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Old 29th Mar 2021, 10:39 am   #121
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Default Re: AVO 163 amp board ~ transformers

With 2N5087, it was just a device I know where the hfe holds up well at low quiescent currents. Yes, it's PNP, but I was still thinking in terms of doing a new board design, so we could go with either polarity if we wished.

The first transistor is a simple emitter follower, and as Craig says, there is little gain variation with hfe.

More significantly, the loading imposed by its base connection on the preceding circuitry changes almost inversely proportionately to hfe.

The preceding circuitry is none other than the resonated transformer. Herein lies a two edged sword. If VT1 had immense current gain and we took advantage of this by making the bias resistors wather large, that second resonating capacitor would look like it went into an open circuit. It would lose its tuning effect. resonance would rely on the capacitor on the primary side, with the ten ohm resistor in series with it setting the Q (if the transformer's unloaded Q is appeciably higher) but that capacitor on the secondary is spec'd as a tight tolerance part. AVO think it's a player in controlling their filter, so the impedance it works into is significant. In an approximation to an ideal world where we relied on passive component values not active component values to set performance, it would be those bias resistors which were important with the loading from the transistor being much lighter.

VT1 may not be providing any voltage gain, but it could be providing appreciable power and current gains. Not to make the circuit voltage gain bigger, but just to make the secondary side Q loading dominated by the resistors.

VT2 onwards is a feedback amplifier, using a series-feedback style which increases its input impedance significantly, so the loading on VT1 should be quite light.

A quick estimate of the resistance which VT1 base appears as is the total load its emitter sees (next stage parallelling bias resistors) plus re, all multiplied by hfe.

The subsequent stages do provide voltage gain, making up the forwards gain of the feedback loop into the 200 Ohm current sensing resistor which is effectively the load seen by VT3. In the case of these last two transistors, hfe directly influences the open loop gain. Not having enough OLG will progressively introduce errors. Having too much can remove the stability margin and leave you with a circuit which may oscillate under some conditions.

The usual approach is to have plenty of OLG available from the active devices, but then to carefully use some local degeneration with, say, undecoupled emitter resistors to reduce the gain back to what you want. This has the effect of reducing the accuracy that active device gains would need to meet.

In the valve world, you get reasonably closely-controlled Gm figures to work with. In the transistor world, all hell breaks loose and you have to handle huge variations in device gains, yet you still want your circuit to work within tight specs. Different approaches to design are needed. Notice that many device specs are open ended, usually giving no upper limit on hfe. The weasel department at the semiconductor firm usually inject a statement about having a policy of countinuous device improvement and that specs can be changed without notice. Yeah, right. That's helpful. Here's the data sheet, and by the way, you can't trust it!.

This progressive improvement has led to circuit designs that once worked respectably becoming untenable with more recent devices. It's a long time since anyone made 2N3055 devices bad enough that you could rely on them to work stably in a Quad 303 amplifier.

Life's easier at lower power, and we can use emitter degeneration to rein in local gain variations, and thereby have enough control over OLG that we can design a stable loop.

If you get involved in the more serious end of circuit design, you sometimes can wind up with loops within loops... something which can blow people's minds, even some experienced people, but it can be done. It's a bit like a general plotting out tomorrow's battle. You don't know what the enemy's going to throw at you, but you need a resilient enough plan to be able to handle it.

Back to that VT1 emitter follower stage.... anothe valid way of taking signal from the resonator and controlling its Q would have been not to try to make a huge impedance, for high Q, but to try to make a very low one. To have the total resonating capacitance shared between primary and secondary (appropriately scaled by inverse turns ratio squared) and to thereby sample part of the circulating current and to dump it into an amplifier with very low Zin. Something like a grounded base amplifier, with a loading resistor in series with the signal path setting the Q (hopefully dominating re)

When you're trying to control the Q of a resonator you can pick between making a high resistance parallel load, or a low resistance series load, and it all comes out in the wash.

There are all sorts of places in electronics design where you can stand things on their heads this way and sometimes one approach gives an easier result than the other.

Back to AVO's original design: I don't see any need to redesign it. A transformer can be wound to replace the original, there may be a difference in the core and an adjustment to the number of turns but that should be OK. I suspect there are relatively few transformers which have failed. The amplifier circuit works, you can get away with a fair amount of variation in transistor performance, and I don't see the PCB itself as a failing component. Damage during refurb like pads coming loose can be fixed as needed.

So it all comes down to those precision capacitors. They are awkward, no kidding. Replacement would amount to a bundle of standard values in parallel, and a bit of careful selection with a good LCR bridge. Best stick to polystyrene dielectric, but polycarbonate might be OK.

