Thursday, October 31, 2013

Vitamin "V"..

..get it? Thought not.

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.

So there!

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:

Left-to-right is a Knight KG-620, picked up at the TRW Swap Meet in Los Angeles towards the end of September 2013, a Heath-Schlumberger picked up at the Chino Hills Swap Meet in the middle of september, and a Heathkit V-7A I got off of eBay from a lady in 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
If you read Bob's piece or do a little research, you will find that is surprisingly standard among kits of the mid-fifties on. The V-series meters (from V-1 through V-7) settled on a circuit using a 12AU7 dual triode and a 6AL5 dual diode for the AC measurements with a pair of voltage dividers for volts and ohms. (The ohms divider used a 1.5-volt D cell battery for power.) After the V-7A, Heathkit went into the IM series with the IM-10, IM-11, IM17, IM-18 and thence to the IM-52xx series with the newer paint jobs. But they all had pretty much the very same circuit.

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.

Another thing I liked about the V-7A was fact that it had a printed circuit board inside and far less point-to-point wiring than the previous models. Never mind that the PCB was absolute shit (hey, it was the fifties) and the traces lift off when heated up above anything more than room temperature.

That said, when I started my VTVM 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


Tuesday, October 29, 2013

Signals, Set, 1..2..3..

I am skipping over the Ugly Weekender to post an effort last weekend that yielded some interesting results. Be advised that I am not abandoning it -- far from that -- I am only time-slicing while I gather parts. Also, I have a couple of other irons in the fire like putting the VTVMs to bed (a post will be up on that adventure shortly) and doing some fix-up on the NesCafe CW filter kit from NEQRP. (Ditto.)

When family, work, and the USAF Auxiliary allows it, I frivolously waste time on ham radio and electronics projects and, when doing so, I am in one of two modes: gathering projects at swap meets or building, fixing, or operating the treasures I haul in.

But, alas, my hunter-gatherer skills far outstrip my builder-operator skills and I have built up quite a backlog of these projects. To shame myself, I order my "acquisitions" into a FIFO list so that there is some throughput at least and the scenery in the workshop garage changes over time. Anyway, somewhere back in the past six months, I went on a jag and started accumulating Heathkit IG-102 signal generators. You know, these beasts on the left there. (For that matter, I always go on these jags. As Dennis Kidder, W6DQ famously says, "Why buy one of something when you can have two at twice the price?" The VTVMs were the result of such a bender.) I amassed three of these generators, two in good shape and one having no power cord and generally requiring more attention. Well, last weekend, as they were gathering dust on the shelf, I decided it was time to act and started in on refurbishing them.

Now, for those looking for a low-end signal generator at a reasonable cost, the IG-102 is a good one. It's about $35-75 (or more) on EvilBay or $20-50 (or more) at a local electronics swap meet. (I prefer the latter venue because of the cheaper prices and the fact that you can eyeball the product INSIDE and OUT.) Also, the fact that the entire manual can be found as a .pdf (think is a definite plus as the build "how-to" is definitely more informative than the troubleshooting chart in the back of the manual.

As for refurbishing the units, I rescued the two and breathed life into 'em in a far shorter time than I thought it would take. Initially, both did not produce any RF out whatsoever. So, after poring over the schematic for a while, I poked and prodded from where I thought the RF should originate and was delighted to find a signal. I proceeded to follow it to the RF out connector but I lost a little bit somewhere between the plate of the pentode side of the 6AN8 and the wiper of the fine attenuator pot. The rest of the RF fled on the input side of the course attenuator switch.

Judicious application of contact cleaner at various points on that path materialized the RF at the output connector and we were in business. One of the signal generators had a "lumpy" fine attenuator pot so I replaced it with a 10 KOhm one I had lying around. (The original was a 3 KOhm audio taper jobbie.) The 10K worked fine; it was a little tight but, with practice, I was able to set my signal level right enough.

The only problem was that I only had one set of tubes so I ordered another 12AT7 and 6AN8 off of eBay. Just to let you know, they ain't cheap and, surprisingly, the 6AN8 is a little more dear than it's cohort. If you want to rebuild one of these machines, budget another $20 for the tubes if yours are shot or missing. Also, here's a re-cap of the recommended debug steps:

(1) Don't hesitate to clean the switch contacts on the attenuator pot and the band switch (where the coils are).

(2) Trace the obvious signal path from the grid of the pentode half of the 6AN8 through the fine attenuator pot through the coarse attenuator switch. If you are using a scope, mind the DC on the plates of the tubes and such. There are convenient DC blocking caps where you can pick up the RF if it is available. One prime spot would be at pin 8 of the 6AN8 as I recall.

(3) You might find the fine attenuator pot "lumpy" -- it can be replaced by a 3 KOhm pot or a 5 KOhm one or even 10 KOhm pot -- which I used on my "keeper" unit.

(4) Replace those useless mic connector plugs on the chassis with BNCs. It's very easy to do and makes your unit more compatible with coax links you might have laying around.

The remaining unit of the trio lacks an AC power cord and will probably end up becoming "transistorized". I actually was going to do this with all three, except the first two work so great when refurbished, I did not want to go to the extra effort. (If it ain't broken, don't fix it.)

In any event, to convert this unit, I'll have to strip out the AC power supply and the tubes and replace the latter with a pair of MPF-102s each. When done, it will be powered by a 9V (or 12V) battery. Notes on the project (it came from the May 1978 73 Magazine) advise soldering the MPF-102s to the actual tube socket lugs underneath the chassis. But, I am thinking about getting some tube-pin-diameter copper wire and stick those into the appropriate tube socket pins and then wire the JFETs to the copper wire. That way, if I blow up a JFET, I can pull it and insert another.

Looking at the pin-outs and how one wires the drain, gate, and source, I surmise that the two JFETs will face each other and the pin-outs will be:

FET #1 - 1=Drain, 2=Gate, 3=Source
FET #2 - 6=Drain, 7=Gate, 8=Source

FET #1 - 1=Drain, 2=Gate, 3=Source
FET #2 - 6=Drain, 8=Gate, 9=Source

..but DOUBLE CHECK; I could be wrong!

I'll get back on this last project and post some pictures.

Thursday, October 17, 2013

Whooped with a ugly stick..

Jeff Tranter, VE3ICH, produces some of the best You Tube ham radio videos going. I absolutely love his style: polished, matter of fact, and everything is neat ~~ as in tidy, cleaned up, organized. Anyway, Jeff built Roger and Wes Hayward's "Ugly Weekender" transceiver as he shows you above. Of course, nothing Jeff does is ugly but you get the idea.

So I was doing a little research and found that there were PCBs -- by Far Circuits, of course -- and they were reasonable at about $13 for the set plus $5.50 shipping. So I tumbled for them and will see how this turns out.

Perusing the two Hayward articles reveals that the components required are surprisingly commonplace: a handful of 2N3904s, an MPF 102, and SBL-1, and an LM-386 audio amp. The rest are resistors and capacitors and easily-wound toroid coils and transformers. The circuits themselves are fairly prosaic (said the electronics whiz kid) with the MPF 102 VFO and a direct conversion receiver. However, there are some interesting features like semi-break-in, a frequency spotting mechanism, and sidetone.

Anyway, so there it is. Probably my next project, the first phase of which will be to roam through my "well-stacked" junk box (as Jeff Tranter describes it) and stuffing the parts into the boards -- or hoarding them and building it up in stages.

"Hmmmmmm, that broad sure has a well-stacked junk box!"
So there it is, too many notes!