AVT - AVO Two Panel Valve Tester?

[ms660]

The only 500 ohm fixed resistor I can see on the schematic I'm looking at is the one that limits the anode current on the signal diode test.

On a layout drawing I'm looking at there is a 100 ohm resistor that's connected to pin 7 of the B7 socket, pin 7 being the anode, could be an anode stopper to prevent parasitic oscillation, on the same schematic as the one mentioned above it shows a 0.001uF capacitor connected between pin 7 and pin 6 of the B7 socket which are the anode and cathode pins.

According to the layout in the manual the upper octal socket of those two is Mazda Octal and the lower one is International Octal.

Lawrence.

[Electronpusher0]

As far I know the capacitors and the resistor in the valve base panel are to try and stop parasitic oscillations.
I have rewired two of these now and I have fitted ferrite beads to every valve base pin and removed both the resistor and capacitors. I have no seen no signs af parasitic oscillations even with high mu valves.

Peter

[Robert Gribnau]

In my experience (I own one and tested hundreds of valves with it) oscillations are not likely to happen.

It is not so much the height of the amplification factor but the height of the mutual conductance of a valve type that highers the risk on oscillations. Since this tester is not made for testing valves with high mutual conductance, I hardly see the point in rewiring (unless the connections are bad) and/or adding ferrite beads.

Just my opinion.

[Stevie342000]

I had wondered about adding ferrite beads and whether there was a benefit to using them?

The only valve that I know of that I have and use is the ECC88 which will cause this tester to oscillate - is that assumption correct?

I can not upload the file with the socket panel on which consists of 2 sets of instructions one early the other later. Sandwiched in between are some schematics and socket panel layout.

In that socket panel there is a 500R resistor referenced as Y, the capacitors are V & W. I did study this further and figured out the Mazda socket is the upper one and IO is the lower socket.

My socket panel has a B9A where the diagram has a blank - this is the only modification.

I am loathe to redo the wiring do not wish to damage any of the socket pins - but it would make circuit tracing and pin tracing much easier - than the monochrome brown varnished cloth.

As far as I can tell it is possible to measure all the resistors without having to remove one end. The fuse like dubilier resistors look to be very high quality and I suspect they will be on target. They look like fuses see earlier pictures.

Is there a shock potential from the shorts test if you come across the test probes? I suspect it is current limited by the resistance.

[Stevie342000]

Sorry it has been a while since I have done any more work on restoring my Avo Two Panel.

It occurs to me now and not for the 1st time that there is the potential to plug the valve holder panel in the wrong way with the B9 socket.

As I think there is no orientation to these sockets or am I wrong not looked for a while. Assuming that the pins are all the same size.

I am currently reading the patent and need to get this item up and running.

If I went down the route of taking out the capacitors/resistors in the and replacing them with ferrite beads what size beads should I be thinking of using?

Onwards and upwards......

[Electronpusher0]

You would have to really try and plug it in wrong. There are two different spacings between adjacent pins.

http://www.r-type.org/static/baseb9.htm

Peter

[Stevie342000]

Thanks for that I really did wonder if that was the case or one of the pins was of a larger diameter.

I am still toying with the idea of ferrite beads but really do not wish to take of the cable from each socket and solder it back on. My eyesight obviously given the above is not what it used to be.

The reason for my reluctance to desolder the wires and put in ferrite beads is that all the cables in mine are covered in what I can only assume is some kind of cloth heatproof insulation all the a lovely red brown beige colour.

The chance if I do one at a time of not putting them back on the right pin is minimal. But then the chance of doing damage to the socket pins by disturbing them may be higher.

The only reason I am toying with this is that I want to test the odd ECC88 and I know there are issues with parasitic oscillation. Am I right in my thinking?

Is there anything else I am missing? Other than where to place a fuse and a value? I know the latter may have been covered in another similar thread.

Any Help Much Appreciated.....

[Ed_Dinning]

Hi Steve, on my 2 panel unit there is a small arrow beside the socket to indicate the 2 close together pins.
You could try adding the ferrite beads to the B9A valve socket end

Ed

[Stevie342000]

Thanks Ed I will keep my eyes open I thought there might be an alignment key as well which I think is what I was trying to get at if there was no difference in pin size or alignment.

Yep that sounds like an option on the B9A socket, as far as I can recall it was a later addition on mine any how along with a B7G socket.

Any ideas on the size of the fuse in the HT side of the circuit? My assumption is 100 - 120mA. With 1 amp fuse in the mains socket as I assume the sizes are adequate enough to protect the transformer and any other parts which aside from donors are unobtainable.

[Ed_Dinning]

Hi Steve, Yes I would think 100mA to 150Ma with a 1A mains fuse should work OK.

The last thing you want is a burnt out transformer. They are a total pain to rewind.

Ed

[Stevie342000]

Thanks Ed my assumption is that I would need to put the fuse in the 0 return lead in the HT circuit, is that correct?

I was looking at breaking the lead and putting an inline sealed fuse holder in there, can you still get those?

I have ordered the capacitors today from Cricklewood on eBay but Houston we now have a problem.

