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Old 20th Mar 2021, 12:40 pm   #41
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Default Re: AVO 163 amp board ~ transformers

Somewhat on topic for this is that I've recently helped two gentlemen to get their VCM163s up and running after problems with the oscillator and amplifiers not working properly.

Both gentlemen were a little bit reluctant to replace components as they wanted the original look, but in the end it turned out in both cases that replacing all of the electrolytic capacitors was enough to get them both up and running perfectly - all of the old electrolytic capacitors measured ok on capacitance and ESR I was told as they were tested, but they obviously didn't work properly in the circuit as replacing them got the testers up and running.

Some of the resistors that were out of tolerance and the transistors had been replaced previously with no effect. So even if you test these electrolytics for capacitance and ESR they obviously didn't work well in the real circuit.

Putting the transistors back and some of the resistors that weren't far out of tolerance had no effect as the oscillator and amplifier still worked after replacing the electrolytic capacitors, but the more out of tolerance resistors made them stop working.

So as I've said many times before I regularly replace the electrolytics first, then check for out of tolerance components and replace those and as a last resort replace transistors - of course you need to check the transformers so they are ok as a first step.

If your meter isn't working you'll either have to replace it with a new meter or build an opamp to counter the magnetic flux it has lost over the years.

I already have new meters working in the VCM163, with new scales, but Covid-19 hit me early last year and stopped me in my efforts to finish that project and write an article about it. I will get back to it when I can but I don't know when that will happen, probably later this year, maybe after the summer if I am well enough.

I'll help out where I can, if time and well being permits me, if someone want to build and test something new. I think that the test specifications for the transformers will be enough for Ed to wind some new transformers that can be used for experiments.
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Old 20th Mar 2021, 1:39 pm   #42
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Default Re: AVO 163 amp board ~ transformers

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Originally Posted by Radio Wrangler View Post
In the VCM163, that transformer isn't really working as a current transformer. R1 is the shunt resistor converting anode current into voltage, so it's a voltage transformer.
Now, here is something that I wonder if you could explain somewhat in more detail so I can understand it better and learn something - When does it become a voltage transformer and not a current transformer (there are AC-current transformers) and why doesn't it work as an AC-current transformer here as the capacitor is blocking the DC-component?
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Old 20th Mar 2021, 8:51 pm   #43
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Default Re: AVO 163 amp board ~ transformers

All transformers transform both current and voltage at the same time. A transformer with say a 10:1 turns ratio, driven from the more turns winding will perform a voltage step-down of 10:1, and at the same time will perform a current step-up of 10:1.

Whether a transformer is viewed as a voltage transformer, a current transformer or an impedance transformer is therefore a matter of intent. And the intent is reflected in the sort of circuitry in which the transformer is embedded.

A mains transformer is an example of what's thought of purely as a voltage transformer. Your local mains voltage is connected across a primary winding and that sets a number of turns per volt. You can have secondaries wound with the appropriate numbers of turns to get the voltage you want out of each. You will get copper losses in them, so maybe you compensate with a small increase in the secondary turns. But these losses should be small, and the mains supply is low impedance... so the secondaries take whatever currents the downstream circuitry wants, the transformer handles it and the voltages don't much waver. We check the wire diameters are up to the currents, and that the core is big enough for the total power, and we are mostly there.

An example of na impedance transformer is on the output of almost all valve amplifiers. We look at the valve characteristic curves and decide what impedance we want to present to the anode. We know the nominal impedance of our speaker and we need a transformer to reconcile the two. The impedance ratio is the square of the turns ratio So a 10k Ohm presented to an anode by a transformer and a 3 Ohm speaker says the turns ratio needs to be Root(10,000/3) which is 57.73:1 turns ratio. THis explains why a lot of simple transformers are around 80:1 ratio. You'll also find impedance transformers in a lot of RF work.

Finally, to the current transformer.

There is a small current transformer in AVO 8 multimeters It's there for the higher AC current ranges. It does several good thing at once.

Let's say we have a good 100mA full scale AC ammeter. It'll have a more sensitive meter movement than this and a bridge of rectifier diodes. It will use the meter movement, and a burden resistor to measure the voltage across a sensing (shunt) resistor through which the current to be measured is passed. To keep the diode drops of the rectifier diodes, trivial, a relatively larger voltage needs to be dropped across that shunt. So we have our 100mA FSD ammeter and it's voltage drop could be better, but we're stuck with a passive instrument and real world diodes. So we live with it.

But now we want a 10A meter. We could just scale down the shunt resistor. But the voltage drop will waste more power at the higher currents it will be running. This isn't good, but we can do better.