Just musings!

David
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Old 29th Mar 2021, 10:43 am   #122
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Default Re: AVO 163 amp board ~ transformers

I'm a slow typist, and a bit inclined to ramble, so quicker people get posts in while I'm still thudding away. Good job David Simpson and myself seem to agree on the conclusions

(Can't refer to him as David S, because so am I)

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Old 29th Mar 2021, 11:44 am   #123
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Default Re: AVO 163 amp board ~ transformers

If I remember correctly now I have been asked about 4 times each about damaged amplifier transformers, damaged oscillator transformers, damaged 1.99F capacitors and damaged thermistors (either Th.1 or Th.2 or both) over the last ten years. Now, if that reflects the geeral failure rate of these parts or not I don't know but it shows that these parts sometimes fails.

I think that if we focus on rewinding the transformers and then try to find a good replacement for the 1.99F capacitor we have come a long way in keeping these testers running for a long time.

Replacing the thermistors might be possible but then we would probably have to ask current manufacturers if they can match the old types when it comes to their curves - if that's not possible I think that a small circuitboard with an opamp or two in between the output transistor of the oscillator and the output transformer can fix the amplitude stability.

The plastic roller selector on the top is probably the only thing we can't replace easily.

I also agree with keeping the circuit as original as possible along the lines of "don't fix it if it ain't broken" and also since it has worked just fine for all these years. It might (will?) be interresting to try to replace it with something new but it won't add to the functionality of the tester in my opinion.
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Old 29th Mar 2021, 12:04 pm   #124
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Default Re: AVO 163 amp board ~ transformers

There is no real need to stick to a wien bridge oscillator. A simple squarewave oscillator followed by a filter would make a good sinewave with very much better dependable amplitude.

Also there are plenty of wien bridge designs stabilised with an FET as a variable gain element controlled by a detector.

The advantage of the thermistor wien bridge is in minimising the number of transistors, which at about 10p per transistor is crucially important.

One other lateral thought. That 1.99uf capacitor need not be exactly that value. THe filter frequency might be off a bit, but just tune the oscillator to match it.

Even a little DDS board and a lowpass would do nicely.

David
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Old 29th Mar 2021, 12:16 pm   #125
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Default Re: AVO 163 amp board ~ transformers

There is this nice PDF by Jim Williams at LTC where Wien Bridge oscillators are discussed on page 29 and forwards: https://www.analog.com/media/en/tech...otes/an43f.pdf
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Old 29th Mar 2021, 1:49 pm   #126
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Default Re: AVO 163 amp board ~ transformers

One of the best app note guys that ever lived.

Sadly missed.

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Old 29th Mar 2021, 3:47 pm   #127
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Default Re: AVO 163 amp board ~ transformers

All we had back in my RAF T/Eq. servicing days in the 60's was the thumping big Wayne Kerr B221.
Back to heffing basics with the 163's transistorised modules -all I've got is the 5 A4 page article on the 163 from "Radio Bygones No140,Christmas 2012" by Kurt Schmid. A question for the Moderators - Can this be attached to a thread post without raising problems with their publishers ? Its written in straight forward understandable terms & would be ideal for all but raw beginners to follow, electronics-wise.
As I said, a million posts ago( poor David's & poor Martin's wrists must now be as weak as a Sailor's on a long sea voyage) - last week, Gm dynamic(which the 163's use) = Gm static(which all other AVO VCM's use) only if Ra is jolly low. R1 from the 163's diagram ? At 10ohms - spot on. Kurt tells us - only 44.8mV at 14.68KHz scoped.
So - 24Vrms 50Hz from a T/F winding, hwr'd by D1 & held at + 12V by D2. A 2uF Capacitor to keep Peak (not RMS) 10mS Anode HT pulses at bay from T1's primary - that worries me. Someone(with 5 thumbs) poking about there with screwdrivers or pliers - wollop - as there is no fuse or sacrificial resistor I can see.
So - back to basics - what components are phooked & how, and why ?
Its not rocket science(sorry Bazz for the pun).

Regards, David (Simo, not Stocko)
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Old 29th Mar 2021, 4:05 pm   #128
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Default Re: AVO 163 amp board ~ transformers

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I think that if we focus on rewinding the transformers
Yes, I agree this should be the No.1 objective. Every other issue depends on Ed's response to this.

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Old 29th Mar 2021, 4:21 pm   #129
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Default Re: AVO 163 amp board ~ transformers

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AVO's original design: I don't see any need to redesign it. A transformer can be wound to replace the original.
So it all comes down to those precision capacitors. David
Where are we with respect to the two thermistors on the original osc brd? I'm pretty sure that the original components are unobtainable.

