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swordholder 1st May 2010 9:33 am

AVO VCM163 - Some faults I have had
Hi Guys,
Over the years I have had to repair several VCM163's for my former company and its customers.
The most common faults I have encountered are.
Anode current incorrect. Usually on 100ma range, check shunt resistor R4 (2R4) situated on the meter range switch. Can be checked by removing anode link and inserting AVO , should read 1/2 of the VCM meter reading.
Grid voltage incorrect, check R21 mounted on Grid voltage switch.
Incorrect Gm readings.
If the Gm meter can be set to "cal" position, but valve gives low Gm.
Check R33 (2k4) on Gm switch. Very often looks charred and reads High.
Other Gm faults, but not so common.
Large poly cap on amplifier board goes S/C . 1.99uf is unobtainable, fit 2uF (not electrolytic) in its place. This has very little effect on performance.
I once had an EL34 giving very strange Gm readings (low) and none of the usual suspects cleared the fault. This was traced to C1,C2,C3 & C4 on the mains transformer going O/C. Can't see why this should cause the problem, but replacing all four cleared the fault.
This may help some members with repairs.
How about similar threads for other common test gear.

Regards to all

Pamphonica 1st May 2010 9:57 am

Re: AVO VCM163 - Some faults I have had
A very useful (and timely!) list. I will think about my "favourite" faults on other VCMs!

swordholder 1st May 2010 12:01 pm

Re: AVO VCM163 - Some faults I have had
A very good idea, it would be nice to have some point of reference on the Forum.
Some have AVO expertise, others Taylor etc

Brian R Pateman 1st May 2010 12:08 pm

Re: AVO VCM163 - Some faults I have had
This is excellent stuff and useful for anybody restoring or repairing one of these instruments.

I'm applying "glue" to this thread and making it a sticky.

Dekatron 2nd Jul 2010 9:31 am

Re: AVO VCM163 - Some faults I have had
I think there is a type in the text regarding R33 - it should be R35 according to the schematics - I have also seen this one charred on more than one occasion.

Dekatron 22nd Aug 2010 3:34 pm

Re: AVO VCM163 - Some faults I have had
1 Attachment(s)
Now I found the picture I took of R35 before I repaired my VCM163, you can see it as the silvery looking resistor with the text ZD printed on it, it is charred and also bulging.

Dekatron 22nd Aug 2010 6:00 pm

Re: AVO VCM163 - Some faults I have had
1 Attachment(s)
And here is a corrected schematic of the VCM163, the corrections are:

* Connections for C2, C3 & C4 corrected, previously on wrong transformer tags
* Wires added for S3E connections to S3D for Ia positions 3, 10, 30 & 100 mA
* Meter M1 & M2 correct resistance values

* Meter M1 contains compensation resistors and one NTC to adjust for temperature drift, otherwise the meter movement is identical to M2 with a coil resistance of 1500 Ohm +/-1%

ColinTheAmpMan1 29th Feb 2012 12:23 pm

Re: AVO VCM163 - Some faults I have had
OK, not actually a fault, but an observation.
On looking at the switch positions for testing double-triodes, I noticed that there was a difference between those for American valves of the 12A_7 type and the British equivalent ECC8_. This is, of course, down to the fact that the American valves are quoted as having 12.6 v heaters and the British ones working on 6.3 v. Although of limited value, the "heater continuity" test would show a "fail" on the American valves if one triode's heater was o/c; the same test on a British valve would only fail if both filaments were o/c. So, it would make sense to test such valves' heater continuity in both series (American 12.6v) and parallel (British 6.3 v) heater configurations, wouldn't it?
I bet some of you out there that have used valve testers over the years have already worked this out, but maybe not.....

Pamphonica 29th Feb 2012 1:20 pm

Re: AVO VCM163 - Some faults I have had
What a very good observation I must admit I had never considered this. Of couse, a failed heater would lead to rather poor readings on the dead side! As I use the full test regime (leaks. Ia and gm), that would rapidly become apparent. But on a quick heater check regime it would be highly misleading.

ColinTheAmpMan1 2nd Mar 2012 11:40 am

Re: AVO VCM163 - Some faults I have had
Of course, another way of doing the heater continuity test would be to treat each 6.3v filament individually. Pins 4 and 5 would be treated as "no connection" in turn. I am not too sure whether the two filament pins are actually physically close to the pins of their respective triodes, but a lack of continuity in one would be obvious; then which triode it applied to would also become plain when Ia and Gm figures were taken.