Keep the 100mA ammeter as-is and put a 1:100 (turns ratio) step-up transformer in front of it. 10A in its primary gives 100mA into the 100mA meter. Result!

But the voltage drop in the 100mA instrument's shunt is scaled down 100:1 making the drop at the primary winding 100 times lower... Another Result!

Note that there is no resistor in the primary of our 'current transformer' What the circuit being measured sees inserted in it is the 100mA instrument shunt resistor divided by the turns ratio squared. 10,000:1 reduced. if we'd done it by changing the resistance, the circuit would have seen it go down by only 100.

If we'd put the shunt resistor across the primary of our transformer, we could get a similar effect, but it might require an awkwardly low value of resistor. It's nice to run a transformer in a low inpedance environment because you don't need as much winding inductance. and so you can get better bandwidth.

I've done some very broadband RF transformers for impedance measurement systems, and padding the system Z down low costs in insertion loss but improves the bandwidth wonderfully. We had more sensitivity than we knew what to do with, so it was a very successful trick.

So the VCM163 puts the shunt resistor BEFORE the transformer because they want a DC path for the valve anode current, they want to stick a capacitor in before the transformer so that the transformer is protected from the larger DC current component flowing in the anode, so they don't have to worry about it contributing to saturation.

Also, the way they've got it runs the transformer in a higher impedance environment which makes it easier to resonate the thing and to get a reasonable Q in order to filter out mains frequency.

David
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Old 20th Mar 2021, 9:35 pm   #44
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Default Re: AVO 163 amp board ~ transformers

Thanks for the explanation, I understood the idea for most of the different design criteria and how it helps/works.

Could the reason for the amplifier input transformer becoming damaged be due to oscillating valves, this as a much higher AC-current then flows in the circuit than usual, this AC-current would then flow through the amplifier transformer possibly damaging it?
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Old 20th Mar 2021, 9:58 pm   #45
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Default Re: AVO 163 amp board ~ transformers

I'm not yet convinced that the transformer is damaged. That 10 Ohm resistor gives a lot of protection, and there is the diode clamp on the secondary. Wasn't the transformer only suspect because it was felt that everything else had been eliminated?

An oscillating valve could do all sorts of things, but not exceed the thermal limited max cathode emission. So there's a current limiting function. Voltage-wise, things could get nasty, but the resonator capacitor in series, and those shunt diodes will help.

Common mode transient breaking down pri-sec insulation might be the likeliest way to kill one. Has it had a megger test?

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Old 20th Mar 2021, 10:20 pm   #46
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Default Re: AVO 163 amp board ~ transformers

I'm not sure what the reason for the faulty amplifier in HBWOODY's thread is but I have seen damaged amplifier transformers but never really come to any conclusion on why they were damaged. The 10 Ohm resistor have always been ok in these circumstances.

So I was wondering if it could be due to oscillations as that is the only possibility for an AC-current/voltage to reach the transfomer, via the DC-blocking capacitor, and possibly damaging it.
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Old 20th Mar 2021, 11:56 pm   #47
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Default Re: AVO 163 amp board ~ transformers

It's quite possible. The DC blocker/resonator will look like a short ar RF and there's an awful lot of intermingled wiring in the socket pin selection department and the looms to the boards. Ferrite beads help, but how do you know you've got enough in?

Pri-secondary brekdown is a possibility, shorted turns ditto.

Trouble is that if you unwind one, it's awfully easy to miss damaged enamel or to not be sure it wasn't damage in unwinding.

Let's see.... 10 Ohms at 15khz Pick Xl as 1000 ohms (just a guess and we'll see how it goes) 1000=2*Pi*15000*L so L= 10.6 mH that's high but I've forgotten that it's resonated.

1.99uf into 10 Ohms and 1990pf into about 10k so we can just call it 1.99uF and 15kHz

1/2*Pi*root(LC)= 15000 so LC = 1.126E-10 so L=56.572uH Much more reasonable

Now that pot core ends -56 in its markings. Could AL be 56nH? I'm not sure the units Mullard marked Al in. but N^2 = 1000 to make 56uH out of 56nH so the turns on the input side would be root(1000) call it 31, 32 or 31.5 depending how you use core windows.

If this really is a step down transformer, then the output winding becomes half that so 32 to 16t would sound fair.

Just playing with fag packet numbers.

