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Old 20th Feb 2017, 10:05 pm   #1
trsomian
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Default AVO All wave oscillator

I have just been trying to get my one of the above working after something like 20 to 30 years inactivity. I took the valves out, and started it gently on the variac, winding it up to 240V over about 45 minutes. The smoothing capacitors are running cool. The HT supply is a transformer delivering 95V AC, a half wave metal rectifier, and after the rectifier it is getting about 65V. Before the smoothing choke there is considerable ripple, but after the smoothing choke is a perfectly clean 57V or so.

The heaters are glowing, with about 6.2V across them. All voltages measured with my AVO 8

I have a circuit diagram (from AVO in 1983!), which shows a transformer voltage of 100V, but does not quote an HT DC voltage. I know metal rectifiers are lossy compared to silicon diodes, but are they really this bad, or is it needing to be replaced with a 1N4004 and a suitable series resistor, and if so what value resistor would be reasonable. The circuit diagram lists the valves as L63, which means nothing to me because I don't normally dabble with valves

The reason I ask, is that I am getting no RF output, and I don't think the AF modulation oscillator is working either
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Old 20th Feb 2017, 10:23 pm   #2
Ed_Dinning
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Default Re: AVO All wave oscillator

Hi, I have an instruction sheet plus circuit for this Osc, but won't me able to access it for 2 weeks. Google the valve type and see what the valve pin outs are, then measure some voltages. Also check the continuity of the modulation transformer.
These are normally a good stable oscillator, ideal for vintage radio work.

Ed
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Old 20th Feb 2017, 10:34 pm   #3
trsomian
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Default Re: AVO All wave oscillator

Never thought of googling for valve data, but it certainly worked

It seems to be rated up to 250V anode voltage, but mine has 57V, which presumably could be a much greater peak if it was oscillating. As stated before, I don't have any "feeling" for valve voltages, so I don't know what to expect, but the HT supply loosing about half the available volts seems wrong, but maybe that was normal with metal rectifiers??
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Old 21st Feb 2017, 10:03 am   #4
ms660
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Default Re: AVO All wave oscillator

Some info on forward voltage drop tests of the rectifier here if it's any use:

https://www.vintage-radio.net/forum/...read.php?t=748

Lawrence.
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Old 21st Feb 2017, 10:51 am   #5
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Default Re: AVO All wave oscillator

Post #4 in the link that Lawrence put up states that your HT voltage after the metal rectifier should be between 88-119vdc so unless that you have something pulling excess current and dragging the voltage down then it looks like you need to replace that rectifier, a 1N4004 would be suitable and I would try a series resistance of say 2.2k to start with and see what you get on the HT rail, if the thing works and the HT is a bit high you can always increase the resistance value or if it is still a bit low you can always decrease it.

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Old 21st Feb 2017, 10:52 am   #6
trsomian
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Default Re: AVO All wave oscillator

I was aware of that thread already, and it is useful, except that I deduce that the unit being tested is far from new, so it may have already aged, hence may not indicate what AVO expected the unit to behave like when new
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Old 21st Feb 2017, 11:07 am   #7
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Default Re: AVO All wave oscillator

I'd aim for around 100 volts DC with a silicon rectifier and see how it performs.

Lawrence.
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Old 21st Feb 2017, 11:23 am   #8
John M0GLN
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Default Re: AVO All wave oscillator

I've just measured mine and there's 95V coming out of the choke into the modulation transformer, it should oscillate with a lot less than that though, I can't remember what was wrong with it but when I first got it about 30 years ago the HT was only about 45V and it still worked.

John
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Old 21st Feb 2017, 12:39 pm   #9
John M0GLN
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Default Re: AVO All wave oscillator

I just had a thought and rechecked the HT, it is 95V CW and 71V when modulated.

John
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Old 21st Feb 2017, 1:57 pm   #10
trsomian
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Default Re: AVO All wave oscillator

Thanks for the help all of you. I will use a 1N4004 and select a resistor such that the voltage without the modulation oscillator operating is 100V or just a bit less. It seems like this might not be the only fault, however.
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Old 21st Feb 2017, 2:09 pm   #11
Craig Sawyers
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Default Re: AVO All wave oscillator

FWIW GD rectifiers still make selenium rectifiers as transient suppressors. I had custom full wave bridges made to my specification as part of a Bletchley Park restoration.

http://www.gdrectifiers.co.uk/produc...t_suppressors/
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Old 21st Feb 2017, 11:46 pm   #12
trsomian
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Default Re: AVO All wave oscillator

I tried a 2K7 in series with a 1N4004, which increased the HT voltage, but still no output. Upon further investigation the oscillator was working, but poor contact in the output attenuator switch was preventing the signal getting out. Having got it working I disconnected the above parts across the original rectifier, and it still works, albeit with a lower output amplitude.

It is interesting to watch the output waveform after switching it off, the RF is still going after the modulation oscillator has stopped, and the RF must be going at quite a low HT voltage.

Thanks for your help, folks
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