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Old 5th Mar 2013, 8:33 pm   #1
Analogman
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Default Avo VCM MK2 coil meter not moving!

Hello from Vienna Austria,

Recently I purchased an Avo VCM MK2. I had to change the plug to a European standard and I changed the input voltage to 220-230, (accessed through the back panel).
When I fired it up for the first time there was no light and no needle movement. I turned some knobs and the needle started to vibrate in the lower end but did not rise. Then it rose quickly and briefly to the maximum and fell down again, however there were no lights. I then turned it off, had a closer look inside, which looks quite clean, made sure all the voltages and fuses were okay, and turned it back on, hit the reset button, and the lights came on.

The settings were all in the proper place for starting a check, but the needle did not budge. I checked the voltages between the anode and cathode of one socket and they seem okay. I decided to make some measurements across the bridge for the coil meter (1 blue and 1 green wire) and measurered the following:

Circuit selector set to Check(C) = 14mV
Circuit selector set to Test
Set Zero set all the way clockwise = 0.8mV / 60 Ohms
Set Zero set all the way counterclockwise = 47.8 mV / 160 Ohms

However no needle movement, well actually it does go down below zero a little bit when I back off the Set Zero control and goes back to null when I turn it up.

Before I proceed any further I wanted to get some tips from the experts here. As you may have surmised I am not an experienced AVO user, and not an electronic genius, actually I am a musician with a lot of tube amps and vintage equipment that I like to repair myself. So please be gentle with me!

I love the looks of this machine and I really want to get it into useful working condition. I am hoping that the coil meter is not broken but am fearing the worst. What should I do next? Any tips?
I will try to post some pics.

Thank you and look forward to your responses.

Lucas
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Old 5th Mar 2013, 11:23 pm   #2
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Default Re: Avo VCM MK2 coil meter not moving!

Hi Lucas,

The first thing I would do is to remove the original meter so you don't destroy it if something happens, if you for instance short something.

You can then replace the meter with a resistor with a value of approximately 100 Ohm (or if you know how to check the meter resistance without damaging it you could replace it with the actual value).

Then you can hook up your DMM across this resistor and use it instead of the original meter, that minimises the risk of damage.

There are two very nice articles written on the AVO Mk I / II valve testers, one by Gary Tempest in the Summer & Winter 2000 issues of Radiophile and another written by Euan MacKenzie in Radio Bygones Christmas 2011 #134 which I recommend that you try to get hold of. You should also check the EMER documents which can be found here: http://www.vmarsmanuals.co.uk/archive/files_index.htm scroll down a bit (or search) for the AVO EMER manuals.

If you can't find these I recommend that you start by checking all of the components - especially the diodes/rectifiers to see if they are working correctly. You can put a standard 1N4007 in parallel to each diode and see if there is any difference. There is an error in the direction of the diodes shown in the circuit diagram which is described in the Radio Bygones article.

/Martin
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Old 6th Mar 2013, 12:44 pm   #3
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Default Re: Avo VCM MK2 coil meter not moving!

Hello Martin and thanks for your reply.

How do I remove the meter? I do not see any obvious solution.

Lucas
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Old 6th Mar 2013, 11:04 pm   #4
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Default Re: Avo VCM MK2 coil meter not moving!

You can just unsolder the wires from the tags on the meter or unscrew the meter from the case. There are two ways this meter can be hooked up to the front panel, one by just the screws which connect the electrodes and a second way where one or more extra bracket has been screwed to the meter case behind the panel.

If you search this forum you will find some threads on this.

/Martin
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Old 7th Mar 2013, 9:31 am   #5
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Default Re: Avo VCM MK2 coil meter not moving!

When you have unsoldered, or removed, the wires from the meter tags you can solder a wire across the meter tags to short the meter so that you don't accidentally destroy it if you happen to drop some wires across the meter tags.

/Martin
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Old 7th Mar 2013, 10:02 am   #6
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Default Re: Avo VCM MK2 coil meter not moving!

