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Old 9th Dec 2013, 2:58 pm   #1
Pilotltd
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Default CT160 calibration problem

Hi - I've managed to get a CT160 valve tester. It appears to be mostly working well. Meter calibrates nicely on set with only a volt or two error at 235V.

However, on testing valves it's very optimistic on the GM, for example 1.9 to 2 on ECC83's.

I've removed it from the case, cleaned all the wipers etc and checked the Heater and Anode voltages and they are all within spec. On trying the calibration process I removed the link and put my meter across RV2. Got a reading of 26.4 Volts, tried adjusting RV3 but the lowest I can get it is 22.4 rather than 20.8V?
at 40V setting. Checking at 13 and 4 I get 7.1 and 2.237 respectively.

I've not touched the dial to set it as the 22.4 worries me.

Both my meters are digital true RMS and agree with each other, so not sure if this is throwing an error in too?

Any ideas?

Thanks

Steve
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Old 9th Dec 2013, 7:38 pm   #2
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Default Re: CT160 calibration problem

Have you checked the resistance values in the grid voltage and calibration circuit?

You can read more on the calibration circuit here: https://www.vintage-radio.net/forum/...ad.php?t=86262 if you want to understand the inner workings.

/Martin
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Old 10th Dec 2013, 1:14 am   #3
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Default Re: CT160 calibration problem

Hi Martin, thanks for the reply and the information, when I went back to it later I could then get the 20.8v ! It had been turned on a while then, so then I checked voltages across R5 and it was way out.

I guess I'm going to have to check R1, R2 and R5. Hope it's not R1 as I can't find one, nearest is 2.32k at .6W . Maybe V2 playing up having an effect?

Steve
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Old 10th Dec 2013, 10:45 am   #4
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Default Re: CT160 calibration problem

Follow Martin's advice above for sure, but one thing I will add is that good ECC83s typically produces a gm of 1.9 to 2 mA/V. I measure these most days when checking customers' valve amplifiers in my workshop, and if I see a valve sitting on the 1.6 mA/V gm figure (the reference gm shown in the Avo data book), I assume that it's still good, but probably on the way down. So 1.9/2.0 is not at all an optimistic figure for an ECC83 - if you build the Avo calibrator, you can check this yourself - or send me your valve and I will check it for you on my calibrator. Cheers, Mark

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Old 10th Dec 2013, 1:27 pm   #5
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Default Re: CT160 calibration problem

You can always use ResCalc (http://www.pmillett.com/rescalc.htm) to calculate resistor values that you can use for substitution, the only thing that you will have to take into consideration is if you split a resistor value into two resistors that they can withstand the voltage across them (except for power and current of course).

Poor rectifier valves, the EB91's, might of course affect the readings. From my paper you can see that they are used a bit beyond their specified limit so they wear out over time. You can always replace them with silicon diodes like it was done in the CT160A, just don't forget the 100K resistors to common ground then as you will get strange readings with some multimeters if the silicon diodes are not loaded so some current flows through them.
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Old 11th Dec 2013, 12:56 am   #6
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Default Re: CT160 calibration problem

Quote:
Originally Posted by fastflyer99 View Post
you can check this yourself - or send me your valve and I will check it for you on my calibrator. Cheers, Mark
Thanks Mark - I'll pm you on that in a few days.

There is definitely something unstable in it. Couldn't get it to set today. I've ordered some resistors and diodes so will start going through and replacing them and I think while I'm at it do the diode conversion and meter protection. I'll order some caps tomorrow and do those while I'm at it, almost a certainty they've gone bad.

Steve
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Old 11th Dec 2013, 1:44 pm   #7
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Default Re: CT160 calibration problem

have you checked the contacts on the cut-out relay, if they have high contact resistance the whole tester is affected and the voltage varies. You could also check the voltage selector. An easy way to see if any of those affect the circuit is to bypass them with two wires, you'll have to check carefully where to place those wires though on the voltage selector, bypassing the relay is much easier.

/Martin
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Old 11th Dec 2013, 3:41 pm   #8
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Default Re: CT160 calibration problem

Hi Martin - Just checked and it was 10ohms - the contacts were ok but the pivot point was bit corroded. After contact cleaner it's now 0.4ohm.

