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Fix my VTVM

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John1397

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I have a VTVM that works but scale is way off. 12 volts dc reads 3 volts, 120 volts ac reads 20 volts, and resistance cannot even get meter to go to infinity or 0 when touching probes together. When turning unit on meter neadle deflect all the way to right then returns to left and you can get 0 adjustment on dc and ac, so what ever is wrong seems to be affecting all ranges and functions. My belief is that it must be a resistor between tube 12au7 and power supply any thought?
 

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kchriste

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Have you checked that the power supply (-62Vdc, +60Vdc, and 6.3Vac) has the correct voltages?
Have you tried another 12AU7 in the circuit?
I would guess that the 1.5V battery is for the ohms range only.
 

mvs sarma

Well-Known Member
I have a VTVM that works but scale is way off. 12 volts dc reads 3 volts, 120 volts ac reads 20 volts, and resistance cannot even get meter to go to infinity or 0 when touching probes together. When turning unit on meter neadle deflect all the way to right then returns to left and you can get 0 adjustment on dc and ac, so what ever is wrong seems to be affecting all ranges and functions. My belief is that it must be a resistor between tube 12au7 and power supply any thought?
WELCOME John
Being a testing equipment, I don't expect the Valve to go weak so soon. In Valve based circuits, it is very common that HIGH MEG resistors give way. If you have a tester, better have a check and replace. I am suspecting the resistor deterioration around the V2. as resistor range also is not behaving.

By the way, as Kchriste has suggested, have a check of voltages. It is probable that you might be in 230V area and keeping the power switch in 120V range. Please ensure this. Even C1 can be replaced.
 
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John1397

New Member
I have checked all resistors and pot associated with pin 3 and 8 that connect directly to the meter and they all test ok but unit still does not have enough meter movement on all ranges and functions. I even lowered the 3.3 meg resistor to 800 k but on resistance measurement it just move the meter movement further from 0 when toching leads together. I was trying to change resistor values to get more current flowing so meter would move more but did not succeed. The one thing I cannot test is the meter itself, if the circuit is working and the meter windings are partially shorted that would account for the lack of meter movement and to design a new circuit around a bad meter could take 2 months. Who thought something so simple would be so hard!
 

kchriste

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The one thing I cannot test is the meter itself
Does the meter have anything printed on the scale near the movement? Something like 50uA FS, etc? If so, it'll be relatively easy to test the movement.
I wouldn't go randomly changing resistors just to "make it work". You need to find the root cause of the problem and avoid bodging it.
 

mvs sarma

Well-Known Member
I have checked all resistors and pot associated with pin 3 and 8 that connect directly to the meter and they all test ok but unit still does not have enough meter movement on all ranges and functions. I even lowered the 3.3 meg resistor to 800 k but on resistance measurement it just move the meter movement further from 0 when toching leads together. I was trying to change resistor values to get more current flowing so meter would move more but did not succeed. The one thing I cannot test is the meter itself, if the circuit is working and the meter windings are partially shorted that would account for the lack of meter movement and to design a new circuit around a bad meter could take 2 months. Who thought something so simple would be so hard!
As suggested by Kchriste, why not measure the voltages 6 to7 V secondary AC for filament and 60 to 70V rectified DC and indicate the readings, John?
 

AlexR2

New Member
Have you tried changing the 12AU7 yet?
The symptoms point to one half of the valve being sick or dead rather than a resistor problem. You could also check the resistors and capacitors connected to the grids (R24, C5, R32, C6).
 

John1397

New Member
Still does not work... I have 3 tubes to choose from and the only difference is a little more pointer movement with new tube and I even tried some 12AX7 and works as good as 12AU7. If you can zero out meter on AC DC then that part is working, then you switch to resistance mode then not enough current flows through meter as you cannot adjust to infinity and touching leads together does not make needle go back to zero. I am just wondering with such high resistance if something could be shorting out signal like maybe to much flux? Also if tube socket were not making contact or one side of 12AU7 were only working meter needle would go crazy to one side or the other and putting wire accross the ohm's adjusting pot does not help. I found another 200 micro amp meter and I applied a volt ohm meter in resistance mode to both meters and they both move to a little past center so that should mean that meter is good.
 

