AVO CT 160 voltage measurements

[retailer]

Having put my Avo CT160 aside to work on the kit guitar amp restoration, I'm now back on it - the defunct meter is back in and working with a new 200uA movement sitting inside the original Avo case, driven by a good old 741 op-amp.
I started by following the service manual and checking the voltages - filament voltages are all fine but when I try to check the anode voltages using the service manual procedure - electrode selector is at A1 and circuit selector at TEST - the meter pegs at full scale, on removing the meter and replacing it with a DVM I see a dc voltage that corresponds to the setting of the anode current switch - with switch at 0mA I measure 0 volts at the meter, at 10mA I measure approx 0.1V, at 20mA it is approx 0.2V and so on right up to approx 0.9V when the anode current switch is set to 90mA, so the only way to check the voltages (with the meter in place) is to set the anode current switch to zero - does the service manual omit to mention this or do I have a fault that I need to track down.

I did run a valve through (6V6) for a quick check and while it didn't agree with my other tester it was very close - I didn't bother with a GM test but the meter balanced at zero with anode current set to 45mA and bias at 11.5V which is about right for a 6V6, so it looks as if that part of it ok.

[John_BS]

Can you please post the full schematic of the 741/meter mod together with it's associated power supply arrangement?
John

[retailer]

I used the original Crowthorne Tubes meter amp circuit and made a post about it here https://www.vintage-radio.net/forum/...d.php?t=190237.

I attached the circuit I drew up in Fusion360 and the PCB board layout, there are many ways it can be done, in my case I designed the board to fit onto the back of the original meter case, held in place with 2 nuts, I slipped a short length of heat shrink over the original meter terminals to insulate them and tucked them out of the way and soldered a flying twin lead across the CT160 10k resistor, this lead plugs into the board input, the output to the meter consists of the 2 pcb pads, a small 9v-0-9v transformer mounts near the power switch and carries the ac supply to the board via a 3 pin plug/socket. I haven't marked component values on the circuit I drew up but you shouldn't have any problems working this out by referring to the Crowthorne circuit with a couple of slight differences - Crowthorne used a trim pot set to 3250 ohms to mimic the original meter resistance - I used a pair of resistors in parallel and went through my stock and selected a pair that measured 3250 ohms when placed in parallel - I got within 2 or 3 ohms, also I omitted the 0.1Uf capacitors.
I haven't marked the trim pots on the pcb layout, so if you decide to use this layout - R5 sets the FSD and R6 is the DC offset this should be set so the meter reads zero when the meter has no input ie shorted.
It's a pity you live on the other side of the globe - I did not end up using the first board I made and you would be welcome to it but the cost of postage to the UK is around $AU25 which is probably 5 or 6 times the cost of the components, this first board has been tested working and fits ok but at the time I was experimenting with CNC board milling so I decided to design and make another that looked just a bit neater, they are shown side by side in one of the pics.

[Dekatron]

I spotted one error in your circuit diagram and board.

The bridge rectifier is not connected correctly in your circuit diagram and neither on the first circuit board - the two AC sides have been swapped with the + and - connections. This can bee seen as the connections with the same polarity of each pair of diodes go to the AC connections whereas they should go to + and - respectively.

If you compare your circuit to the Crowthorne design you'll see the difference in how the bridge rectifier is wired.

I can't see the underside of your last design but you might have corrected it there.

[retailer]

You jogged my memory Martin - yes I did do a dodge with the rectifier - when doing the board/circuit in Fusion I couldn't find a suitable component with pinouts to match the bridge rectifier I was using so I used one with the correct footprint but incorrect pinouts and wired it accordingly, I was at that time new to Fusion360 and was trying to come to grips with creating and editing components, since then I have given up on Fusion360 for PCB's as the free version has a size restriction, I have gone back to KiCad and use a Gcode generator called FlatCam to mill boards as there is no restriction on the size.

The new smaller board is working fine the CT160 however I believe has a fault, that needs to be tracked down.

[retailer]

I think these little beggars were the cause of the issues I was having - with the meter out and a 3k3 resistor in it's place I could measure approx 10mV across the resistor(ie meter) on all of the insulation test switch settings, in the final test mode set to either A1 or A2 I could measure anything from 100mV up to 1V across the resistor depending on the anode current switch setting. I wasn't sure at what point in the test procedure the capacitors were in circuit but thought that I'm going to replace them anyway it might as well be now. With the capacitors replaced a quick check with a DMM across the 3k3 resistor and I now measure zero volts on all of the insulation test switch settings, I'm 90% confident the current issue is fixed, I'll keep my fingers crossed there are no others apart from calibrating.

[Dekatron]

Good that you corrected the rectifier.

I always replace those capacitors when refurbishing a CT160, usually using Polypropylene capacitors as they are a known problem when they get old, they work better as a resistor than a capacitor then.

[retailer]

Both halves of the case of my CT160 are in poor condition so I have removed the electronics in preparation for painting the case halves, the valve panel is also in need of a heavy clean so I have separated this from the main unit. As part of the calibration, I followed the service manual procedure to check the heater, anode and screen voltages, however when I switch the Electrode Selector to A1 and the Circuit Selector to Test as stated in the service manual, the meter indicates full scale to the right, this does not happen if I switch the anode current controls to zero, to prevent damage to the meter I've removed it and placed a temporary 3.3k resistor in its place along with a DVM, it's been almost 20 yrs since I last had a CT160 and I can't recall having to do this - the service manual doesn't mention removing the meter or setting the anode current switch to zero so I suspect I have some sort of fault.

[Craig Sawyers]

I repainted my case. Took it to Halfords and used a book they have of all of their custom paint shades. Found one that was an excellent match, and they made it up on the spot.

The casework looks great!

Craig