I've cleaned the contact pin and bar on the voltage selector too.

Changed R1 - it was 2.32 rather than 2.34

Just trying to figure out the easiest way of removing the board on RV1 with R14, R5 and R2 on

Steve

Last edited by AC/HL; 11th Dec 2013 at 6:20 pm. Reason: Unnecessary quote of the entire previous post removed
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Old 11th Dec 2013, 3:49 pm   #9
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Default Re: CT160 calibration problem

To decrease the risk of the pivot point getting corroded again you can bypass it with a flexible wire, like the ones used in multimeter leads, you can solder on the connection on the stud on the relay and then directly onto the lever. I've done that myself on a few of these relays and I know that Karsten Simon did so too (he wrote about the meter protection with Schottky diodes).

I usually cut of the wires off the resistors, close to the body, and then untangle those wires with just enough heat from the soldering iron so that they can be unwound from the soldering studs with a pair of pliers. This usually works just fine as long as you don't put too much heat on the soldering point so that you don't destroy the isolation on the wires.

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Old 11th Dec 2013, 5:37 pm   #10
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Default Re: CT160 calibration problem

Here's a weird one - the values installed for R3 + R4 dont add up. R4 says 660K R3 has a 5.1M and a 660K or 650K resistor (can't read it properly) in parallel. It doesn't equal 1.32M? Here's a photo.
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Old 11th Dec 2013, 7:38 pm   #11
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Default Re: CT160 calibration problem

The important thing is that the total resistance of R3 + R4 results in a current that makes the needle land on the red calibration line when the voltages are correct.

The total resistance should be close to 1.32M and since the black Welvyn resistors are known to increase their resistance over time I guess that the 5.1M resistor was put there to lower the resistance of the 650/660K resistor.

You can use modern metal film resistors to get 1.32M, you can also add a small 10 turn potentiometer to the combination so that you can adjust the calibration resistance when the correct voltages have been set.

Before you try to calibrate your tester you should make sure that the meter has the correct internal resistance of 3250 Ohm and correct FSD of 30uA.

/Martin
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Old 11th Dec 2013, 8:50 pm   #12
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Default Re: CT160 calibration problem

My AVO CT160's also had an additional parallel resistor to R3+R4, but these change over time, so as Martin indicated they often need to be changed. If you do replace the th EB91's with solid state diodes, you also need to change out R6 to get the correct anode current reading. I used a 1K 10 turn precision variable resistor and adjusted it until the values tracked with those across the A1 link terminals (something in the range of 710-740 ohms if I recall). Based on information from Martin, this link describes the conversion to solid state diodes https://www.vintage-radio.net/forum/...777#post467777 .

With regard to calibration and the AVO meters, I have probably worked on 8 or 9 of these meters, and most measure low (usually 5-10%) with 30uA applied. I have been able to readjust some of the meters using the meter flux bar, but in some cases have had to add an extra pole magnet. Without this adjustment it is hard to get everything calibrated correctly.
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Old 11th Dec 2013, 9:41 pm   #13
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Default Re: CT160 calibration problem

Thanks guys, I'll get some multiturn pots ordered tomorrow and check the meter before I change anything else. Not looking forward to adjusting the meter if it needs it

Steve
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Old 17th Dec 2013, 6:19 pm   #14
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Default Re: CT160 calibration problem

Tested my meter today, as near as I can tell it takes 31uA to drive it full deflection. I don't know how accurate the meter used is, as both my Digital Meters only do mA, nothing smaller. I borrowed an old Avo Panclimatic to do the test.

What do you reckon - is it near enough? I'm of a mind to leave it, rather than play inside the meter.

Thanks

Steve
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Old 18th Dec 2013, 12:13 am   #15
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Default Re: CT160 calibration problem

To get your meter to work properly in the circuit you have at least these four options:

*. Use a meter amplifier with an opamp to adjust for the extra current.

*. Adjust the internal swamp resistor with a resistor in parallel and also add at least two external resistors and possibly two external trim potentiometers for easier adjustment.

*. Add a small neodynium magnet two the magnet and then adjust the flux with the magnetic shunt.

*. Have it sent to Herts Meters Ltd for a repair/refurbishment.

The last two means that you'll have to open the meter which means that you might damage it if you aren't careful, bought on the other hand you'll also be able to se the condition of the parts and also be able to clean the meter housing.