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Old 29th Mar 2021, 5:33 pm   #130
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Default Re: AVO 163 amp board ~ transformers

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Spice gives 1.5 which now agrees well across the board.

Craig
Could you share the Spice files? I might learn something from them. I use LTSpice and hopefully I could get your files to work there if you used anything else than LTSpice.
Hi Martin

I just put the schematic into Tina-TI, minus transformer and overall feedback. Takes about five minutes to input the schematic. So the analysis is for the open loop gain.

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Old 29th Mar 2021, 5:43 pm   #131
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Default Re: AVO 163 amp board ~ transformers

I've never heard of Tina-TI but looked it up now, seems like a tool that is way above my head - so is LTSPice sometimes too, but I use it as best I can but never for advanced circuit analysis like it looks like the Tina-TO tool can handle after a first glance.
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Old 29th Mar 2021, 5:59 pm   #132
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Default Re: AVO 163 amp board ~ transformers

It's quite amazing what you can bend LTspice into doing (microwave stuff? Speaker development?)

It's worth persevering with Martin. There's a bit of a step at the beginning, but from then onward the learning slope is reasonable, and you start to be able to use it to do things. This gives you a firm foundation so you can go further as you wish.

If you learn a lighter simulator, you soon run into its limitations and have to dump it to start again on something more capable. Instead of several dead-ends, just go for one that goes all the way.

Various semi manufacturers have their own sim software available. Some of it a bit crippled, some of it only available in their cloud via alive webpage, and some of it is pretty good. LT pice is one of the best and is a full blooded spice variant into which you can bring other spice models and twiddle them into working. Very much in the spirit of Jim Williams. I suspect he had a hand in it.

David
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Old 29th Mar 2021, 7:47 pm   #133
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Default Re: AVO 163 amp board ~ transformers

I will not give up LTSpice anytime soon, most probably never, it has helped me so much in understanding circuit/component behaviour. However I still have a lot to learn on how to get it to work with transformers like these in the VCM163.

I've used LTSpice to crack a few old designs when it comes to Trochotrons, Dekatrons and Polyatrons and I was able to piece together something new with its help to run Polyatrons properly.

Now I just have to wrap my head around Tina-TI too!
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Old 29th Mar 2021, 7:50 pm   #134
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Default Re: AVO 163 amp board ~ transformers

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Quote:
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AVO's original design: I don't see any need to redesign it. A transformer can be wound to replace the original.
So it all comes down to those precision capacitors. David
Where are we with respect to the two thermistors on the original osc brd? I'm pretty sure that the original components are unobtainable.

B
The thermistors are unobtanium unless you scrap some other equipment that contains them.

When it comes to capacitors I have been in contact with a few "audio" focused companies and they can select capacitors from their normal ranges to get 1.99F at +/-1-2% at the required voltage (I don't know their minimum order though) but they can only supply Polypropylene and some other metallized types but not Styroflex.
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Old 29th Mar 2021, 8:01 pm   #135
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Default Re: AVO 163 amp board ~ transformers

Me being a simple analogue guy, thinks that simple analogue valve testing & valve tester repairs is much simpler than complicated add-on's to 163's or other VCM's. ( Apart from the valuable op-amp mods for 30uA meter's replacements).
So, you have a 163 - its basic common-cathode electrode voltage supplies are sourced the same as say MK3's &4's & the CT160. With those, without using the bridge balancing pots, you can still determine Gm(static) by the simple formula delta Ia divided by delta Vg - close to the VDM's working Ia recommendation. But hey - you can do the same on a 163, as you have a permanent Ia meter & a Vg pot. But then, the 15KHz Vg modulation method for determining Gm (dynamic) has seemingly been chosen by AVO to hopefully give a slightly better result independent of mains voltage fluctuations or climatic conditions affecting components. But, as I said a million words ago, with a diddy wee Ra in the 163's anode circuit Gm(dyn.) = Gm (st.). That means careful 163 owners, attempting to calibrate their precious testers can double check their results, so to speak. Even if both boards are phooked, as long as the anode voltage can be safely applied to the anode via the 10 ohm resistor - you can still surely test valves, but in a long-winded way, so to speak.
I agree with Bazz that Ed's findings are crucial. As are HPW revealing exactly what the heck he was doing when his 163(or is that 163's) went U/S ?
Quite frankly, AVO's large circuit diagram for the 163, like all the previous V/T's c/d's is akin to a massive complex of tracks in a railway yard. God awful to try & follow. That's why I've suggested that all interested parties "sing from the same hymn sheet" & refer only to Kurt Schmid's excellent RB article.

