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Old 18th Nov 2017, 6:37 pm   #1
Mike Noble
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Default Roberts R66 low HT and low heater voltage.

Hi, I have an R66 which was completely dead. There was no grid voltage on V4 and no heater voltage getting to the valves although voltage was present on C30. A dose of switch cleaner and there is now heater voltage and after being on for a while a grid voltage of -4.5V appeared. In fact some stations can now be heard. The heater voltage is low at 1.7V with 2.6V present on the output of MR2. Also the HT is low at 49V with 86V at the output of MR1.

I read in previous posts I can change MR1 to a 1N4007 diode in series with a 220 ohm resistor but what about MR2? Can I just swap this for two 1N4001's which I already have?
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Old 18th Nov 2017, 9:26 pm   #2
G0HFCFrank
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Default Re: Roberts R66 low HT and low heater voltage.

The HT does seem rather low, Mike, so why not start with the 1N4007 + 220Ω. The LT side should OK for nominal 1.4V filaments, they ran off a single 1.5V cell in battery mode.
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Old 18th Nov 2017, 10:04 pm   #3
Mike Noble
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Default Re: Roberts R66 low HT and low heater voltage.

On reading the service sheet in more detail, 2.6V is the correct voltage on MR2 so just the HT to sort out. The service sheet gives the AC voltage across MR2 which I failed to notice.

Last edited by Mike Noble; 18th Nov 2017 at 10:16 pm. Reason: Not very clear what was meant.
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Old 18th Nov 2017, 10:10 pm   #4
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Default Re: Roberts R66 low HT and low heater voltage.

You stated that the heater voltage is low at 1.7V, that's high.

Lawrence.
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Old 18th Nov 2017, 10:21 pm   #5
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Default Re: Roberts R66 low HT and low heater voltage.

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Old 18th Nov 2017, 10:24 pm   #6
Mike Noble
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Default Re: Roberts R66 low HT and low heater voltage.

Yes, I can now see this is high but probably ok. I misread the data sheet and was expecting a higher voltage. I'll get a 1N4007 and 220 ohm resistor to sort out the HT side and see if this helps.
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Old 19th Nov 2017, 2:05 pm   #7
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Default Re: Roberts R66 low HT and low heater voltage.

If you are replacing MR1, I would go the whole hog and use a silicon bridge. This would eliminate DC in the transformer and double the ripple frequency which is easier to attenuate.
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Old 20th Nov 2017, 3:15 pm   #8
Mike Noble
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Default Re: Roberts R66 low HT and low heater voltage.

Just thought I'd let you know how I got on with the repair.
After swapping MR1 for a 1N4007 and 220 0hm in series the radio was dead.
There was now no heater voltage on the valves as when the radio was first tested. This was traced to a dry joint on the switch but even with this corrected the radio was still dead. Swapping the DL96 output valve restored some sound but the radio was very microphonic and the volume would change if the case was tapped. The most sensitive valve was V3, a DAF96, and with this swapped for a known good valve the microphony and varying volume level were cured.
Hooray.
All I need to do now is source some valves as the replacements were pinched for my Bush MB60.

If I was going to use the radio I might consider a bridge rectifier but it's sounds fine as is. Thanks for your help.
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Old 20th Nov 2017, 4:12 pm   #9
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Default Re: Roberts R66 low HT and low heater voltage.

Both the DAF96 and DL96 are on Langrex stock list, plenty in stock, you will need to phone, email for a price.
https://www.langrex.co.uk/public_html/valvenov.pdf
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