped over the set to usually inject enough signal. I’m not saying that’s your problem as you have now done further testing. Obviously confirm that your chassis is on the neutral regardless.

[David G4EBT]

Originally Posted by Sideband

There are two possible problems:- either one of the tuning capacitors inside either transformer is O/C and thus grossly affecting the IF tuning or one of the ferrite cores are broken with a similar effect. The second problem can usually be proved fairly easily by gently turning the chassis upside down and listening for any clicks from the transformers, try it a couple of times and see. If this is the case, the offending transformer can be removed and opened and hopefully the core can be glued back together. If successful it will need to be peaked up afterwards.

Just a point of clarification about the IFT design in the DAC90A.

The cores lie horizontal and are not prone to breakage as they have a brass adjustment screw (which I guess is 8BA) which has two flats on it rather than a slot, and locknut. The cores are not accessible, removable or replaceable. To remove the IFT from the can, the cores have to be completely screwed in or the IFT cannot be slid out of the can. The only pictures I took of the suspect IFT on mine is of the rear where the capacitors are located. The two circular Bakelite housings of the coils/cores on which the two silver mica capacitors sit are similar in appearance on the other side, but with the adjusters. (The upper is the secondary - the lower one the primary).

I didn't have a suitable trimming tool for these rather unusual adjusts so I made one rather than resort to using small priers or whatever, which would be asking for trouble.

The alignment instructions are to screw the cores right out, then first adjust the secondary for maximum output, then the primary, first of IFT2, then on IFT1. Though the instructions are to do that with the generator connected directly first to the control grid of V2 (Pin 6), then to the control grid (pin 6) of V1, as has been said, a loop of wire draped over or near the set will do the job just as well.

It should be done with the volume control at maximum, and the attenuator on the signal generator turned down to a comfortable level.

As to whether the low volume fault lies in the audio stage or the preceding (IF/detector/RF) stages, as has been alluded to, that should become obvious by injecting an audio tone into the slider of the volume control. If the audio stage is good, the tone from the speaker will leave no room for doubt. The preceding stages can be checked by seeing if tunable stations can be heard at good volume on a signal tracer amplifier at the V.C. slider. (Velleman K7000 will suffice for both those tests).

The two IFT capacitors are both 110pF. 500V Silver Mica +/- 5% capacitors are available from 1pF to 10,000 pF from HiFi Collective at £2.00 each. +/- 5% tolerance is fine as they're in a parallel tuned circuit with adjustment to resonance via the inductor. (The ones I bought were bang on 110 pF anyway).

https://www.hificollective.co.uk/com...lver_mica.html

More expensively, 500V +/- 1% are available at £4.40 each, but aren't ideal as they're physically larger than the 5% ones and the space is tight.

https://www.hificollective.co.uk/com...apacitors.html
#Hope that might be of help and interest.

I feel rather chastened by my own DAC90A experiences, so the last thing I want to infer is that I'm a DAC90A 'guru'. Had I been so, I would have bottomed the problems I had much sooner. That said, to quote Rudyard Kipling's poem 'The Comforters' (written when mourning the loss of his son Jack in WW1), I have 'trodden the road' and 'borne the load' so can at least empathise as a 'kindred spirit!':

Until thy feet have trod the Road
Advise not wayside folk,
Nor till thy back has borne the Load
Break in upon the broke.

More succinctly and less poetically, 'I feel your pain'.

Best of luck with it.

[Leon Crampin]

David, a good trimming tool for these cores can be made from brake bundy tube, squashed slightly in the vice so that it fits the flats on the adjusters. A right angle bend at the other end of the tube makes it easy to adjust critically.

One good Bush detail is the fitting of rubber washers under the locknuts. This means that the assembly can be locked as "stiffnuts" by partial tightening of the 8BA brass nuts. As the adjusters are, I think threaded in the bakelite formers and will not stand overtighteneing, this is a good scheme. Tippex locks the cores pretty well and looks authentic.

Leon.

[G0HFCFrank]

I hope you will pardon a silly question but have you wired in the replacement output transformer the right way round?

Frank C.

[ChristianFletcher]

Hi David

That was a good description on the construction of the IF transformer thanks as I had the same issue recently. I only had a high voltage ceramic to hand so used that rather than the correct mica capacitor it appears to work for now but if it causes a problem I will change it out but not easy to reach in a DAC10. I did wonder if enough leg remained on the old capacitor to make a repair.

