First radio (DAC90A)

[davek0974]

Hi all, new member, looks like a fantastic site

Many moons ago, I used to dabble with electronics etc, collected a shed full of old equipment, then came marriage and out went the equipment. Then spent many years model engineering and recently wanted to start massing with electronics again, mostly vintage.

I had built a few Tesla coils in the past, the last was driven by a massive GU81m tube and a valve oscillator I built, this was less than brilliant so it was boxed up and I moved on.

Recently I dug the oscillator out and have been messing with it, it's a simple twin triode oscillator with an el34 amp stage to drive the main tube, currently suffering poor waveform, work in progress

Anyway, recently I came by a Bush DAC90A which needed work, presented with a loud hum of fixed volume but still receiving.
I have recapped it completely plus all the resistors and the rubber wiring etc.

Sadly it still hums, disconnecting the joint group on pin 4 of the ul41 made it far better but not great, my suspicion is the ul41, would that sound right?

The HT is running 175v, the low side is 96v, these sound a bit low?
The mains tap is on 250v as my supply runs 239v so that sounded best, dropper resistor seems quite hot.

UL41 or not

[paulsherwin]

Yes, probably a heater-cathode leak in the UL41. It is probably drawing excess current too, which will pull down the HT. Don't continue to use it in this state as it may be stressing the output transformer.

There are lots of threads on UL41s and their possible replacements if you search the forum.

[Nickthedentist]

Hello and welcome.

The DAC90A is a lovely set to work on, so long as you're au fait with live chassis design and its risks.

I agree with Paul.

What's the voltage on the grid of the UL41?

Nick.

[G6Tanuki]

The UL41 is known for various leakages.

If you've changed "that capacitor" and you still have excess current/hum issues in a UL41 then I've had success by 'zapping' the accumulated conductive deposits that link the heater/cathode to the grid.

See https://www.vintage-radio.net/forum/...d.php?p=722140 - the UL41 in my daily-listener Eddystone 840A has done several hundred hum-free hours after such a treatment.

[davek0974]

Thanks,

Voltage on grid was about 0.6v measured with a DMF

I just spent an hour rummaging through the loft looking for an old box of valves I stashed, found it and found a UL46 which I understand plugs straight in?

It works but still has hum unless a strong signal is tuned in, then it sounds reasonable. Trouble is I can only find two stations? I thought there more left?

Looks like the alignment trimmers are still sealed so probably not worth trying a realignment.

Not really sure what to look at, I'll measure the voyages again tomorrow with the new valve in to see if it's altered them at all.

[paulsherwin]

The UBC41 may also be a bit leaky.

Remember that the DAC90A uses a frame aerial which is directional. Try rotating the radio to a different alignment.

[davek0974]

I don't have a spare one of those, have to see if I can find one

Will also try moving it around a bit

[davek0974]

Ok, did a full voltage survey, here's the results, tech sheet values in brackets....

UCH42
Pin 2 89v (98)
Pin 3 54v (47)
Pin 5 60v (47)
Pin 7 0.67v (0.8)

UF41
Pin 2 90v (74)
Pin 5 60v (47)

UBC41
Pin 2 59v (74)
Pin 7 0.33v (1.3)

UL41/6
Pin 2 155v (190)
Pin 5 92v (98)
Pin 3/7 4.6v (5)

UY41
Pin 7 170v (205)

Seems my voltages are all over the place, but lack of experience means I don't know if that matters in a 60 year old radio

I have found a UBC41 on eBay and it's on the way, whether that will help or not I don't know.

Any other suggestions??

[Nickthedentist]

Yes, also try it in a different location, to avoid potential interference from SMPS (yours or neighbours').

Nick.

[Herald1360]

Making the right allowance for the different resistance load of whatever meter was specified in the datasheet and the modern DMM used may clear up some of the odd readings- otherwise check resistors (these tend to drift high, especially higher value ones) and replace leaky paper dielectric capacitors. If components are generally OK and the set works well enough, values within about 20% will be fine except for filament/heater voltages which should be better than 5%.

[davek0974]

I'll check the heaters but all the resistors have been changed plus all wax and electrolytics.

The UF41 voltages seem high while the others are low, does that appear normal?

[Silicon]

Check the volume control wiring. Someone might have wired the 'low volume end' terminal to chassis.

This would mess up the bias on V1 and V2.

Is there any dirt or charring of insulation that could let the AC mains leak through to valve grids?

[paulsherwin]

Low voltages are what you would expect given that the HT is down, which may just be caused by a tired UY41. The UF41 voltages are the unusual ones. This is unlikely to be connected to the hum problem though.

How loud is the hum? Practically all domestic radios of this era had some hum, and people just considered it normal. Nowadays we're used to audio equiipment having no hum at all and are much less tolerant. Other DAC90A owners might want to comment (I don't own one).

[davek0974]

That's the problem, never having owned one, I dont know if it's normal or not?

There does seem to be a lack of stations though, the wife uses MW in her car (don't ask ) and there still seem to be quite a few stations on it. I can only get about two on this set, LW seems just about dead but I have no idea what to expect on that nowadays.

Originally Posted by Silicon

Check the volume control wiring. Someone might have wired the 'low volume end' terminal to chassis.

I'll check the volume pot, it's suffering with a rough track by the sound of it, crackly and unsteady control of output.

I'll recheck the wiring.

[Nickthedentist]

I'd say the amount of hum on a good DAC90A is minimal. Certainly no worse than any other valve set of the period.

HOWEVER, many have much higher hum levels, usually due to UL41 problems. Even with "new" UL41s, the difference can be quite marked.

I wonder whether a local member would allow you to pop round with your set and try one of their UL41s for a few minutes to see whether it made a difference?

Nick (owner of at least 6 DAC90As).

[paulsherwin]

In your location you should be able to receive BBC R4 on LW, plus a couple of French stations and possibly RTE R1 from Dublin. On MW you should have BBC R5, Talksport, BBC Three Counties, Absolute Radio and probably some local commercial music stations.

[Silicon]

There should be several strong stations on medium and long wave.
Check the continuity of the aerial coils. Have you cleaned the waveband switches?

When I get a 'new' vintage radio I usually replace one capacitor and reform the power supply electrolytics and then see how well it works before replacing anything else.
If it initially works but then stops working after I have replaced all the capacitors, I can assume I have made a mistake somewhere.

The low HT voltage could be caused by a weak rectifier but it could also be due to a fault which is taking too much current from the power supply.
If you measure the voltages across the UL41 cathode resistor and the 'smoothing' resistor (R13?) you can calculate the current taken by the circuit.

You could replace the UY41 rectifier with a 1N4007 diode and a 500Ohm resistor wired in series if you think the rectifier might be weak.

[davek0974]

Thanks,

I'll check the coils and also the wave change switch, as well as getting the current drawn as mentioned.

[tom.seeger]

The low HT in itself would not be the cause of poor sensitivity although both problems may be related. I have a DAC90A here in Canada where the line voltage is 120VAC and so the HT is only around 130VDC. The radio is very sensitive on MW. I would even say better than average.
In North America there are no LW stations, so that band is very quiet except for a few weak beacons.

[davek0974]

Ok, latest readings,

The main dropper, R13 is reading 79v over 10k which I think gives 7.9mA?

V5 cathode, R17 is reading 8.6v over 150 ohms which I think is 17.4mA?

Do they sound reasonable or not?

The volume pot is not grounded but wired as per diagram.

Dave