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Old 9th Jan 2017, 1:11 pm   #1
Diabolical Artificer
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Default Solartron VF252

Having got this off Frank here on the forum quite some time ago, I finally gave in to the feeling of guilt I got every time I came across it whilst rooting around for something in the shed, and got it on the bench. As Frank says in his thread here - https://www.vintage-radio.net/forum/...d.php?p=745491 - it's a smashing meter and well built.

Powered it up slowly on a LL/variac, tested for HT etc. The needle isn't zeroing. At turn on it goes to FSD, oscillates a bit, then comes to rest at FSD.
I'm reading 37v DC on the meter. Currently i have left it turned on with about 100v AC on the variac.

At first the voltage regulator valve wouldn't fire. I looked for the usual high value R's, found a couple and changed a couple of coupling cap's. The reg valve is now igniting.

It's going to need a good going over at some point, but in the meantime, how do these old valve AC meter's operate? I've looked at the schematic which isn't brilliant and have found the usual attenuation input side, as well as a LTP differential amp on the OP. Should there be a reference oscillator circuit for the meter to compare the IP to?

Any help much appreciated, Andy.
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Old 10th Jan 2017, 7:24 am   #2
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Default Re: Solartron VF252

I found more off spec R's yesterday and have replaced most of the coupling caps as well as cleaning the switch's. It seem's to be the higher value R's that have gone high or OC.

I'll try and clean up the schematic as it's tricky to see what's what.

Andy.
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Old 10th Jan 2017, 11:29 am   #3
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Default Re: Solartron VF252

VF252. For Andy, I attach an up-to-date scan of the circuit diagrams and the component parts list. This will save you struggling with the poor version I posted on 23 Feb 2015. It is nice to see a bit of equipment without electrolytic capacitors on the power rails. Certainly improves the reliability. wme_bill m0wpn
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Old 11th Jan 2017, 7:01 am   #4
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Default Re: Solartron VF252

Cheers Bill, much appreciated, that really helps.

Andy.
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Old 11th Jan 2017, 11:07 am   #5
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Default Re: Solartron VF252

Just having a look at this and trying to figure out where the DC ( approx 40v) on the meter is coming from. The meter is weird as it has three terminals, I presume 0v or ground and +/-. there is + 40v DC on both the -/+ terminal's. I've traced this back so far to the EB91 diode and V10.

I can't figure it out. V10 has 22 ohm anode R's, and V10 is being fed (signal's ? ) onto it's cathode and grid. I'm stumped, would someone kindly explain what this circuit does? V8 & V9 are confusing too, as their cathode's go to the restive divider network. My best guess is some sort of compensation circuit.

My assumption about an AC VOM needing a reference oscillator seems to hold no water either as I can't see one here.

A.
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Old 11th Jan 2017, 1:59 pm   #6
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Default Re: Solartron VF252

Still stumped. Have found 5v AC on the meter terminal's and there is about 2v P-P of what I can only describe as rubbish there too. The meter needle bob's up and down and seems to react to an input from a sig gen.

I've tried shorting the input and shoving a 1/4 guitar jack in and out into the "ext meter" socket in an effort to clean the contact's. Have replaced several leaky coupling caps and off spec resistor's, but without any input from more experienced forum member's, the meter will go back into storage or stripped for parts, which would be a shame.

Someone suggested that plugging a mono jack into the "ext meter" socket will ground a/the multi vibrator. Tried this, the meter zero's out but still show's no reading. Whether this mean's that this device isn't intended for use as a straight AC meter I've no idea. I presumed it double's signal strength meter for radio transmission as it has a "trans input". Another thing I'm uncertain about.

A.
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Old 11th Jan 2017, 2:21 pm   #7
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Default Re: Solartron VF252

I'm pretty sure the meter is intended as a straight AC millivoltmeter. I've used one exactly the same and it did measure AC voltages. The 'trans input' is simply a transformer-isolated input for use when when measuring voltages from a balanced source, or one with a DC offset.

If you apply an input signal, can you trace it through the stages of amplification with a scope? The signal ought to appear on the 'CRO' output in magnified form. If it doesn't, then it should be possible to figure out where it's disappearing. The major gain stages seem to be V1/2/3A, then V4A/5/4B, V6A/7/6B and finally V8/9/10 to drive the meter.

Chris
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Old 13th Jan 2017, 6:21 am   #8
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Default Re: Solartron VF252

Ah, transformer input, not transmitter. I've started tracing the signal through but have had no joy finding why the meter isn't zeroing. With other meter's I have, the needle shoot's to FSD then return's to "0". This one hover's, slowly oscillates then sits at FSD.

I've still not much idea how a three wire meter works, but my guess is it act's like a see saw. I wondered if the EB91 diode was shot, if it is supposed to rectify the OP from the amp section, might try clipping in a couple of silicon diodes to see.

Thanks for your input Chris, Andy.
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