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Old 9th Dec 2017, 11:55 am   #41
Station X
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Default Re: DAC 90A Smoothing Capacitor replacement

Jason.

I can let you have a NOS boxed Mullard UL41 for 10 including postage. I'd test it before sending it out.

PM me if you're interested.
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Old 9th Dec 2017, 12:16 pm   #42
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Default Re: DAC 90A Smoothing Capacitor replacement

Good offer of a new UL41. You would be very lucky to get another even used/tested for this price.

You could just be expecting too much of this set, a slight background hum was quite normal and accepted of mains radios at this period.
Though extremely popular, the Bush DAC90A was never one of the best radios around.
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Old 9th Dec 2017, 12:42 pm   #43
JasonKillbourn
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Default Re: DAC 90A Smoothing Capacitor replacement

Oh, no worries, I wasn't going to throw the current UL41 away, but I figured a spare would be a good idea in the long run anyway, and, if the replacement cures the hum, I would box the old one, pending a future attempt at flashing it back to life, so it could serve as a viable spare.
As regards the UBC41, thank you Kalee, I'll certainly have a go at identifying pin 3 and will short it to the chassis with a crocodile clip lead to see if that works.
I've also had a good listen to YouTube vids and my DAC90A sounds quite a bit more hummy, so I'd say it does need further work.
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Old 9th Dec 2017, 12:48 pm   #44
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Default Re: DAC 90A Smoothing Capacitor replacement

Oh dear, I had a load of phone calls that delayed me while I was writing that last reply and just saw Graham's kind offer, and I'd actually just bitten the bullet and committed to buy an NOS UL41 for quite a few quid more. That'll teach me to be impulsive.
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Old 9th Dec 2017, 3:42 pm   #45
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Default Re: DAC 90A Smoothing Capacitor replacement

Hi Jason, as the volume appear to increase when you added the caps, can you check the voltage out of the UY41 cathode. My data says it should be about 205 VDC at this point and about 100 VDC on the smoothing cap (ie voltage readings at either end of the big 10K resistor with respect to chassis).

If the voltage is say 10 or more volts low, then this may be a sign of a tired rectifier and could possibly also increase hum slightly.

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Old 11th Dec 2017, 7:05 pm   #46
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Default Re: DAC 90A Smoothing Capacitor replacement

Thank you for the suggestion Ed, I was away over the weekend, but have just had the chance to get the multi meter out and took a reading from either side of that 10k resistor (R15) of approx 171 VDC and 90 VDC, so the UY41 is possibly looking a bit iffy.
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Old 11th Dec 2017, 7:29 pm   #47
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Default Re: DAC 90A Smoothing Capacitor replacement

Hi Jason, also worth doing an AC voltage check on the dropper resistors as these can get bad joints (they rarely increase in value as they are wirewound)

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Old 11th Dec 2017, 7:37 pm   #48
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Default Re: DAC 90A Smoothing Capacitor replacement

I will buy that UL41 Graham if still available as I have 3 Dac90.s that belong to me.
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Old 12th Dec 2017, 10:15 am   #49
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Default Re: DAC 90A Smoothing Capacitor replacement

OK HB.

I'll PM you once I've tested it.
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Old 12th Dec 2017, 9:11 pm   #50
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Default Re: DAC 90A Smoothing Capacitor replacement

Just a quick update, as I had the chassis back out earlier and ran pretty exhaustive tests on all resistors and nothing startling stood out. I also remembered to try Kalee's suggestion to short pin 3 of the UBC41 to the chassis, but it had no effect on the hum.
The current state of play is: there is a question mark hanging over the UY41 following that low voltage reading the other day, so I am pondering sourcing a replacement, and I am awaiting the imminent arrival of a NOS UL41. Thank you all again for your help, and hopefully I should have more news soon.
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Old 12th Dec 2017, 10:45 pm   #51
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Default Re: DAC 90A Smoothing Capacitor replacement

