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Old 22nd Oct 2019, 10:57 am   #21
Sideband
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Default Re: Bush DAC90A - power but no sound

You're right. With no feedback it doesn't (or shouldn't) matter which way round you connect the transformer primary. As to how the transformer is mounted....not sure. Maybe it doesn't matter. All you can do is try it and see.
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Old 22nd Oct 2019, 11:27 am   #22
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Default Re: Bush DAC90A - power but no sound

I mostly use an analog meter. This has a battery inside to measure resistance. This will give a click when connected across the speaker or transformer primary. Use the lowest range to get the most current.

If you do not have an analog meter then use a single cell or battery with a wire brushed over the battery terminal. A scratchy sound when the speaker isenergised shows the peaker is probably OK. If no sound is produced when the transformer primary is energised, then the transformer is faulty.

I always measure DC voltages early on to find serious faults. You could do this if you connected a 1k 2W resistance in place of the transformer primary.

As said before, do not run the set with no anode current, the current all goes to the screen which will object.
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Old 22nd Oct 2019, 2:36 pm   #23
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Default Re: Bush DAC90A - power but no sound

My own DAC90A restoration Thread is here - see Post #9 from David G4EBT in particular : -

https://www.vintage-radio.net/forum/...d.php?t=146402

I also had an open circuit on the Primary of the Output Transformer - contact Ed Dinning on this Forum for a rewind or replacement.

Please ensure you don't turn it back on without working your way through all the preliminary checks and essential replacements e.g. C18, the audio coupling capacitor.

Make sure you obtain the Bush Service Instructions before proceeding.
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Old 22nd Oct 2019, 3:08 pm   #24
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Default Re: Bush DAC90A - power but no sound

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Originally Posted by DonaldStott View Post
Please ensure you don't turn it back on without working your way through all the preliminary checks and essential replacements e.g. C18, the audio coupling capacitor.

Make sure you obtain the Bush Service Instructions before proceeding.
For Justamo….you might ask why? As the transformer primary is open circuit, there is no connection (load) to the anode of the output valve (UL41). This also means there is no voltage to the anode since the primary of the output transformer feeds the high tension to the anode. Without anode voltage, the next electrode within the valve (called the screen grid) also fed from the high tension supply will try to do the job of the anode for which it was never designed to do, neither is it rated to. As a result, the screen grid will get red hot which it should never do and if run like this for any length of time, the valve will quickly fail or will be so severely damaged as to be useless.

UL41's are becoming very expensive and hard to find so you certainly don't want a new one to be subjected to any kind of overload. Make sure you change C18 (audio coupling capacitor) as a priority as well as replacing the output transformer. Even with a new transformer, don't think of switching on the set without changing C18 at least. You should change all of the waxy paper caps at any rate but first things first, lets get the main problem sorted and go from there.
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Old 22nd Oct 2019, 8:14 pm   #25
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Default Re: Bush DAC90A - power but no sound

Looking at the picture in post #7, C22 (0.1 uF) - the RF Bypass capacitor across the mains, is still in place and still intact. Before any live testing is carried out a wise precaution would be to snip the capacitor out or it will become very annoyed and will almost certainly go off with a nerve jangling bang. The set will function perfectly well without it, but if it's desired to replace it, the replacement should be a Class X capacitor specially designed to be across the mains.

Not easy to tell from the picture, but to my eyes, on what wiring I can see, the insulation looks brittle and crumbly. Replacement of the wiring with heat resistant silicone rubber insulated flex, including the speaker connection, dial bulbs wiring and around the voltage selector panel would be high on my list of priorities. These ubiquitous sets are now some 70 years old, so inevitably, have suffered the ravages of time. Do heed the warnings about removing and discarding the spring retaining clips from the valve-holders before pulling out any valves. The springs serve no useful purpose and if left in place pose a high risk of breaking the glass pips on the valves.

Good luck in your endeavours.
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Old 23rd Oct 2019, 10:31 am   #26
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Default Re: Bush DAC90A - power but no sound

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Originally Posted by David G4EBT View Post
The springs serve no useful purpose
They did and might still do in some receivers....Don't forget that they changed the type of valve holder during the production run, the construction of the valves was also changed.

