Heathkit V-7AUK VTVM HT help needed

[pentoad]

I swopped out the original 16uf filter cap for a 22uf and changed the rectifier (which I thought was big diode but turned out to be a selenium rectifier) for a rectifier diode ( it's half wave )
The voltage is higher than it should be by a few volts ( should be between 50v and 70v on the positive side but has increased to 74v and between 60v and 85v on the negative side but has increased 93v.It has calibration presets but will the meter accuracy be affected by the difference in voltage?
Do I need to add a dropper resistor of about 100ohms or so to bring the HT down?

[Chris55000]

Hi!

A resistor of 1k ohms 5W in series with the rectifier should be suitable!

The wattage rating I suggest is quite a bit higher than is really needed for the h.t. current being drawn, but a 5W ceramic–cased or one of the aluminium–clad type that will bolt to the chassis will run relatively cool in operation – there is quite a larger component of a.c. ripple–current flowing into it for the reservoir capacitor, to allow for, as well as the d.c. component of anode–current of the two valve sections!

The double–triode balanced valve–voltmeter circuit is relatively insensitive to h.t. and l.t. (heater) voltage variations, and this why it is so popular, finding use in virtually all commercial manufactured instruments, however you still don't want more h.t. voltage than Heathkit specified if you can avoid it, as keeping the h.t. to a minimum in these low–current valved circuits will minimise heat dissipation and warm–up drift.

Chris Williams

[ColinTheAmpMan1]

Originally Posted by pentoad

I swopped out the original 16uf filter cap for a 22uf and changed the rectifier (which I thought was big diode but turned out to be a selenium rectifier) for a rectifier diode ( it's half wave )
The voltage is higher than it should be by a few volts ( should be between 50v and 70v on the positive side but has increased to 74v and between 60v and 85v on the negative side but has increased 93v.It has calibration presets but will the meter accuracy be affected by the difference in voltage?
Do I need to add a dropper resistor of about 100ohms or so to bring the HT down?

It is quite normal that a selenium rectifier will give a lower voltage than a silicon rectifier. Have a look at the current being drawn and use Ohm's law to decide what resistance is needed to drop the volts. Don't forget to work out the wattage needed for the resistor (that will be something more than the simple arithmetic tells you, you need some leeway).
Was the selenium rectifier faulty? When I rebuilt my V-7AU I decided that the rectifier was fine, so I left it in circuit. You could also just go with what you have now got and see if the VTVM will calibrate properly. It might just be ok.
Colin.

[pentoad]

I tried 1.5kohms 3watt and it knocked it down a few volts to 69v+ and -89v so maybe ok for the time being.

Are those test sockets standard 4mm (banana) as I need to make up some leads?

I'm assuming I can calibrate it with a 1.5v battery (in the absense of the calibration battery) and 110v ac adapter as I have an accurate dmm.

[pentoad]

I believe the calibration battery is 1.55vdc which I should able to replicate with an adjustable power supply.

[ColinTheAmpMan1]

Originally Posted by pentoad

I believe the calibration battery is 1.55vdc which I should able to replicate with an adjustable power supply.

Heath were being a bit ambitious when they stated that they had supplied a "calibrated flashlight cell" with the kit. This cell was no more than an everyday zinc chloride cell, which will give 1.56V when relatively fresh. If you can tell the difference between 1.56V and 1.55V you'll be doing well. The important bit is to use the 1.5V range and approach the red dot just beyond the far end of the scale from a lower point.

As it happens, the sockets on my V-7AU are 4mm and I simply bought a couple of standard test-leads from Maplin. Some banana-plugs may be a bit long for the sockets, but this may not bother you too much. Don't forget that the the DC socket is a 1/4" jack and needs to have a 1M0 resistor fixed so that it is close to the tip of the prod. The lead itself is screened and the screen earthed at the VTVM end. I am assuming that you have a manual for this, but in case you don't, I can let you have a pdf copy, if you wish, just ask.

Colin.

[pentoad]

I have an alkaline cell but its 1.609v not 1.5

[pentoad]

Originally Posted by ColinTheAmpMan1

Heath were being a bit ambitious when they stated that they had supplied a "calibrated flashlight cell" with the kit. This cell was no more than an everyday zinc chloride cell, which will give 1.56V when relatively fresh. If you can tell the difference between 1.56V and 1.55V you'll be doing well. The important bit is to use the 1.5V range and approach the red dot just beyond the far end of the scale from a lower point.

As it happens, the sockets on my V-7AU are 4mm and I simply bought a couple of standard test-leads from Maplin. Some banana-plugs may be a bit long for the sockets, but this may not bother you too much. Don't forget that the the DC socket is a 1/4" jack and needs to have a 1M0 resistor fixed so that it is close to the tip of the prod. The lead itself is screened and the screen earthed at the VTVM end. I am assuming that you have a manual for this, but in case you don't, I can let you have a pdf copy, if you wish, just ask.

Colin.

