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Old 16th Sep 2008, 12:57 pm   #1
Doris The Diode
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Default DAC 90A - Radio stopped working

Hi everyone,

I've just bought a DAC 90A yesterday in working condition.

So in my true technical spirit I had to get the back off and have a look, it's very clean inside and has the paper capacitors replaced. I could see this by shining a torch inside underneath, the chassis as not been taken out.

I pulled the valves out, one by one and cleaned the oxide off each leg as the rectifier and the UL41 were in particular badly oxidised.

I put them back in and the radio now produces a low noise hum and does not receive any stations. In the frist 30 seconds of warming up there is a crackle of a station but then stops and just the low level hum.

I'm shooting myself in the foot for taking out the valves and cleaning them (dust as well) but had never thought cleaning all the valve pins would stop the radio from working, nothing else has been touched.

I suspect I've damaged a valve and would appreciate some tips where to start fault finding.
I have a very poor jpg copy of the service sheet and I am quite good with a soldering iron.
I guess I could start with measuring the volts, putting in a 1KHz signal at the junction of C17 & C18 to see if V4 works and audio stage I guess if that works it's time to find a 465KHz signal and put that in.

What Wattage value are the resistors as it does no say on the service sheet. I intended to replace most components and probably all the resistors.

Any help welcomed, I've noticed this is a very nice forum with great people at hand to help.

Regards to all,

Doris The Diode.
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Old 16th Sep 2008, 1:10 pm   #2
Station X
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Default Re: DAC 90A - Radio stopped working

I'd start by taking the valves out again and cleaning the pins with servisol. The chances are one or more is making bad contact with the valveholder.

You can get a copy of the manufacturer's service sheet for this radio by clicking the link at top right.

Don't bother with injecting signals yet. Check the various voltage starting with the HT.
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Old 16th Sep 2008, 1:14 pm   #3
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Default Re: DAC 90A - Radio stopped working

Have any of the valves gone milky inside? if so, you have cracked the vacuum seal and the valve is junk. Apart from that you are very unlikely to have damaged them.

You may have cleaned the oxide off the pins, but you won't have cleaned it off the sockets. Spray the pins of the valves with contact cleaner such as Servisol then reinsert them and waggle them about. You can use a SMALL amount of WD40 if that's all you have, but don't spray it directly on the valveholders.

If that doesn't work you should start by taking some voltage readings and comparing them with service data. You can order good quality docs for £1.99 by clicking on the link at the top of the page. The money helps fund this forum.

There is no point in changing resistors unless they are significantly out of tolerance.

Bear in mind that this is an AC/DC radio with one side of the mains connected to the chassis, so be very careful if you are new to vintage radio work.

Good luck, Paul
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Old 16th Sep 2008, 5:36 pm   #4
Doris The Diode
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Default Re: DAC 90A - Radio stopped working

Hi,

Thanks for the advice.

I took the chassis out and gave it a good hoovering and found R9 to be cracked at one end so I replaced this. Also replaced R5 which showed 3M instead of 2.2M.

None of the valves are milky and there is no sound coming out apart from a few clackles now and then.
Transformer reads 600 Ohms on primary and about 1 Ohm on the secondary.

The voltage at V5 should be 220V but is 180V and the radio is drawing 130mA measured at the mains plug, once stable and warm.

I have downloaded both DAC manuals, which are very good so I've got really stuck in

Any advice or help will be much appreciates

Cheers,

Doris The Diode
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Old 16th Sep 2008, 6:39 pm   #5
paulsherwin
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Default Re: DAC 90A - Radio stopped working

The mains current sounds about right, though it is more helpful to measure the HT current. The HT is probably down because the UY41 is a bit tired, and the radio should be working well even with this lower voltage.

If you touch the volume control wiper with a screwdriver, you should hear a loud cracking from the speaker with the volume control turned up. If you don't hear anything, try touching the UL41 grid (pin 6).

If the crackles are there at the volume control, you have a fault in the RF or IF sections.

Given the history of the fault it's very likely that you have disturbed something without noticing while exploring inside the radio.

One thought - have you checked the wavechange switch is in the correct position?

Paul
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Old 16th Sep 2008, 10:51 pm   #6
Doris The Diode
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Default Re: DAC 90A - Radio stopped working

Thank you for all your help!

The reason why it went totally quiet was because when examined it closely the secondary winding of the transformer had one of the strands of wire (lead which goes to the speaker) shorting to the other terminal. It was very difficult to see as the wire was oxidised and black, I shone a torch there and then it was clear to see.
As soon as I touched it, it burst into life so I then cut it off.
How it shorted by itself I don't know and of course it had to go wrong when I got my hands on it :-(

A few components had to be replaced as mentioned above and whilst listening to The Arches on LW (but without the family gathered around the wireless listening tentatively) I measured the voltages, which were down but acceptable.
Current drain measured across the 10K resistor feeding the HT was 9.5mA as advertised so I deemed the set working correctly.

I'm pleased to say that Bush DAC 90A serial nr 73/64023 is up and running

Thanks once again,

Doris The Diode
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Old 17th Sep 2008, 9:11 am   #7
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Default Re: DAC 90A - Radio stopped working

Just out of interest, I have a DAC34 which is very similar to the DAC90A, and this too has a lower than expected HT at the output valve anode. It rises rapidly to around 264V, but once everything else warms up, drops to 195V and sits there nice and stable. According to the data sheet it should be around 210V, but as everything seems to work, I'm not going to worry about it. I've tried a replacement UY41 rectifier, but this hasn't made any difference.
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Old 17th Sep 2008, 11:32 am   #8
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Default Re: DAC 90A - Radio stopped working

Quote:
Originally Posted by Doris The Diode View Post
I'm pleased to say that Bush DAC 90A serial nr 73/64023 is up and running
Congratulations on getting the set working. I suspected it would be something simple like this.

Don't worry too much about the low HT. It's normal for the voltage drop of valve rectifiers to vary between samples even when new, and they drop more and more as they age. You probably couldn't tell the difference if the HT was 20V higher, and the valve may soldier on for many years. They aren't too difficult to find if you need another one.

Paul
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Old 19th Sep 2008, 8:17 pm   #9
ColinB
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Default Re: DAC 90A - Radio stopped working

Sounds about right to be honest.

The HT voltages for these types of radios were always a bit optimistic. I've got a DAC10 on the bench now. The Trader sheet claims 210v at the UY41 output, I've got about 180v. OP stage in good shape (no H/K leak in UL41 ) and all new caps - I'm not worried.

FWIW,

Colin

Congrats on finding the fault too
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