Uher CR240 Troubleshooting Best Practice

[ms660]

The 8 multiplier isn't set in stone, but usually between 5 and 8 covers a lot of bulbs and also valve heaters, remember that the filament is non ohmic.

Lawrence.

[Uncle Bulgaria]

That's really handy, Lawrence. I was trying to check the resistance cold and hot, and the voltage drop. That rule of thumb is a little more useful!

Could my BD241 be blown? (T601).

DMM Ohms: Negative terminal on B: B-E and B-C are 'OL'. Positive terminal on B: B-E = 3.2M, B-C = 2.1M.

With it in circuit I have 4.1V across the 2200µF capacitors on the output of board 600, and if I leave it turned on for more than a few seconds, IC2 starts smoking.

[ms660]

Can't comment on the DMM transistor ohms readings, I've never used one for testing transistors.

Since the last report about the pre-amp stages it seems that something drastic has happened for whatever reason, I would think how you want to proceed given what's happened.

The important thing to remember about transistors/IC's etc is that they don't like it up 'em, their survival rate (unlike valves) is slim.

Lawrence.

[Uncle Bulgaria]

I'm not convinced the DMM is all it should be, as I've put the BD241 back in after the junctions were OK on my AVO 4.

I was looking at that as it's the first sizeable component in the PSU path and instrumental in getting an output, but now with the PSU board unsoldered from the motherboard I'm getting 9.5V.

I'm not going to be defeated having got so far, I just hoped to be putting it back together now.

On with the diagnosis with my new-found skills.

[ms660]

It looks like the regulator might have survived, disconnect the feed to the smoking IC2 and take it from there, making sure those bulbs you got aren't in circuit and that T10 hasn't succumbed.

Lawrence.

[Uncle Bulgaria]

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.

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.

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 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.

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?

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

I have a spare motherboard, so I can get another TBA 820P from it, and another T10 if necessary.

[ms660]

In bold:

Originally Posted by Uncle Bulgaria

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.

[Uncle Bulgaria]

Originally Posted by ms660

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

I forgot to mention that pin 1 shows a slightly lower voltage than pin 14 on IC1, so the 100 ohm resistor is doing its job.

Originally Posted by ms660

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.

I don't follow this. With S1002 open, the only connection between 1013 and 607 is the 2.2k R81. This is a new resistor, checked before putting in. With the mains out, I get 2.2k between 1013-607. With the mains in but the unit off, I get 0R. With the mains in and the unit on, I get 2.2k.

Originally Posted by ms660

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.

The bulbs remain OOC. I suppose the reason they're incandescent is because white LEDs had not been invented. As the bulbs don't bias anything and just go to ground, replacing them with white LEDs and suitable current limiting resistors (or single resistor, as they all light simultaneously) would be easier? Finding a low-current 3V bulb appears impossible as the circuit specifies 30mA.

Originally Posted by ms660

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.

The Zener 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.

Very sensible and clearly explained. I was groping at the avalanche breakdown voltage explanation in my understanding, thinking that if it rose to 9.5V, the Zener would drop it down, which having read your link I see it does. So it's a simple form of regulation for that base of T10.

With T10 replaced (there are a number of BC548Bs on the spare board, so I can afford to lose another), Z131 plugged in, unit 'off' at volume control/on switch:

1013: 5.6V
1014: 900mV
607: 9.5V


So ~2mA passing the 2.2K resistor, the Zener doing its job. 900mV on 1010, the jumping off point for the panel lamps (OOC).

However, when switched on (but S1002 off):

1010: 9.1V
1013: 9.5V
1014: 9.5V

This would match the situation before replacing T10, when the lamps were lit with S1002 off.

EDIT: Zener is good tested for resistance both ways on the AVO...

[ms660]

Originally Posted by ms660:

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.

"I don't follow this. With S1002 open, the only connection between 1013 and 607 is the 2.2k R81. This is a new resistor, checked before putting in. With the mains out, I get 2.2k between 1013-607. With the mains in but the unit off, I get 0R. With the mains in and the unit on, I get 2.2k."

S1002 switched on means contacts 5&6 of S1002 make.

Zero ohms to me means something approaching 0 ohms, eg: a short circuit/perfect or near perfect continuity.

Lawrence.

[ms660]

Connect a resistor of say 82 or 100 ohms between the emitter of T10 and ground making sure the bulbs are not in circuit, then do T10 voltage checks when mains powered and switched on with the bulb switch on (contacts 5&6 making) and then with the bulb switch off (contacts 4&5 making) then do the same on battery power, that will tell you what's what in that department.

Lawrence.

[Uncle Bulgaria]

Yes - I got that your point was about S1002 being on. I was confused by varying measurements with the switch remaining off. I couldn't see how the mains power pack being inserted could affect the link between 1013 and 607 as the switch remains open.

I'm investigating T10. Latest news is that with the switch closed, C37 replaced with a 100n film type I had lying around after thinking the original was suspect, and the mains connected, it does not turn on and R617 is burning hot.

Interestingly, in the circuit diagram C37 (IC1/IC2 pin decoupling) is 1µF, and on the 1:1 board layouts it's 0.1µF. Similarly, R617 is 1R on the diagram and 2.7R on the layout. In the flesh, it's 0.1R! (Brown, black, silver, silver?)

[Uncle Bulgaria]

Finally found it. A short on the motherboard. Fixed and the PSU is now working.

