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Old 15th Feb 2018, 11:24 am   #1
MikeViking
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Default AVO CT160 - relay activates at power on

Hi. My name is Mike. First time poster.

I picked up an AVO ct160 recently. Case is a little sorry and the panels need a clean but the electronics and internals all look clean. I cannot see any modifications or burnt out components. The 8uF filter cap isn't bulging or leaky. Yet to test that. Fuses were good when it arrived.

I meant to put it on my variac to start it up first time but forgot to turn down the voltage. So I accidentally dropped 240vac onto it. Oops.

Nothing went bang. No smoke. The red power lamp came on. But the relay activates immediately. Only a split second buzz of the really then it activates. Panel meter lights up red.

I have the rotating dials all set to zero. I rolled the pots and switches down to the lowest volts or amp settings. The test is in the SET mode as per the procedure to verify the mains. The needle never moves before the protection relay kicks in.

Open to suggestions. It would appear I have a short somewhere in the screen or anode volts (switch perhaps?) drawing too much current thereby activating the relay. Could be the SET Ia switch as well that's tied to the relay and V1b and the 50v tap.

Or perhaps the diode valve in V1a has an issue. I was going to pull the 2 valves and then power on but it looks like the relay relies on current through the diode to activate.

Thoughts??
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Old 16th Feb 2018, 9:00 pm   #2
Philips210
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Default Re: AVO CT160 - relay activates at power on

Hi Mike.

There is some info in this thread https://www.vintage-radio.net/forum/...d.php?t=130745

Hope that's useful.

Regards
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Old 17th Feb 2018, 6:36 am   #3
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Thanks Symon.
I might disconnect the meter or try some of the protection circuits as well. If I have the meter disconnected, can I test/troubleshoot the AVO without it? Or should I find myself a 3k resistor to put in place of the meter?
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Old 17th Feb 2018, 8:51 pm   #4
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Default Re: AVO CT160 - relay activates at power on

Hi Mike.

I can't honestly say for sure as I've not had any experience of the CT160. I can't see any harm in substituting the meter for a suitable value resistor for test purposes. Perhaps another more experienced member might be able to provide better advice.
I assume that you have the manual including the circuit diagram but if not it can be found here https://www.kevinchant.com/avo.html

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Old 25th Feb 2018, 1:53 pm   #5
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Well the meter may be busted.

The relay wouldn’t let go. Took it apart and didn’t find anything. Moved it manually a few times to ensure it was moving. Started it up again, this time in the variac, and all good.

We now have 240vac, a glowing red power lamp and 2 good fuses, protection relay is not activating and, oh, no meter movement. The meter should move in the ‘set’ position for calibration/confirmation that the input voltage is ok, but it doesn’t move.

It could be the rectifier diodes. The 2x GEC D77s glow, but I haven’t checked they are producing the expected dc voltages. The paralleled meter and R9 shunt read 9kohm so they are not open circuit. I am hoping that it simply a matter of retubing the rectifiers or replacing with one of the silicon rectifiers in this thread:

https://www.vintage-radio.net/forum/...ad.php?t=74490
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Old 25th Feb 2018, 4:59 pm   #6
Craig Sawyers
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Default Re: AVO CT160 - relay activates at power on

If the paralleled meter and shunt read 9k your meter is open circuit. There is basically a 10k resistor in parallel with the meter which has a resistance of 3.25k. So the parallel combination should be about 2.5k

It is highly unlikely that the D77's are duff, and that would not explain the resistance readings across the meter.
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Old 25th Feb 2018, 9:58 pm   #7
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Thanks Craig.
I will remove the meter and R9 to check. Probably on the weekend. Cheers
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Old 26th Feb 2018, 8:42 am   #8
Craig Sawyers
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Default Re: AVO CT160 - relay activates at power on

If the meter is working, be very careful testing the continuity with a meter, because it takes less than 100mV/30uA to peg it. So a DVM can easily destroy the meter, depending on the meter. Put something like 47k in series. If it is already open you'll clearly read an open circuit. If the meter is OK, you'll read around 50k and the meter will read something less than full scale.
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Old 26th Feb 2018, 10:42 am   #9
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Thanks Craig.

It is open. Removed from the tester and tried with and without the series resistor. Nothing.

It looks like someone has tried to open the meter before. There are marks on the 4 small circular nuts that hold the meter together. I feel like I should open it up but I am wary about how sensitive the meters can be.

Any suggestions on what to do next? Pry the case apart or is there a technique needed for getting the meter open? Or call it a day and source an alternative meter?
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Old 26th Feb 2018, 2:28 pm   #10
Craig Sawyers
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Default Re: AVO CT160 - relay activates at power on

It comes apart - just remove the slotted circular nuts (IIRC) and it just comes apart.

I was lucky when I repaired mine that I chanced on an AVO MK IV testmeter free. This has physically the same meter, with a different scale and different sensitivity. Once you have them open you can swap the scales over, and then screw it together. You can then use an op-amp circuit so that the CT160 innards think they are looking into a real meter, and the opamp drives the replacement meter with the correct current.

