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Old 30th Apr 2022, 9:27 am   #1
hammom02
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Default Dansette HiFi Mk2 smoking amplifier

I recently aquired a Dansette HiFi Mk2 from 1964 via an auction. After unpacking I switched it on and got a continuous buzzing noise and little else (a bit of a smell though and some crackling), so I immediately turned it off and thought it was the usual suspects. I've restored about a dozen Dansettes including a HiFi, so I replaced all of the capacitors including the huge 50+50 can. Switched it on again and the same buzzing. This time I noticed a bit of smoke as the amp warmed up! Switched off and headed for those that know a lot more than me. From a previous thread (https://vintage-radio.net/forum/showthread.php?t=95222) I'm assuming that it could be a faulty transformer or rectifier (where is the rectifier on this model?). The poster (Caled2013) resolved the issue by replacing the transformer, but there is no indication as to what the spec might be. I have a schematic, but that doesn't say either. This version of the amp has two ECL86 valves (one is the original Mullard, the other is an unbranded 'foreign' made replacement). Any suggestions would be very welcome as I'd love to get this going again. Picture of the amp is attached. Thanks.
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Old 30th Apr 2022, 9:29 am   #2
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Default Re: Dansette HiFi Mk2 smoking amplifier

The valve on the left seems to have lost its getter, therefore its vacuum unless the getter is somewhere else on that valve

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Old 30th Apr 2022, 9:32 am   #3
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Default Re: Dansette HiFi Mk2 smoking amplifier

I believe the valve which has lost it's getter is the rectifier an EZ81 probably
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Old 30th Apr 2022, 9:38 am   #4
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Default Re: Dansette HiFi Mk2 smoking amplifier

Both valves are ECL86. The one on the left is original Mullard and the one on the right is an unbranded foreign make.
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Old 30th Apr 2022, 9:40 am   #5
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Default Re: Dansette HiFi Mk2 smoking amplifier

Excuse my ignorance, but what do you mean by 'lost it's getter'?
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Old 30th Apr 2022, 9:43 am   #6
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Default Re: Dansette HiFi Mk2 smoking amplifier

There is no EZ81 in this. The amplifier uses a bridge-rectifier which I think may have failed. It may need replacing with a 4 diode configuration.,
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Old 30th Apr 2022, 10:09 am   #7
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Default Re: Dansette HiFi Mk2 smoking amplifier

According to a very video on YT, it does indeed use a 4 diode bridging rectifier. Would it be that Westinghouse EC20 unit on the left of the image?
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Old 30th Apr 2022, 10:10 am   #8
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Default Re: Dansette HiFi Mk2 smoking amplifier

Forgot the image!
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Old 30th Apr 2022, 10:14 am   #9
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Default Re: Dansette HiFi Mk2 smoking amplifier

The valve on the left has definitely lost its vacuum.

I've been working on a slightly different version of this amplifier recently and had to replace the rectifier for the second time. Unfortunately in failing it took out the mains transformer.

https://www.vintage-radio.net/forum/...d.php?t=128996

Post 35 onwards.
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Old 30th Apr 2022, 10:17 am   #10
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by hammom02 View Post
Excuse my ignorance, but what do you mean by 'lost it's getter'?
Mike
See the pictures below.

If its like the one on the left of the picture.

The mirror shiny bit is the getter it mops up residual gas. If its milky white then the valve is useless.

Not all valves have a visible getter though.
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Old 30th Apr 2022, 10:19 am   #11
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by hammom02 View Post
Forgot the image!
That's the rectifier on the left.
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Old 30th Apr 2022, 10:39 am   #12
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Default Re: Dansette HiFi Mk2 smoking amplifier

Ok, thanks guys. One of the valves has definately had it then and probably the rectifier. They next question is - how do I replace that rectifier?
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Old 30th Apr 2022, 10:44 am   #13
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Default Re: Dansette HiFi Mk2 smoking amplifier

I suggest that you test the rectifier to make sure it's actually faulty before replacing it.

You could replace it with four diodes on a tag strip or with a bridge rectifier as shown in the attached picture.
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Old 30th Apr 2022, 12:07 pm   #14
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Default Re: Dansette HiFi Mk2 smoking amplifier

Ok, I'll test first. Thanks for the image and advice.
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Old 30th Apr 2022, 12:12 pm   #15
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Default Re: Dansette HiFi Mk2 smoking amplifier

The Mk1 version of the Dansette Hi Fi has a more visually substantial drop-thru mains TX. But that in the Mk2 version appears to be able to well handle the increased power demands of the ECL86s.
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Old 30th Apr 2022, 12:33 pm   #16
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Default Re: Dansette HiFi Mk2 smoking amplifier

The valve which has lost its vacuum won't have been emitting smoke itself, so there will be some damage elsewhere. Transformers can be got smoking hot. IF you're lucky you may have caught it before permanent damage was done. Solid state rectifiers like that bridge can also go up in smoke. In this sort of fault finding, you have to look not only for the cause, but also any victims damaged as a result.

Those valves are not cheap, but not terribly expensive, they are available. The transformers are more serious if damaged, but there are people who can re-wind them. All the other components are somewhat easier to source. So it can be repaired, if it's thought worth the cost.

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Old 30th Apr 2022, 1:14 pm   #17
Edward Huggins
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Default Re: Dansette HiFi Mk2 smoking amplifier

You soon know if a metal rectifier goes by that acrid smell! Yes, transformers can be replaced, but a Push-Pull, Ultra-Linear OPTX can be surprisingly expensive. One wound for EL84s should be a very good match - unless Ed Dinning advises otherwise!
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Old 30th Apr 2022, 8:39 pm   #18
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by Edward Huggins View Post
There is no EZ81 in this. The amplifier uses a bridge-rectifier which I think may have failed. It may need replacing with a 4 diode configuration.,
I stand corrected.
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Old 18th May 2022, 1:54 pm   #19
hammom02
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Default Re: Dansette HiFi Mk2 smoking amplifier

Well, I've replaced the old Westinghouse rectifier with a modern Bridge rectifier as suggested and switched on to see what happens. It now has a quieter buzz which, after a while, is getting slightly louder as it warmed up. A HiFi Mk2 should be much quieter than this.
Could it have been the transformer after all?
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Old 18th May 2022, 2:18 pm   #20
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Default Re: Dansette HiFi Mk2 smoking amplifier

It is my eyes or are there dry solder joints on the main reservoir / smoothing electrolytic? That could cause hum.

It is probably time to start taking voltage readings for diagnosis. HT feed (AC and DC) & pentode electrodes would be informative.
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