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Old 25th Mar 2021, 12:32 pm   #81
Dekatron
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Default Re: AVO 163 amp board ~ transformers

Shouldn't it be possible to build a small test-jig with a circuit with an oscillator and amplifier similar to the one used in the VCM163 and then use this circuit to adjust a new transformer with, apart from checking the transformer inside an original VCM163 circuit?

A working transformer could be inserted as a control element to check against as a sort of calibration.

One thing to remember is that the oscillator in the VCM163 runs at approximately 15kHz but the Marconi TF1313 only runs at 10kHz in the test specification (which I think was its maximum rfequency).
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Old 25th Mar 2021, 1:21 pm   #82
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Default Re: AVO 163 amp board ~ transformers

10kHz is probably close enough for strays to show up, and likely AVO's figure done at this freq was a little off of the ball because of this.

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Old 25th Mar 2021, 1:58 pm   #83
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Default Re: AVO 163 amp board ~ transformers

Well, the tolerance of the Q-factor is +/-10%, the resistance +/-20% for the test specification, so its not exactly a high precision measurement that needs to be made for them to pass.
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Old 25th Mar 2021, 2:25 pm   #84
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Default Re: AVO 163 amp board ~ transformers

I hesitate to suggest this, (just in case nice Farnell stuff gets phooked as well as 163's) - - - but an FG2 or an LMF4 would lend themselves to a decent source of 15KHz for such a project suggested by Martin.
When I built my DC Valve Tester several years back, I also made a plug-in rig for Dynamic testing. I needed to "modulate" the pure DC Vg source applied to the Grid. And indeed, did have a good lewk at the 163's circuitry, but decided to try something much simpler. In the end I simply passed the Vg through the sec winding of a wee transformer, and externally sourced an AF of 1KHz from my LMF4 & fed it to the primary. Trial & error, and monitoring voltage loss & sinusoidal purity on a Hameg - for several "AF" T/F's - - eventually plumbed for a 1:1 "Linolite" Isolation T/F out of a gash bathroom light/shaver unit ! Lo & behold - less than 5% of losses, and the signal's waveform was ok up to 5KHz.

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Last edited by David Simpson; 25th Mar 2021 at 2:30 pm. Reason: Add'l info
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Old 26th Mar 2021, 1:30 pm   #85
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Default Re: AVO 163 amp board ~ transformers

Let's not lose the momentum you guys have achieved . Are you now in a position to write a first draft of a specification for the amp transformer that would be sufficient for Ed to work from?

Then we can move on to the osc o/p transformer; I think that should be an easier task .

B
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Old 26th Mar 2021, 9:11 pm   #86
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Default Re: AVO 163 amp board ~ transformers

Just in case anyone following this thread has never seen it, there is an interesting review of the workings of the 163 published in Radio Bygones, No140, Christmas 2012, Kurt Schmid, DH3PJ.

He presents five figures taken from a dual channel scope looking at the AF voltages measured on the grid and anode circuits of the tester, but he does not say precisely where the measurement were taken from. He reports the oscillator was running at 14.68kHz (IIRC, I think I measured mine at 14.4).

I'm not sure where I got this article; perhaps it's been presented on the forum at some previous time?

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Old 26th Mar 2021, 9:58 pm   #87
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Default Re: AVO 163 amp board ~ transformers

So, does anyone know if there are any reasonably priced Q-meters that I can buy or any reliable kit that I can build for this very purpose?

The measurement in a VCM163 frequency can vary quite a lot and still give you the same measurements as long as the amplitude is calibrated.

I do think that we have what we need, but we might get more information if it turns out that the transformer on the amplifier board that HBWOODY was going to ship to Ed is open circuit and needs a rewind, it will then be possible for Ed to unwind it and count turns for each winding.
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Old 27th Mar 2021, 12:51 am   #88
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Default Re: AVO 163 amp board ~ transformers

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Originally Posted by Dekatron View Post
So, does anyone know if there are any reasonably priced Q-meters that I can buy or any reliable kit that I can build for this very purpose?
Hi Martin

I hadn't realised that the measurement of the Q is something that is still needed; I had, perhaps naively, thought we could get the turns number from L.