Hello Lucas,
I have sent you a PM.
Regards Kevin
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Old 13th Mar 2013, 11:16 am   #7
Analogman
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Default Re: Avo VCM MK2 coil meter not moving!

Thanks for all your help and support,
I can see that this will be a longer restoration project but I am determined to get it done. Doing some research on the internet I may have found a replacement meter for the VCM Mk2. There is a german website that offers replacements and repairs for all Avo meters, but they are ridiculously expensive! So I did a search for moving coil meter 500uA 100 Ohms, which is what I believe the AVO VCM Mk2 to have as opposed to the MK3 and 4 which have a value of 30uA. In any case I found this website which offers many different types of meters;
http://shop.griederbauteile.ch/en/pr...ducts_id=16608
My question to the experts, would this meter work as a replacement for the AVO VCM Mk2??
Thanks,
Lucas
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Old 13th Mar 2013, 3:21 pm   #8
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Default Re: Avo VCM MK2 coil meter not moving!

Hi Lucas,

The original meter has an FSD current of 440uA and a resistance of approximately 100 Ohm. Since the meter is paralelled with a potentiometer there is some leeway in what FSD and internal resistance the meter can use. Unfortunately I don't have the articles and schematics with me right now so I can't say if the 500uA FSD meter will work right away but I think so.

I suggest that you try to measure your original meter to see what FSD and internal resistance it has. Measuring the FSD can be done with a DMM, a battery and a few resistors plus one or more potentiometers. You will have to be careful so you don't blow the meter when you test it. There is another thread describing this measurement for the 50uA AVO meters for the VCM163 in this forum: https://www.vintage-radio.net/forum/...id=2&styleid=2 in post #4 you can see the circuit diagram. You'll have to re-calculate the series resistors for a current of at least 440uA.

Before hooking up your original meter in the test circuit you can use your DMM to check that the current is not too high!

My next suggestion is that if you are going to replace the meter with something new that you use the OPAMP solution described by Gary Tempest in his article and then use a meter which fits somewhat better in size compared to the smaller 84*64mm meter in your link. It will be a little bit more complicated with the meter amplifier but hopefully your meter is OK so you won't have to replace it.

/Martin
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Old 13th Dec 2016, 1:38 pm   #9
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Default Avo VCM MK2 questions.

Hello,
This repair has been on a back burner for some years but now I am finally determined to get it done. Here is an update of the faults I have found and what I have done, as well as some question for the experts.

First off i bought the Radiophile article from Gary Tempest (back issue # 86 and 87) on "Restoring the AVO MK1 and MK2 Valve Testers". Secondly I bought the article from Euan Mackenzie in RadioBygones issue # 138 on "Servicing and Calibrating AVO Valve Characteristic Meters MK1 and MK2" These are of course a must have if any one is repairing one of these VCM's.

In the old thread i was having problems with my moving coil meter. I decided to remove the meter and to build a circuit for testing the FSD. In my case to remove the meter I had to remove all the front panel knobs (including the small push knobs), in order to remove the metal face-plate. The meter was held by two large screws left and right. I unsoldered the 4 wires on the back strip terminal and pulled the meter out. Some (inconclusive) testing showed that the meter is indeed working, which I was very happy about as finding a new meter is near impossible, and fitting a meter with an op amp because of different sensitivity or resistance would have been a lot of work. However these meters often loose magnetism over the years and there are some good threads here about adding magnets to adjust the sensitivity. At this point I would like to offer a link to an article i found about old analog meters that is very helpful, "Analog Meters - Getting the most out of your meter" by David Ashton http://www.embedded.com/electronics-...of-your-meter-

So here is my first question for the experts and the reason why my measurements are inconclusive, is because I am not sure to which point on the scale I should deflect to. The meter has three scales, the bottom scale reads from 0 to 100uA, the middle (replace - good) scale reads mA/V. and the top scale reads from 25Mohm to 0 (short). FSD means full scale deflection so I assumed to deflect to the farthest point on the scale which is the 100uA point. Adjusting the variable resistor in my circuit and using a fixed voltage of 1.3v from a AA battery and assuming an internal resistance of 100ohm in the meter, I read 437uA on my MM. I hope this is correct. If I deflect to the little red symbol on the top scale (short) I got a reading of 376,6uA. I will do some more conclusive testing and find the exact internal resistance but I wanted to clear up this question first.