The alternative is to buy a 100uA movement with about the same needle length from Farnell (I think I used Anders for another valve tester). You basically take the movement out, then cut the case down so that it fits in the original AVO meter movement recess, and fix everything in place with hot melt glue. Then do that op-amp trick.
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Old 27th Feb 2018, 10:19 am   #11
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Default Re: AVO CT160 - relay activates at power on

You can contact the people at www.hertsmeter.com and ask what they charge to check your meter to see if it is possible to repair it or not, if you want to keep the oroginal meter.
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Old 27th Feb 2018, 10:55 am   #12
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Thanks Martin. The needle still moves freely and sways so the springs are still connected and doing their job. Hopefully it’s a simple fix
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Old 27th Feb 2018, 11:07 am   #13
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Default Re: AVO CT160 - relay activates at power on

Mike,
you can check it with a good magnifier for broken coil ends-solderings_possibly you can do yourself some diagnosis...
Karl
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Old 28th Feb 2018, 1:42 pm   #14
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Well, somewhat successful.

The meter is open. The coils looked intact and while the whole thing looked a little dusty, it wasnít 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. Thatís 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.
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Old 28th Feb 2018, 2:23 pm   #15
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Default Re: AVO CT160 - relay activates at power on

As a temporary solution you can remove the bobbin and put a metal film resistor, or two, with the same total value in its place to test the meter, then you can try to resolder the joint while the bobbin is out of the meter.

You should protect the moving coil and magnet from dust and metal swarf while you have it in the open, also protect it while soldering as the solder flux (rosin) might sprinkle on the movement that might damage its balance or stick in unwanted places dampening or hindering its movement.
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Old 28th Feb 2018, 3:26 pm   #16
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Default Re: AVO CT160 - relay activates at power on

Fine Mike!
Dont be nervous; if you are some excited, drink please a small glass beer-its really reassuring for the hands too
Be successful, Karl
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Old 9th Mar 2018, 8:58 am   #17
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Well we tried the beer trick to settle the nerves, but that didn’t work. I couldn’t find the soldering iron, flux or meter. I did find the dance floor at the disco though...

Serious note. Is there a thread on how to clean the panels and valve sockets. They are pretty dirty. I will add some pics tonight.
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Old 9th Apr 2018, 12:00 am   #18
MikeViking
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Default Re: AVO CT160 - relay activates at power on

Well I havenít got to the meter yet. I did check a few voltages around the transformers. The heater and anode voltage are close. The lower voltage values are closer than the higher voltages, but within 10-25% on all taps.

Measuring the voltages on the top panel (via the test sockets) works for heaters and anodes, but the screen voltages are lower than the value selected by the screen switch. I need to investigate that further. If I set the screen voltage rotary pot to 100v, I would expect 100v at the test socket right? Is that not the case?

I also check the barrel switch. I set up the barrel for a 12at7 and was able to measure the heater, anode, grid and cathode voltages on the pins of the socket. I guess a little bit of De-OxIt may be needed to clean oth dockets ad they look very sorry indeed. At least I get the right voltages to the pins. Replacing the socket shouldnít be hard if required.

Looking promising. Attached is a pic of the top panel you can see the barrel switch and test points in the left. In the right, the noval socket (McMurdo brands) and the dirty, corroded goodness that I need to clean up!
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Old 9th Apr 2018, 9:36 am   #19
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Default Re: AVO CT160 - relay activates at power on

Quote:
Originally Posted by MikeViking View Post
Measuring the voltages on the top panel (via the test sockets) works for heaters and anodes, but the screen voltages are lower than the value selected by the screen switch. I need to investigate that further. If I set the screen voltage rotary pot to 100v, I would expect 100v at the test socket right? Is that not the case?
The screen volts control is not a rotary potentiometer, it's a multiple position switch. The anode and screen voltages measured on the valve sockets will not match those selected on the switches. This is because the CT160 works using AC voltages. You are advised to consult the service manual.

Measured AC voltage should be 1.1 x selected voltage -2 +6%.
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Old 9th Apr 2018, 10:18 am   #20
David Simpson
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Default Re: AVO CT160 - relay activates at power on

I'll echo what Graham says. The voltages for both Screen & Anode are sourced from a single secondary winding of the CT160's multi-functional AC 50Hz mains transformer. From memory I think there are 18 tappings. Have a good look at the multitude of circuit diagrams available via "Search". The Maintenance/Calibration Manual also gives a range of expected & acceptable voltages for each tapping.
I've attached a very basic diagram of how a CT160(& more or less it's siblings the MK3 & MK4) works. Its been available via "Search" for a number of years.
As Graham says, once the thumbwheel code has been set as per the AVO Valve Data Manual, the recommended voltages are just representative RMS Values. Nothing like the effing great peak AC voltages used internally. With that in mind - remember - the peak anode voltage is on the Meter's Terminals ! So take extreme care. Also - take extreme care with the -ve Grid Volts setting (RV2), that & that only controls Anode Current. The Coarse & Fine Current controls have a completely separate function - Bridge Balancing.

Regards, David
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