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Defective 7106 multimeter

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j G Veilleux

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Hello,
A fiend of mine has a BK2860 multimeter that didn't work after a bad measurement with probably high ac voltage (200-300vac). He doesn't really know what happened nor what scale he was using. I changed a few components U6 and ZD-2 but there's still something wrong.

As the DCV scales implies only resistors voltage divider and ICL7106 itself, it's certainly the best scale to start looking for problems. I include the 2860 schematic I got from BK. It seems they don't have a service manual. I identified the 20V network in red. I'll also be giving you some readings with 200mv which implies only 4 X 500 Kohm resistors.

With DCV scales, input shorted, readings are 4,82V for 20V scale and 100.2 mV for 200mV scale.

As seen in some threads, ground problems seem to give such strange readings with ICL7106, I ensured every ground was really ground. So problems is probably not there. Few components reading may give you more hints :
- - - - - - - - - - - - Inputs shorted - - - Inputs shorted
- - - - - - - - - - - - 20 V scale - - - - - - 200 mv scale
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
BK Reading - - - - - 4,82 V - - - - - - - 100,2 mv
Pin 36 Ref Hi - - - - 102 mv - - - - - - - 102 mv (should be 100)
Pin 35 Ref Lo - - - - 0 mv - - - - - - - - 0 mv
Pin 31 In Hi - - - - - 44,6 mv- - - - - - 87 mv (should be 0)
Pin 30 In LO - - - - - 0 mv - - - - - - - 0 mv

Pin 31 voltage is the strangest reading. Where is it coming from ? Could the voltage difference for these 2 scales be explained with the divider's resistors ?
Between R6-R7 - - 26,1 mv - - - - - - - 70,7 mv
JA2 - - - - - - - - - - 5,8 mv- - - - - - - 54,1 mv

As voltage is going down with resistors going away from pin 31, it looks like voltage comes from 7106 itself. Voltage could also comes from JA3, but it connects to U3 pin 3 and 2, 2 input pins that's perfectly working. Bad ground ? Few more readings for grounds :
Q2 - C 0,036 V
Q2 - BK 0,599 V
Q1 - E 2,943 V
Q1 - B 2,265 V
Q1 - C 2,896 V

So, 7106 power and ground look good.

I did more testings in reading few DC voltages.
20 V scale : BK reads 12,5 volts as 16,98 V (+4,48 volts gap)
: BK reads -12,5 volts as -7,34 V (+5,16 volts gap)
200 mv scale : BK reads 71 mvolts as 169,2 mV (+98,2 mvolts gap)
: BK reads -71 volts as 31,5 mV (+102,5 mvolts gap)

So voltage gap average (4,82 V and 100,35 mV) is almost right on shorted input readings (4,82V and 100,2).
Could a 7106 defective capacitor creates this ? I did some test with an octopus circuit and none is shorted neither open, but one could be bad. As I hope not changing everyone, which one would you suspect? Could the 7106 be defective ?

Thanks in advance for your comments. If you'd like specific reading, don't hesitate, I'll do them.

J Guy

BK2860 A.jpg
 
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