Heathkit V-7AU restoration

[m0xte_chris]

Hi all. I obtained this gem from a radio rally at the weekend for a far too reasonable fee. I've been after one for a number of years but have passed on many in not great condition. From the same seller, I also obtained an RF-1U in good condition. There's not much in the way of restoration required by the looks. It's all original, the battery hadn't leaked in it, it was well constructed and came with the instructions. Minus all probes of course.

I intend to use this and the signal generator after restoration rather than end up with a museum piece with no utility. I am planning to replace the selenium rectifier, filter capacitor and main AC coupling capacitor for preventative reasons. Then replace the mains lead with a three core one and earth the chassis. This is a compromise of course which will mean floating DC measurements are not possible. I will then make up some new leads for it and see if I can find a Heath 309 RF probe.

If anyone has any wisdom to impart I would be grateful

[David G4EBT]

Originally Posted by m0xte_chris

Hi all. I obtained this gem from a radio rally at the weekend for a far too reasonable fee. I've been after one for a number of years but have passed on many in not great condition. From the same seller, I also obtained an RF-1U in good condition. If anyone has any wisdom to impart I would be grateful

Can't comment on the V-7AU Chris, but I recently restored an RF1-U on which I wrote a forum thread here, which might be of interest:

https://www.vintage-radio.net/forum/...d.php?t=144716

There isn't a full manual on internet for the RF1-U as far as I know - just an extract, but if you'd like a copy of the full manual, drop me a PM and I'll e-mail it to you.

[m0xte_chris]

Thanks for the info and the offer. I will review your notes when I approach the RF-1U.

I actually have a full original copy of the RF-1U manual with that generator. I will scan it, clean it up and put it on my web site and link it here for the benefit of anyone else who comes across one. Same with the V-7AU. I haven't seen a full decent copy of the manual yet.

[m0xte_chris]

Ok first session complete. I have replaced the selenium rectifier with a 1N4007 and the capacitor with a new one (BC 22uF 160V axial) I had lying around. Measuring the peak across the capacitor ended up at 175V which is a bit high for that capacitor so power was switched off pretty sharpish. With some trial and error I found a 3.3K series resistor reduced this to a reasonable 125V which means the test points at both legs of the capacitor to ground are right in the middle of the acceptable range according to the manual. I used a 1% 0.25W metal film capacitor. This drops only 18V under load which is around 100mW so good enough for the job.

Valves are good and all functions appear to be working after this.

Have ordered a probe kit from KK4HXJ for £4 more than I paid for the meter. Such is life.

[Skywave]

Heathkit V-7AU: I have one on those in my ever-expanding 'round-to-it' tray. One of the reasons why it has been there for such a long time is that its mechanical design is such that trying to figure out how to get it to bits for a re-furb. seems almost impossible! I must confess that I have been tempted to completely re-house its entire innards into something that will make future maintenance somewhat easier!

Al.

[stuarth]

The probes supplied originally for the version of the V7-AU you have were standard 4mm test leads for the Common and the AC/Ohm inputs, and a special lead for DC which plugged into the 1/4 “ jack.

The DC lead was nothing more than a length of screened cable with a 1 Meg resistor between the probe tip and the inner of the cable in the probe body. The screen was earthed at the jack end, and floating at the probe end. The input impedance of the instrument on DC ranges was 11Meg Ohm, 10Meg in the range resistors inside the box, and 1Meg in the probe.

This arrangement is excellent for probing around RF circuits because the 1Meg resistor reduces the effective capacitance at the probe tip, and, coupled with the cable capacitance, gives an RC filter to signals going into the box.

The down side is that you have to use different test leads for different functions, and the DC probe does not take crock clips or other ways of hooking onto test points.

There is a very simple mod for this - just connect a second 1Meg resistor inside the box between the AC/Ohms socket and the DC jack, then the AC/Ohms input also becomes the DC input.

I used mine for many years like this as my default bench multimeter with no problems. It made it a much more useable meter, I could use it with any of my 4mm test lead collection. If I needed to measure in the presence or RF, I could use either the original DC probe to reject RF, or the Heathkit RF probe to measure the RF.

