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Old 9th Sep 2013, 12:49 am   #11
Dekatron
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Join Date: May 2009
Location: Linkoping, Sweden
Posts: 1,463
Default 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.
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