Regards, David
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Old 29th Mar 2021, 8:25 pm   #136
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That's why I've suggested that all interested parties "sing from the same hymn sheet" & refer only to Kurt Schmid's excellent RB article.Regards, David
Kurt Schmid's article uses the corrected circuit diagram that I made many years ago, a few years before his article, and it very nearly destroyed my chances of getting the rights to scan and re-publish the AVO material by Megger Ltd. I told Kurt explicitly when he asked me that he couldn't use it in his article as I was in negotiations with Megger just then - but he used it anyway just removing my name and the small box with what corrections I had done and he also removed my name from the reference section in the article. Now, it was published here on the forum before his article, free for anyone to use, but Megger first didn't like it being used in magazine articles as they still had the copyright to the base material, but it fortunately worked out well in the end anyway. I'm not sure if RB wrote a small note in later magazines that the circuit diagram was drawn by me and where to download it but I discussed it with them, I just never checked if it was published or not (perhaps someone here owns the later magazines and can have a quick look?).

The original corrected circuit diagram with what corrections were done can be downloaded here: https://www.vintage-radio.net/forum/...81&postcount=7

If you wonder it really p****d me off at the time, just as much as when my AVO CT160A document was sold to another company that sells service manuals - I contacted them too to remove it but don't know if they did, it is easy to see if you know where to look if they still sell my document as the photos/pictures are not all original but drawn by hand and stitched together from other manuals by me as they were missing from the original material I got hold of.

That felt good to get off my chest - if it upsets the moderators please just remove it!
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Old 29th Mar 2021, 9:05 pm   #137
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Default Re: AVO 163 amp board ~ transformers

Put me down for a good dose of sympathy, Martin. I think everyone in the field gets their toes trodden on at some time or another, though maybe not quite so comprehensively.

David
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Old 29th Mar 2021, 9:43 pm   #138
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Default Re: AVO 163 amp board ~ transformers

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Even if both boards are phooked, as long as the anode voltage can be safely applied to the anode via the 10 ohm resistor - you can still surely test valves, but in a long-winded way, so to speak. David
Exactly right! I should have mentioned that much earlier when someone questioned the accuracy of the 163. Apart from using "standardised" valves, you can quite simply measure the changes of Ia as you twiddle the Vg knob and plot the line, (which takes seconds if you use Excel or similar) and when I've done that, the Gm values have been very close to the ones read off the Gm meter after all that "14kHz stuff". Maybe we should just forget the boards .

The serious mis-use of your materials, Martin, must be very "annoying" indeed.
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Old 29th Mar 2021, 10:57 pm   #139
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Default Re: AVO 163 amp board ~ transformers

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It's quite amazing what you can bend LTspice into doing (microwave stuff? Speaker development?)

It's worth persevering with Martin. There's a bit of a step at the beginning, but from then onward the learning slope is reasonable, and you start to be able to use it to do things. This gives you a firm foundation so you can go further as you wish.

If you learn a lighter simulator, you soon run into its limitations and have to dump it to start again on something more capable. Instead of several dead-ends, just go for one that goes all the way.

Various semi manufacturers have their own sim software available. Some of it a bit crippled, some of it only available in their cloud via alive webpage, and some of it is pretty good. LT pice is one of the best and is a full blooded spice variant into which you can bring other spice models and twiddle them into working. Very much in the spirit of Jim Williams. I suspect he had a hand in it.

David
Tina-TI has quite a lot of limitations, because it is a pruned down version of a costly Tina package. I found one of the limitations over the weekend - no logic devices.

Fortunately I'm kind of up the learning curve for a really full-featured simulator - Micro-Cap. This used to be a punishingly expensive professional grade package, until the software writer retired, and then he made it freeware http://www.spectrum-soft.com/index.shtm . It is a tricky package to learn. As an example, again over the weekend, I was simulating a circuit that used the CD-4000 logic family, which can cope with Vcc of 15V. I wanted to run at 12V. The software defaults to 5V, so you have to learn how to change that (not straightforward or obvious), which you can find by searching the blog.

Micro-Cap, it has to be said, is not simulation software for the novice or the fainthearted.

Craig
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Old 30th Mar 2021, 8:07 am   #140
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Default Re: AVO 163 amp board ~ transformers

My apologies to Martin for inadvertently opening old wounds. 'Fraid the politics of circuit diagram copywrite issues are way over my head. I had just thought that something simpler & shorter might aid folk reading this heffing long thread or delving into AVO's thick 1960's/70's 163 Manual.
The displays from KS's Tek scope can easily be replicated by anyone with a decent 2 channel scope. Displaying such 10mS pulses on anodes & grids has long been used by folk calibrating all AVO VCM's & the CT160.

Regards, David
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