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Sencore PR57 Repair
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Old 14th October 2019, 12:57 AM   #1
pwdiya12 is offline pwdiya12  United States
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Default Sencore PR57 Repair

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?


Peter
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Old 14th October 2019, 02:55 AM   #2
PRR is offline PRR  United States
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When you "check the resistance of the meter", does the needle move? Does it move when you jiggle the case?
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Old 14th October 2019, 06:47 PM   #3
pwdiya12 is offline pwdiya12  United States
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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!
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Old 14th October 2019, 06:55 PM   #4
pwdiya12 is offline pwdiya12  United States
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There are 4 1N695 Germanium diodes- can I replace these with 1n4001 or similar-or is there something special about germanium diodes?
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Old 14th October 2019, 06:59 PM   #5
Enzo is offline Enzo  United States
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Ge diodes have about half the voltage drop of silicon.

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

http://bama.edebris.com/download/sen...ore%20pr57.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.

Last edited by Enzo; 14th October 2019 at 07:05 PM.
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Old 14th October 2019, 07:42 PM   #6
pwdiya12 is offline pwdiya12  United States
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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


Peter
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Old 14th October 2019, 09:23 PM   #7
PRR is offline PRR  United States
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C105 is suspicious. Take it out: it won't read on-calibration, but if it reads "some" now, then C105 has gone short.
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Old 14th October 2019, 10:29 PM   #8
Enzo is offline Enzo  United States
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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.
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Old 14th October 2019, 10:39 PM   #9
pwdiya12 is offline pwdiya12  United States
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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
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Old 14th October 2019, 10:45 PM   #10
pwdiya12 is offline pwdiya12  United States
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Thanks Enzo- I think it may be a combination of dead meter driver circuit and potentially faulty switches-The other diodes and resistors all measure correct I will adopt your plan!
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