|
Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
|
Thread Tools |
25th Jul 2018, 8:15 pm | #1 |
Pentode
Join Date: Jul 2009
Location: Brightlingsea, nr. Colchester, Essex, UK.
Posts: 209
|
Avo VCM mk2 puzzle
Could anyone tell me which flavour of the Mark 2 I own? I'm puzzled by the apparent absence of an EA50, shown on all the circuit diagrams I have. It has three mains transformer, again not documented on my drawings.
The meter serial number is 455-948, if that's a clue. Thanks in advance. Bryan |
26th Jul 2018, 6:08 pm | #2 |
Tetrode
Join Date: Jul 2006
Location: Montréal, Canada.
Posts: 78
|
Re: Avo VCM mk2 puzzle
There is some information floating around on this tester but it seems to be somewhat succinct. You may have already found this thread:
https://www.vintage-radio.net/forum/...ad.php?t=99311. I did carry the restoration of a MK II (regular 2 tranformers & EA50 diode unit)and would not have been far if it was not the generous help of a few members of this forum who are "authorities" on the subject. Hopefully someone will come up and shed more light on this special tester you have. Cheers. |
26th Jul 2018, 8:41 pm | #3 |
Pentode
Join Date: Jul 2009
Location: Brightlingsea, nr. Colchester, Essex, UK.
Posts: 209
|
Re: Avo VCM mk2 puzzle
Thanks, vaslejean, for that very helpful reply. I hadn't come across David Simpson's thread in my searches, and it is clear that he and I share the same VCM model. I'd opened mine up to clean a switch contact (and also discovered what seems to be a common problem of all models, namely the un-ganging of the set zero pots).
Reverse engineering my VCM would require far more dismantling than I'm prepared for, so as I have a functioning machine I'll leave it at that. I'd be interested to know however what part the non-existent EA50 would have played. Bryan |
27th Jul 2018, 12:42 pm | #4 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
|
Re: Avo VCM mk2 puzzle
Hi Bryan, I think you would find the article in Radio Bygones Christmas 2011 Issue No. 134 by Euan MacKenzie informative. You have a PM. It says "VR92/EA50 is used to rectify the screen voltage...… It should be replaced by a modern silicon diode. It is well worth fitting soft recovery types eg Philips BYW96E, as these minimize circuit switching oscillations and EMI". Cheers, Jerry
|
27th Jul 2018, 5:15 pm | #5 |
Pentode
Join Date: Jul 2009
Location: Brightlingsea, nr. Colchester, Essex, UK.
Posts: 209
|
Re: Avo VCM mk2 puzzle
Thanks very much, Jerry. I have downloaded the Radio Bygones mag but done little more than glance at it. However the photo it shows of a Mk1 looks like mine (eg no lifting handles on the front face), so I may own a Mk1 after all. I'll report back when I've digested the article.
Bryan |
27th Jul 2018, 5:18 pm | #6 |
Tetrode
Join Date: Jul 2006
Location: Montréal, Canada.
Posts: 78
|
Re: Avo VCM mk2 puzzle
Thank you for your kind word Bryan, as Jerry is suggesting there is some highly worthy information gathered in the article he is referring to. I went a long way to attempt restoring some of the controls (SET MA/V & SET ZERO) with little success. Some of the switches needed replacement...
I eventualy went for the suggestions made in the RADIOPHILE article (another article to have!) signed by Garry Tempest and replaced both SET MA/V & SET ZERO assembly with a NOS tektronix scope dual rotary switch that has a concentric inner shaft. It allowed me to install a pot at the rear of the switch for fine adjustment... ..I would like to find (now this is a long shot) full specs of the transformers You might want to see where the leads of that 3rd transformer go to ...? These instruments are worthy of restoration. |
1st Aug 2018, 3:15 pm | #7 |
Pentode
Join Date: Jul 2009
Location: Brightlingsea, nr. Colchester, Essex, UK.
Posts: 209
|
Re: Avo VCM mk2 puzzle
I've attached some pics of my VCM. The centre transformer appears to supply heater volts and the connections to the 'Set ~' switch. The larger of the other two supplies anode/screen volts.
I'll try calibrating it according to the article in Radio Bygones, and make any changes that might throw up, but apart from that I'll leave a more-or-less working tester alone. If any more detailed pics are needed I'll try to provide them. Incidentally the lid doesn't contain either operating instructions or retaining clip for the plug-in valve cap connector. And the mains lead exits inconveniently through the bottom plate, with a sharp bend. Thanks all. Brtyaqn |
1st Aug 2018, 4:22 pm | #8 | ||
Tetrode
Join Date: Jul 2006
Location: Montréal, Canada.
Posts: 78
|
Re: Avo VCM mk2 puzzle
Quote:
Quote:
I am attaching blue prints of the transformers that have found last night where there is a mention made of transformers assembled by outside contractor ... |
||
1st Aug 2018, 4:44 pm | #9 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,853
|
Re: Avo VCM mk2 puzzle
Hello Bryan, 'Twas my thread that Vaslejaen mentions in post 2. I never did locate a circuit diagram for a three transformer Mk2. It had been 'got at' at some stage previously. Someone had tried repairs without much knowledge of AVO VCM's & no circuit diagram.
As I had a couple of later VCM's at the time, I completely stripped it out to all its separate parts. The Anode/Screen Transformer was used to make a variable switched HV DC supply for a bench PSU, and likewise the Heater/Set ac/ Mains Adj. T/Former was used for an LV DC supply in the same PSU. I've still got the 3rd T/F somewhere. The front panel & a load of switches & pots, & paxolin tag boards, plus the duff C/out - I gave to another Forum guy. Regards, David |
8th Aug 2018, 2:45 pm | #10 |
Pentode
Join Date: Jul 2009
Location: Brightlingsea, nr. Colchester, Essex, UK.
Posts: 209
|
Re: Avo VCM mk2 puzzle
Thanks for your reply, David - slightly delayed because I'm now in danger of becoming a bit obsessive about these VCMs, having read several articles including the Bygones article, your thread (quoted above) and your (later?) one explaining the explanation of the 160.
So thanks to all for confirming that I have a rather unusual flavour of VCM Mk1. Bryan |