I have the user manual but not the construction/build instructions that would have come with the kit

[ColinTheAmpMan1]

Originally Posted by pentoad

I have the user manual but not the construction/build instructions that would have come with the kit

I didn't know that there was anything other than the Construction Manual. Perhaps yours was originally purchased ready-built? Anyway, here is the copy that I have.
Incidentally, some of these manuals are tricky to find. This one was produced for the US market and mentions 105-125 VAC and "tubes".
The issue over the battery is based on the chemistry - a zinc chloride battery gives 1.55V, but an alkaline gives a slightly higher no-load voltage, as you have found. If you have problems finding a fresh zinc chloride battery, try Poundland (who sell packs of Kodak ones at - surprise - £1 each). Maybe Poundland isn't open yet, but you could try Wilko or somewhere similar. A supermarket might be a possibility, too. Just make sure that the chemistry is zinc chloride. Batteries in local convenience stores may be a bit old and rather expensive.
Colin.

[pentoad]

Mine had a large wax capacitor 0.05uf 600vac in the middle which had melted. I replaced it with a 600vac 0.068uf, however I noticed in the manual it states 0.01uf 1600v (must be DC) but mine is rated at 1250dc.

[ColinTheAmpMan1]

Originally Posted by pentoad

Mine had a large wax capacitor 0.05uf 600vac in the middle which had melted. I replaced it with a 600vac 0.068uf, however I noticed in the manual it states 0.01uf 1600v (must be DC) but mine is rated at 1250dc.

That capacitor has a high working voltage so that the meter can be used up to 1500 VAC with no worries. Mine looks a bit sweaty, but is otherwise OK. Given the fact that the instrument can get a bit warm inside, it is unsurprising that the capacitor looks a bit sweaty, but the VTVM calibrates just fine and I don't think the cap is leaky (I have just checked it and it is 100M at 1000 VDC). I realise that Canada is a long way away, but Dave Cantelon of Just Radios has 0.01uF 1600 VDC tubular axial metallised poly film capacitors:

https://www.justradios.com/cart.html

I have bought stuff from Dave and he is good. No connection other than a satisfied customer.

Something I should have added is the strong suggestion to change the mains cable for a proper three-conductor type, so that you have an earthed test instrument. While you are doing that, fit a proper cable restraint, too. The original Heath arrangement is truly lethal (just a knot in the cable that goes through a rubber grommet). Unless, of course, you have a liking for getting mains shocks off your test-gear.

Colin.

[pentoad]

I understand that earthing the chassis can cause some problems.
Have you seen this article?
http://www.heathkit.nu/heathkit_nu_V-7A.html
BTW he puts the 1 Meg resistor inside the case but shouldn't it be nearest to the tip of the probe?

[ColinTheAmpMan1]

Originally Posted by pentoad

I understand that earthing the chassis can cause some problems.
Have you seen this article?
http://www.heathkit.nu/heathkit_nu_V-7A.html
BTW he puts the 1 Meg resistor inside the case but shouldn't it be nearest to the tip of the probe?

I hadn't seem the article; thanks for bringing it to my attention. The modifications that he has made are not too clear on the circuit diagrams, but it looks like something to do with the earthing to mains and the moving of the DC probe resistor inside the case. He appears to show the chassis (and therefore casing, which is fastened to it) as a single horizontal line and there is no indication of a three-conductor plug. Other than being able to make floating measurements, I can't see much purpose in this, but I can see the potential danger in not having a safety earth . I wouldn't do it unless it was really important to me to do floating measurements and even then, I think there might be other ways.

Regarding the DC probe, yes it is normal practice for the probe resistor to be close to the probe tip. I can only think that this is still concerned with floating measurements, but I might be wrong. Consider this; if you are making measurements on a 1500 VDC point using the 1500 V scale and a normal, unscreened test-lead and without the probe resistor at the probe tip, then the voltage under test is running right from the probe tip to the instrument input with only the insulation of the lead to stop you getting a shock. I don't think this is either good or sensible modern practice, but that might just be me wanting to live a few more years. I tend to take stuff like this with a pinch of salt when there is no rationale given as to why the changes are made. That is not to say that some modifications shouldn't be made, such as the horrid original Heath cable restraint (a knot indeed!).

If this chap had wanted to make floating measurements with a high-impedance input meter, why didn't he just buy a decent modern DMM?

Colin.

[pentoad]

"While you are doing that, fit a proper cable restraint, too"
What type do you favour for a 3 core cable? I have a couple of heathkit cases to do.

[ColinTheAmpMan1]

Originally Posted by pentoad

"While you are doing that, fit a proper cable restraint, too"
What type do you favour for a 3 core cable? I have a couple of heathkit cases to do.

For my AV-3U, I used one of the black plastic things that are pretty ubiquitous. I can't find the receipt, so I can't tell where I sourced them from, but I think it may have been an eBay seller. You could try this one:

https://www.ebay.co.uk/itm/Strain-Re...cAAOSwLVZV4HqR

The hole might need to be enlarged and and it should ideally be a round hole with two flats, one on each side. If the cable is at all large, they can sometimes need some persuasion, but you can probably rest reassured that if it requires that much effort to assemble, it will need just as much to let go of the cable.

I should probably admit that I haven't yet used one of these on my V-7AU yet, but I did make an improvement to the original. Perhaps that should be one of my next jobs.

Stop Press: I have just sorted my V-7AU with one of these restraints and it was a pig of a job, but much better and safer. The hole needed altering and enlarging and re-assembling the battery-holder with the earth-connection isn't easy.

Colin.