AVO CT160 - relay activates at power on

[ms660]

According to the manual the screen grid is DC, the screen grid rectifier should be shorted out for the screen grid test AC measurement.

Lawrence.

[MikeViking]

Ah screen is DC voltage. Been checking everything with the meter in AC as the heaters and anode switches were attached to the PT secondary.

Will wander out to the shed and recheck my work around the screen.

[David Simpson]

Hi Lawrence, for measurement purposes - yes AVO say that one of their recommended old multimeters can give a DC reading of the half-wave pulse on the screen. However, all AVO's quoted voltages are just representative for calibration purposes. Actually, they are jolly confusing, but the whole aspect of AVO's range of AC operating Valve Testers/VCM's has been exhaustively explained (See "SEARCH") over a number of years on this Forum by a number of valve tester enthusiasts.
The only true way of reading & understanding Va, Vs & -veVg is by using an oscilloscope. Then measure the P - P waveforms by selecting the correct centimetric Volts per Division & using a x10 probe for the higher voltages, then apply the RMS maths.
Take my word for it(& other folk's) there is not one jot of DC Voltage applied to any valve electrode in a CT160 or any other AVO Valve Tester. Its either full sinewaves or half wave pulses(which mathematically have an RMS, or DC equivalent, value).
Whatever you do - take extreme care - for an anode voltage switch selection of(nominally)200V - getting on for 600V Peak to Peak of present on the anode(& the meter) under "Test" conditions. (Actually 2 x Root 2 which = 2.8 x 200V approx.).

Regards, David

[ms660]

The screen grid supply rectifier needs to be shorted out when doing the screen grid voltage test prescribed in the manual on page 9.

Lawrence.

[Dekatron]

All voltages produced in an AVO VCM are either AC (heater) or half wave rectified AC voltages. Anode & screen being positive half-waves and the grid-voltage a negative half-wave. In all models (except the Two Panel, the CT160A and the VCM163) the grid voltage is a combination of two negative half-wave periods out of phase from each other. There is only one instance when a DC-voltage is present and that is when a rectifier/diode valve is being checked as then a reservoir capacitor is used for smoothing of the half-wave rectified voltage.

In early models the valve under test also works as a rectifier itself and in later models valve rectifiers/diodes or silicon diodes were introduced as both rectifiers and also as protection against currents flowing in reverse direction via valve elements.

To measure the voltage with a DMM when silicon diodes are being used in the CT160 you sometimes need to load them with a 100K resistor to cathode, this as some DMM and valve voltmeters don’t load them enough for current to flow, sometimes you need that when rectifier valves are being used too - AVO introduced these resistors in the CT160A and Mk IV tester. But if you use an AVO 8 the instrument itself loads the circuit enough for correct measurements.

You should set your meter to AC and expect the voltages described in the service manual."

[David Simpson]

Hi Martin, min Ven, If you look at page 9 of the Maintenance Manual, and indeed the Circuit Diagram itself, Full P - P AC is fed from the HV Sec winding to Ra(R10, 200ohms) and hence to the Anode. V1 & V2 a & b's, are all occupied elsewhere in the circuitry.
In general terms, folks, all a h/w rectifier does is produce a 50Hz half-wave pulse 10mS long. For that pulse to become a steady DC(as in a radio's HT for example) it has to pass through filtering components such as a couple of hefty electrolytics & a choke. Nasty "Mains Hum" which often is present in old vintage radios in need of TLC, is usually due to failure of one or more filtering components, thus the DC HT reverts to AC, to put it simply.
AVO, in their wisdom, and certainly in the cramped clam-shell casing of the CT160, decided not to fit any such filtering circuitry.
Any doubters - get busy with an oscilloscope, as previously advised.

Regards, David

PS. As stated several years back - that CT160 Diagram of mine is just a simplified explanatory diagram. Not in any way a copy of AVO's working diagram. The emphasis is on "simple".

[MikeViking]

Originally Posted by MikeViking

Well, somewhat successful.

The meter is open. The coils looked intact and while the whole thing looked a little dusty, it wasn’t bad.

What I did find is that this may simply be a bad solder joint.

The meter connects to the outside world via the large bolts on the back. Internally they connect to some tags/lugs with a wire soldered to them. That wire then runs to a coil. Looks like a bobbin. The connection from outside screw to coil is fine. You can see the coil hanging around in the bottom of the attached picture.

The coil is terminated on the plastic bobbin. Another wire then runs up to the meter. That’s where I have flaky continuity.

So, bolt to coil at red circle is ok. Red to green through the coil is ok. Green joint via wire to to blue is intermittent. It really looks like that green joint has fatigue and become intermittent. See the attached pic.

Given the size of the wire in the coil/bobbin, I am really scared of putting a soldering iron near it. Not a lot of room to work with. Stripping that wire to reveal something to solder onto makes me nervous.

Well looks like the coil is open. Bypassing the bobbin swamp resistor didn’t work. Still no continuity through the meter.

Is there anyone in Australia that can service these AVO meters? Does Euan McKenzie do that type of work??

[Dekatron]

The only ones that I know of that can refurbish a meter is Herts Meters in the UK, but I doubt if they can repair a broken movement, possibly replace it with another movement. You can contact them at this email address: info@hertsmeter.com

[MikeViking]

Thanks Martin

I might explore some of the meter replacement threads as well.

Cheers

[Craig Sawyers]

We were in Brisbane a couple of weeks ago - forgot that is where you were. Could possibly have popped in. But back in UK now.

One radical surgery option is to remove the movement, then buy a 50uA or 100uA movement, and take the actual movement out. Then dremel off the the case, just leaving vestigial remains. With suitable packing pieces and hot melt the new movement can be put into the original meter case, and retain the meter scale. Usually the needle is a little shorter than the original, but it is still workable.

You need to do the op-amp trick to match the new movement to the AVO guts.

[MikeViking]

Cheers Craig. There is an excellent thread here on how to use a 100uA meter with an op amp to replace the meter.

I wonder if anyone has made a kit of this replacement? I have very limited knowledge of op amps but I reckon I could retro fit one to my ct160 in place of the current meter.

[Craig Sawyers]

I did exactly that in my CT160, which had the dead meter syndrome.

In fact I recall, and then found in my files, that I corresponded with Gerry Horrox in 2006 about the CT160 meter vulnerability - it is exceptionally easy to blow up the meter. The problem is that you have to know the Ia and Vg values, and set these up before you advance to TEST. Looking at my notes, in the parked position of the gm dial the FSD of the meter is 10mA. If the emission of the valve is duff, and you have set the Ia controls to the datasheet suggested values (for an 6080, 100mA) you peg the meter with a 10x overload, leading to a toasted meter very quickly indeed.

The solution is simple. You replace the 22k value of R14 with 1M in parallel with 330k to make 248k. That sets the basic meter sensitivity to 100mA FSD. So the "new" method of use is to set the two Ia controls to zero and advance to TEST - at which point the meter reads the anode current with 100mA FSD. You then use the two Ia controls to get close to zero, and advance the gm dial to SET ZERO. The meter sensitivity is now 1mA FSD (R14 is shorted out in this position) and final nulling is possible. All without pegging the meter.

Oh and add back to back diodes across the meter. 1N4148 does the trick. And an 8uF non-polar in parallel too. The Mk IV had both of these by design.

[MikeViking]

Hold the phone! You are suggesting a few resistors and a new 100uA meter will get me rolling again? I need to check this out! Thanks for the info!!!

[Craig Sawyers]

Remember that you need to do the opamp trick if you use a different meter movement, whether of not you wangle it into the original AVO meter case, or otherwise.