The lights on boards 800 and 1000 light continuously the moment the unit is switched on, regardless of whether S1002 is depressed or not.

T10 has 5.6V on the base, 4.9V on the emitter (with resistor) and 9.5V on the collector.

The switch pins: 1014 has 9.5V and 1013 has 7.6V with S1002 open.

I don't have enough C cells to test on batteries.

[Uncle Bulgaria]

This morning's reading has been on BJT switches, though the accepted circuits I've found have the emitter grounded.

I'm guessing that the biasing on T10 is meaning the switch is not turning off properly. The current on the base means it is not biased into cut-off when the switch is open.

Does this mean the S1002 acts to allow current to flow between the 9.5V rail on the collector, through the 2.2K R79 which should turn the switch on?

In which case, should I be increasing the values of R81 and R79 to decrease base current, even though the Zener diode is holding the base to 5.6V?

[Uncle Bulgaria]

With all other lamps off T10 out of circuit, the current through the 82R resistor is 60mA whether S1002 is depressed or not.

What am I missing?

[ms660]

Originally Posted by Uncle Bulgaria

This morning's reading has been on BJT switches, though the accepted circuits I've found have the emitter grounded.

I'm guessing that the biasing on T10 is meaning the switch is not turning off properly. The current on the base means it is not biased into cut-off when the switch is open.

Does this mean the S1002 acts to allow current to flow between the 9.5V rail on the collector, through the 2.2K R79 which should turn the switch on?

In which case, should I be increasing the values of R81 and R79 to decrease base current, even though the Zener diode is holding the base to 5.6V?

Battery operation....Bias is supplied from the battery to turn on T10 via switch contacts 14&15 on the 12/24 volt input socket assembly>>>the on/off switch contacts>>>K4 switch contacts (on record) or contacts 37&38 of S1 (on playback)>>>contacts 5&6 of S1002>>>R79.

Mains operation....Bias is supplied from the regulator T606>>>contacts IV&VI (mains unit aka Z131 inserted)>>>the on/off switch contacts>>>K4 switch contacts (on record) or contacts 37&38 of S1 (on playback)>>>contacts 5&6 of S1002>>>R79.....

….Also from the regulator T606>>>R81>>>R79 when contacts 5&6 of S1002 break....So far as I can make out.

I wouldn't alter those resistors.

EDIT: Post crossed.

Lawrence.

[Uncle Bulgaria]

Originally Posted by ms660

Battery operation....Bias is supplied from the battery to turn on T10 via switch contacts 14&15 on the 12/24 volt input socket assembly>>>the on/off switch contacts>>>K4 switch contacts (on record) or contacts 37&38 of S1 (on playback)>>>contacts 5&6 of S1002>>>R79.

Mains operation....Bias is supplied from the regulator T606>>>contacts IV&VI (mains unit aka Z131 inserted)>>>the on/off switch contacts>>>K4 switch contacts (on record) or contacts 37&38 of S1 (on playback)>>>contacts 5&6 of S1002>>>R79.....

….Also from the regulator T606>>>R81>>>R79 when contacts 5&6 of S1002 break....So far as I can make out.

You have a great gift (and patience) for making out blurry diagrams, Lawrence! That tallies with my tracing with the meter, but put rather more clearly than the scribbles in my book!

However, that makes it weirder that there isn't anything wrong with the circuit - I've just done a Circuit Lab model of the switch and T10 and it agrees the lamps are lit for both positions of the switch.

1014 will always be at 9.5V as it comes off the same rail whether batteries or mains are connected. 1013 is 7.6V having only the 2.2k resistor between it and the 1014 jumping off point on board 600.

In this state, with the switch off, there's current flow across T10 (because there's always a voltage at the base, so the NPN 'switch' is on?) There should of course be zero until S1002 is closed to turn the lights on.

[ms660]

When mains powered, T10 is turned on regardless of the setting of S1002, as I already explained.

Lawrence.

[Uncle Bulgaria]

Am I just being very dim?

Are the lamps supposed to be on all the time on mains?

There is always an output on BD241 with the mains connected >>>> R81, R79, T10 ON

There is also an output on 608, then to 1014 via K4 as you've noted.

However, on battery I/II is open, and 15/16 is open, so there is no output on R81, so T10 is OFF. If the battery pack Z215 is inserted, there is a connected back to BD241 for recharging.

Yes, I'm just being dim.



Thanks for setting me straight, Lawrence.

I see RS sells LEDs with a 30mA current rating. Can I drop them straight in, or will I still need a current setting resistor?

[ms660]

Can't really advise on substituting bulbs with LEDs, they were pretty dim back in my day and I expect the voltage has changed on them since as well as the current.

I'm sure others will be able to advise.

Lawrence.

[ms660]

Originally Posted by ms660

Mains operation....Bias is supplied from the regulator T606>>>contacts IV&VI (mains unit aka Z131 inserted)>>>the on/off switch contacts>>>K4 switch contacts (on record) or contacts 37&38 of S1 (on playback)>>>contacts 5&6 of S1002>>>R79.....

Correction to the above (blurry schematic)…..contacts IV&I/II

Contacts I to VII info:

Contacts I&II make as do contacts IV&I/II when mains unit Z131 is fitted.

Contact III is battery -ve and Z131 (mains unit) -ve.

Contact V is battery +ve.

Contacts IV&VI make when battery unit (Z215) is fitted.

Contact VII is Z131 (mains unit) +ve.

So far as my eyes can tell...

Lawrence.