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Old 6th Jun 2021, 12:29 am   #1
resurgance
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Default Avo CT160 great kit. Test Points?

Hi there,
I am a ham and started into my vacuum tube journey as a foundation for rebuilding and using a Collins S-Line setup.

I ended up accidentally acquiring a few testers with boxes of tubes that I had bought as spares for my radios. These testers were a Hickok 539b without working meter and in need of some TLC, an Avo MkIII in almost perfect condition (seems to have been pulled working from NZRAF in 1987) and then more recently due to having been smitten with the lovely MkIII I bidded on an Avo CT160 that was condition unknown, and completely missing a meter and no power cord.

Well. The Avo CT160 is a great bit of kit.
So I seem to have gone sideways as I have not yet started on resurrecting the S-Line radios. But wow have I enjoyed rebuilding these valve testers and it has been a focus now for a while starting with the 539b which although quite different to the Avo's was also an enjoyable project.

So far huge thanks to Martin, aka Dektron here for many emails and valuable advice, protection board and new scale for a Simpson Taught Band meter I purchased to re-meter the CT160. I have to date;
- Rebuilt the almost completely seized selector wheels
- replaced the 0.02uf caps and 8uf caps
- tested resistors, and other components - finding an almost perfect bias pot!

Thoroughly enjoyable project. Last week I received an original plessey cord connector from ebay (fairly hard to find!) as I like trying to keep things as original as practical - and this morning powered this up for the first time and was pretty excited to see the meter move via the tester and not my meter test rig.

My Question and reason for posting (in addition to sharing my enjoyment of rebuilding this great kit) is to ask if anyone had come up with a set of test points using either a selector combination and valve socket OR test points within the tester to verify all voltages before plugging in a tube?
I am very slowly working through the wiring diagram but it is a very slow process so please let me know if there is a good method here.

Attached Pic is first power up this morning, exciting stuff I tell ya. Meter is not properly home I am still to make up a packer from black acrylic with a lightbox side tab on the left to show fault light. I intend to shift the red lamp to top left part just under meter with a light guide as the new scale is not opaque.

Thanks for reading
Stew Thorp
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Old 6th Jun 2021, 1:54 am   #2
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Default Re: Avo CT160 great kit. Test Points?

Hi have tested so far;
Heater voltages between H- and H+ they are in spec.
Anode Voltages between A1 and C when Circuit selector in 'Test' these are a little on low side but OK.

Not sure about shorting Anode and Cathode between pin 2 and 5 to test screen voltage, as I would have thought this was dependent on selector switch settings. So so far have not tested Screen voltage.

Also the meter deflects backwards when under test - Now that I have 4 diodes protecting the meter I am not too concerned about this, but why does the meter do this under 'test' I have also had this on the MkIII and seems like something really basic that I am doing or not doing.

THanks
Stew
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Old 6th Jun 2021, 9:39 am   #3
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Default Re: Avo CT160 great kit. Test Points?

Sounds like you are well on the way on the slippery slope!

A question please. How did you restore the selector wheels? And what estimate of force needed to rotate the selector is now needed? Did you use any chemical cleaners? How would you rate your success in restoring the selector?

I am still looking at mine. Really don't want to dismantle it, and there have been warnings about chemicals used, and without the selector the whole tester is pretty useless. It is so much easier to glance at the selector switch that a collection of rotary switches, they really are useful.

Good luck.
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Old 6th Jun 2021, 10:11 am   #4
resurgance
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Default Re: Avo CT160 great kit. Test Points?

Hi don't worry too much about the selector wheels with care it is not too bad and 100% worth doing as now mine are easy and locate on clicks nicely.
I took photos of wires before desoldering and laid everything out in order of disassembly.
Some points are;
Be super careful of brown insulator joining all terminals along width of wheels near solder joins as this is what holds terminals when rotating wheels and its super easy to break.
Make sure there no burrs on end of shafts as then when removing them or removing wheels off them they don't catch on anything.
I used deoxit carefully on the individual parts and then when i reassembled turned the plates around in some as the plating was in better condition.
I cleaned other parts with isopropyl as most of buildup was green goo or straight goo that was causing friction just from surface area. Seems to come from areas where the silver coating had come off. This coating is very thin be careful.
Then used very light coating of vaseline on all friction parts that are not electrical contacts. Less is more.
Then when reassembling i turned all shafts around so fresh silver was facing outwards to electrical contact points. Mine had perfect conductivity when done.
Highly recommend doing it.
This is my experience and not gospel so if anyone has anything else to add or corrections to the above please speak up!
Cheers
Stew
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Old 6th Jun 2021, 9:41 pm   #5
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Default Re: Avo CT160 great kit. Test Points?

