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Old 31st Mar 2021, 7:09 pm   #1
60 oldjohn
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Default Black DAC90

I restored this first generation DAC90 with smooth knobs about 5 years ago and used it in our house most weekends. Last Christmas it was not used for three weeks, when I finally turned it on all I got was a loud hum, quickly switched off.

When I first restored it I used an autotransformer from ED Dinning and disposed of the original asbestos shielded dropper resistor. This effectively turns the DAC90 into an Bush AC90. At the time the only Electrolytic capacitors in the set were the Smoothing and Reservoir caps which reformed ok.

Five years on and the set hums loud regardless of volume settings. I looked at ordering the small Rubicon caps from Farnell but they are out of stock.

A few days ago I got round to removing chassis, checked the double electrolytic,, 32uf, spot on and 20uf should be 16uf, nothing to worry me.

I had read that the EBC33 can give similar results (HK short) so I measure the heater to other pins on all valves and eventually found the CCH33 had a measurable HK short. Lucky I had valve out of another DAC90 to try and it is working well while I get another from my valve supplier David Thompson.

While I had it on my bench with the aerial laid on the bench I was listening BBC Radio Wales, I am on the east coast of Yorkshire so a few miles away!

John.
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Old 31st Mar 2021, 9:14 pm   #2
Half a Mullard
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Default Re: Black DAC90

Hi John
Well done and Iím glad you got that sorted out. Iíve found those electrolytic cans fitted to Bush radios to be pretty reliable. Just shows the sensitivity of those sets picking up Radio Wales, nice one! Hopefully it will serve you well again! Good idea fitting an auto transformer too, Iíve never liked the look of those droppers!
All the best
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Old 31st Mar 2021, 10:08 pm   #3
60 oldjohn
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Default Re: Black DAC90

Thanks Howard, with the set out of the case I had no idea of what part of the MW I was tuned to. When I heard a faint but very clear signal I was expecting Radio Caroline, Not BBC radio Wales.

John.
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Old 1st Apr 2021, 5:52 am   #4
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Default Re: Black DAC90

Good diagnosis John, many people would have gone straight to the conclusion that the smoothing can was suspect.

What sort of reading did the CCH33 return?
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Old 1st Apr 2021, 6:19 am   #5
John M0GLN
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Default Re: Black DAC90

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Originally Posted by 60 oldjohn View Post
. When I heard a faint but very clear signal I was expecting Radio Caroline, Not BBC radio Wales.

John.
I get a similar result to you, BBC Radio Wales from Washford in Somerset is about the strongest MW station I can receive here in Southport,

John
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Old 1st Apr 2021, 10:57 am   #6
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Default Re: Black DAC90

That would be the Radio Wales tx at Penmon (10kW), or are you listening on 657kHz? (2kW)
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Old 1st Apr 2021, 11:03 am   #7
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Default Re: Black DAC90

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What sort of reading did the CCH33 return?
I did not do any tests with CCH35 (my mistake in #1) in circuit. When tested out of circuit pins 2 - 7 heater a few ohms, pins 7 - 8 short circuit. I found a used CCH35 it also had a HK short, pleased I checked it before trying it in the set.

John, I was a bit surprised, I believe it is over 220miles over land to Washford.
Its been a few years since I have been there to see Neil at the now moved Radio Museum.

Also worth mentioning, when removing the two screws the secured the chassis both were reasonably slack. But one took a massive force as the spring washer was scoring deep into the radio bracket. I have lost the filling that it cut out of the chassis but have taken photos. I did try easing the washer with a thin screwdriver but that did not work, it kept biting for about 1.5 turns.

I am having trouble with photos I will try later.

John.
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Old 1st Apr 2021, 11:21 am   #8
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Default Re: Black DAC90

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That would be the Radio Wales tx at Penmon (10kW), or are you listening on 657kHz? (2kW)
When I first heard Radio Wales the only tx sites I could find were Washford on 882kHz which is the frequency I'm listening on and a couple of low power ones on a lower frequency, now you mention this tx at Penmon it must be the one I can hear, the BBC don't make it easy to check, on their website I get this,

"The FM and AM Frequencies table has been discontinued.
To find the frequencies of BBC Radio stations being transmitted near you, visit this PDF download of our Transmitters, "

and the PDF link returns 404 Not Found.

