Restoring a 1947 Philco 7001 Vacuum Tube Volt Meter (VTVM)

Find

Walking toward the parking lot at the end of the 2003 Rochester NY Hamfest, I found this VTVM on the ground in a free giveaway pile. I didn't "need" an antique VTVM, but this one visually stood out from the usual junk items: it was only a bit grimy and dusty, not rusty. It was missing 3 of 12 front panel screws, but it had all the knobs and controls, as well as the probe and ground cables. The gleaming chrome engraved front panel and meter had nary a scratch. I told the guy monitoring the pile I would restore it, and it was mine.

Inspection

Once home, I looked inside before trying to power it up on the variac. The power cord (apparently original) was intact, but cracked stiffly as I uncoiled it. The inside was dusty, and flakes from of the mouldering cotton insulation on the power transformer secondary wires fell away with the slightest touch. All 4 of the tubes were present, and three of them were loctals (locking octals). I observed no signs of burning or other distress inside, but I concluded that the mains cord and power transformer leads should be fixed before applying power. A quick search of the internet yielded no schematics or operating manual. While it wasn't safely usable as-is, the VTVM's engraved front panel alone justified the space it would occupy on my mad scientist lab shelf until I felt competent to tackle its issues. I put the cover back on, cleaned the exterior, and put it on the shelf.

Resurrection

Eight years later I had gained the experience and patience I thought I would need to restore the antique VTVM to safe operation. This time, an internet search for "Philco 7001" brought me to this Philco Phorum article. Chuck Schwank, who curates The Philco Repair Bench, has schematics for almost every piece of antique Philco gear, and will send you a well-made copy of the operation manual for $7. Several days later Chuck provided me with my own copy and I was ready to begin. The document included the current Philco test equipment price list. The 7001 VTVM was the highest priced item in the list, at $104.50 1947 dollars!

Priorities

My goal for this restoration was to obtain a working VTVM. The best restoration would preserve all the functional original parts, and use cosmetically accurate substitutes for anything that needed to be replaced. While I believe the effort to do this is justified for rare or sought-after articles, I didn't feel this unit qualified for that level of attention. My personal priorities were to restore safe operation, to simplify future maintenance, and to make no mechanical alterations (e.g. no holes drilled or structural parts filed) that would complicate a future "perfect" restoration by someone with more patience, more skill, and less sanity than I have.

AC mains power

The instrument left the factory with a two-wire power cord, non-polarized plug, no mains fuse, and "death caps" between both legs of the AC line and ground. I believe it is best to use a modern 3-wire power cord, to fuse the hot lead before the power switch, and to remove the "death caps" on the AC line. I also prefer to be able to replace the power cord without soldering. To this end, I snipped the power cord and death caps at the terminal strip where these met the power transformer primary and power switch leads. I installed a fuse clip in the space vacated by the death caps, attached by a simple bracket made from scrap steel to an existing structural screw. Inside the case, I terminated the hot and neutral legs of the AC line with a two position molex connector, with its mating with connector on the mains cord. I installed a rubber grommet in the existing line cord entry hole, fed the new 3 wire cord through the grommet, and secured it to an existing screw for the carry handle with a nylon cable clamp . I soldered a ring terminal to the mains cord safety ground wire and secured it to one of the other carry handle screws. I could have used a modern Heyco-style strain relief, but that would have required enlarging the existing line cord entry hole. The rubber grommet looks more "period", and the internal nylon clamp provides the required anti-yank resistance.

Power transformer leads

I snipped all the power transformer leads at the terminal strip and rectifier tube pins. Removing the power transformer required disassembling nearly everything attached to the front panel, as the nuts securing it to the mechanical structure were inaccessible. This was more than I had intended to do, but provided me with the opportunity to do more thorough internal cleaning down rthe line.

