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Old 7th Feb 2019, 2:18 pm   #1
mickm3for
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Default Sinclair meter DM235.

Hi this meter has a thermistor missing posibly giving errors in reading
the part th1 is only listed in part list as high voltage current limiter no other details apart from part number
looking for anyone with any ideas etc thanks in advance Mick
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Old 7th Feb 2019, 2:46 pm   #2
SiriusHardware
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Default Re: sinclair meter dm235

I see what you mean, the otherwise decent service manual is critically short on detail about that part.

From my reading of the circuit description it looks as though that may only be involved in the resistance measurement circuit and it seems that if excess voltage - enough to overcome the threshold voltage of D8/D9 - causes unacceptably high current to flow through TH1 / D8/D9 then TH1 heats up and goes high resistance to limit the current.

This being the case, you should be able to replace it with a modern self-resetting 'semiconductor fuse' of suitable value. You could try just linking it out to see if you then get reasonably accurate resistance readings but if that works then obviously don't leave it that way, fit one of the above mentioned semiconductor fuses.

If you still have problems with TH1 linked out, look for damage to the resistors on the little 'resistor pack' PCB.
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Old 7th Feb 2019, 5:38 pm   #3
mickm3for
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Default Re: Sinclair meter DM235.

Hi linking this causes major errors in voltage and resistance with nothing in circuit (oc) the error is less ie 12v dc reads 9.8v etc and 1k reads 600 ohms
thinking about a 4m7 pot to see if i can get it to read corectly this poss would give the cold resistance ?
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Old 7th Feb 2019, 10:15 pm   #4
SiriusHardware
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Default Re: Sinclair meter DM235.

Yes, that sounds like a good idea. I would think the resistance would be at the lower end of that scale but you can always try smaller value pots to make the adjustment easier once you find out the approximate resistance required for the right readings

I'm wondering if there was already another fault when that thermistor was removed - it's difficult to see why anyone would have removed it, other than to use it to replace that part in a similar instrument with fewer problems. Hopefully I'm wrong about that too.

If you're lucky someone who has one might drive by and be kind enough to measure the 'cold' resistance of the thermistor in theirs. I have quite a few Sinclair / Thandar instruments but not one of these, unfortunately.
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Old 13th Feb 2019, 5:48 pm   #5
TonyDuell
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Default Re: Sinclair meter DM235.

Since nobody else has done it, and I had a spare half hour in the workshop today...

I took my DM235 apart, desoldered TH1 amd measured the resistance. It's around 1.5k at room temperature. A bit higher (say 1.6k) immediately after desoldering (as it was a bit hot), falling to perhaps 1.47k in the cold workshop.

I'd try a 1.5k resistor and see if your meter will calibrate.
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Old 13th Feb 2019, 6:20 pm   #6
mickm3for
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Default Re: Sinclair meter DM235.

Thanks for the info Mick
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