Help!!! b&k precision 820 capacitance meter.

[Napalm2002]

Hello Everyone

I have a b&k precision 820 capacitance meter that I love and used all the time until I had to replace the batteries. As I read the warning to beware to install the batteries polarity correct imade a huge mistake. Now only a dot comes on the display. I am a newb however I have a lot of test equipment. Can someone help me with where to look. I checked all the transistors and diode and they show ok. Did I blow the display drivers?


Thanks in advance
mike

[radhaz]

Do you have a schematic?
http://elektrotanya.com/bk_precision_820_sm.pdf/download.html
Make sure you only click this text:
This file is downloadable free of charge: Get Manual

[Napalm2002]

I have a schematic. Just need some Insight thanks.

[retiredcaps]

Can you explain what you did wrong in the first place?  In addition, can you list the things you already checked and what their readings were?

[dacman]

D201 & D202 should have shorted out and provided some protection to the rest of the meter.  If they burned open, then I'd secondly suspect some of the ICs directly connected to the battery, such as IC103 and IC104, which happen to be display ICs.

[Napalm2002]

Tester has no input protection from reversing the battery polarity. I checked all of the diodes and they r ok. I also checked all of the transistors and they r ok. I suspect blown led driver ic's which I suspect are 74 series logic but I am having trouble finding any substitutions. They are as follows RCA 118 307-072, RCA 036 307-061, two RCA 049 307-146. Also 75491pc which I can cross reference and a hctr6010ap that I can cross as well.

[radhaz]

What do the ICs read in ohms VDD to ground?

[Napalm2002]

D201 d202 are good

[Napalm2002]

i cannot check ic vdd to ground bec i cannot identify the ic's bec i cannot cross numbers.

[Napalm2002]

Anyone help I'd ing the RCA ic chips please. All my voltages are good. Power supply is working. Ic chip dead? Do u think that I took them all out?

[Napalm2002]

Ic102 vdd to ground in ohms reads 20k
Ic103.                 14k
Ic 104                  15k
Ic 105 &ic 106.       20k

[Napalm2002]

Anyone have any ideas

[dacman]

There are two ICs that are directly powered by the batteries, and they are IC103 and IC104.  Either one could cause the display to be blank.

There should be 10.3 Vdc on IC102 pin 8.  Check the signals on IC102.  IC102 is isolated with resistors from IC103 and IC104, and it may be OK.

IC103 looks like a CD4511BE.  That’s a CMOS IC, static sensitive.  I’d install it in a golden IC socket.

IC104 looks as if It may be a NOR gate wired as an Inverter.  I don’t know of one with the pinouts of IC104.  One could be cross-wired, but what I would do is install a golden socket, and then install four N-Channel MOSFETs, one for each gate.  (For each digit, the MOSFET would be attached as follows: Gate to signal from IC102, Drain to the Digit, and Source to Common.)

[Napalm2002]

Thanks dacman any clue as to which part no mosfet. and what is a golden socket

[dacman]

What I mean by a golden socket is an IC socket with gold plated contacts, such as P/N: 1-1825094-4 for IC103 (DIP-16) or P/N: 1-1825094-3 for IC104 (DIP-14).

For the N-Channel MOSFET, most should work, such as the BS170.  (A high beta NPN should also work.)

[Napalm2002]

Thank u very much dacman!