AVO CT160 Valve Tester. Meter Replacement.

[Station X]

The meter in my AVO CT160 valve tester became so erratic in operation that I decided to replace it.

I did this by following the instructions in an article published in Radio Bygones issue 141 February/March 2012. As the magazine has now ceased publication I have attached a copy of that issue to this post.

The RB article describes replacing the complete meter or just the meter movement with a Simpson 03591 25uA fsd taut band meter or meter movement. I obtained my meter from Mouser. It wasn't cheap.

I opted to replace just the meter movement, fitting the new movement into the existing meter case. This meant that I didn't have to modify the CT160's front panel and the ability for the overload lamp to shine through a window in the meter case was retained.

[Station X]

The existing meter case had to be drilled to accept the new meter movement. The case was opened up and stripped of its contents. It is important to retain the wire wound swamp resistor and its wiring as these will be reused.

The RB article has a drawing showing the position and size of the new holes. As it can be difficult to accurately mark out the case I decided to use a drilling jig or template to allow accurate positioning of the holes. This was made by coordinate drilling a piece of steel plate in the milling machine. It was temporarily attached to the meter casing using screws and nuts through existing holes. The jig can be attached in two ways, four if you want to be pedantic, allowing small diameter pilot holes to be drilled first.

[Station X]

Here are two pictures of the drilled case.

I also drilled a small diameter hole between the two existing holes for the terminal posts. For reasons to be explained later, one of the new meter movement's leads will pass through this hole.

[Station X]

Two extension nuts were needed to hold the new Simpson movement in the AVO case.

These were simple turning and tapping jobs done on a lathe.

[Station X]

The new Simpson movement was removed from its case leaving the leads as long as possible. The metal scale was removed for later modification. The movement is very delicate in this state.

The original swamp resistor is soldered to one of the movement's leads as shown, the other end of the swamp resistor being connected to one of the existing terminal posts. Nothing is connected to the other terminal post. The other lead from the movement passes through the hole drilled in the casing between the terminal posts.

I had to make brass spacers to prevent the movement tilting in the case, but unfortunately neglected to record their dimensions. Nuts or washers could also be used as spacers.

I found the easiest way to slide the movement into the case was to grip the magnet in a large pair of plastic tweezers.

In the rear view note the lead protruding from the hole between the two terminal posts and the brass extender nuts.

[Station X]

Artwork for new meter scales to suit the Simpson movement is included in the RB issue.

Originally I used a laminated scale on its own, but found that it was prone to bend and contact the needle.

The original metal scale plate from the Simpson movement was modified to fit the AVO case. This meant trimming it along the sides and top. Forum member Dekatron supplied me with some single layer scales and one of these was glued to the metal scale using UHU adhesive. The scale was then screwed to the movement.

Having checked, by blowing on it, that the needle was free to move I fitted the front "glass" to the meter, making sure that the lug on the fsd setting screw was located in the 'Y' of the new movement. The most difficult part of the job was over.

[Station X]

The original AVO meters had a total resistance of 3250 ohms and a full scale deflection of 30uA. To give the new 25uA meter the same characteristics both shunt and series resistors are required. These are adjustable to match individual meter movements and swamp resistors.

The resistors were mounted on a printed circuit board along with the two diodes and a capacitor forming the protection circuitry. Printed circuit board is a bit of a grand description. I simply milled away narrow strips of a piece of copper clad board leaving I think four lands. The meter terminal studs contact the copper through brass washers. A short piece of stiff wire, soldered to the copper, sticks through the board and the wire leading to the movement is soldered to it.

[Station X]

I've forgotten exactly how I calibrated the new meter.

From what I remember I connected a variable voltage PSU, a high value resistor and a 50uA meter in series with the new meter. The PSU was then adjusted to give 30uA as shown on the 50uA meter and the shunt resistor on the PCB adjusted to give fsd on the new meter.

To set the total resistance to 3250 ohms I believe I adjusted a multi-turn 10k pot to a value of 3250 ohms as measured on my DMM. This was then connected across the new meter's terminals. With the PSU still supplying 30uA the series resistor on the PCB was adjusted so that the new meter read 50% of fsd.

There is just room for the capacitor in the CT160's case, I had checked before fitting it to the PCB.

With everything reassembled it was time to adjust the CT160's voltage selectors so that the needle pointed to the '~' symbol on the scale and start testing some valves.

[Station X]

I'd be willing to pass on the drilling jig to a forum member needing it. I don't want it back, but you'd have to pass it on to the next member needing it.

[ChristianFletcher]

Outstanding work and I enjoyed reading the magazine article.

Thanks Chris

[deliverance]

Very interesting and excellent work Graham.

[Dekatron]

Really nice to finally see the result from this modification, it looks very nice!

Ever since Euan and I wrote that article I don't think I have heard from anyone who made this "surgery" so I am glad that there finally is some feedback.

[Station X]

Thanks Martin.

I'll try to draw out the PCB layout to complete the story. I don't want to dismantle the meter again just to photograph the copper clad side.

[woodchips]

Thank you to all, I just hope I don't have to make use of it!

[Electronpusher0]

I have just checked the price of the Simpson meter, as you say not cheap!

However its still an excellent repair.

Peter

[Station X]

One off topic post moved to new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=176412