Dekatron 8th Sep 2013 11:49 pm

Re: AVO VCM163 - Some faults I have had
I thought I would share some information on common faults that I have seen over the last five years.

*. Dry joints on the wiring of the rotary switches. When I first came across this I thought "oh well, one soldering spot that has been missed", but in the last VCM163 that I repaired/restored there were six (!) dry joints of which four where on the electrode selector switch. I have mostly found dry joints on this switch but also on the two push button switches.

*. Dry joints on the valve sockets, sometimes the plastic insulation has been stuck through the hole and solder applied to the pin but there was of course no contact between the pin and the wire!

*. RV1, RV2 & RV3 either open circuit or in some cases with noisy tracks. I replaced these with 10-turn precision potentiometers of twice the value which I then put a resistor of the same value in parallel so that the value would be the same. The 500 Ohm potentiometer RV1 would be replaced with a 1K potentiometer with a 1K resistor in parallel, the 1K potentiometer RV2 with a 2K potentiometer with a 2.2K resistor in parallel and the 2K potentiometer RV3 with a 5K potentiometer with a 3.3K resistor in parallel. This way you also ensure that if the track in the potentiometer ever breaks you will still have current flowing through the resistor (which hopefully works then) meaning that you will not loose grid voltage during a test, which was what happened when I first was told about this problem.

*. If you can't clean the relay contacts carrying the mains voltage (240V to the Anode/Screen transformer) you can easily bypass them by using another relay that is hooked up to the blue lamp LP1. When LP1 is shining you can either use a relay with an AC coil of 6V or rectify the voltage and drive a DC coil of some 8V. I rectified the voltage and used a pair of "normally on" SSD-relays, not as easy to find as "normally off" types but well worth the search. Or if you are lucky you can find relays on eBay and scrap them to use their contacts in the repairs but they are becoming hard to find.

*. After a friend suffered a short in his tester which blew most of the transistors and diodes (but fortunately not the thermistors nor thw two small transformers) on the amplifier and oscillator board I installed a small toroid transformer on a piece of PCB above the Heater transformer. The short was a result from a wire that had been accidentally stuck underneath the metal bar that holds the two push button switches on the front, and the wire was carrying the mains voltage from the voltage selector switch - the short then meant that full mains power was shoerted to the Anode/Screen transformer secondaries which was then fed to the amplifier/oscillator boards. The toroid transformer is fed from the mains winding on the Anode/Screen transformer - after the relay contacts, so it too is switched off in the case of an over current situation that triggers the cut-out relay. This transformer can then supply 20V AC to the oscillator board, you will just have to de-solder the old wires and insulate them and install two new wires from the toroid transformer. When I was doing this modification I also tested a similar circuit but where I replaced the zener diode and single diode rectifier on the oscillator board with a full wave rectifier and an LM7812 12V regulator. You can use the existing electrolytics for smoothing and just add a pair of small 100nF capacitors across the pins on the LM7812 and stick it in the holes where the zener diode and resistor was placed earlier, this results in a somewhat more smooth voltage to the amplifier and oscillator boards but it does not change the measurements in any way. You can also use any small kit available which contains the full wave rectifier, smoothing capacitors and an LM7812 and place it close to the transformer, you will then only have to remove the zener and the resistor feeding the zener (R17) and hook the 12V wire to the positive electrode of C7 (100uF).

*. On my first VCM163 the grid volts potentiometer was bad, noisy track and not as linear as it once had been. While searching for a replacement I decided to hook up a 10-turn precision potentiometer manufactured by BEI duncan which I found on eBay. It worked very well and I used a 10-turn locking knob with a scale so I could lock the grid voltage. At the same time I modified the resistors used in the grid voltage selector switch so that I could adjust each range independantly. I used the same approach here as with RV1-3 above, use a 10-turn potentiometer of twice the value and put a resistor in parallel to get the desired resistance value. Now I could adjust each range 0-3V, 0-10V, 0-30V and 0-100V with ease. The hardest thing here was to find a ten turn precision potentiometer for the grid volts potentiometer that actually got down to 0 Ohm on the end of the track, the datasheets show that this is not guaranteed but a small resistance is acceptable. I finally found a few that went all the way down to 0 Ohm which I could use.