Whilst I checked the holder panel there were no issues however I thought I must check to make sure that the capacitor in the tester is 0.05uF for which I got 0.047uF polypropylene the others are all polyester 0.001uF 630V, which I assumed was high enough?

But in trying to put the tester back down on the floor without breaking anything and it was was my fault as I normally hold it on the transformer but this time I caught the little paxolin precision resistor with my little finger and I broke it.

Well the paxolin at least, it looks like the wire is intact and I am pretty sure I should not use it in this condition.

So I am now going to rewind that resistor, at least I would be happier doing that. As my assumption is that I bet this is the 40R resistor in the meter circuit and my fear is that the paxolin is a heatsink and now it is broken it is a weak point.

If it fails my other fear is what it will take out when it does fail.

Any ideas on where to get the wire and the paxolin to rewind it?

I could probably use the old wire at a pinch. My concern is my eyesight is not what it was and this is fine wire.

Can anyone help with that? Yes I suppose I do mean you Ed as you're the go to man for coil winding/transformers.

A coil winder is one of the projects on my list. My assumption is you get the paxolin and measure a known length of the wire to get the resistance you need? winding by hand for now?

Oh yes I will measure it but when there is some light so I can handle it more safely just to find out what the resistance is. Oh yes its location it is the small the small transformer right hand side when out of case. Will take some pictures later as well.

Having an Arghhh moment.....still......

[Ed_Dinning]

Hi Steve, a quick first answer, more later
Small fuseholders can be hot melt glued to the transformer core and wired direct to the common AC outputs, thus giving direct protection to the transformer windings.

Cap voltages should be fine

Before rewinding the resistor try measuring it as accurately as you possibly can. It is a multiplier resistor for the meter and may be specific to each meter. Or, you can apply a small voltage across meter and multiplier and adjust voltage for a set point (say full scale). then use a length of resistance wire to get the same reading; then wind wire onto pax or similar former. It should not get that hot in use. If it fails you will lose meter readings.
Yes I should have suitable wire here

Cheers, Ed

[Stevie342000]

Thanks Ed
Now I need to find some paxolin of course I could take the old one off and glue it but not sure what glue to use long term.

I thought it might be tailored to the meter to calibrate it from what I have read.

I was going to measure it given that it is flapping about it has the potential to fail sooner or later.

So it does need to be fixed, I took a quick look at the patent but was not sure which wire it was will study it more in depth soon....hopefully.

As for the paxolin it does not really matter how thick it is but for originality I would prefer to find some 1mm paxolin. Will check sources to see what I can find.

I did find a picture and I have studied that and the schematic and I am pretty sure it is the 50R resistor but not in detail. I did a quick check of the meter unit and one end is connected to mA/V potentiometer and the other I think is connected to one side of the grid transformer or a coil of some description. If that makes sense.

Still kicking myself. Not had enough good natural light to check it out properly will do over the next few days.

[Ed_Dinning]

Hi Steve, Paxolin or any similar insulator should be fine as it is not intended to run hot.
Looking at the circuit it is what is sometimes called a swamp resistor and sets the meter calibration. It will be in the 50R region (+/- 10% ?) to suit that particular meter.
First precisely measure its resistance and determine the volts needed to get the meter to read FSD with the resistor fitted.
Then you can start taking things apart
Wire length is needed as is wire diameter, I can then find you some suitable resistance wire, which may not be the same but close enough for the job.

Ed

[Stevie342000]

Thanks Ed I will measure it and then determine its length. I have a digital set of calipers(I think that is the right name) which I can use to determine the diameter of the what I think is silk covered resistance wire - constantan.

Just a thought could it not be replaced with a modern metal film 1% 2 Watt? If not why not?

My assumption is the tolerance is not the same and there may be an issue with inductance? Anything else?

[Ed_Dinning]

Hi Steve, measure the wire diameter with the silk covering removed, ie the bare diameter

A fixed resistor would certainly work but what value would you use?
This resistor is a "set on test" part and is calibrated to the meter, hence my instructions on how to set up in earlier posts

Ed

[Electronpusher0]

For reference I redrew the meter and resistor schematic.
Attached

[Stevie342000]

Originally Posted by Ed_Dinning

Hi Steve, measure the wire diameter with the silk covering removed, ie the bare diameter

A fixed resistor would certainly work but what value would you use?
This resistor is a "set on test" part and is calibrated to the meter, hence my instructions on how to set up in earlier posts

Ed

Hello Ed thanks for that not forgotten gotten side tracked as we all so often do. It is on my to do list to measure the resistance and the wire diameter.

Will get back to you shortly with the data.

Thanks my guess is 37swg it's very fine wire.

[Stevie342000]

Originally Posted by Electronpusher0

For reference I redrew the meter and resistor schematic.
Attached

Thanks for that very useful I should study it more to understand how it relates to the hard physical objects. So I can better understand the function as layed out.

I do understand how the tester works but have not made all the links to the drawing and the product.

I know there have been many other threads on the AVT Two Panel Tester is there not a sticky that could be useful in some way so others do not ask all the questions that I and others have asked in present and the past. It is difficult to sift through it all....Ed?

[Ed_Dinning]

Hi Steve, try a request to the mods for a sticky to be set up

Ed