Heathkit V-7AUK VTVM 12AU7 substitute
 

I'm finding it difficult to AC balance, It's different every time it's switched.
It could be the 12au7,
I'm wondering if I could substitute an ecc81 or ecc83 which are similar but not the same as a don't have an ecc82 (although I have one I think maybe one but as the numbers have rubbed off in places it's difficult to tell)

Re: Heathkit V-7AUK VTVM 12au7 substitute
 

Hi!

The A.C. Balance control is used to tap off a small positive voltage from the potential–divider across the lower portion of the +h.t. supply, and the slider of this control connects his 5 X 22M resistors to the anode of the second diode section of the 6AL5/EB91.

The purpose of this voltage is to "back off" the small negative potential developed across the two separate valve diode electrodes, by "contact–potential" due to the effect of the electron stream in the 6AL5 valve, that would otherwise potentially cause an inaccurate reading..

Make sure these five 22M resistors are all OK, and there is no surface leakage across the pins of the 6AL5 due to dirt and grot, etc. If your DVM won't read up to 22M, it's best to replace the five resistors with new.

The positive voltage divider resistors across the h.t. supply should also be checked in case any have gone high in value or open–circuit, an ordinary DVM can be used to check these.

If all the above test OK, then the 12AU7 may be running into grid–current, in which case it will need replacement – a 12AT7 or ECC81 should be tried as a replacement first, as the higher–gain 12AX7/ECC83 may cause wandering–zero problems!

Chris Williams

Re: Heathkit V-7AUK VTVM 12au7 substitute
 

I have a couple of tested used 12AU7'sI could send you for postage from the U.S.
Frank

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

More commonly known as ECC82 in Europe, although Brimar used the American designation.

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

Pretty sure I have a couple of 12AU7s in the collection. I'll have a look tomorrow.

Cheers,

Frank

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

Yes, Mullard ECC82, tested good and boxed. Fiver +p&p sound reasonable?

Cheers,

Frank

Re: Heathkit V-7AUK VTVM 12au7 substitute
 

Thank you for your help
As far as I can tell the area around the 6AL5 ( EB91) pins are clean
I tested the 5x 22M they were between 24M and 28M and the voltage divider 12.5K, 33.78K and 105R
AC balance and calibration procedure is unsuccessful
I am unable to get the AC to balance - the AC balance preset is wound fully anti clockwise
but I cannot achieve zero shift with DC+ and DC-
Using an 8.25VAC supply the AC calibration preset is fully anti clockwise but only registers 1.75VAC across the probes, but with ONLY the AC/ohms probe connected it's 11.5VAC
I have tried an ecc81 with more or less the same result.

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

Hi!

You also need to make sure the meter movement magnet hasn't lost any of it's magnetic field strength – to test for this, connect a 10k resistor in series with a fresh 1.5V AA battery to the Meter's (+) and (–) terminals with the PCB removed from the back – if the movement is sensitive enough for the circuit, the pointer should deflect ⅔ of the way–upscale, if the deflection is much less than this, the meter movement will need re–magnetising – other members who have more experience with movement problems than me can best advise on this point!

The normal f.s.d. sensitivity of the Heathkit Movements is 200uA nominal!

Chris Williams

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

On an AC range, there would need to be some capacitors around. Worth checking for leaks.

Don't have a schematic but I'm assuming it's nothing unusual as valve voltmeters go.

David

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

Ohms and DC ranges work, it's AC measurements that's the problem, so I think the meter movement is ok

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

I need to extract the circuit board so I have ordered some small thin spanners, as there are two difficult to get to nuts which need undoing.

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

Hi!

There is two capacitors in the a.c rectifier circuit – it's an average reading peak rectifier – one is in series with the input to the 6AL5 and the second is between the anode of the second half (pin 2) of the 6AL5 and chassis–earth, and there is also a 5n (0.005u) from the input grid of the voltmeter valve (12AU7 pin 2) that also needs checking!

As well as the capacitors, please don't forget the cathode "tail" resistors for the 12AU7 – 1 × 150k from pin 3, 1 × 220k from pin 8 and 1 × 150k from the slider of the "Set Zero" control all returned to –h.t., these will affect the balance of the circuit if they wander up in value, so I suggest replacing these with new and trying to rebalance the circuit again!

Chris Williams

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

Hi!

V7a circuit diagram!

Chris Williams

Re: Heathkit V-7AUK VTVM 12AU7 substitute
 

I replaced a the 0.005u grid capacitors, cathode resistors and ecc82.
So now with an ac source of 8.29vac from a transformer winding ( measured on a DMM not true rms ) with the calibration preset wound fully clockwise the needle can just point to 8v but there is no further latitude for adjustment. Maybe brand new ecc82 is required?
I tried measuring AC mains but it tripped the MCB.
This is suggested in the calibration procedure but at American 117vac mains
I have modified it with a mains earth connection with safety in mind.