My original plan was to sleeve all the deteriorated transformer filament and high voltage secondary lead insulation with modern flexible fiberglass "spaghetti" obtained from Radio Daze. However, even the large #11 size would not fit over the ragged breaks in the cotton insulation. I decided to disassemble the transformer end bells, to remove the existing insulation from the secondary wires all the way back to the transformer core, and to sleeve them with #20 size fiberglass spaghetti. This worked out well for the HV leads, but halfway down one of the filament leads, the stranded wire itself broke off, leaving it too short to use. I finished removing the filament lead insulation, sleeved the segment between the core and break with fiberglass, and solder spliced a new pair of filament leads using modern stranded transformer lead wire. I sheathed the spliced area with the #11 fiberglass secured with heatshrink tubing. The primary leads and secondary center tap lead were insulated with PVC that was still in good condition, so I did not have to intervene there. The original and repaired leads emerged from the reassembled transformer end bells in good condition. A full cosmetic restoration might have used modern lacquered cotton-covered wire in the original colors, but I was content to use modern materials. The resistances of all transformer windings were in near perfect agreement with the values shown on the schematic.

In addition to replacing the transformer leads, I also replaced the shielded cable between the terminal strip connecting the AC mains leads to the power switch. It also showed ragged cotton insulation and flying bits of shield braid. I didn't have an appropriate piece of shielded control cable handy for this repair, so I made my own shielded cable. I encased #20 stranded teflon wire with braided shield, covered the braid with the #11 fiberglass, and secured the ends of the fiberglass with heatshrink tube. It was more labor than it needed to be, but it looks at least as nice (though not historically accurate) as the original. Philco's use of shielded wire here showed some care in manufacture. The general quality of soldering and wiring layout is typical for TV's or radios of the time, rather than the higher standards of Hewlett Packard or Tektronix instruments that appeared later, but I didn't observe anything really offensive.

Several views of the VTVM before restoration

Filter capacitor

The power supply filter capacitor was a cardboard-sheathed single section 20 uF 450V twist lock FP type. While there were no signs of leaking electrolyte, I decided to replace it with a modern 22 uF 450 volt radial lead electrolytic attached to a new terminal strip fastened to an existing structural screw near the location of the original capacitor. Alternatively, I could have performed a more cosmetic restoration by attempting to reform the original capacitor, or replacing the contents of the original can with a modern cap. Since I preserved the original capacitor, I still have this option. However, I was more interested in determining whether the instrument would function well before I invested too many hours on appearance.

Internal cleaning

As a side effect of having to detach the entire structure of the instrument from the front panel in order to remove the power transformer, I obtained much better access to the two bakelite terminal boards containing all the internal adjustment pots, wiring, and components that were not attached to tube pins and selector switch terminals. I cleaned the boards with numerous cotton swabs moistened with drug-store 91% isopropyl alcohol (IPA), taking care not to soak the cotton-insulated wire and custom-wound precision resistors on the boards. I hoped that the benefit of removing conductive dust from the boards outweighed the risk of damaging the wiring or components with the IPA.

I treated the front panel zero adjust and range pots, as well as the two four-deck selector switches with Caig Deoxit.

I carefully removed all old solder and bits of wire left on the terminals and tube pins from which I had previously snipped the wires. This is one of my least favorite restoration tasks, as it's a pain to manipulate soldering iron, solder wick, solder vacuum, picks, and tweezers in the cramped confines of point-to-point wiring. It's one reason why I'd rather scratch build new equipment on turret boards than restore old point-to-point wired gear.

I worried that if the instrument did not work at the end of this modest restoration project, I might feel compelled to tear it apart and rebuild it to my semi-mil-spec standards. I imagined the excruciating tedium of desoldering and cleaning every terminal on the selector switches, and the worry vanished.

Things left alone

Other standard steps when restoring antique electronics include checking carbon composition resistors for drift, and replacing all paper capacitors. Philco used good quality resistors in this unit, as all of the carbon comps were still well within tolerance. The manual emphasized the quality of the new (for that era) ceramic encased resistors used for all critical circuit elements. They were all spot-on. However, several tubular capacitors were tucked into relatively inaccessible locations, and I wasn't really sure what would make the best modern replacements. I hoped that Philco paid as much attention to the capacitors as they did to the resistors, so I left them alone until after functional tests and calibration. Also all the tubular caps are in low voltage dividing or compensating positions, so I reasoned that they might be less prone to DC leakage.