Quote:
Just a quick update, as I had the chassis back out earlier and ran pretty exhaustive tests on all resistors and nothing startling stood out. I also remembered to try Kalee's suggestion to short pin 3 of the UBC41 to the chassis, but it had no effect on the hum.
It's an easy test to do - and as it had no effect on the hum, almost certainly the UBC41 is not the culprit. (The signal would, of course, have completely disappeared while you had the short in place).
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Old 13th Dec 2017, 4:17 pm   #52
JasonKillbourn
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Default Re: DAC 90A Smoothing Capacitor replacement

Well, the UL41 arrived today, so I am very pleased to say the hum has gone and the DAC90A sounds great
However, following that 171 and 90 VDC reading from either side of the 10k resistor, I am wondering whether I shouldn't just get hold of a replacement UY41 to be on the safe side?
Also, since I do seem to have pretty poor reception in my kitchen, where I tend to listen to the radio, does anyone ever bother adding an external aerial to these sets?
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Old 13th Dec 2017, 4:25 pm   #53
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Default Re: DAC 90A Smoothing Capacitor replacement

James where did you get your UL41? I got a military spec NOS one off someone on ebay but it hums like my old one despite doing all the recommended mods and replacing both smoothing caps. I've saw another thread here where someone else experienced the same.

Being a live chassis set its not a good ideal to use an external aerial. The internal loop is actually very good, reception of AM is a challenge in today's world, do you have a pantry transmitter?
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Old 13th Dec 2017, 5:03 pm   #54
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Default Re: DAC 90A Smoothing Capacitor replacement

Yes, I did question the wisdom of an external aerial for that reason. Rather annoyingly, reception is much better in my bedroom, at the other end of the flat, but is faintly crackly in the kitchen, and unfortunately I haven't got a pantry transmitter, but would be interested in getting hold of one, so have you any recommendations for an affordable option?
Anyway, my UL41 is a military spec NOS one (CV1977) from an ebay vendor in Billingshurst called dan198745, so perhaps I got lucky?
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Old 13th Dec 2017, 5:13 pm   #55
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Default Re: DAC 90A Smoothing Capacitor replacement

Yeah I had the same product, different seller however. Its probably just luck of the draw

I highly recommend the SSTran AMT-3000, available in kit form from the USA. Not the cheapest to buy (100 ish with duty on top, depending on exchange rate), and expect to wait possibly months before seeing it and having all your emails to SSTran ignored - until you mention a refund/claim

That said, it sounds the best, and documentation on build and setup are outstanding. The audio processing onboard is excellent, and when set up properly should cover your home just fine. There is something similar in the Spitfire (available locally), but its lacking the audio processing, you could do your own I suppose. Aside from that there's loads of others varying in quality and range, or even have a go making your own.
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Old 13th Dec 2017, 10:58 pm   #56
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Default Re: DAC 90A Smoothing Capacitor replacement

The radio sitting inside a large tuned loop or a loop driven from a long wire could give more sensitivity without connection to the live chassis.
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Old 14th Dec 2017, 9:16 am   #57
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Default Re: DAC 90A Smoothing Capacitor replacement

Quote:
Originally Posted by JasonKillbourn View Post
Rather annoyingly, reception is much better in my bedroom, at the other end of the flat, but is faintly crackly in the kitchen...
I presume you've tried rotating it for maximum signal. This often leaves the set at a strange angle to the walls, worktop, table or what have you, though.
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Old 15th Dec 2017, 10:56 am   #58
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Default Re: DAC 90A Smoothing Capacitor replacement

Before condemning the UY41 as a cause of low HT voltages, bear in mind that Bush made some changes to the DAC90A chassis and failed to update the voltage readings.

The surge limiter in the anode circuit of the UY41 was increased in value and the pilot lamp shunt resistor was reduced. These measures were, I believe taken to minimise scale lamp failure on a hot switch-on.

I would expect to see about 95V on the set's HT line and 180V at the UY41 cathode with a reasonable set of valves. This seems to apply to all the sets I've seen (after restoration) despite the mains voltage here usually being 240 - 245V.

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