Other problems found were a broken terminal soldering joint between MR4 (16K9) and R9, and of course a stuck "Set Zero" pot. This is the double stacked potentiometer RV1 RV2, and on mine the back one was frozen resulting in a broken shaft connector. I was able to repair the connector. I cleaned and inspected the insides of both pots and noticed that the stop poles were also bent. i was able to straighten them. Otherwise the wound wires look to be in good condition. One problem I am having however is that at the end of the turn the top metal tongue which is reading the resistance actually leaves the track as if the wound wires don't go far enough. This results in a sudden jump in resistance from about 8ohm to several hundred k.ohm. I was able to fit a small piece of rubber over the stop shaft to keep this from happening but this also keeps the other end from reaching the end of the track.

So to my second question. What resistance readings should I be getting on RV1 when it is in circuit (I have not desoldered the wires from the junctions)? on mine it starts at about 8 ohm then rises to about 119ohm in the middle then falls again to about 8ohm. RV2 is linear and goes from almost 0 to about 1,4Kohm.

My last questions are about bridging and/or component replacement. I have replaced C1 with 2x 16uF electrolytic capacitors in series to achieve 8uF. Is this okay or is there something disadvantageous to this approach? I replaced the 16K9 and KG1 rectifier diodes with 1N40007's. In Mackenzie's article he states to not use these but rather to use soft recovery diodes. Other threads here have discussed using 1n4007's without problems. before I continue to replace the other diodes I wanted to clear this up, and wanted to ask in general about bridging components. I think this is an elegant solution if the space allows. That way the original stays in but the new bridged component takes over the work if the old one should fail or go out of spec. Is this acceptable practice or are there situations when this should not be done. I am talking about diodes especially here as bridging resistors or capacitors will change the value. Mackenzie also says the EA50 thermionic rectifier should be replaced. I would still like to keep this in and if necessary bridge it with other components.

Sorry if this is so long and so many questions, but I hope that may be someone else benefits from these posts as I have from previous posts.

Many thanks, Lucas.

Last edited by Station X; 19th Apr 2017 at 8:38 am. Reason: White space added.
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Old 17th Dec 2016, 4:59 pm   #10
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Default Re: Avo VCM MK2 coil meter not moving!

Your FSD meter reading looks fine, when it comes to the reading at the red line (SET AC line) it looks a little bit high. With an FSD of 437uA the red line is usually around 367uA (some 84% of FSD) but it might be a little bit lower or higher as each meter is individually checked at manufacturing so the red line might be painted a bit more to the right or left depending on the meter. Can you see what line on the 100uA scale the red line corresponds to? A meter can sometimes be nonlinear in some regions so that the current needed to reach that point might vary.

When it comes to the potentiometer I am not of any help as I have never repaired those (never owned an AVO Mk II VCM).

However when it comes to the diodes and EA50 I can recommend that you disconnect the old ones at one end, removing them from the circuit but keeping them for historical reasons, and isolate that end and replace them with a modern diode. Be careful with the old diodes as the ends sometimes comes loose, so sometimes it is better to remove the wire connecting one end and then solder the new diode to the wire and covering it with shrink tubing and then solder the other end to its connection. Bridging sometimes works well but only until the old diode stops working and then it might be a hard work to find the fault.

In my opinion it is also better to use an 8.2uF/630V Polypropylene capacitor compared to two 16uF in series as you will otherwise have to put shunting resistors across them to share the voltage across them so they perform as an 8uF capacitor and those shunting resistors will affect the circuit - just like Euan writes.
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Old 23rd Dec 2016, 9:09 am   #11
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Default Re: Avo VCM MK2 coil meter not moving!

Thank you Dekatron for your reply.