Stuart

[wd40addict]

The 1meg probe tip resistor makes this ideal for measuring all DC valve electrode voltages without electrically disturbing the unit under test.

I was given the larger IM-13u years ago. It sat in the cupboard until I was working on a Volksempfanger, which has particularly high value anode resistors. Using a DMM produced huge bangs and squeaks from the speaker as 3' of aerial (the test lead) was effectively being coupled in.

I suddenly remembered the Heathkit, dug it out and knocked up a 1meg probe. I could now make measurements without problems. The meter has remained on my bench till this day and is my go-to instrument for checking valve DC operating voltages.

[m0xte_chris]

stuarth: thanks for the info. The probe kit I ordered contains all the parts to build a set of "original" style probes so you get a ground clip lead, an AC probe and a DC probe with all the cables, connectors, 1M resistor etc. I did consider adding the 1M resistor inside the case but when I look at my use cases, they are pretty much as it was intended to be used originally so I thought I'd go with the original design even if it is incompatible with the rest of my test gear.

As for the mechanical design, it's not great. It definitely favors assembly over repair. I managed to do the work I have done so far on it simply because the parts were easy to access. I doubt this success will continue when I replace the main wax AC coupling capacitor as that is tangled in the rat's nest inside it. My soldering iron shaft is a bit bulky as it's designed for production soldering so I have to be rather careful I don't melt anything unintentionally as well!

Nice to hear they're generally respected.

[ColinTheAmpMan1]

I have one of these which works just fine, but does require a longish warm-up time. I have made up both DC and AC/Ohms probes easily - the instructions come in the manual. The manual is quite readily available as a pdf, at least the V-7A manual is and the differences are little.

There are two caveats, though:

The switch knobs are made of fairly friable plastic and split easily if the grub-screw is tightened too much. I have tried (unsuccessfully) to try to fix the knobs, but it is probably better to just replace them with something better.

The 6AL5 (EB91, etc.) double diode used can be a bit fragile. Too high a voltage applied can destroy it quickly. I found this out the hard way and bought a job lot of military-grade EB91s for back-up.

Colin.

[m0xte_chris]

Thanks for the tips. It does indeed take a while to warm up. I was pleasantly surprised to find out, once it had got there that it was very stable and even surprisingly the calibration was pretty much spot on at least on DC.

I am going to obtain a backup set of NOS valves for it if some go past for a not too inflated fee.

[ColinTheAmpMan1]

I think you will find that the 12AU7/ECC83 isn't particularly critical. I put a used one in there and it's fine. You should note that the manual/assembly instructions state that the valves should be "aged" by leaving the VTVM switched on for at least 48 hours.
Colin.

[m0xte_chris]

Indeed. Apparently you can still get new ones (I assume made in China) from RS, part number 678-4105. I think i'll hold out for the next radio rally though and see if I can pick up a couple there if someone hasn't pillaged the junk boxes for resale.

[Chris55000]

Hi!

My experience of Valve Voltmeter Designs is that they're not fully documented enough from a theory and constructional point of view in books & magazine articles, nearly all the circuits I've seen are minus component values!

Do be careful of the front meter–cover in Heathkits tho’ – the cover is very fragile and cracks easily, and replacement movements/covers are impossible to come by quickly!

The movements were made by Sangamo I believe but I've tried in vain to find ones the same, in differing sensitivities, etc!

(If anybody knows what range these Sangamo movements came from I can adapt my V7–AU for one of another sensitivity – the originals were 200uA F.S.D.)

Chris Williams

PS!

The H.T. secondary of the mains transformer can get s/c turns, but it can be rewound (speak to Ed!) or try a search for "R Core Tube Pre–amp Transformer – I think you can get them with. 110V & 6.3V secondaries!)

[stuarth]

I think the meters might be Heathkit specials. I’ve seen them marked Pullin and Simpson as well as Sangamo on Heathkits, and I don’t recall seeing that style of meter on anything else.

The Heathkit MM1U (a passive multimeter) uses a 50uA movement, the V7-AU and AV-3U use 200uA movements.

