Heathkit IM-4180 FM Deviation Meter Question

[CaptDon]

I have a pair of these units which I very carefully restored. Both seem to have what I think is a problem. When first looking at a function that has a significant upscale reading and then switching to the 7.5KHz or 2KHz mode it takes a very long time for the meter to settle down to the correct reading. The meter will peg off the top end of the scale and then settle back taking up to 30 seconds to settle back to zero if no deviation is present. They both act the same way. I have measured and substituted nearly everything in the metering circuit and nothing seems to help. I have even taken C143 (10uf) out of circuit since it seemed there was no exact fast discharge path except when the tune position shunts it with 10K to ground. Anyone else seeing this strange behavior on their unit?
Thanks in advance!!

[CaptDon]

For anyone who cares, I noticed the problem was aggravated on humid days or humid conditions (Marine Radio testing on the boats). After some careful investigation it appears that the original corrosion damage has left leakage paths under the push button switches in an area that would be hard to clean without removing the switch assembly (not really an option due to the fragile state of the traces). In the '2KHz' position the circuit nearly acts like a very poor 'sample and hold' that slowly returns to zero. The worst behaviour is when you select 'Battery Voltage' first with a big upscale swing and then select '2KHz' mode. With no modulation of the radio under test the meter will forcefully peg on the high side and slowly drift to mid scale then very very slowly return to near zero. I did bridge a 1 meg resistor across the 15 meg resistor. It helps and only diminishes the true correct deviation reading on the 2KHz position about 100Hz at full scale. The effect is less at lower deviation. It also ONLY effects the 2KHz position. It does not effect any other scale or reading. I have two of these units, both died from severe battery corrosion and both now have the same weird behaviour most pronounced on the 2KHz test position. Anyone else with similar experience? These things are notorious for corrosion death!!

[mag_therm]

Do you mean Heathkit IM-4180 FM Deviation Meter ?
India Mike Hyphen

[CaptDon]

Good catch!!! Sorry! I edited both posts for anyone searching in the future for help. I flushed out under the switches and also flushed the switches with hot soapy water then rinsed clean with hot clear water. After about 15 minutes with a blow dryer to make sure all moisture was gone I sprayed a tiny amount of contact cleaner into the switch bank. This seems to have greatly helped the unit to work correctly. When selecting the 'battery' position it actually charges a couple of capacitors in the metering circuit to battery voltage and also near the 8vreg rail. These capacitors normally only see around 1 volt or less during deviation measurement. When going from 'battery' test mode to the 2KHz mode which is the most sensitive metering mode the meter really slams against the upper end. It seems reasonable to go from battery test mode to step through 75KHz 20KHz 7.5KHz and finally to 2KHz if you need to measure PLL injection on the 2KHz scale (Which has an audio low pass filter, not a good scale for voice). Doing this sequence will discharge the excess voltage on the metering capacitors.