In the meantime I have been checking and cleaning the valve panel. I had to take the 'enigma' switch apart to clean, as it was sticking, and I found one blown resistor as well as other resistors out of value. 5k, 75 ohm, and 130 ohm are the original values. I have ordered some resistors, silicone tubing and a 8,2 uf polypropylene capacitor from Just Radios in Canada. They are on the other side of the pond but have practically everything one needs at reasonable prices.

Until the parts arrive and I get back to the project, I wish everyone a happy Christmas holiday!
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Old 27th Feb 2017, 11:02 am   #12
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Default Re: Avo VCM MK2 coil meter not moving!

Hello again.

Update; I have been working sporadically on the restoration project and am now at the calibration stage, where I am having some problems.

Questions;

1. Which calibration procedure do you think is best?
Mackenzie talks about a chicken and egg problem when using the avo manual calibration procedure.
I successfully got through the first part of his procedure ( adjustment of "S" meter sensitivity) but have had troubles with step two, measuring Vrms and step three, neg. grid voltage. Partly, I think, due to the false line voltage value.

2. How should I adjust the line voltage to a correct value?
On the back I have sockets for 220-230 and 240-250, but my line voltage is somewhere around 233v. Of course this can be adjusted for via the 'Set' button, but this also effects the other values which need to be calibrated, therefor the chicken egg scenario. I was thinking of adding a resistor in series or a CL-90 inrush current limiter to drop the voltage 4 or 5 volts before the primary on the transformer. Then I would be in the 220-230 range.

3. Does changing the old rectifier diodes to new silicon diodes also change the supply voltages? I know a lot of old selenium diodes also have a large voltage drop on them where as new diodes hardly have any. Should this be compensated for?

4. What is the proper way to measure Vrms?

Thanks, Lucas.

PS. for calibration purposes the valve board is not connected at the moment.
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Old 17th Apr 2017, 10:56 am   #13
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Default Re: Avo VCM MK2 coil meter not moving!

Bump, no answers? Should I start a new thread about this?

Lucas
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Old 17th Apr 2017, 12:33 pm   #14
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Default Re: Avo VCM MK2 coil meter not moving!

I have no experience with the VCM as I have only worked on conventional (analogue) Avometers, but I'm sure there will be people with proper insights on here who will assist- bear in mind it is Easter weekend so many have gone away and may be offline.

I must admit in most cases I'm not a fan of leaving failed components in place and bypassing them with new, unless it's something like a wirewound bobbin that can be snipped electrically free from the circuit.

I also don't replace sound components with modern ones unless they are a) likely to fail in the near future or b) very awkward to get to and having taken everything apart this is my best opportunity to get at them!

Replacing ancient capacitors is wise (but changing them all regardless of reputation and serviceability divides opinion on this forum and others.)

As for measuring Vrms a meter which says it can measure 'true rms' is the way forward..?

This is where my knowledge runs out, unfortunately.
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Old 17th Apr 2017, 9:31 pm   #15
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Default Re: Avo VCM MK2 coil meter not moving!

Avo VCM and CT160.

For Lucas "Analogman".

I posted the set-up and calibration proceedures for the CT160 / VCM mkIII and VCM mkII and mkIV some years ago. The original information came from the Avo Test Dept in 1959.
The proceedure suggested had the great merit of not requiring a perfect valve to set everything up against, which is the method which appears in later version of the manual. And gives Mackenzie much thought.

It also included a modification to the MkIII to prevent damage to limit screen current, which was built in in an improved form in the MkIV.

You will find that that posting includes adjustments for the meter correctly for the SET position, which corrects for variations in the mains input and brings all other voltages into their correct ratio.

May I suggest you do a search. May have been in Sept 2009 with later comments upon interpreting readings of a RMS calibrated meter or a peak reading meter when measuring half-wave rectified as in the Valve Tester.

Congratulations on your efforts so far.

wme_bill

Last edited by WME_bill; 17th Apr 2017 at 9:43 pm.
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Old 17th Apr 2017, 10:28 pm   #16
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Default Re: Avo VCM MK2 coil meter not moving!

I think this may be the thread: https://www.vintage-radio.net/forum/...ad.php?t=45173
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