Stew:

If you just want to check you've got the voltage settings correct, you could rig up a jig to the sockets located between the A1 and A2 links. From memory I found the the pins from audio XLR plugs were the right size for those sockets.

The meter will be driven backwards due to the fundamental way in which these instruments work: measuring a voltage (which is developed across a resistor sampling current) by comparing it with a reference which you've set up using the panel controls. In other words, you're looking for a null, as with a Wheatstone bridge. It would have been more satisfactory to use a centre-zero meter for this. If you have a single 1N4148 wired across the meter to handle reverse current you should be OK in terms of protection. I assume you've also got a capacitor across the meter to average out the rather spikey waveform: I use a 6u8 tant.

John
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Old 6th Jun 2021, 10:03 pm   #6
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Default Re: Avo CT160 great kit. Test Points?

Hi John,
Ok great I'll try that. I can measure between C and S pins but i think i need to short cathode to anode to get a measurement on these pins? Just wasn't sure where best to do that.
Re meter protection yes there is a 100uf cap to smooth it all out. There was some debate about 10uf vs 100uf and influence on readings in certain situations?
Cheers
Stew
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Old 6th Jun 2021, 10:22 pm   #7
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Default Re: Avo CT160 great kit. Test Points?

The critical voltages to check would be Grid, Anode and Screen (if pentode etc). They are all refered to the Cathode, so that's the common (reference) terminal. However, on all these terminals apart from the screen there are either ac or mixed rectified ac where one half cycle is present (and different in amplitude) when the valve is "off" (this is on the grid). So you need to have a look-up curve to get from the "Vg measured" to the effective Vg when the appropriate half-cycle is active. You can generate this merely by connecting the DVM between C and G and then plotting the voltage you get as you run the Vg pot from zero to -40V.

The AVO manual gives you the fudge factor to use (IIRC) when measuring the anode voltage with a mean-reading-but-rms-calibrated meter.

John
PS I think you have to be in "Test" mode in order to get the voltages on the valve terminals
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Old 6th Jun 2021, 11:46 pm   #8
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Default Re: Avo CT160 great kit. Test Points?

Thanks John,
Yes the waveform of the voltages are interesting I had seen images but was going to put the scope on this avo and have a good look for myself. The DMM does not show screen voltages but this may be due to my misunderstanding of the tester. I will apply my brain as in truth i am being a tad lazy consulting the forums under the guise of being careful! These old testers do make me nervous due to voltages present and also precious meters and other unobtainium parts as some say but having the meter protected on this CT160 does make me sit easy. I wanted to add some protection somehow to the other parts that can be lost like certain conditions creating arcs in the bias pot for example.

Also present is my general early stages of vacuum tube-ology. I did cheat somewhat initially by building an Automatic Roetester and even managed to take out a main fet and op-amp there by not having a series resistor in a gas regulator test.

I have a very mechanical mind and in-roads into a few sciences but wish I had a few more IQ points.
So much to learn.

Cheers
Stew
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Old 7th Jun 2021, 9:59 am   #9
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Default Re: Avo CT160 great kit. Test Points?

If you haven't read what I wrote here: https://www.vintage-radio.net/forum/...ad.php?t=86262 about how the design of the AVOs was done (the CT160 is used as an example) please do so, it explains the different tests and how the circuit is built and how it measures the different parameters.

Also, have a read of the CT160 Service Manual, can be found on the Internet, here for instance: https://frank.pocnet.net/instruments/AVO/index.html

There is one difference between different versions of the AVO valve testers and that is that in some versions the Anode and Screen voltages are half wave rectified and in some they are "pure" sinusoidal voltages so you have to either check the schematic diagram to see which version you are checking or read the service manual for what setting to use on the DMM (AC or DC). The circuit diagrams for AVI Mk III, Mk IV, CT160, CT160A and VCM 163 are at the end of my article above.