Willbe

New Member
I have a Knight-Kit from 1960 but the schem. looks the same.
Start on the lowest + DC voltage scale and get that working. The multifunction switches make it look complicated. If you have a copy machine you can draw simplified circuits for each function. There might be just a single component that's changed value.

Maybe you should check the meter movement first. The full scale current is marked on the meter face. 50uA is 9v in series with 180k, 200 uA needs 45k.
 
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John1397

New Member
I know meter is a 200uA and is ok. I have checked all resistor by using dc 1.5 volt dc range and 1.5 volt supply, then use dc calibration method which is the meter movemewnt is supposed to go all the way to the right side. Problem is meter needle will not even go half way. I already tried changing the resistor values higher and lower on pins 2 and 7 of 12AU7 tube and does not help. I am thinking of changing pot R1 to higher or lower value and resistor R33 to 100k maybe this will get more current flowing between pins 3 and 8 where the meter is connected. Once I get more difference between pin 3 and 8 it will work.
 

Willbe

New Member
In principle, if you post your simplified schematic with all component values and DC voltage readings with the 1.5v input, if there is only one bad component it should be able to be found, and how far off it is, by inspection and calculation.

It sounds like you are making changes to correct for an as-yet-undiscovered root cause.
 
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mvs sarma

Well-Known Member
In principle, if you post your simplified schematic with all component values and DC voltage readings with the 1.5v input, if there is only one bad component it should be able to be found, and how far off it is, by inspection and calculation.

It sounds like you are making changes to correct for an as-yet-undiscovered root cause.
Schematic was posted and it is simple enough.
John could take some measurements if he has a DMM ( not analog meter) and post .
Yes , it helps analyze the case
 

mvs sarma

Well-Known Member
I know meter is a 200uA and is ok. I have checked all resistor by using dc 1.5 volt dc range and 1.5 volt supply, then use dc calibration method which is the meter movemewnt is supposed to go all the way to the right side. Problem is meter needle will not even go half way. I already tried changing the resistor values higher and lower on pins 2 and 7 of 12AU7 tube and does not help. I am thinking of changing pot R1 to higher or lower value and resistor R33 to 100k maybe this will get more current flowing between pins 3 and 8 where the meter is connected. Once I get more difference between pin 3 and 8 it will work.
please check the meter separately with a pre calibrated 200uA current source

while doing following test be CAUTIOUS NOT to DUMP high current -- just check the potentiometer direction with out the moving coil. then only insert moving coil.
the best is to connect a 1.5V cell with a 1 Meg potentiometer and a dmm in uA range in series. gradually increase the current and when the pointer just comes to full scale, see the reading on dmm. it should be 200uA approx
At times, you may find that the meter is stuck after some travel. I had occasions of tracing dust sticking in the centre of movement obstructing the physical movement. It should be dealt with very delicately and with suitable non magnetic tools af very small size.
 
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John1397

New Member
My tube measurents are with 1.5 volts dc applied to probe -2 volts pin 3 and a little more -2.1 pin 8 and the wiper of R1 has -4 volts which don't tell me anything. If I had good meter or if they gave you readings on schematic it might help. As you apply more dc voltage to input before 3.3 meg resistor voltages on pin 3 and 8 get less negative. Thing works good on until you apply voltage. I had to apply 10 volts dc to make meter show 1.5 volts. I have two meters same readings on both, 5 tubes same readings can't all be bad, power voltages all good, pins in socket all test good or the way they should, resistors test good with 1 volt don't have high voltage resistor tester. I was going to change resistors R33. R34. R35 but it seems like if I can balance meter to zero that part must be working. Probably just have throw it off a cliff.
 

mvs sarma

Well-Known Member
My tube measurents are with 1.5 volts dc applied to probe -2 volts pin 3 and a little more -2.1 pin 8 and the wiper of R1 has -4 volts which don't tell me anything. If I had good meter or if they gave you readings on schematic it might help. As you apply more dc voltage to input before 3.3 meg resistor voltages on pin 3 and 8 get less negative. Thing works good on until you apply voltage. I had to apply 10 volts dc to make meter show 1.5 volts. I have two meters same readings on both, 5 tubes same readings can't all be bad, power voltages all good, pins in socket all test good or the way they should, resistors test good with 1 volt don't have high voltage resistor tester. I was going to change resistors R33. R34. R35 but it seems like if I can balance meter to zero that part must be working. Probably just have throw it off a cliff.
once you restore it, may be you repent for your feeling of throwing it. I am of the opinion, that our knowledge grows only if we face problems and solve them. At times I used feel bad for delay in a simple fault (the simplicity known only after detecting it)

Hold patience, and approach logically and I am sure that you would make it work, soon !!
 
