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Old 25th Nov 2023, 11:53 am   #1
boiss
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Default Battery HT Converter

I was given a Vidor portable to look at and there was a battery HT converter for a B126. I was told it was was a kit from a company in Kent, It is built on 3 pieces of PCB soldered together but not very rigidly. the cct board and the 2 ends, one has a 3 pin Din socket for the O/P. the I/P is a PP3 connector for the 6 x AA cell battery pack. It uses a toroid and 4 transistors, 2x 3904, 3819 and BS170.
The problem with it is that with 6 new alkaline batteries the O/P is only 72V without a load. I was told that it was supposed to be 90V?
Does anyone know anything about this converter and have a circuit diagram for it. I think it was in PW, but I have trawled through many copies of PW for a few hours without finding anything. Pics attached.
M
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Old 25th Nov 2023, 12:25 pm   #2
newlite4
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Default Re: Battery HT Converter

The radio should still operate at 72 volts. Some compact post-war valve battery portables worked on 45 volts so that a smaller battery could be accommodated. If you wanted to increase the output voltage, you may be able to vary the oscillator frequency so that you can get the best match for the resonant frequency of the transformer.
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Old 25th Nov 2023, 1:36 pm   #3
marceljack
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Default Re: Battery HT Converter

There were also 67,5V HT batteries, so probably the value of 72V is intended to replace this type of batteries.
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Old 26th Nov 2023, 12:49 am   #4
boiss
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Default Re: Battery HT Converter

I know the radio wil work 0n 72V especially with 9V on the filaments, But !!
It was purposely built into a B126 (90V) Eveready cardboard casing with the 6x AA battery holder. They fit perfectly. This converter was built by the guy's wife. Maybe if the heaters were run off a PP3 instead of 7.5V and the HT was Low it would have worked the same until a valve gave up the ghost. However I have found enough info to know that this converter was supposed to replace a B126 battery, ie 90V. I have no intention of tracing the cct and trying to work out which R has the wrong value etc. The problem gets better and better, because the owner wants to use this supply on another Vidor (CN426) as well, for picnics!! (How did I get involved?)
Last time I touch this battery stuff?
I have a Romac 136L with power supply, great little set to work on, even rebuilt the chassis because corosion had got at the Ally, I may make a battery converter for this while I am at it. The local Chinese bazar has a 40% sale on. 18650 4500 mAH Lipos, were 2.5E so now they will be 1.5E, time to buy a few for other projects methinks!
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Old 26th Nov 2023, 8:57 am   #5
Bazz4CQJ
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Default Re: Battery HT Converter

You've mentioned the problems with the batteries; have you made any assessment of how much RF noise the unit puts out? Additional screening and filtering is often required to use such devices happily?

B
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Old 27th Nov 2023, 12:03 pm   #6
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Default Re: Battery HT Converter

Thats a good point! I presumed that as it was meant to be used in radios that were usually MW/LW, the frequency used would be in the Hz range and the harmonics would be quite low in the 100s of KHz area. At the end of the day it was only supposed to put out about 1W. I will try it with and without a load later. My Sony goes down to 150KHz.
As for the Chinese bazar someone beat me to the 18650s or they withdrew them. The pp3s are are only 1euro each now, (Goldstar) 10euros to make a battery pack?
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Old 27th Nov 2023, 5:45 pm   #7
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Default Re: Battery HT Converter

Ok I have done some simple checks. I/P 9.6V, O/P no load 68.2V, with an 8K2 load on the O/P the voltage rises to 80.6V.
From 150KHz up to 1.900MHz it has a quieting effect as if it raises the noise floor in some way (AGC?). At 1.910MHz there is a signal not modulated and at then at the following frequencies:- 2.825MHz, 5.665MHz, 8,485MHz, 11.315MHz, 14.130MHz, 16.975MHz.
Obviously this is working at about 2825KHz and the higher results are the harmonics, the higher ones are not so clean, and I didn't bother with going any higher. The signal at 1.910MHz is much weaker and I presume is the Image.
The fact that the O/P voltage increased with a load (About 10mA) suggests that there is some sort of regulation/feedback. Shame it is 10V low!
It appears that it would not cause any interference on MW and LW although it might reduce the sensitivity, I can't verify that as I don't have another MW Rx.
Cheers
M
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Old 8th Dec 2023, 12:57 am   #8
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Default Re: Battery HT Converter

I was told by the owner that the Ht converter switches off when the radio is turned off. Unfortunately this isn't true, the current draned from the 6 AA cells in series is just over 9mA, so with 1500ma cells they will last less than 6 days with the radio off. Battery drain when on and good volume is about 125mA so the converter is fairly efficient. With the regular removal of the AA cells or at least 1 of them, the bottom of the printed battery (B126) back would need to be easily removed or available. I guess some sort of charger could be possible too.
M
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