Sencore PR57 Repair

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I have a Sencore PR57- which provides variable AC with voltage/current and leakage metering. The voltage from the isolated terminal works and changes with the AC voltage control, however the meter does not register any thing. I have checked the resistance of the meter (1900ohms), but how else can I check that the meter is OK?

thanks- disconnected the wires from the circuit board to the meter, and when I put the multimeter on the meter connections, the needle moves nearly full scale- so I guess that means the meter is good!

dumb question- but is this a good way to test all analog meters?

So now I have to diagnose the meter drive circuit (seems to be 5 diodes and 2 15v back to back zeners)

thanks again PRR!
Ge diodes have about half the voltage drop of silicon.

The schematic and board layout drawings are at the end of this manual: pr57.pdf

And may I suggest: FIND OUT what is wrong rather than throwing parts at it. You speak of replacing Ge diodes, but are any of them open or shorted? I'd be more expecting a dirty or failed switch connection, or even a bad trimmer pot.
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Thanks Enzo- I plan on testing all the diodes on the board as the initial step, then each of the other components and replacing the 10uf cap as a matter of course. If everything tests good, then it is the switch bank, else it will be the dead component ( and those 1N695 seems to me to be the most likely suspects).

I will report back after the tests.

thanks again

How about this as a strategy: Turn it on and run the voltage up so you would expect a good voltage reading. Then use a volt meter to trace voltage from the switches over to the meter. Noting that the AC becomes DC after the diodes.

You MAY have a defective part, but never assume all problems are parts needing replacement. Connections and wiring fail as much as parts do.
C105 was OK- but replaced it anyway. Diodes D102,103,105 and 106 all test bad (non connection on peak transistor tester and 0v on Fluke diode test and open connection on fluke ohms). so do I need to find some 1N695 or similar germanium diodes, or is there a common silicon diode I can use (given the difference in voltage drop)? thanks again
Good new- I found some real NOS ITT 1n695- installed them, and now the meter works, there is some small discrepancies with my Fluke DTM, but I will calibrate the unit to address this.

First let me thank you both do much for your help, I always learn valuable information from you all- so thanks again

One final query, are schokkty diodes a real alternative to germanium diodes, or will it always be circuit dependent?
Not all meters will move at all when measured with a multimeter, but most will. As an example, I've got some GE panel meters that are "600A AC" meters, and they look practically like a short. I don't think they're a moving coil meter (note that I am not an analog meter expert), and I suspect they are designed to be used with an extremely low impedance shunt.

But most panel meters, sure.


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> I don't think they're a moving coil meter

If the scale is non-linear, moving-iron is a good bet. Cheap (a spring and a bob of iron near the wire), and works great at the higher current which may be what matters most.

Current meters over 20A are often intended (as you say) to be used with an external shunt or current transformer. I have "100A" meters in the cellar which are 1A (digital) plus a 100:1 current transformer. 100A DC meters often use a milli-Ohm resistor shunt.

You can "test" your 600A AC meters with some #12 wire and tap it on your car battery terminals. TAP, not hold! First the battery can deliver way over 600A for a second, and also because an "AC" meter may not (shouldn't, but might) read steady DC. A tap makes an impulse which should jerk an AC-only meter. If the wire gets stuck (it might weld), #12 wire will vaporize before the car battery is deeply harmed.
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