I have a Wayne Kerr component tester B4225, which is primarily for LCR measurements, and I don't recall using it for Q, but I believe it can do it at frequencies to 10kHz. I'll take a closer look at the manual, which is attached.

B
Attached Files
File Type: pdf Wyn Kr 4225 Man.PDF (3.01 MB, 13 views)
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Old 27th Mar 2021, 4:49 am   #89
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Default Re: AVO 163 amp board ~ transformers

Those are rather low Q values, well within the capabilities of a Wayne-Kerr bridge and not really needing a full-blooded Q-meter of the resonating type. It's rather a low frequency for many of them.

We can get turns numbers from the inductance values and the core Al figure. The available core Al values are not quite the same as the old Vinkor parts, so we have to calculate our own turns numbers anyway.

Knowing N28/N48 material at that frequency range, I'm not expecting Q to be a problem, and with that 10 Ohm resistor on the input, I expect it to be damped in-circuit to well below what the coil assembly alone achieves.

There are trick ways of winding pot cores to optimise Q, such as using tape wound first on the former to pad out the windings away from the gap a little, and away from its fringing flux. This relies on using an oversize core. It's something done in specialised filter designs needing the very highest Q inductors.

If the losses in this transformer come out higher or lower than the original, we can fiddle it in the amplifier circuitry.

David
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Old 27th Mar 2021, 7:28 am   #90
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Default Re: AVO 163 amp board ~ transformers

We are somewhat space constrained as we only have an inch in height, 25.4mm, as the original circuit board is mounted between two aluminium sheets. Since we need some kind of adapter to fit a new core for the pins to match up we can only use some 23.8mm as 1.6mm is needed for the adapter circuit board. If you are willing to drill new holes in the original circuit board we can use the full height.

The P-core from TDK that I suggested above in thread: https://www.vintage-radio.net/forum/...3&postcount=66 seems to have almost identical parameters as the original LA2401 P-core, I've included a screenshot where I've encircled the values. This one will fit with an adapter circuit board.

But hopefully we will know how to rewind the original core after Ed has had a good look at HBWOODYs so we probably won't have to bother with a new design as these cores almost never break and the original core with mounting details can be reused.
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Old 27th Mar 2021, 6:36 pm   #91
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Default Re: AVO 163 amp board ~ transformers

Hi Gents, I'm keeping an eye on the progression of the thread and its getting very interesting.
Cheap Q meter is possibly the Advance one. I have a Marconi TF1245A here that is in need of setting up and has both oscillators.

I don't think cores will be a problem for this application, there are plenty of types available, and if we produce a new board there is no restriction on bobbin pin outs.

With little practical reduction in screening it is possible to adjust the gapping into a range required for the adjuster (heathen yes, but it works).

I'll need to have a closer look at the oscillator output transformer to determine the size of the lams used and if a bobbin is available so good electrical isolation can be achieved.
It will probably be a similar size to a lot of TV blocking oscillator types

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Old 27th Mar 2021, 8:26 pm   #92
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Default Re: AVO 163 amp board ~ transformers

As regards height limits, this all depends on what each of us wants to do.

My preference is to ditch both old boards and try build one new board with the osc, amp and 9-0-9V psu on it (dimension of the original boards are 100x125mm).

The Wien oscillator I have played with has a PCB of 40x35mm and the AF millivoltmeter is on a piece of veroboard about 40x35 mm (coincidence).

If that were done, we wouldn't have any limit on height for transformers.

Using a dual op-amp IC for the amplifier, we could dial in whatever gain we needed; we don't have match exactly the existing amp transformer on the secondary. I'd say that the $64M issue with that transformer is that it needs to cope well with 400V on the primary (relative to the chassis).

However, I do appreciate that the "one new board" strategy may not be good for everyone.