*. The VCM163 models with the grey roller selector and the grey knobs very often have cracked roller selectors, they get brittle with time and break when the "kogs" snag on the protective bar on the top covering them or when snagging on the flat metal leads that connect them to the wiring inside. The "kogs" on these rotary wheels snag on the plastic of the protective bar when they are turned, so I removed the bar and very gently filed the edges of all the tracks where the "kogs" move - there are a lot of edges to file! This helped a lot but some "kogs" where still snagging on the flat metal leads coming out of each roller selector. To correct the snagging on the metal leads you can do it in three ways (there are probably more ways but these are the ones I used) - de-solder all of the wiring and then put a piece of PTFE shrink tubing on each metal lead so the "kogs" can slide more easily, or very slightly bend each metal lead so that the "kogs" don't touch them at all. There is just enough space between each roller selector for the metal leads to pass between them without any contact with the "kogs". The thir way is to loosen the long SRBP strip that hold all of these metal leads and move it very slightly sideways, if you are lucky all of the metal leads then move in unison freeing up some space for the "kogs" - I have never succeeded with this third way but it seems plausible that it will work.

*. It is absolutely possible to replace all of the transistors on both the amplifier and oscillator boards with for instance modern BC547B. I have done so on several VCM163's and while being at it you can also replace the resistors with modern metal film resistors and replace capacitors with modern high stability types. I only kept the two transformers, the two thermistors and capacitors C1 & C2 on both boards.

*. If you have a hard time finding a replacement meter for the right gm-meter you can use a 100uA meter with an internal resistance of 1500 Ohm in its place if you lower R14 to approximately 90-100 Ohm. This will not affect the measurements, only the current flowing in the amplifier meter circuit to go as high as 100uA through a 1500 Ohm moving coil. A friend of mine have tested this on both frequency response and input signal levels and it works just fine. It night be possible to adjust the feedback loop in the amplifier to suit other meters but I have not done any testing on this.

This is all I can remember right now, if anything more surfaces I will write about it later.

Bazz4CQJ 9th Sep 2013 12:22 am

Re: AVO VCM163 - Some faults I have had
Many thanks for sharing that information Martin. I'm sure that lots of effort went in to resolving those problems.

Panrock 9th Sep 2013 5:08 am

Re: AVO VCM163 - Some faults I have had
Just noticed this thread. I too have a VCM163 and this useful information is going into my service file. Thanks.


Dekatron 9th Sep 2013 8:37 am

Re: AVO VCM163 - Some faults I have had
3 Attachment(s)
I just remembered one more failure of the electrode selector switch. On some models the centre pieces of this switch are made from white plastic which only has two plastic pins connecting each side. On this type of switch the white plastic bends over time which means that the rotating metal contacts travel on top of the finger contacts on the switch wafer making poor or no contact, in some cases the rotating piece has bent and destroyed the contact finger on the switch wafer as it snags. In the pictures below you can see that the rotating contacts are riding on top of the finger contacts on the switch wafer and also that the rotating contact snags on the edge, there is also one picture showing one of the unsoldered wires.


Pamphonica 9th Sep 2013 11:41 am

Re: AVO VCM163 - Some faults I have had
I would like to re-iterate the fault advice regarding wires trapped under mounting pillars.

My VCM163 had an odd fault when I got it. The Vg was definitely very low, if not zero, at all settings. It turned out to be the wire from the Vg circuitry trapped under a mounting pillar en-route to the switches. Over time the insulation was crushed and a short developed.

Finding the location of a short to earth can be hard, but if you can make or borrow a low-ohmmeter (say 0-100 milliOhms) it makes the job a lot easier. Mine came from an auction at about 10 or 15 and has been invaluable for finding shorted power supply decoupling caps on tightly packed circuit boards.

- Jeremy

BottleMan 3rd Apr 2015 8:48 am

Re: AVO VCM163 - Some faults I have had

Originally Posted by Dekatron (Post 356181)
And here is a corrected schematic of the VCM163, the corrections are:

* Connections for C2, C3 & C4 corrected, previously on wrong transformer tags

I had a transformer given to me by the guy who actually wound and tested them at AVO. I have measured the windings for my own purposes and find that the taps used for the capacitors should be 14, 18 and 23, if they are to have equal voltages across them. Personally I will be fitting two 1uF "X" capacitors, using tap 18 only.
I can make this data available to anyone who wishes.

