Anyway, wanted to also start up a thread on my recent VTVM jag (see my previous signal generator post) so I can lay down a placeholder and get set for the Ugly Weekender project I and Alan are going to embark upon.
As with the other stuff I do, I pick up one thing at a swap meet (or somewhere) and then determine I need something else to help me with the original item so I run off on that tangent. In this case, it was the need to correctly measure high DC voltages accurately to help me align a Hammarlund receiver I am going to restore and the fact that ordinary 20 KOhm-per-volt VOMs would load down the circuit. Actually, I don't know why I couldn't use a DMM..oh wait! Yes, I do. DMMs only have that bullshit series of tick marks on the bottom of the LCD display and that just would not do, would it?
The real reason, of course, is that it was a perfect opportunity to acquire more neat-looking electronic junk that I mostly will never use. But it's my money, my time, and my pleasure.
Well, of course, the W6DQ syndrome (Remember? "Never buy one thing when you can get two at twice the price") set in and I managed to score THREE of these gadgets in various states of repair. Here they are in various states of undress:
San Burning Doo-doo.
Funny thing about all the VTVMs you will find on eBay and at swap meets: they have pretty similar circuits. It is basically a pair of triodes -- one with a "standard voltage" and the other with a voltage derived from a voltage divider and slammed into the grid. With a constant, identical plate voltage on the two triodes, the differing grid voltages will cause differing currents and, hence, an imbalance in a meter circuit thus causing a deflection in the meter. But that's as far into the weeds as I want to go for VTVM theory. The internet abounds with pages devoted to the theory of these instruments from Ryder's tour de force on the subject to a marvelous compendium on Heathkit VTVMs -- principally the V-7A -- by Bob Eckweiler, AF6C, of the Orange County (California) Amateur Radio Club. Bob has a regular series he runs over on the OCARC site on Heathkits that are superbly written and beautifully laid out. They are must-visit treasures from a ham who clearly knows and loves his subject.
|Various VTVMs with Heaths among them|
I kinda fell for the V-7A because that was extant when I was a kid in the late fifties and, although my dad bought and built the venerable Eico 232, I remember my grade school buddy, Jeff Smith, had a V-7A and I always lusted after the gray box and gray wrinkle paint finish on the case. I especially loved the knobs being used for the zero controls -- as opposed to the cheesy pots with the plastic slotted shafts on the IM-11s, IM-18s and other later models. Oh sure, you could turn then, but for any zeroing accuracy, you had to use a screwdriver and even then it was difficult.
Jones, I was dead set on getting a working, pristine unit for the bench. So, I was thrilled when I nailed the eBay auction for the unit pictured above -- but crestfallen when it arrived. It was in so-so shape inside. Nothing too grungy but nothing to write home about either.
Anyway, it did not work 100% and the symptoms were variously an inability to zero, the wandering meter needle and a frustrating problem with adjusting the "AC BAL" control. So I joined the Heathkit Yahoo group (a heartfelt recommendation), took it apart, waited to hook up with wise gentlemen like Bob Eckweiler, and went stem to stern on the thing, poking and prodding.
About the best piece of advice I got was from Bob. He told me to plan on scouring the switches and controls with contact cleaner and re-heating every solder connection on the PCB as well as any suspicious looking ones on the switches. He also advised checking out the precision resistors on the voltage divider as well as the other resistors on the ohms divider.
Also consider cleaning any corrosion from the battery cup, checking the resistors on the PCB (most will ohm up to their marked value despite being in circuit), and replacing the power supply cap with a 22 uF electrolytic and the rectifier with a 1N4007 would not be amiss if you suspect ripple.
After I got the meter apart -- I was able to remove the front panel from the PCB and rest of the guts -- I could work more freely and accomplish these tasks. And, what do you know, the little beauty waked right up!
That was the good news. The bad news was that the meter needle seems to have been sprung and prying off the plastic cover and inspecting the innards revealed that the movement was irretrievably bonked. Someone had measured about 500 VDC with the switch on -DC Volts. So, while the unit innards may be copacetic, the problem with the "AC BAL" control and problems consistently zeroing seemed to be because of the sprung meter.
Which brings me to my first rule of thumb: run like the wind from ANY meter that has a droopy or non-zeroed meter. If it is on eBay or you cannot physically lay your hands on it to verify otherwise, if the meter is not zeroed where you cannot lay hands on to verify the condition consider it trashed and move on.
Here is my second rule of thumb: grow a severe case of patience and DO NOT buy a VTVM on eBay. You will eventually find one at a swap meet and you can verify its condition up close and personal. Chances are it is trashed and the guy either knows what he is doing and wants to bend you over or he does not know jack squat and you will get bent over.
Now, if you look closely on eBay over the next two weeks, you will see an auction from me for a Heathkit IM-11 "for parts or not working". That's the meter in this unit swapped out and with really clean parts and switches and controls. IT IS FOR PARTS ONLY. Did you get that? JUST FOR PARTS. The meter does sit at zero now because went inside and moved it to zero. But the little mechanical needle will not do that externally with screwdriver adjustment.
We clear on that?
O.K. That's all for this evening. Gotta grab some rack time. Will be back with more stories on the revitalized V-7A and my spiffy SM-20 and the Knight Kit KG-620 -- the really neat swap meet find