David
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Old 21st Mar 2021, 11:07 am   #48
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Default Re: AVO 163 amp board ~ transformers

WOW.
I will have to read this thread a few more times as I am obviously out of my depth here.
Fascinating.
Will send my amplifier board to Ed as soon as I have his address.
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Old 21st Mar 2021, 11:30 am   #49
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Default Re: AVO 163 amp board ~ transformers

@David (Radio Wrangler), the text on the ferrite core reads AL2401-B6 and according to the datasheets found for its replacement LA1224 it has these parameters:

LA1224 Pot core 21mm A13 (3h1) e = 100 with adjuster at mid range position, Adjuster LA1502

Since AVO have these Test Specifications for the amplifier transformer:

----------------------------------------
Amplifier Transformer:

Use Universal Bridge TF 1313, Marconi, for the following measurements.

PRIMARY:

Resistance: 0,2 Ohm +/-20% at 20C, Inductance at 10KHz 99.5uH, Q-factor: 2.4 +/-10%

SECONDARY:

Resistance: 45 Ohm +/-10% at 20C, Inductance at 10 kc/s 99.5mH +/-2%, Q-factor: 14 +/-10%
----------------------------------------

Would it be possible for you to adjust your calculations with these values in mind?
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Old 21st Mar 2021, 12:37 pm   #50
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Default Re: AVO 163 amp board ~ transformers

This extensive thread re. Avo VCM163 amp board transformers does point up the potential for errors in the gm test especially when the frequency and waveform of the oscillator is taken into account as well and may explain why different results are often obtained when testing valves on 2 different testers of the same make/model.
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Old 21st Mar 2021, 2:01 pm   #51
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Default Re: AVO 163 amp board ~ transformers

It is possible to get most AVO VCM testers to show very similar measurements as long as they are calibrated properly and set up properly for each test, but all measurements will have an error. It also differs with what valve you test as some are more sensitive to the differences between two testers.

External noise is a problem nowadays as many switched power supplies and motor drivers emit frequencies on the mains, automatic electricity metering systems often communicate over the mains wiring so they also add to this. Even poorly designed LED lamps add to this (even big name manufacturers have these problems).

Even modern semiconductor based testers have this variation when testing valves, mostly due to heating of components during tests and most of these testers don't take that into account. So far I haven't seen a single modern semiconductor based tester that has taken this into account, nor have I seen any that has presented a proper calibration procedure that takes this into account, nor have I seen any proper "calibration valve" for these modern testers. Calibration and getting the same measurements between two testers is still complicated and it seems that no one has done anything in this area for these modern testers. Modern testers do deliver good measurement results but they are not without fault and tests vary from day to day on the same valve due to the internal heating in many of these modern testers.
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Old 21st Mar 2021, 3:42 pm   #52
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Default Re: AVO 163 amp board ~ transformers

Quote:
Originally Posted by Dekatron View Post
the text on the ferrite core reads AL2401-B6 and according to the datasheets found for its replacement LA1224 it has these parameters:
LA1224 Pot core 21mm A13 (3h1) e = 100 with adjuster at mid range position, Adjuster LA1502

Since AVO have these Test Specifications for the amplifier transformer:
Use Universal Bridge TF 1313, Marconi, for the following measurements.

PRIMARY:
Resistance: 0,2 Ohm +/-20% at 20C, Inductance at 10KHz 99.5uH, Q-factor: 2.4 +/-10%
SECONDARY:
Resistance: 45 Ohm +/-10% at 20C, Inductance at 10 kc/s 99.5mH +/-2%, Q-factor: 14 +/-10%
---
Would it be possible for you to adjust your calculations with these values in mind?

OK, this is interesting. There is a biggie just shown up.

Both the primary and the secondary have inductances quoted and they aren't in the 4:1 ratio I'd expect in the 2:1 volts ratio in Avo's test info for this transformer. Something seems to have got crossed and we may have data on two different transformers or two different generations.

Al =100 and L=100uH for the primary would mean root(100uH/100nH) = call it 32 turns.

now to get 99.5mH (call it 100mH) then it's root(100mH/100nH) = 1000 turns.

Now that's a heck of a lot of turns on a pot core and needs very fine wire for that size of core so the vast length and fine wire is going to make quite a lot of resistance 45 Ohms up from the 0.2 Ohms of the primary. Yes, I'll buy that as a reasonable estimate.

So we now have a VERY different transformer to what the first data suggested. That was a 2:1 step down, which surprised me. This is now a 1:31 step UP.

I expected a step-up, but maybe not as dramatic as 1:31

Let's twizzle the numbers backwards.

The emitter resistor on VT2 is 10k if I remember, less the loading of the following stage, which is in series feedback mode and should be fairly high Z, so the input to VT1 should see well over 10k. The bias resistors make 10k, so the transformer secondary is loaded in 10k Ohms.