Thanks for correcting me.

John
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Old 1st Apr 2021, 11:25 am   #9
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Default Re: Black DAC90

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John, I was a bit surprised, I believe it is over 220miles over land to Washford.
John.
I was as well and now I know why thanks to Julesomega .

John
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Old 1st Apr 2021, 2:39 pm   #10
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Default Re: Black DAC90

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Originally Posted by 60 oldjohn View Post
[

Also worth mentioning, when removing the two screws that secured the chassis both were reasonably slack. But one took a massive force as the spring washer was scoring deep into the radio bracket. I have lost the filling that it cut out of the chassis but have taken photos. I did try easing the washer with a thin screwdriver but that did not work, it kept biting for about 1.5 turns.


I've managed to get the photos. In future I think I will be using a flat washer instead.

John.
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Old 1st Apr 2021, 2:48 pm   #11
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Default Re: Black DAC90

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That would be the Radio Wales tx at Penmon (10kW), or are you listening on 657kHz? (2kW)
I'm not sure, just had a listen and after a lot of waiting said BBC radio Norfolk and more waiting around Radio Caroline. If I get a chance will listen later to both frequency's.

John.
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Old 1st Apr 2021, 6:27 pm   #12
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Default Re: Black DAC90

Quote:
Originally Posted by 60 oldjohn View Post
Quote:
Originally Posted by 60 oldjohn View Post
[

Also worth mentioning, when removing the two screws that secured the chassis both were reasonably slack. But one took a massive force as the spring washer was scoring deep into the radio bracket. I have lost the filling that it cut out of the chassis but have taken photos. I did try easing the washer with a thin screwdriver but that did not work, it kept biting for about 1.5 turns.


I've managed to get the photos. In future I think I will be using a flat washer instead.

John.
Wavey or Crinkle washers work well and are a bit kinder on the mating surfaces.

John
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Old 11th Jul 2021, 2:53 pm   #13
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Default Re: Black DAC90

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Good diagnosis John, many people would have gone straight to the conclusion that the smoothing can was suspect.
Looking back, I should have replaced the Electrolytic Can first "the only electrolytic,s in the set"
After a lot of valve swapping I decided to swap out the Can only put off by the lack of availability from Farnell either on back order since Christmas, or + £14.95 postage on certain items.
Set has been working for about two weeks with two large old Electrolytic caps I had in stock, unfortunately not been able to keep the original Can I have been able to mount the 2 caps on two small pieces of strip board without drilling extra holes.
Original fault Hum at switch on for 10 seconds, then works perfect. Change valve, perfect... for an hour then loud hum, another faulty valve I thought I had another valve IIRC it lasted a little longer before stating hum Yes I know I should have taken voltage readings.

John.
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Old 12th Jul 2021, 10:13 am   #14
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Default Re: Black DAC90

John, l had a terrible hum problem which was resolved by replacing a phenolic valve mount, it had suffered a lot of heat and become conductive.

Looking at the service sheets it appears that C20/C21 are held by a P-clip. I would have lifted the two positive legs and used croc clips to temporarily insert two new ones.

Isn't hindsight marvellous! 20/20 vision!

Hopefully I'll see you in November...
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Old 26th Jul 2021, 7:07 pm   #15
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Default Re: Black DAC90

Good job working out the fault, Just an observation I've made and I'll be interested in what others might think. I have 2 Dac 90's and 2 Dac 90A's all of which I've fixed.
Listening to them I'm convinced the early models actually sound better, a fuller deeper sound.
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Old 26th Jul 2021, 8:20 pm   #16
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Default Re: Black DAC90

I have a DAC90 which suddenly started humming while it was on - I ran over to switch off, assuming the big electrolytic was going to vent.

Long story short, the EBC33 had suddenly developed a slight leakage, heater to GRID.

I still have it - if I use the DAC90 on DC mains ever, it'll go back in.
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