I'm in the process of getting this Eico 221 VTVM working again.
Schematic:
http://www.stevenjohnson.com/manuals/mfiles/eico221.pdf

I replaced the electrolytics and paper caps but left the maroon caps that look like the orange drops.

The problem; I can't get it to zero out on step B of the calibration procedure. The meter tacks to the left. The internal battery is new. Here are some voltages

B+ coming out of pin 8 of the 6X 5 is 296v

6SN7 pin voltages
2=109v
3=4.5v
4=.8v
5=109v
6=4.7
heater voltages ok

6H6 voltages
4=.7v
5=neg.5v
heater voltages ok

Any ideas?
Thanks,
Ben

Try a new 6SN7, and test all the resistors around that tube.

I swapped the 6SN7 with a known good one. Still the same. I'll be checking resistors etc asap.

If Johnnysan's suggestion does not fix it, start by grounding both 6SN7 grids and try to zero. If it will not, check the 1K cathode resistors. That leaves nothing else in the circuit except the 6SN7.

If it does zero with both grids grounded, then ground pin 4 only and try to zero. If it does, then some voltage is sneaking in through the switching circuits. That may be a miswire or dirt/corrosion on the switches. The plates are tied together so no imbalance can occur there.

Don

Question: should the grid, pin 4, be a slight negative voltage? Also, on the schematic there is no pin number for the other side of the grid. Is it also pin 4?
Ben

Pin four should be close to ground. There may be some slight negative voltage at same potential as pin one. Pin one is the other grid.

Pin 1 is grounded through a 5 meg so it may have a half volt or so, contact potential. Same for pin 4 since it is grounded through the divider network, about 11 meg including the series grid resistor. If the instrument works correctly, the range switch position will not affect the meter zero.

Right now the range selector makes the needle swing pretty wildly. I have used DeOxit on all contacts but haven't checked wiring against the schematic. From the looks of it this one was a kit built.

Those maroon colored caps--should they be replaced? They look like the newer orange drops.
Ben

Forget the caps for now.

If the range switch affects the meter, something is miswired.

Ground 6SN7 pin 4, set to zero and go through the ranges and functions to confirm.

Then check all the wiring. Since it was a kit, that is most likely.

Don

Thanks for the info. I won't be able to try it until after work today. The DC/Ohms test probe shows it should have a 15M resistor in series with it. I've noticed a lot of test probes on test gear have this or recommend a cap in line with the lead. Why aren't they just built in? If I use the meter without this will it hurt it? Just some noob questions.

I've been staring at the schematic, which I thought was complicated at first. It's actually pretty simple compared to a superhet schematic. Now I really need to study the math. I have a huge mental block with that, but it kills me not knowing more about circuits and understanding the math is important.
Ben

The resistor is in the probe to provide isolation from the circuit under test. The circuit will not see probe capacity and load a sensitive circuit. The probe is shielded to prevent acting as an antenna for stray AC and RF voltages.

Thanks Don! The suprising thing is that I understand what you said. Four years of banging my head against the wall with this subject might be paying off...

Grounding pin 4 makes it so I can adjust to zero on all ranges and settings. I will check and replace resistors. Thanks for the help!
Ben

I do not believe replacing parts alone will solve the problem. There is some voltage on pin 4 that should not be there. It is most likely coming from a miswire, not a defective part. Carefully check the wiring according to the schematic. Also look for solder globs and shorts on the switches and the 6SN7 socket.

You can try working back from 6SN7, pin 4, by grounding the resistor divider at various points until you get to the test probe. The meter will return to zero until you get one step past the trouble spot.

The kit was put together pretty clean --no globs of solder anywhere--so I don't think its that. Could you explain in more simple terms how to trace back from pin 4 to the source of the stray voltage? I'm not sure where the resistor divider is. Also, since the tube sockets are phenolic, could there be enough of a carbon track to be causing this?
Ben

I was playing with grounding the grid of the 6SN7 last night and found something interesting. When grounded, the needle zeros fine through all ranges and selections. I removed the jumper while still powered up and the needle still zeroed. I repeated to make sure what I was seeing was right and the results were the same with the exception that when ohms was selected the needle went erratic again.

I'm still trying to understand how to find the offending component.
Ben

Ben-
To zero the ohms scale, you need a good battery, inserted with the correct polarity, and need to have your test leads shorted together. If you have a single test lead with a switch, the switch needs to be set to AC/Ohms, not to DC.
Set the zero with the leads shorted, then set the OHMS Adjust for full scale with the leads apart.

--Chuck

Ben-

I recently went over my 221 and was having similar problems. Turned out to be a dirty range and function switches. I had to clean them multiple times with DeOxit and scrub the contacts Q-tips. Spraying alone wasn't enough.

Matt

It worked -- at least got some good results.

I'm still having zeroing problems on the first two ranges and it still won't test or zero ohms, even with a fresh battery etc, but I can zero through most settings now. I also used Deoxit in all of the pots. Any Ideas why a few of the settings are not zeroing?

The rotary switch on the left was cleaned with Deoxit and Q-tips. The right hand one is what it looks like before cleaning.