The AM Forum

Hi All:

I picked up a EICO 232 VTVM meter at an Estate Sale.  It is a newer one in that it has one tube (a 6FG7 I think?) and looks clean.

I went ahead and swapped out the one electrolytic in the unit to be safe with a new one same value.

After about 10 minutes of warm up in DC mode the meter just jumps around for no reason, with or without the probe connected.

The zero set and ohms calibration both seem to be jumpy..... suspect dirty or bad pots?

Any ideas on where I need to be looking at troubleshooting.... Tube?  Some other issues?

I have a schematic and manual on the way from vintage manuals.

73--Wally W9BEA

Hi Wally  I do a lot of repair on old Heathkit meters.
 I always clean the pots (DeoxiT D5 works well), replace the tube(s) with a known good tube, replace the power supply caps, put in a new
battery ( I use duracell, the base is raised more than some
brands) and be sure the battery contacts are clean.
When on the low ohm scale, if the needle wonders around
it's a good chance the battery is not making good contact.
 These old VTVMs are still very good meters. Good luck.
 Bare in mind that luck is better than skill. It takes no effort.
Bruce

I have an Eico 232 VTVM and it has 2 tubes, a 6AL5 dual diode and a 12AU7 dual triode. I can see possibly replacing the 6AL5 with silcon diodes but replacing a dual triode with a triode/pentode 6FG7? Are you sure the tube is a 6FG7?

73
Mike W5RKL

Without pulling the manual, I believe it's a 6CG7.

Hi All:

I will check what tube it is when I get home from work tonight.

I know it is a later unit, has a single tube and a white looking diode underneath.

The face is a light blue, and not the silver I know from my signal tracer.

I think it was factory wired.  Looks awfully nice if it was a kit.

I replaced the Electrolytic already.  I ordered a manual from vintage manuals and the lady at the order desk indicated that she would send me the correct one (apparently she knew about the earlier unit with 2 tubes and mine has 1).

I didn't clean the pots yet,  I wasn't sure it was needed.  I am for whatever reason concerned that spraying stuff in there might cause other problems.

I will write back later tonight.

73--Wally W9BEA

Wally also clean the tube socket pins and the pins on the tube.

Hi All:

Hit the tube socket and pots with a little deoxit.  Put some on a Q-tip and wiped the rotary contacts.

I was wrong.  It was a kit.  The tube socket would have been likely riveted to the chassis and the wire dressing/placement is a little different on some wires.  Still good construction though.

The tube is an Eico marked 6FQ7, made in Japan.

There is a fresh battery in the unit, and the contacts are clean.

I also put a 3 wire cord on it.

Not sure I like the probe.  It is cracked and just cheap feeling.  Might make two probes, one with a 1 meg resistor and one without.  Might even swap out the front panel connector with either a BNC or something else.....  Havent decided.  Either way, first I want to ensure that the meter works okay.

NOW, after about 10 minutes of warm up in the DC+, DC- or AC position and no voltage applied to the probes, the meter will start to deflect to 1/3 to 1/2 scale.  Then it backs down with a  little "bump" to the zero knob.

I'll test the tube on the TV7 when it cools off.

73--Wally W9BEA

6CG7 or 6FQ7 is basically the same tube.
https://www.tubedepot.com/t/tubes/preamp-tubes/6cg7-6fq7

Hi All:

I think the issue lies in the adjustment pots.  They should be 2K each and I am measuring 1.2 to 1.4 K.  This alone is not an issue, but the adjustment is skittish as I move the wiper across the resistor.

The tube measures 85 or 90 depending on which triode I am testing on my TV7.  The book calls for a minimum of 50.

Also, I replaced a 4.7 meg resistor that measured 6 megs in and out of the circuit.  It went to one of the grids and the other end went to ground.  I also double checked all the screw to chassis mechanical connections.  Everything good and tight.  I loosened the connection and re tightened them all.

The meters seems to have settled down.... just a bit.   We'll see what happens when I replace the zero pot.

73--Wally W9BEA

Bad pots will drive you crazy.  Just when you think you've clean them, they act up again.  I've completely taken some apart and worked on them, with some better results.  Some pots in old vintage equipment can be hard to replace.  That Eico meter should have standard type pots.  I built one about 50+ years ago but I can't remember what pots were in it.

Fred