For the V7-AU, with an amplifier before the meter, the precise sensitivity is less critical than for a passive multimeter. You could use a 50uA or 100uA movement, possibly with a shunt resistor if the calibration pot can’t bring it back into calibration. You might get away with less sensitive meters if the amplifier can be persuaded to provide the extra current.

Incidentally, I don’t think I’d add a mains earth to the V7AU, it was originally designed for use without an earth, and earthed test gear can give problems including hum loops.

Stuart

[m0xte_chris]

Mostly concerned with accidentally making the chassis live with the common lead or the live breaking off and making the chassis live. While unlikely, safety first.

Thanks for the tip about the front cover. One of the reasons I bought this one is it’s in good condition which is a rarity. I’ve been working on top of a “borrowed” tea towel just in case I crack it.

[ColinTheAmpMan1]

Mine has an earth connection on the mains lead. The earth wire goes to a solder-lug which is fastened to chassis by the screw fastening the battery-retaining spring. This isn't shown in the service manual/assembly instructions, but then again, this document is for a US instrument and they were less rigorous about earthing. There is a sticker on the VTVM which suggests that repairs, reconditioning and calibration were done in 1975. Perhaps the earthing was added then?
My meter cover is undamaged as well. The meter bears the words "MEASURING INSTRUMENTS (PULLIN) LTD" across the top, as well as the usual "Heathkit" and "Daystrom Ltd".
Colin.

[m0xte_chris]

That is exactly how I earthed mine. It came with figure of eight lead and no earth.. I used an M3 solder tag and opened up the hole a tiny bit with a deburring tool. This is attached to the battery holder. Earth lead is soldered (should be crimped really but I don’t own a crimp tool) and then heatshrink applied as strain relief.

Same pullin text on mine.

Probe kit dispatched from US and replacement capacitor arrived from Farnell. Surprisingly the original 1.6kv capacitor is about 3/4” thick and a good 2 1/2” long. New one about the size of an AAA and 3kv!

[wd40addict]

Colin, what colour code is used in the mains lead? This will give a clue when the earthing was done. The Pullin meters seem to be fitted to the earliest units with the grey front panels. What colour scheme is your front panel?

[m0xte_chris]

Quick update. I replaced the wax capacitor. The infernal thing had melted pretty badly so it took a while to clean out the goop that was left. They certainly make a mess!

Capacitor was made by Hunts. Does anyone know what sort of wax these are?

[ColinTheAmpMan1]

Originally Posted by wd40addict

Colin, what colour code is used in the mains lead? This will give a clue when the earthing was done. The Pullin meters seem to be fitted to the earliest units with the grey front panels. What colour scheme is your front panel?

The mains-lead colour-code is the up-to-date brown/blue/green-yellow.
The metal case is a light-grey crackle-finish and the front-panel is a rather darker grey and has a smooth finish. Having just looked again at m0xte_chris's, mine could be its twin. Looking inside, I can't see anything that might date it particularly, except maybe for the silk-screening on the PCB which reads "MODEL V7-A/UK 85 9F 102/K". The mains transformer has a "HINCHLEY" label and "592 3U" ink-stamped on it, but it may, of course, have been replaced at some time. Internally, it shows evidence of having been a kit originally, with perfectly adequate, but non-professional-looking soldering and lead-dress. The strain-relief on the mains-lead looks a bit unfortunate, with what looks like several layers of self-amalgamating tape preventing the mains lead from pulling through the rubber grommet, rather than a better solution such as a knot, or even better, a proper strain-relief grommet.

I must apologise for giving the duff information regarding the availability of manuals for this bit of gear. I had forgotten that Heathkit/Daystrom had come down heavy on all the web-sites that had copies of their manuals and required them to remove all of their Heathkit manuals a while ago. I also thought that I had copies on one of my hard-drives, but this isn't so. I have another hard-drive which was replaced some time ago when my computer was upgraded. Maybe all of the Heathkit manuals are on there. I will take a peek sometime soon and if I do have a copy of the manual (I must have at some time, because I printed it out), I will post it here. I may just have pdfs of some other Heathkit manuals, too. I seem to remember something about them all being archived together......

Colin.