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mvs sarma

Well-Known Member
A search later reveals following info and the site where i had to go is linked here along with excerpts of comments. Hope it serves. I again repeat that , this type of restoration calls for enormous patience.

.........................
www.heathkit.nu

IM-18

The IM-18 VTVM was one of my favorite tools. It was one of those basic designs that couldn't be improved upon. The circuitry was essentially the same since it was introduced (under a different model no) back in the '50s.

The circuitry hadn't changed in the late '70s when it was later given a new paint job and the model number changed (Marketing moves in mysterious ways).

Anyway, it was a rugged tool. In the '70s, Heath came up with an upgrade (the IMA-18-1) that replaced the two vacuum tubes with solid-state equivalents so that it would be ready the instant you turned it on. I did a lot of warranty service on the modified units as they tended to blow up when brought near the high-voltage circuits in color TVs.

I think most customers who bought the mod went back to the vacuum tubes.
* Needle pegs on DC or AC on first turn-on: Suspect a leaky DC-cal or AC-cal potentiometer.

IMA-18-1 (almost same as IM-18)
* 6AL5 = TR1002 = #150-57 (solidstate counterpart )
* 12AU7 = TR1119 = #150-58 (solidstate counterpart )

...............................
Hope this helps. Perhaps the potentiometers can be removed cleaned and lubricated without letting the insulation down and restored back, as per the service technician whose statements I had quoted above.
 

John1397

New Member
I unhooked wire to 3.3 resistor where it enters pin 2 and put 10 meg resistor to ground so both triods are equal and it is really balanced good as meter is on 0 and pot is in center of rotation, then you put 3.3 resistor back and apply 1.5 volts dc hardly any reading almost seems like tube is not amplifing. Suppose there is no way of testing short of a tube tester or would instaling a 12AU7 in guitar amp which I have work as a tester?

Has anyone know of schematic of a 12AU7 transistorized tube as I cannot even find these transistorized tubes with search engine?


$150.00 per hour times 30 repair hours = $4500.00
 

Willbe

New Member
Your VTVM circuitry is the tube tester. Measure the transconductance of the tube, but the tube is probably good based on what you've said so far.
 

tunedwolf

Well-Known Member
Pretty much most of the VTVM's I have seen or used, all use a similar, if not identical, circuit to that used in the RCA Senior VoltOhmyst that I own.
I have attached scans of the manual I have for mine, it has a pretty good circuit description, maintenance and calibration sections that might shed some light on your troubles with your Heath instrument. :)

rgds
 

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mvs sarma

Well-Known Member
I unhooked wire to 3.3 resistor where it enters pin 2 and put 10 meg resistor to ground so both triods are equal and it is really balanced good as meter is on 0 and pot is in center of rotation, then you put 3.3 resistor back and apply 1.5 volts dc hardly any reading almost seems like tube is not amplifing. Suppose there is no way of testing short of a tube tester or would instaling a 12AU7 in guitar amp which I have work as a tester?
Has anyone know of schematic of a 12AU7 transistorized tube as I cannot even find these transistorized tubes with search engine?
$150.00 per hour times 30 repair hours = $4500.00
Perhaps we , at hobby level, cant calculate the cost per hour spent by us.by this approach, we should not spend money for purchasing old equipment from e-bay etc.

By the way, i feel that the transistorized replacement sold by HP those days is nothing more than matched pairs of high voltage FETs. later we may not require 60 or 100V DC . even 12V or 20V would suffice. i could imagine FEts like 2N200 of Philips make of those days. they also made 6 pin versions of matched FETs.
 
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