A 31 to 1 transformer has an impedance transforming factor of 1000 to 1, so neglecting the tuning capacitors that would look like 10 Ohms at the primary. Is this a coincidence or what? In reality those capacitors and operating in resonant mode will modify this a fair bit and it will be higher, but likely to affect accuracy. THis transformer really really needs to be resonated, that 1000 turn winding will have appreciable distributed self capacitance and the resonator capacitor is needed to absorb this in its calculated value.

I don't think this circuit was designed in a particularly mathematical fashion, I think there has been a fair bit of playing about on the bench involved.

So the original 2:1 step down test spec seemed silly for the measurement amplifier board.

The 1:30 step up is in the right direction, but does seem more than I'd expect.

Martin, can I ask you to double check everything, please. It could really be 1:30 but it does seem to be making a harder job of it than I'd expect.

David
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Old 21st Mar 2021, 3:46 pm   #53
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Default Re: AVO 163 amp board ~ transformers

I'm having doubts over whether I've understood the Ue value properly. It might not be in nH/(turn squared) which is usual for the brands of pot core I'm used to. If so, all the turns numbers could come down, but the 1:30 ratio would remain and still seem a bit big.

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Old 21st Mar 2021, 5:16 pm   #54
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Default Re: AVO 163 amp board ~ transformers

I measured my transformer again with my Fluke 87V and it measures 0.3 Ohm on the low resistance winding which connects to the 1.99uF capacitor and 41 Ohm on the high resistance winding connecting to the 1990pF capacitor.

ow I only have a caliper and no micrometer to make the measurements of the diamater of the wire, but it looks like 0.3mm on the low resistance side and 0.15mm on the high resistance side.

The data I have found while Googling on the ferrite is here in the thread and it says that the LA2401 was substituted with LA1224 and those values are consistent with other data on the LA24XX-series pot cores I have found. The value given in the datasheet is permeability (e) at 100 for the LA1224, not inductance AL (nH).

I also saw that AVO says that the value of 99.5uH should be adjusted to exactly that value with the trimmer.
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Old 21st Mar 2021, 7:51 pm   #55
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Default Re: AVO 163 amp board ~ transformers

Quote:
Originally Posted by Radio Wrangler View Post
Something seems to have got crossed and we may have data on two different transformers or two different generations.
David

When I posted the photograph of my amp board (post #31), I did notice that the PCB is Marked "ISSUE 2" (and there is a part no. on the board). I think I'd assumed this was maybe a batch number thing, but maybe there was a significant build revision?

Re accuracy of the 163's, firstly, I have spent more hours than I would like to admit to characterising "standard valves", "by hand" and have always found that the 163 comes up with tests results very close to those. Secondly, when David Simpson very laudably organised his "round robin" exercise of standard valves which he had prepared a couple of years ago, the outcome seemed to suggest that the 163's owned by forum members all came up with very close measurements. I, for one, am convinced that the 163's are pretty accurate, it's just that they've reached an age where reliability is declining,... rather like myself!

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Old 21st Mar 2021, 8:20 pm   #56
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Default Re: AVO 163 amp board ~ transformers

I've never seen an issue 1 of these two PCBs but I have some information that says that it only had to do with some component value changes/corrections and also the addition of electrolytic capacitors to stabilise the circuits.
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Old 21st Mar 2021, 8:41 pm   #57
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Default Re: AVO 163 amp board ~ transformers

Quote:
Originally Posted by Dekatron View Post
I measured my transformer again with my Fluke 87V and it measures 0.3 Ohm on the low resistance winding which connects to the 1.99uF capacitor and 41 Ohm on the high resistance winding connecting to the 1990pF capacitor.

I also saw that AVO says that the value of 99.5uH should be adjusted to exactly that value with the trimmer.
That's good. Your measured transformer looks to match that in the second document, and we have a solid statement of the target inductance.

Could that earlier document be fore the output transformer on the oscillator board?

David
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Old 21st Mar 2021, 8:49 pm   #58
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Default Re: AVO 163 amp board ~ transformers

Yes, I wrote "Oscillator transformer" here: https://www.vintage-radio.net/forum/...8&postcount=22 and "Amplifier transformer" here: https://www.vintage-radio.net/forum/...7&postcount=21
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Old 21st Mar 2021, 10:47 pm   #59
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Default Re: AVO 163 amp board ~ transformers

Ah, that's where I got them crossed...

David
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Old 22nd Mar 2021, 12:45 am   #60
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Default Re: AVO 163 amp board ~ transformers

Sorry for confusing you, I thought it was important to have both test specifications and both diagrams in the same thread.
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