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Old 10th Mar 2015, 7:28 pm   #21
mark pirate
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Default Re: Dustiest DAC90A ever?

The DAC90A was the most popular and long lived british radio of all time.
In my experience, Bush products were extremely reliable and long lived. My parents had a Bush AC11 radio and a T57 console TV from new in 1955, both were utterly reliable and the TV ran for nearly 18 years without ever needing any repairs. I know now that it was probably just a valve needing replacement, but it got taken away when the new TV was delivered

They also had a Bush SRG91 stereogram that was still as good as new when it was finally given to a relative 20 years later, it too never needed any repairs!

Being my favorite make, I now have an example of all of the above (and more).
My very battered Bush TR82B radio is my daily set, it is 54 years old and is still working well with all original components.

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Old 12th Mar 2015, 6:23 pm   #22
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Default Re: Dustiest DAC90A ever?

I have just discovered that some work has been done. the volume control is almost new but marked 1M Log instead of 500K. Will this matter much and is there any reason not to fit a shunt resistor of 600K say?
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Old 12th Mar 2015, 6:38 pm   #23
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Default Re: Dustiest DAC90A ever?

It shouldn't make any difference.
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Old 27th Mar 2015, 9:28 pm   #24
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Default Re: Dustiest DAC90A ever?

Progress has been slow but it works well but rather quietly after some cap replacements -not ALL the waxies yet .The tuning cord has snapped and I have some replacement but wonder the best way to do it . The notes make it look simple but I am not so sure . Before I start , has anyone done it and got advice?
I have checked the valve voltages and all are about 20~30% down eg the rectifier anode reads 170( should be 222ac) and cathode 160 (205dc). Would this be caused by the rectifier itself or the original double smoothing electrolytic which also reads low? It doesn't get warm and there is no hum.
I did note a small improvement in volume with a new UL41 but saving that till later.
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Old 28th Mar 2015, 12:14 am   #25
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Default Re: Dustiest DAC90A ever?

Replace all the wax caps, then try it. The rectifier may also be a bit tired, but the smoothing/reservoir cap sounds as if it is ok.
The tuning is quite easy to re-string, just follow the instructions on the trader sheet.

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Old 31st Mar 2015, 6:19 pm   #26
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Default Re: Dustiest DAC90A ever?

I was monitoring the voltage on the G1 of V4 after changing the cap to it. It was slightly unstable and increased to +.13v but oscillated around this? I removed the UL41 valve and cleaned the pins again - problem disappeared and now stable at 0.05v. Cathode is 5v so I think that is OK now.
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Old 29th Apr 2015, 8:08 pm   #27
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Default Re: Dustiest DAC90A ever?

I recently fitted 2 new 6V 0.06A bulbs which work quite well, after trying 3V 0.2A which were very dim. I have noticed that the bulbs brighten when tuning to a loud station, and then settle down again. A bit like a magic eye. Why is this happening? Is it to do with AGC?
Also what is to stop me wiring them in parallel?
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Old 30th Apr 2015, 10:19 am   #28
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Default Re: Dustiest DAC90A ever?

Most of these sets date back to when almost all houses had open fires and when an awful lot of people smoked, I wonder what effect that had on them?
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Old 30th Apr 2015, 8:36 pm   #29
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Default Re: Dustiest DAC90A ever?

The dial lamp bulbs are connected between chassis and mains neutral, so they will be carrying the combined current of the AC heater chain plus the DC HT current. Any change in HT current, such as with AGC action, will result in a change of brightness. I don't have a DAC90A but the lamps on my Marconi T18DA glow brighter as the set warms up and HT current begins to flow. However I haven't noticed whether the brightness changes when stations are tuned in.

Technically, the change in HT current with AGC should be pretty small. Is the brightness change linked to audio output volume? With a class A output stage the HT current should be more or less constant, assuming the valve is correctly biased. The correct lamp rating is 3.5 volts, 150mA per bulb. Check that the shunt resistor hasn't changed in value.

Regarding dusty houses, I have a friend who spent his career as a TV engineer, and he recalls being able to tell whether the house was vacuum-cleaned by the amount of dust accumulated within the set. Tobacco smoke usually makes its presence known by a dark area on the grille cloth where the 'speaker aperture is.
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Last edited by Phil G4SPZ; 30th Apr 2015 at 8:49 pm. Reason: Info added
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Old 8th May 2015, 7:02 pm   #30
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Default Re: Dustiest DAC90A ever?

Yes the bulbs only brighten when volume is over halfway setting and also on LW , I noticed. What does this indicate?
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Old 8th May 2015, 10:15 pm   #31
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Default Re: Dustiest DAC90A ever?

In my experience (with which others may disagree) in a set with a class A output stage, the average HT current, and hence the dial lamps' brightness, should remain pretty constant irrespective of audio volume. If the brightness varies with volume, it suggests that the output stage is not correctly biased at the mid-point of the valve's operating characteristic. However, you have checked the cathode voltage and deemed it to be correct; has the cathode bias resistor changed value? This would normally be accompanied by audible distortion, possibly more noticeable at low volume.

I can't for the moment think of a reason why switching between LW and MW would cause a change in lamp brightness.
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Old 11th May 2015, 4:01 pm   #32
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Default Re: Dustiest DAC90A ever?

Phil I meant that the brightness changes when tuning through R4 on 1500m not when I change the switch over.
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Old 11th May 2015, 4:49 pm   #33
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Default Re: Dustiest DAC90A ever?

Quote:
In my experience (with which others may disagree) in a set with a class A output stage, the average HT current, and hence the dial lamps' brightness, should remain pretty constant irrespective of audio volume
As power is a current squared function (it gets brighter quicker than it dims) I would expect them to brighten with more change in anode current, not by much though. Take this limiting example, 0.5A bulb at 0.5A, lovely, put 0A and 1A at 1Hz through it, off and blown, still 0.5A mean.

Last edited by Guest; 11th May 2015 at 4:50 pm. Reason: spelling (not an exhaustive look though)
 
Old 11th May 2015, 11:17 pm   #34
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Default Re: Dustiest DAC90A ever?

Merlin, you're technically correct about what would happen at 1 Hz, but the variation here is taking place at something between 50Hz and audio frequencies, so the bulb filament will simply smooth over the peaks and troughs and respond to the average.

However... I've just tried out my Marconi T15DA, which is a similar AD/DC circuit, and it exhibits exactly the same behaviour. Mind you, I have to turn the volume up until it's painfully loud and distortion sets in, before the bulb begins to flicker.

All valves produce distortion due to the inherent non-linearity of their Vg/Ia curve, so as you apply a larger signal swing to the grid, the average anode current will rise because the positive-going current peak will have a slightly greater amplitude than the negative-going current peak. But, as I said, the average anode current and hence the lamp brightness should only increase once you start driving the valve into distortion.

If you're still using 60mA bulbs that might account for some of the brightness variation. Try using the correct 3.5 volt 150mA bulbs and check that the shunt resistor is the correct value. Also, try a different speaker.

Have you still got low HT voltage?
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Last edited by Phil G4SPZ; 11th May 2015 at 11:19 pm. Reason: Clarification
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