In the first option you will have to build the meter amplifier and put it inside the valve tester which will mean some construction work, but it will also give you the chance of adjusting the amplifier in the future if your meter looses more magnetic flux.

Which option you choose will probably have to do if you want to open the meter or not - I would go for the meter amplifier if the meter is well balanced and also linear in its deflection, otherwise i would have it repaired/refurbished.

/Martin
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Old 18th Dec 2013, 11:15 am   #16
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Default Re: CT160 calibration problem

Hi Martin, I thought I read somewhere that Herts meters had stopped repairing them?

Your paralleled shunt resistor solution got me thinking, maybe incorrectly R9 (10k) is in parallel to the meter giving a total resistance of 2452.83 ohms in circuit. In my case R9 is reading 9.93 so total is down to 2448.59. Altering the value of R9 should make it possible to get the meter to read correctly in circuit shouldn't it?

The meter seems balanced nicely and is linear across the scale.

Steve
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Old 18th Dec 2013, 11:46 am   #17
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Default Re: CT160 calibration problem

Maybe Herts Meters have stopped servicing these movements, last time I heard anyone using them was perhaps a year ago or so. Last time I used them was well over two years ago. You can always ask them, their webpage is here: http://www.hertsmeter.com/ .

If your meter has an internal resistance of 3250 Ohm and needs 31uA to reach FSD the voltage drop across the meter is 100.75mV, the original meter only has a voltage drop of 97.5mV which you must reach with the compensation network in place - unfortunately this is only possible by lowering the internal resistance of the meter which is done by using a resistor in parallel with the swamp resistor, then the compensation resistors will bring the voltage drop back to its proper value.

You can read more on how the resistor value is calculated here: https://www.vintage-radio.net/forum/...8&d=1342875214 in chapter 5.2 .
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Old 20th Dec 2013, 4:49 pm   #18
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Default Re: CT160 calibration problem

If you take into account the accuracy of analogue meters, including your Avo Panclimatic, being off by 3% is quite acceptable. Three of my AVO 30uA meters I tested read slightly low ~3-5% percent, and I was able to adjust this with the meter flux bar mounted on top of the magnet. This assumes you want to break into your meter and risk contaminating and/or damaging it, but it only requires removal of the face plate (also a good time to remount the glass). The meter flux bar is usually lightly cemented in place, you can gently pry the bar to break the old glue, you may need to slightly loosen the screw it pivots on. If I recall correctly, moving the bar down toward the coil, increases the meter sensitivity. I adjust the flux bar, then lightly put the faceplate on the meter, check it and then repeat as needed. Then put a small dab of glue to secure the bar. Then secure the faceplate with the mounting hardware. I do not like to using any magnetic (ferrous) tools around the meter.

In my experience, it is common for the front glass seal to have failed, so as a general rule I always remove it and reseal it. I have also had problems with the white paint in the back of the meter housing flaking and getting into the meter movement. The meter bearing adjustment is also a problem.
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Old 22nd Dec 2013, 6:02 pm   #19
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Default Re: CT160 calibration problem

Hi Mark - I'm probably going to leave it until after the holidays. My meter looks pristine. The glass is well sealed and the meter looks to be very clean internally from what I can see through the glass.

The neg grid volts control is now spot on calibration and the meter is lining up pretty well on the set mark for the requisite mains voltage. However the mA/V dial needs adjusting. Do you know what size the locking nuts are? I'm guessing they are BA or imperial sized. I'll have to have a look through the shed for something suitable later in the week. All the spanners to hand are metric and too fat anyway to hold one nut and release the locking one Looks like they have shellac on them too!

Steve
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Old 23rd Dec 2013, 4:56 pm   #20
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Default Re: CT160 calibration problem

Hi Steve, would definitely leave the meter be. I did adjust my mA/V setting many years ago, but do not recall the sizing of the wrenches. I use thin ignition or midget wrenches which work well in these circumstances. As a side note if you have the tester apart, you may consider installing a 5x20 mm fuse holder and 50mA fuse in the grid line to protect the irreplaceable grid pot from overload damage, and added meter protection diodes/capacitor to protect the meter https://www.vintage-radio.net/forum/...988#post538988 .
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