[DonaldStott]

Thanks again for all the helpful replies with suggestions of things to try.

First things first - the Output Transformer has been replaced the right way round. This is a rewound job from Ed Dinning and measures 702Ω on the Primary and 2.0Ω on the Secondary. I have fitted a new 0.01μF 1kV ceramic capacitor across the Primary.

One tag of the O/T Primary i.e. the one closest to the Dropper, is connected to Pin 2 (Anode) of V2 (UL41) while the other tag has three connections - one goes to the top of R13 (HT Line Smoothing), one goes to Pin 7 (Cathode) of V5 (UY41) and the last one goes to C21 (Reservoir cap). See here: -



The loudspeaker has been repaired (detached suspension) but measures ok at 3.0Ω

I also checked the replacement 470K log switched pot from Bowood Electronics and that measures 463kΩ

Checked all my work again, especially those connections that have been disturbed when replacement components have been introduced - mainly new resistors and caps along the tag board. Only thing that jumped out was that R13 (10kΩ) measured 13.22kΩ - the voltage drop across R13 is 102.5V which means that the HT drawn is 7.75mA - seems a bit low to me?

Anyway it's confession time as I have to admit to completely forgetting about my earlier tests using my Velleman K7000 - thanks to David G4EBT for that reminder. When an audio tone was injected into the slider of the volume control a tone from the speaker was present but very weak, pointing towards an issue with the output/audio stages. When using the K7000 as a signal tracer to detect tunable stations on the volume control slider the volume was very loud indeed, making me jump! This points towards no issues with the IF/detector/RF stages?

And just when you thought that things couldn't get any more challenging, valves V5, V4 and V3 have now become microphonic when tapped - not sure what this means.

This Bush DAC90A may be on borrowed time ... !

Bit of a long shot, but have you checked the valveholders? The last DAC90a I worked on had multiple fractured pins in the bases. Yours may be different but mine had the 'tuning fork' style pins which seem very fragile. If it's just one pin then you may be able to affect a temporary repair by 'borrowing' one from the rectifier base.
Arthur

[Station X]

Have you checked the voltage between cathode and g1 of the output valve just to make sure the valve is biased?

[mark2collection]

Donald,

Once all the voltages and sanity checks have been completed, it may be worth checking the yellow screened/braided audio lead from the volume control centre, to the tag board. I had one where the internal rubber insulation was so poor, and disturbing the lead by replacing the volume control, was enough to cause issues as you describe.

In the end, had to make a new lead using the original outer sleeve/braid & some heatshrink. Works a treat & looks the bizo

Mark

[DonaldStott]

Originally Posted by Shuttle

Bit of a long shot, but have you checked the valveholders? The last DAC90a I worked on had multiple fractured pins in the bases. Yours may be different but mine had the 'tuning fork' style pins which seem very fragile. If it's just one pin then you may be able to affect a temporary repair by 'borrowing' one from the rectifier base.

I've looked carefully at all the valve holders and tested their "grippyness" with a resistor lead. No apparent problems here with fractured pins but some of them could do with being tightened up. I'll try to rectify this by using a fine screwdriver to push the valve base connectors in towards the centre but I don't fancy the pin bending bodge

Originally Posted by Station X

Have you checked the voltage between cathode and g1 of the output valve just to make sure the valve is biased?

Assuming I'm connecting my DMM test leads to the correct pins on the UL41 i.e. pins 6&7, the voltage between the cathode and g1 is 4V?

Originally Posted by mark2collection

... it may be worth checking the yellow screened/braided audio lead from the volume control centre, to the tag board. I had one where the internal rubber insulation was so poor, and disturbing the lead by replacing the volume control, was enough to cause issues as you describe.

I've checked for continuity along the outer shield and inner conductor and just to be sure I double-checked for any leakage between the outer shield and inner conductor - all seems well.

I gave the audio lead a good wiggle when the set was live and there were no problems?


***


In my Post #14 above I detailed the measurements for the Output Transformer Primary and Secondary - do these seem ok?

I'm considering the replacement of R13 (10kΩ) which measures 13.22kΩ and which may be contributing to the low HT across this set? It only seems to be drawing 7.75mA - is this too low?

[Station X]

4VDC between cathode and grid is good. Also the fact you have 4VDC between cathode and ground shows that the O/P valve has emission.

Your signal tracer has shown that the RF, IF and detector stages are OK.

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

[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?