Graham Holloway

Dekatron 3rd Apr 2015 9:34 am

Re: AVO VCM163 - Some faults I have had
Hi Graham,

Thank you for the information. Please make the information available here in this thread, with photographs if you have the possibility.

Have you read what Swordholder wrote on this in the first post: "I once had an EL34 giving very strange Gm readings (low) and none of the usual suspects cleared the fault. This was traced to C1,C2,C3 & C4 on the mains transformer going O/C. Can't see why this should cause the problem, but replacing all four cleared the fault." - this is the only case that I know of where these capacitors have affected the testing. I checked this by unsoldering one side and then the whole tester became susceptible to interference from the wall warts and other signals on the mains voltage. By using a DC-power supply with a pure sine wave inverter these problems went away, without these capacitors in place so I think that they are there to dampen/remove interference as well as having a stabilising effect. So maybe you should consider using capacitors on all taps like AVO did.

In the three VCM163s that I own and one that a friend owns the capacitors are wired to the connections that I changed them to in the circuit diagram, I made a full reverse engineering of my VCM163s when I corrected the schematics, by measuring and checking every connection and also asked my friend to check his.

I guess that this is one more of the famous AVO changes or errors in their schematics and how they actually wired the VCM163s - either an error in the wiring or an error in the schematic, or both.

I can't say which one is the correct one but checking the specification datasheet for the transformer the approximate voltages 100V, 200V and 300V are found on tags 14, 18 and 22, but on all three of my VCM163s those voltages are found on 15, 19 and 23. AVO or the person winding the transformer might have changed that over time due to updated specifications, so on my transformers the voltages are found on different pins, or it might be an error in the design specification.

Could you check your VCM163 and see how it is wired, or do you just own the transformer?

If you still have contact with the person who gave you the transformer it would be very good if you could ask if he remembers anything on this matter!

When we know more on this I can make a new schematic that reflects this.


BottleMan 3rd Apr 2015 10:40 am

Re: AVO VCM163 - Some faults I have had
1 Attachment(s)

I only have the transformer. The guy who wound/tested them was not aware of any changes that occurred while he worked for AVO. I suspected the capacitors were there for electromagnetic immunity considerations. It might be worth checking the 163 with a pair of "Y" capacitors added - Line to Chassis and Neutral to Chassis - say 4n7F/250V.

I've attached the file for the transformer test.



Dekatron 3rd Apr 2015 12:30 pm

Re: AVO VCM163 - Some faults I have had
Hi Graham,

Thank you for the file.

Avo says that the transformer voltages should be measured at 240V RMS to get proper readings, this is probably due to that higher voltages will affect the magnetisation in the core. The only thing I know is that the core will be saturated quite easily on these and that affects the sine wave of the output voltage a lot. The specification sheet I have has been amended and is the second revision, but there might have been more which I am not aware of.

I've usually seen lower voltages than the rated ones when measuring the transformers at 240V RMS, but that has almost always been due to the tip of the mains voltage not having a proper sine wave form but a kind of slanted tip. When measuring the voltages with a pure sine wave inverter they have always been within tolerance. So if you get any readings that doesn't match up you should use a scope to see what the mains voltage as well as the output voltage looks like - don't forget to use an isolating transformer for the scope though or you might make a short circuit.

I just had a look at my photos of my first VCM163, the one I reverse engineered, and the capacitors are connected from pin 6 to pin 15, 19, 23 and lastly to 25, that is also how all three of my VCM163s look.


BottleMan 3rd Apr 2015 4:42 pm

Re: AVO VCM163 - Some faults I have had
261 Volts is within UK tolerance. As the testing was done with zero load, the only current was magnetising current and it didn't even register on my ammeter. Quite apart from this, I am looking at the secondary voltages, which will always have the same relationship with one another. I suspect that at the time the 163 was manufactured the choice made regarding filtering of the input may have been 4 X 1uF was cheaper/easier to fit/readily available than a 400V ac capacitor.
There is a screen on the transformer. Has anyone tried seeing how effective it is?

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