Some people install a fuse in series with the grid volts potentiometer, some have replaced one of the mains fuses on the panel with this fuse, only using one mains fuse. If you accidentally blow your grid volts potentiometer there are a few ways to replace it, either using another potentiometer adjusting it with external components like Mike Phelan did: https://www.vintage-radio.net/forum/...ad.php?p=47426 or with a linear potentiometer and a range switch like Eino (einovaan) did here: https://www.vintage-radio.net/forum/...86&postcount=6.

The AVO patent describing the way that AVO tests a valve with sinusoidal waveform AC voltages (half wave rectified and normal sinusoidal wave form) described here: https://vintage-radio.net/forum/show...4&postcount=13 is quite easy to understand when you have penetrated the equations well enough. The pivot point for me was to understand that all of the frequncy dependent parts of the integral equation disappear so that you are left with an equation that is frequency independent and you are then left with something that is equal to the general vacuum tube equation but with some constants that determine the AC voltages you need to perform a measurement identical to DC voltages, equations and voltages shown in the portion taken from the CT160 Operating Instructions in simplified form in the included picture.

The "fudge" factor that AVO wrote as 0.52 is nothing else than a scaling factor used to make the scale for the Grid Volts potentiometer conform to the law they used in the circuit. Some people say that 0.515 or 0.525 is better than 0.52, and in some circumstances it is and that is due to that their scale sits a little bit to the right or to the left compared to the 0.52 value and that could be due to several reasons, either their Grid Volts potentiometer and adjusting resistors make that a better fit or that the potentiometer has been worn down and nowadays it doesn't fit the 0.52 value as well as something lower or higher. You can read about the "fudge" factor in the document I wrote above.

I hope this helps in understanding the AVO testers, if not ask away!
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Old 7th Jun 2021, 8:47 pm   #10
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Default Re: Avo CT160 great kit. Test Points?

It was perhaps too simplified when I wrote that "The "fudge" factor that AVO wrote as 0.52 is nothing else than a scaling factor used to make the scale for the Grid Volts potentiometer conform to the law they used in the circuit.".

I think I made a somewhat better explanation in the comparison document where I wrote: "It is just a simple way of offsetting the grid voltage and at the same time obtaining a non-linear scale for the mA/V dial, which results in an increased resolution for low mA/V measurements!".

The Grid Volts potentiometer works in conjunction with the mA/V (Gm) potentiometer to achieve the scaling factor.

I hope I didn't confuse you more now, but please do read the circuit comparison document as all the maths and details are there!
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Old 7th Jun 2021, 10:02 pm   #11
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Default Re: Avo CT160 great kit. Test Points?

Martin that is a great explanation thanks!
Am back into the working week so this will be digested later this week when i can claw back some of my own time
.
Cheers
Stew
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Old 11th Jun 2021, 7:19 pm   #12
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Default Re: Avo CT160 great kit. Test Points?

The picture below gives you some idea of the reading and images you would see on a scope. The plate and screen voltages are pretty much fixed based on the accuracy of the line voltage, so no real reason to monitor those if the transformers are in working condition. I did build an attached unit that plugs into the top cap connection panel and anode links to give plate current, bias, heater and line voltage. The Simpson digital meters have high low ranges and are scaled to give the correct readings. I use SS diodes on both of my CT-160's and a few other mods mentioned by Martin and others, they give quite accurate and comparable tube readings to tubes with known characteristics (calibrated on both bench and curve tracer). Reading Gm is a bit less so because of the reading of the dial. Most of my valve/tube testers have sat in the closet for many years, as I primarily use a uTracer 6 curve tracer these days.

I should mention that I added heater volts because I found the settings do not accurately correlate with the actual heater voltage per the tube specification and I also have a 3 way switch for the heater volts transformer that gives 3 ranges instead of two (I used a mid voltage tap). This allows me to get the heater voltage within 0.1V of spec.
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Old 7th Aug 2021, 4:51 am   #13
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Default Re: Avo CT160 great kit. Test Points?

Hi Mark,
Thanks for the great reply, nicely laid out.
I have really enjoyed getting this ct160 going again, the meter has taken some time to figure out due to me wanting to properly understand the why, rather than just doing it.
Like you I have another tester I use but thoroughly enjoy the fantastic build quality of these as well as the cunning test methods and electrick-ery.
Have not been on in a while I had only seen your reply today!
Cheers
Stew
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