Newly acquired Simpson series 3 VOM AC function is inop.

[craft99]

Hello Folks, New member here. So, as the title says, I recently acquired a Vintage Simpson 260 series 3, in which all functions are working except  AC, I'm wondering if anyone else has run into the same problem with this model?--- Simpson manufactured a whole slew of variations of the 260 VOM meter in several different series. Mine happens to be a series 3.
But I digress, I'm going on day 3 troubleshooting . The laundry list is long of every thing I've done so far, so I wish not to bore anyone by going into an endless rant right out the gate so to speak. Just a small snippet of what I've done so far --- Checked all resistance values within the AC measurement circuits, all are either close or right on spec. Replaced 2 geranium  diodes in the modified rectifier bridge circuit, D1 and D2 in the schematic ,also, resistors R29 and R30. These 4 components make up the full wave bridge ( though not a conventional one ). After the current finishes running through all these  series resistances and trimmer pots what's left at the  rectifier bridge when on the 250VAC scale, is something like .648mV. From research, should be around 1.2mV. Maybe I can try changing out all the series multiplier resistors ? IDK

[w2aew]

Have you verified that the forward voltage of your replacement diodes is similar to the 1N100 type diodes that Simpson used?  There are a lot of "fake" semiconductors out there, and someone might have sold you ordinary silicon diodes instead of proper germanium diodes...

[Stray Electron]

Have you verified that the forward voltage of your replacement diodes is similar to the 1N100 type diodes that Simpson used?  There are a lot of "fake" semiconductors out there, and someone might have sold you ordinary silicon diodes instead of proper germanium diodes...

   X10!   Most of the parts on the market today are FAKE.  I wouldn't use any parts in a repair until I had tested them first to be sure that they worked and they had the proper specs.

    You should probably go over to simpson260.com and see what information they have.

     I have a model 261 series 3. It's missing the case but seems to be complete but I don't know if it works. I keep it around because it was rebadged for Miller. 

[craft99]

Thanks w2aew. the diodes were replaced by these.---1N34A ITT GERMANIUM GOLD BONDED-- found them on ebay and kind of pricey too. Been knocking my self out trying to figure this meter out. Wished  I had another voltage source to check the lower AC ranges.  I'm thinking maybe the 1meg multiplier resistor that handles the 250V range checks good but nevertheless defective? I'm getting approximately 207mV right after the germanium diodes which supposedly is too high. AI says 120mV would be correct on the 250VAC scale with 120V out of the wall receptacle. Forgot to add , those 207mV are not showing up at the internal meter terminals

[w2aew]

Thanks w2aew. the diodes were replaced by these.---1N34A ITT GERMANIUM GOLD BONDED-- found them on ebay and kind of pricey too. Been knocking my self out trying to figure this meter out. Wished  I had another voltage source to check the lower AC ranges.  I'm thinking maybe the 1meg multiplier resistor that handles the 250V range checks good but nevertheless defective? I'm getting approximately 207mV right after the germanium diodes which supposedly is too high. AI says 120mV would be correct on the 250VAC scale with 120V out of the wall receptacle. Forgot to add , those 207mV are not showing up at the internal meter terminals

If there isn't continuity to the meter, then that sounds like the bigger problem.  If indeed the connection to meter movement is open somewhere, that would explain the higher-than-expected reading after the diodes because the meter resistance (plus the resistor in series with it) is not very big.  Have you checked continuity through the wafer switch?

[Mark_T]

Forgot to add , those 207mV are not showing up at the internal meter terminals.

Thanks god for that because at F.S. deflection should be 90 mV max.
I don't understand how you did the measurement, so look at how i would do...
Measure V1,V2 and V3. I expect V3 = 0.734 V
(When you measuring resistors disconnect the movement or short it!)
Hope it might help.

[craft99]

Forgot to add , those 207mV are not showing up at the internal meter terminals.

Thanks god for that because at F.S. deflection should be 90 mV max.
I don't understand how you did the measurement, so look at how i would do...
Measure V1,V2 and V3. I expect V3 = 0.734 V
(When you measuring resistors disconnect the movement or short it!)
Hope it might help.

in regards to the wafer switches from what I can see, are  layered in 4 sections physically and in the schematic as well.  Problem is, I cant gain  access to to all of them to confirm their making good contact. The outer "rivets" is what I'll call them, are no problem.
 Is there a workaround  to access  those underside contacts?  To the extent I can see if those  contacts which ride against the those bus bars and are actually making good contact or maybe even damaged to some extent.
    I  will run through those testing points you mentioned  and report back

[craft99]

For got to mention, while tracing the circuits I was not getting the 4K resistance reading  as marked on the housing of R31. Only when I removed it from the board did I get the correct 4K reading. But as w2aew mentioned, I was not aware of the need to take the internal meter out of the circuit while checking resistances . After reassembly for the 3rd time the meter still seemed to work  correctly on the other modes. 

[craft99]

Hey w2aew. I got it!! there was a tiny trace which comes off R29 and 30 that looked suspect ,so I cleaned it up, and reflowed the solder a bit at that spot. I was in the middle of your tests, got distracted for a bit ( I'm 70+ LOL) Then plugged it into the socket. Surprise, Surprise, it worked!. A huge Thankyou to you w2aew and all. Seriously appreciate your help and replies.. Regards