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Old 30th Mar 2021, 8:20 am   #141
Dekatron
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Default Re: AVO 163 amp board ~ transformers

No problem David, Iíve bern thinking about mentioning it on several occasions but held off until this time, it feels good to have let it out and being over it - so it was actually very good that you brought it up!
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Old 30th Mar 2021, 10:13 am   #142
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Default Re: AVO 163 amp board ~ transformers

Quite frankly Martin, the issue of access to vintage documentation has been a sore point with many VR folk in the UK. VMARS have a great archive, but there used to be issues with Royal Signals/WS19 folk over free access to their stuff, for example.
Back to reality - can someone with better eyesight & patience than myself - please sum up(briefly) the origins of the 163 woes(real or imaginary), and how things stand just now.
My belief is that eddy currents around the treacherous Cornish coast have somehow affected the ferrite cores of wee transformers. However, in reality, some genuine and careful 163 owners do have concerns about their aging but loved hexpensive valve testers.
Over the years I've communicated with a number of AVO VCM owners & not one of the 163 folk have raised issues about the semi-conductor circuitry. Pretty bomb-proof I would have thought. My hero, and all-time foremost Valve Tester UK expert - Denis Tabor - speaks very highly of the 163.

Regards, David
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Old 30th Mar 2021, 6:58 pm   #143
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Default Re: AVO 163 amp board ~ transformers

Quote:
Originally Posted by David Simpson View Post
Can someone with better eyesight & patience than myself - please sum up(briefly) the origins of the 163 woes(real or imaginary), and how things stand just now. I've communicated with a number of AVO VCM owners & not one of the 163 folk have raised issues about the semi-conductor circuitry. David
My particular experience: apart from the shorted turns Gm meter, everything was fine until 2 years ago, switched the 163 on and it's not working. All indications point to the boards, and sure enough, the old electrolytic caps are in various states of distress, especially the 1uF's which have become resistors. Changed them all out for some good ones, put the boards back, but still not working. I was not overly impressed with any aspect of the build quality of the boards, the soldering looked pretty "average". I'd have to dig out my old notebook to find out what else I had to do, but basically, it was the point at which the caps went where pandora's box seemed to get opened, even though I treated the boards very gently. After that time, I've had some other problems.

HBW's amp board resisted Martin's best (on-line) efforts to get it working, and the transformer fell under suspicion. The board has been sent to Ed, who has characterised the transformer, which appears to be working, but he has identified other issues with the board. The old 2N2926 transistors seems to have aged badly. I think that's where we are

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Old 30th Mar 2021, 7:16 pm   #144
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Default Re: AVO 163 amp board ~ transformers

Old epoxy potting technology was not that good, long term. The 2N2926 in various colours were the go to device - in the very early 70's when I was first fiddling around with electronics. I would not be at all surprised that device performance had drifted due to mechanical stress on the chip in the half century that has passed.
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Old 30th Mar 2021, 8:33 pm   #145
David Simpson
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Default Re: AVO 163 amp board ~ transformers

Don't know if anyone has noticed the Thread raised by by "paulsherwin" back on 25th May 2016 regarding the 2N2926, might be of interest.
You've certainly got 163 woes Bazz, and I'm sure forum folk will endeavour to help you. I've recently been in Transistor Testing(& Ic/Ib graphing) mode, I'll see if I've any replacements or alternatives to the 2N2926.

Regards, David
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Old 30th Mar 2021, 9:36 pm   #146
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Default Re: AVO 163 amp board ~ transformers

I'm not sure if mine is any worse than "average". They are ~50+ years old, mine spent a long time unused in less-than-perfect storage conditions. I don't feel unhappy with it, but I intend to recover its reliability, and if that means complete replacement of the two boards with one or two new boards, I'm happy to do that. A new osc board (reusing only the old transformer, or anything that Ed can offer) may be here sooner than you think! In prototype form, it uses two SS9014's, which are somewhere close to BC547C, NTE2672, BC337−40, BC635, MPSA18, 2N5962, MPS6571, 2N2222, S8050, 2N4401, BC537, SS9013.

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Old 31st Mar 2021, 9:32 am   #147
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Default Re: AVO 163 amp board ~ transformers

Folk can hardly avoid the "Sticky" section whenever they open these pages. We can see that "swordholder" had a thread(now closed) back on 1st May 2010 (nigh-on 11 years ago) on VCM163 Faults. Apart from heaps of great advice, he proposed a dedicated VCM 163 Section which could help owners over many years. But it was never taken up. Likewise, for several years now I've proposed a similar Section for the lesser AVO VCM's & CT160's, but zilch !
So this thread has taken up 8 pages, much to the determent of Martin's & David's wrists.
But there are already presidents accepted by Paul & his Moderators - the giant "Mullard HSVT Card" project, and the AVO Multimeter project. Both have ran for years & never closed !
VCM163's are just about the most expensive bit of kit folk can aspire to. So if the two pcb's & one or two other components are now threatening their continuing excellent service to forum members - lets get everyone singing from the same hymnsheet in one super thread, whilst also avoiding more & more repetitive threads being raised.
"SOR" - - Save Our Wrists" !

Regards, David (wearing his Carpel Tunnel elasticated support)
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Old 31st Mar 2021, 8:18 pm   #148
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Default Re: AVO 163 amp board ~ transformers

Hi Folks, after digesting the above thread and doing a little calculating on my own I believe I can produce suitable transformers that while not being "drop in" should be easy to fit.
To save confusion I have defines as follows:

Amplifier Input transformer, a ferrite device wound on a pot core that is part of a tuned circuit to resonate at 15KHz and reject the 50Hz 1/2 waves thumping through the valve.
Good insulation pri sec and needs "tuning " to the oscillator for best results.
LA1224 is a pair of cores with adjuster and is fitted onto a baseboard PCB. This is a ferrite good to 750KHz and a mid range Al (T/nH) of 295.7. It is 21mm dia.