I chose not worry about the tubes (7Y4 recifier, 2 x 7B5 bridge amps, and 6SN7 bridge) before testing the instrument. None of them are operated anywhere close to a limit, so apart from mechanical shock or a disasterous electrical stress, they should last forever in VTVM service.

The meter movement was made by Simpson, another sign that Philco chose quality parts for this unit. The meter rectifier was a stack of four tiny copper oxide diodes. I hoped it was still in good shape.

The outside of the case is finished in a hammertone dark turquoise enamel, with one signifcant chip missing from the paint, and plenty of smaller ones. It would be tempting to strip and refinish it back to factory, but not justifiable for this level of restoration. Once of the four felt-covered bottom feet was missing, and replaced with a washer and a #10 screw by the previous custodian. There are also 3 missing front panel screws, #6 x 1/2" slotted round head. Modern galvanized steel screws look too shiny. Stainless looks good with the chrome front panel, but I think that I will just scavenge vintage replacements from some non-restorable discard. These things can wait for my next outbreak of boredom, or a more fanatical restorer.

Pictures of the VTVM after restoration

Philco
7001 VTVM side view after restoration

Philco
7001 VTVM tubes and filter cap after
restoration

Power on

I checked all the wiring from the mains cord though the PT primary winding with my DVM, then brought the instrument up slowly on my 2 amp variac while the VTVM lay uncased on the bench. No magic smoke escaped, no sparks flew, no smells emerged, and all the tube filaments glowed. The B+ showed 137 VDC (135 on schematic), and the meter movement fluctuated a bit, but did not crash violently into either stop. The filaments read 5.3 VAC (5.7 VAC on the schematic), but I know my DVM is off significantly on low AC voltages. The purple-jeweled pilot light indicated readiness to proceed.

Basic function checks

Following the operating manual instructions, I tested resistors and capacitors in each of the ranges, and tested the +DC voltage on the 3V range with an alkaline battery. Good to better than 10%. It would need a run though the calibration procedure to do better, but I was satisfied that the instrument was pretty close to functionally restored. I spent another evening playing with the calibration procedure, but I will need a better reference instrument than my cheapie DVM to do a proper job of it. Ahhh, the days ahead....

Conclusion

I'll call this restoration complete after one more pass though the calibration process with better reference sources and an HP bench DVM. I'll leave that for a future winter evening's recreation. With properly calibrated AC ranges, it will be much more useful than either of my cheapie DVM's. I re-learned how to read analog scales quickly enough while performing my casual calibration. This VTVM can take its place in with my other working vintage instruments, two Hewlett Packard 201CD signal generators and a Tektronix 556 dual beam oscilloscope. I also learned something about the theory of balanced bridges that are at the heart of VTVM's, LCR bridges, and so forth.

One of the things I learned while researching capacitor reforming was that most, if not all cheapie DVM's and panel meters use the same analog chip that provides only 1 megohm input impedance, regardless of what the manual might say or imply. See this article at tubesound.com for all kinds of useful information about this. So the 15 megohm input impedance of this VTVM could actually be useful when I need to debug high impedance grid circuits (I use the 10 M ohm probe on my oscilloscope for these tasks). I'm tempted to buy or build a capacitor reforming rig also capable of measuring ESR and leakage at rated voltage. However, I don't really enjoy repair or restoration work as much as I do scratch building, so I'd just as soon not go there.

Completing this "simple" project was also a way for me to gauge my level of interest in restoring a Heath 10-12 oscilloscope like this one, also a hamfest giveaway. It works, so I already know that it's not completely dead (that means it's partially alive!). Replace the bad caps and drifted resistors and it's likely to be as good as new. Which isn't very good, as far as modern oscilloscopes are concerned. Restoring it would be another stupid labor of love, aside from the fact that I would learn more restoration skills that I don't really need, because I work too slowly to ever make money doing this for hire. Another useless thing I could do with this scope is install one of these in it, transforming it into a digital clock that consumes a 100W or so. But I digress...

Note that I didn't necessarily perform each restoration task in the order presented on this page. For example, I finished installing the mains cord last. I skipped around whenever I became stuck figuring out how to proceed.

Next project: restore my mad scientist lab so at least this stuff stays off my dining room table.