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Old 23rd Oct 2019, 11:07 am   #27
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Default Re: Bush DAC90A - power but no sound

There are no springs in the valve holders of the DAC90A I'm in the process of repairing. It's serial no. is 73/204886 by the way, if that helps to date it.
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Old 23rd Oct 2019, 12:57 pm   #28
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Default Re: Bush DAC90A - power but no sound

My DAC90A was 73/ 56971 - it was an easy job to remove the spring retaining clips.
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Old 23rd Oct 2019, 1:50 pm   #29
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Default Re: Bush DAC90A - power but no sound

Is it not so, that the springs on the valve bases were originally designed in to ruggedise the valve mounting when used in the stressful environment of car radios, which was one of the primary design applications for some of that series of valves?
Thus redundant in the currently discussed use. - Just another unfortunate trap for the inexperienced and unwary restorer. Tony
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Old 23rd Oct 2019, 4:11 pm   #30
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Default Re: Bush DAC90A - power but no sound

Quite often the springs will have rusted and attached themselves to the skirt of the valveholder which means that as you try to pull out the valve the little locating pip on the side of the glass will shatter and the valve will be no longer any good. Removal of the spring ensures that it ought to be possible with care to safely pull the valve.
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Old 23rd Oct 2019, 4:16 pm   #31
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Default Re: Bush DAC90A - power but no sound

The springs can serve a useful purpose when someone ships a set without first removing the valves and wrapping them in bubble wrap or W.H.Y..
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Old 23rd Oct 2019, 7:17 pm   #32
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Default Re: Bush DAC90A - power but no sound

My DAC90A does not have removable springs, it has riveted skirts as illustrated.

My instinct was to release the tension from the pips using my thumb nail.

Interesting that the UL41 and UY41 are valve bases are secured with nuts & bolts, rather than rivets.

Only the rectifier is all glass.
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Old 23rd Oct 2019, 7:47 pm   #33
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Default Re: Bush DAC90A - power but no sound

Here is a close up of mine before restoration and removal of the spring retaining clips: -

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@rambo1152 - Not sure what you mean by "Only the rectifier is all glass" ?
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Old 23rd Oct 2019, 8:00 pm   #34
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Default Re: Bush DAC90A - power but no sound

The UCH42, UF41, UY41 etc etc started off with metal rims and pips.

Here's a Philips article about them from 1946 describing the low temperature sealing technique used to seal the bulb to the base:

https://www.dos4ever.com/EF50/PTT_ok..._large_eng.pdf

Lawrence.

Last edited by ms660; 23rd Oct 2019 at 8:14 pm. Reason: added link
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Old 24th Oct 2019, 9:08 pm   #35
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Default Re: Bush DAC90A - power but no sound

I usually lift the spring out of the slot and rest it on the lower part of the Valve holder and then slide it back up into the slot when the Valve has been replaced.
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Old 25th Oct 2019, 9:04 pm   #36
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Default Re: Bush DAC90A - power but no sound

I hope 'Justamo' hasn't become disheartened with the radio or thinks it is beyond his ability.
I was eagerly awaiting the next installment.
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Old 25th Oct 2019, 9:39 pm   #37
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Default Re: Bush DAC90A - power but no sound

I wouldn't worry too much about that, Sideband, as from his previous posts I think he just wants to get these radios going as cheaply and as easily as possible so that he can sell them on for a fast buck. I would suspect that a rewound or replacement transformer would work out more than the radio is worth and perhaps a bit of a lesson about the potential issues that can be encountered with these sets.

As a second point, I'm against removing original springs from valve bases in these radios. I've never yet broken the pip off a valve while removing one. Obviously it can happen if you're a bit 'ham-fisted', but if this is the case, then what I always say is that perhaps you shouldn't be working on vintage radios. You just have to release the spring carefully - I sometimes treat them with a very light oiling if they're a bit rusty. We shouldn't pander to peoples carelessness, either get used to handling these old components properly and with care or don't do it at all. Not having a go at anyone - just saying!
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Old 26th Oct 2019, 8:23 am   #38
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Default Re: Bush DAC90A - power but no sound

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As a second point, I'm against removing original springs from valve bases in these radios. I've never yet broken the pip off a valve while removing one. Obviously it can happen if you're a bit 'ham-fisted', but if this is the case, then what I always say is that perhaps you shouldn't be working on vintage radios.
I agree to some extent. However I HAVE broken a pip off (an EF41) and I wouldn't regard myself as ham-fisted since I've been restoring sets for 30 odd years and was in the Trade for nearly 40 years (and the valveholder wasn't rusty). When every corner TV shop had valves for sale, the odd accident didn't matter. Now they are a somewhat limited resource and much more expensive it does no harm to pass on previous experience and mishaps to others.
I remove the spring since I had my 'mishap' and have had no further problems in doing so. I never really liked the rimlock method and glass pip and I wouldn't mind betting that many a good valve met an untimely end when being removed for test.

I hope the DAC90A in question doesn't get scrapped.
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