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Old 27th Mar 2021, 11:28 pm   #93
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Default Re: AVO 163 amp board ~ transformers

I think that we should see this as different steps in a solution where one step is to replace each of the transformers with something new, another step rewinding damaged ones and then also look at a separate step with a new solution with a more modern design. As long as the two thermistors arenít blown Ed should be able to rewind and adjust the transformers if they are damaged, with a cracked pot core a new one could be used and placed on the old base, and if that piece needs to be replaced too an adapter PCB can be made.

When I tried new designs I found it easier to make new circuit boards that fit with the original wiring harness compared to rerouting the wires inside the tester. There is also one more aspect of the design that AVO mentioned somewhere in the design documents for the VCM163 (or it could have been for the VT161) where they mention magnetic coupling between the transformers as a reason for placing them apart from each other, I donít know how big this effect could be if they are placed side by side but it is possible that it gives rise to some crosstalk.

With a modern design I do think that we can find small ferrite signal transformers to experiment with. I used ferrite pulse transformers for some of my tests as I had a few lying around and I didnít have any proper signal types. It worked well with the pulse transformers although they werenít designed for it, nor did I do any calculations to check if they were running within their design limits, I just adjusted tje signal level until it was correct and then checked the waveform with my scope. I also hand wound a few transformers on RM10 & RM8 cores and some small EI transformers that I had for other purposes (blocking oscillators for Trochotrons).

When it comes to the oscillator design I think it is most important to get a temperature stable design so the amplitude is stable like in the AVO design. The amplifier however can be designed as one wishes as it is not ctitical as long as it can be adjusted to give the correct figure.
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Old 28th Mar 2021, 12:04 am   #94
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Default Re: AVO 163 amp board ~ transformers

Not having a VCM163, I'm not intending building anything. It's not something I intend to buy and getting one just for this job, at those prices, is a no-no.

David
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Old 28th Mar 2021, 12:09 am   #95
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Default Re: AVO 163 amp board ~ transformers

OK Martin, so would you say that the right approach would be for Ed to consider whether he can duplicate the original transformers as closely as possible?

If that's the case, what info is still lacking? I don't know for sure if HBWOODY sent his board to Ed.

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Old 28th Mar 2021, 12:11 am   #96
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Default Re: AVO 163 amp board ~ transformers

A lot of AVO valve testers of all models ended up in the skip from the 70's until valves became fashionable again. There is a guy I know who got rid of ten VCM163's in a skip in the mid 90's. He really, really wishes he kept hold of them!
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Old 28th Mar 2021, 12:12 am   #97
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Default Re: AVO 163 amp board ~ transformers

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A lot of AVO valve testers of all models ended up in the skip from the 70's until valves became fashionable again. There is a guy I know who got rid of ten VCM163's in a skip in the mid 90's. He really, really wishes he kept hold of them!
Yes, mine was given to me.

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Old 28th Mar 2021, 7:56 am   #98
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Default Re: AVO 163 amp board ~ transformers

Hi folks, I only have a MK3 Avo tester here of my own, but HB Woody has sent me the measuring transformer (ferrite). I don't as yet have an actual oscillator output type (laminated)

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Old 28th Mar 2021, 8:49 am   #99
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Default Re: AVO 163 amp board ~ transformers

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Quote:
Originally Posted by Craig Sawyers View Post
A lot of AVO valve testers of all models ended up in the skip from the 70's until valves became fashionable again. There is a guy I know who got rid of ten VCM163's in a skip in the mid 90's. He really, really wishes he kept hold of them!
Yes, mine was given to me.

B
Lucky you. In fairness my Mk3 and Mk2 were given me. Both with faults, both fixed.

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Old 28th Mar 2021, 9:49 am   #100
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Default Re: AVO 163 amp board ~ transformers

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Originally Posted by Ed_Dinning View Post
Hi folks, I only have a MK3 Avo tester here of my own, but HB Woody has sent me the measuring transformer (ferrite). I don't as yet have an actual oscillator output type (laminated)

Ed
Have you had time to check the amplifier transformer yet?
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