Working backwards from the measured figures this gives a Pri of 18.34turns of 0.25mm wire and a Sec of 569turns of 0.12mm wire. These give a good fill of the bobbin.
2 layers of type 56 transformer tape is good for more than 2KV.
Turns ratio 31:1

These agree with Davids figures closely.
Other cores can be substituted easily; I may go with the Mullard FX series as I have some of them.
Probably better sticking to a similar material to A13 and the ue of 100, if an increase of size then adjust turns for the same L

Oscillator output transformer, a laminated device about the size of a very small valve portable or TV blocking osc type that is untuned and looks to be a simple design
It is a 2:1 step down ratio
Assuming a max swing of say 4v RMS, something like an EI 35 core (35 * 30mm lam)
(centre limb 9 * 6mm); Stalloy material running at 0.6 Tesla well away from saturation, and an interleaved construction as Dc is blocked by C6.

We get a pri of about 2 turns. Clearly not an ideal figure to wind, so we can either use a much smaller core or we can drop the flux density.
The bobbin has plenty of space so simply go to 200 turns pri and 100 turns sec.
From the circuit constants the primary current will be of the order of 1mA, so 0.15mm wire will give a good fill.
This gives a transformer that is overkill but is easy and cheap to manufacture.

Your comments please folks. I'll then try and get some wound up if anyone wants to try them in their tester. I'll probably do the oscillator output one first as it is the most empirical design

Ed
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Old 31st Mar 2021, 8:38 pm   #149
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Default Re: AVO 163 amp board ~ transformers

If you want the oscillator transformer to be a drop-in solution you will need to make it so that the resistances (DC) are equal to the original ones as the rest of the resistors in the mA/V rotary switch depends on that resistance. There is a calibration potentiometer there which can be used to adjust for some change but as close as possible to the original resistances is always best, especially since the potentiometer sometimes end up close to its end of the track - the potentiometer could of course be replaced with another value to fit the variation.

I'll see if I get some time in the coming weeks to desolder my oscillator transformer and perhaps give you some more measurements.

If you can get hold of the RM8 or RM10 cores with adjuster they are a good substitute too.

Whatever core you choose, please send me information on the footprint and if you use a bobbin with pins so I can add it to the small circuit board adapter I've drawn.

You are welcome to send me a pair of them when they are finished so I can test them (I'll pay for shipping of course).
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Old 31st Mar 2021, 9:31 pm   #150
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Default Re: AVO 163 amp board ~ transformers

Regarding the osc transformer, I've just figured out in recent days that we can get measurements from it without removing it from the board (i.e. de-soldering 4 connections and one hunky retaining tag).

On the primary winding, one connection goes to ground and one connection goes to one of the preset pots. The layout will permit taking a scalpel to the track that goes to the pot, and cut out a ~1mm length and thereby fully isolate the primary and get the measurements.

The secondary winding only connects to the two output tags on the PCB. Put a meter across those tags and you get the readings. I agree that the need to "match" the secondary to the grid circuit, as best possible, is important.

At this point, the gap in the primary track can be repaired.

I may be able to do this task myself quite soon and will keep you posted to try avoid duplication of effort.

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Old 1st Apr 2021, 12:18 pm   #151
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Default Re: AVO 163 amp board ~ transformers

@Ed_Dinning: The nominal output voltage from the oscillator transformer is some 81-82mV RMS in circuit and the DC resistances are important to get the original circuit to work properly.
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Old 1st Apr 2021, 3:43 pm   #152
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Default Re: AVO 163 amp board ~ transformers

Hi Martin, thanks for that, so there is only about 160mV on the primary and the transformer is very lightly used.

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Old 1st Apr 2021, 7:12 pm   #153
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Default Re: AVO 163 amp board ~ transformers

Attached is a close up of the osc transformer, taken from the secondary (output side). Note the thin wires from the winding, wrapped around the thicker silver wires which form the connections to the PCB..

Her Majesty looks on...and is not too sure about the quality

B
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Old 2nd Apr 2021, 7:59 am   #154
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Default Re: AVO 163 amp board ~ transformers

Hi Baz, thanks for that , it looks like a 1" lamination popular for small transformers at the time and smaller than the one I was looking at.

Are the lams gapped, it looks a bit like it from the picture and it could also be using large and small E lams, which would be difficult to get, but not to rewind.
There should be no DC in the primary winding, but there may be in the secondary winding, which would require a gap.

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Old 11th May 2021, 9:30 pm   #155
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Default Re: AVO 163 amp board ~ transformers

Today I tore down both the oscillator and the amplifier transformers and counted the turns and made some more measurements and wrote it down in the included PDF-file.
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Last edited by Station X; 12th May 2021 at 11:53 am. Reason: Attachment removed at poster's request.
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Old 11th May 2021, 9:49 pm   #156
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Default Re: AVO 163 amp board ~ transformers

Thanks for posting that detailed info Martin. I'm not sure where Ed has now got to with the work he has been doing, but he did report that the oscillator transformer he was working on was intermittent, so presumably needs a full re-wind.

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Old 12th May 2021, 11:51 am   #157
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Default Re: AVO 163 amp board ~ transformers

Spotted an error in the document, a little cut-and-paste fault made by me, the primary and secondary winding instructions for the amplifier transformer got wrong, wire diameter has been corrected and also which is named primary and which is named secondary to be consistent with AVO test specifications.

@Moderators, please delete the old PDF.

For all of you who downloaded the old PDF, please download the new one and throw away the old one!
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File Type: pdf AVO VCM 163 Amplifier and Oscillator transformers-V2.pdf (911.4 KB, 19 views)
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