Heathkit IM-18D VVM servicing hints?

[avocollector]

Have this and it is being uncooperative about repair. Basically a 6AL5 and 12AU7 rectifier and double triode pair. The 6AL5 is to rectify ac input signals and the 12AU7 is used as a balanced bridge with the output of the 6AL5 feeding into grid 2 of the 12AU7, or dc levels fed into the same grid via a resistor network/switching. Symptoms are it works on ac signals and 'sort of' on ohms, but nothing on DC positive or negative input voltages. Have substituted a 12AU7 in case the gain is too low but no change. Therefore thought the resistor network is the likeliest cause (usually someone checks mains on the ohms setting etc) but checking all resistor values has me stumped as all read OK. Rectified ht voltage is about right so the electrolytic, diode and mains transformers seem to be OK. No paper caps to go awol so I'm beginning to think problems with the wafer switches. Any thoughts, hints suggestions??

[wd40addict]

Do you realise the jack socket is used for DC positive voltage input, whilst the red banana socket is used for AC and ohms? In all modes the black socket is common.

The DC probe connected to the jack should also contain a 1 meg series resistor. This allows DC measurements on typical valve stages without the disturbing the AC conditions. This means readings can be taken without provoking big bangs from a radio's loudspeaker for instance.

[MrBungle]

I’ve got an IM-13U on the bench and it had wonky resistors in it. Carbon comps on the main board. Worth lifting them and testing if you can. Mine were in the bias / divider network. Think I had a 2.7k reading 39k.

[David G4EBT]

There an interesting youtube video concerning restoration of an IM-18 VTVM at this link. At 4.5 mins in, he mentions the test probes, and replacement kits that can be bought on e-bay from an American amateur KK4HXJ. (I hadn't appreciated that the original probe had a built-in switch to switch the 1 Meg resistor in or out of circuit):

https://www.youtube.com/watch?v=WHmKTEo6gN8

Probe kits:

https://www.ebay.co.uk/itm/VTVM-Prob...-/331900401753

Another youtube video on the IM-18 here which might be of interest:

https://www.youtube.com/watch?v=UQpk8R0qcAw

Good luck in sorting it.

[MrBungle]

I've actually bought several probe kits from that supplier - they are really very good. I've got one on the way for my IM-13U at the moment.

[stuarth]

Do the zero setting pots work correctly (one sets zero on dc voltage ranges, one sets fsd on ohms ranges with a good battery). If they do, then the basic valve and bridge circuits are ok.
If your meter has 3 sockets, black, red and 1/4” jack, then you should use black and red for AC and ohms, black and the jack for DC with the special lead with a 1 Meg resistor in the probe tip, but if you connect a 1Meg resistor between the red socket and the jack, you can use black and red for all measurements. Then you’ll only need the special lead for DC measurements where RF is present.
Stuart

[ColinTheAmpMan1]

Do you have a manual for this? Heathkit manuals are difficult to find, but if you don't have the correct one, I have a pdf of the manual for the v-7A, which is probably very similar. I could send a copy to you. I also have a circuit-diagram for the IM-18, if you are interested (no manual). It looks from the circuit-diagram as though the IM-18 only uses one probe, which has a switchable 1M resistor.

It is possible that the double-diode has blown, but that shouldn't give the symptoms you mention. The 6AL5 in the V-7A is decidedly delicate.

Colin.

[avocollector]

many thanks to all for their help, suggestions and hints. Basically I had not realised/forgotten about the jack plug for DC measurement and originally you could zero the needle on start up. I do have the schematic and most of the manual for it. However now after dissembling it and checked all the resistors on the pc by lifting one end, I now have another problem. Turns on then the needle goes briefly across to the right then hard left and cannot be zeroed. Which means the balance is way out etc etc. Ho hum as they say (=taking to bits again) but many thanks again to you all for your ideas, hints and offers of help.

[MrBungle]

Good luck with it. My IM-13U is similarly misbehaving at the moment. I’m replacing all the resistors in mine as it’s that far out. The power supply biased itself at -100 and +30V which is well off!

[ColinTheAmpMan1]

Originally Posted by MrBungle

Good luck with it. My IM-13U is similarly misbehaving at the moment. I’m replacing all the resistors in mine as it’s that far out. The power supply biased itself at -100 and +30V which is well off!

I have decided to replace all the 10% tolerance resistors in the AV-3U that I am trying to fix (the 2uF capacitor feeding the meter bridge-rectifier was horribly leaky). Some of the resistors are out-of-spec, so most will be replaced with better (though not original) ones.
I am wondering about the precision resistors in the two divider-chains. I am hoping that I won't need to replace them, even though they were 1% tolerance originally, because they are very odd values that aren't in any of the E-series that I am familiar with (216.2 R, anyone?). If you have replaced any precision resistors in your VVM, where did you source them from? I have looked at one or two of the usual suppliers, but searching for some of these values brings up 0.1% or closer tolerance parts at eye-watering prices!

Colin.

[MrBungle]

I’ve never had to replace them. If they’re the black cylindrical ones they are usually dead on.

To get precise values in the past I wrote a computer program that works out equivalent resistance combinations from an input value, an E-series and tolerance and desired network size. Takes a while to run even in my E7 Xeon at work when you get to three resistor combinations.