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Old 27th Jul 2019, 5:41 pm   #47
ms660
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Join Date: Apr 2011
Location: Cornwall, UK.
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Default Re: Uher CR240 Troubleshooting Best Practice

In bold:

Quote:
Originally Posted by Uncle Bulgaria View Post
You're reassuring me that I'm managing to consider this along the right lines! I realised pin 14 (that is the feed to IC2? I can't find a datasheet of the TBA 820P - it is in a larger package than the 820Ms I have found) was directly connected to the bulbs so there could be some link if excess current was at play, though why IC1 is unaffected I don't know.

Yes, pin 14 of IC2 is connected to B+ as is pin 1 via the 100 ohm resistor.

I had looked at T10, which measured all right, but I was getting a resistance of zero between pin 1013, where the cable leaves the instrument board and the 'illuminate meters' switch, and pin 607 where some supply leaves the PSU. As there's a 2.2k resistor between those points with the switch open, I was sceptical.

Pin 1013 and pin 607 should have zero ohms (or near as damn it) between them when S1002 is switched to on and the mains connector is plugged into the machine so far as I can make out.

T10 seemed to have intact junctions, but I replaced the 2.2k from 607 as it was measuring a variable resistance.

Now with the lamps OOC, I have 9.5V on 1010 with the switch open. This suggests T10 is actually fried, as surely the switch had to actuate T10 to allow the lamps to light.

I would replace T10 and also the bulbs with the correct type or something of similar ratings, I wouldn't just replace the transistor and leave those replacement bulbs you got in circuit as they would draw too much current through T10.

I guess (though there's a fold in the circuit diagram scan) that 607 joins up with the 9.5V rail, but that means there's 9.5V on the base of T10 whether the switch is open or closed.

There should be 5.6 volts on the base WRT ground irrespective of S1002's position when on mains and ditto voltage when on battery with S1002 switched to on.

I've never understood Zener diodes, but would I be correct in thinking that it's supposed to divert that to ground until the switch is closed? In which case why have the extra supply from board 600 anyway?

The Zenner diode pegs T10's base voltage at 5.6 volts WRT ground, a typical base-emitter junction voltage drop for a silicon transistor would be 0.6 volts, that would make the emitter approx. 5 volts WRT ground, that would be a sensible supply voltage for the series connected 3 volt bulbs.

And if the bulbs are supposed to be two 3V, why have 9.5V on their supply?

See above

I have a spare motherboard, so I can get another TBA 820P from it, and another T10 if necessary.
Don't forget that a Zenner diode only conducts when connected in reverse when it's Zenner voltage has been reached:

https://www.electronics-tutorials.ws/diode/diode_7.html

Lawrence.
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