AVO CT160 Valve Tester problems.

[HBWOODY]

CT160 LOW ANODE VOLTS
When checking the anode volt test (A1) the voltages measure low:-
60volts measures at 10volts
100 " 12.3
150 " 15.1
250 " 20
400 " 62

When doing Section2 - Final test Detail (c) leakage of 1 megohm

Circa Sel SW Elec Sel Sw
C.H/R A1
I was unable to get a reading when using A1 on the cap board could this be part of the same problem?
Hope the above makes sense.

[Radio Wrangler]

Hello and welcome!

How did you measure it?

Most valve testers do not use DC at the voltages written on the switch scales. You may get a half-wave rectified, unsmoothed, waveform. Most multimeters on their DC range will read very definitely wrong on this waveform.

The scaling on the valve tester's own meter, and the scales on the switches etc, and the data in the data books take this into account.

Some basic valve testers even feed AC to the valve under test and let it rectify the power itself!

David

[HBWOODY]

Thank you for the reply to my garbled questions. I will try and ask them again in a more coherent way.

In the AVO maintenance information Chapter 3, Section 2 (C) there is a test done with a 1 megohm resistor which should set the meter at the 1 megohm (midpoint) on the Avo panel meter. I was able to complete them successfully except with the A1 position on the top cap board.

Secondly during Anode volts check test 2 (E) in the same section I can only get the correct readings across A2 and C as you can see in my first post measurements across A1 and C are well down.

Today I will try and get to know the circuit a bit better.

I apologise to those who do not possess the "Section 3 Maintenance Information" I have referred to.

I did also set up a valve test to enable me to check the A1 voltage and found that the reading echoed the low results I had when checking the Anode voltages in the service procedures.

I hope this post is clearer than my first.

While trying to sort out this problem last night I decided to swop round the Diode valves in the AVO circuit which resulted in correct anode voltage readings at A1 but proved to be a brief success as the problem returned almost immediately. I did have my meter set on the AC range.

Haydn

[Station X]

It sounds to me that you may have a problem with the wiring or switching of the A1 lead. Possibly just a dirty switch contact.

There's a simplified diagram showing how insulation tests are carried out on page 9 on the manual.

[David Simpson]

Hello Haydn,
Best bet - use "Search" & have a good shufti at the massive amount of helpful info on CT160's.
Their use of full P-P AC sinewaves on the Anode & half wave pulses on the screen & grid often confuse folk.

Regards, David

[HBWOODY]

Thank you all for your input.

Checked the wiring of A1 from the ELECTRODE SELECTION switch to the TOP CAP board and found that the A1 Link was not making good contact (one of the plastic screws was loose). Now much relieved I can continue.

Haydn.

[Station X]

That's good.

Most faults are caused by quite simple things easily found by using a meter and consulting the circuit diagram.

BTW. It's only the heads of the screws that are plastic.

[HBWOODY]

Have successfully completed "SECTION 2" in CALIBRATION procedure.
How is the meter connected when doing "SECTION 3" Checking SET mA/V control?
Also in SECTION 4 I am unable to adjust the voltage to the required 47V as I cannot find RV6 to make the adjustment.
My CT160 uses valves and the calibration board looks different to ones I can find looking on the internet.

Haydn

[Station X]

Regarding Section 3, what is it you don't understand? The test meter is connected across R5.

Section 4 makes no mention of RV6 or any other variable resistor. I don't think it exists as the variable resistors only go up to RV4.

Perhaps we're looking at different versions of the Servive Manual? Mine is dated February 1963.

[HBWOODY]

Graham
I will connect the meter across R5 to check SET mAv control.
Manuals must be different. The actual wording in mine is:-
SECTION 4 Checking the Set~ Indication
(A) Standardise the testmeter at 47V DC. Set the panel control as follows:-
Circuit Selector to Set~, and Electrode Selector to A1.
(B) Connect the test meter across RV2 and switch on.
(C) A reading of 47V should now be obtained, whilst the meter on the instrument panel should indicate within the "~" zone. If the voltage reading is correct but the panel meter indication is outside "~" zone adjust RV6 as required.
Hope above makes sense.
Haydn

[Station X]

In place of adjust RV6, my version says check resistors R3 and R4.

I assume RV6 replaced R3 and R4.

What's the date of your manual?

[HBWOODY]

I have no idea of the date of the information I am using, it says on the front
Section 3, Maintenance Information.
Thank you for your answer which will help me proceed.
Where can I get a copy of your manual?
Haydn

[ms660]

Scroll down to the last page in the 1st link below for that CT160 adjustment:

https://frank.pocnet.net/instruments...s/AVO-EMER.pdf

Some schematics with amendments:

https://frank.pocnet.net/instruments...d_modified.pdf

Lawrence.

[HBWOODY]

Lawrence
Thank you for that information. Will continue my learning curve tomorrow. I wonder how many variations of the CT160's there are, thought this was going to be straight forward. The challenge will keep my old brain cells well exercised.
Haydn

[HBWOODY]

Lawrence
I see you are based in Cornwall like myself.
If I fail in my efforts to calibrate the CT160, is it a task you could complete for me, as dropping it off and picking it up would not be a problem. I live in wonderful wet Camelford.
Haydn

[ms660]

Originally Posted by HBWOODY

Lawrence
I see you are based in Cornwall like myself.
If I fail in my efforts to calibrate the CT160, is it a task you could complete for me, as dropping it off and picking it up would not be a problem. I live in wonderful wet Camelford.
Haydn

Unfortunately not, sorry.

Help (where I can) via the forum is no problem though.

Lawrence.

[David Simpson]

Haydn,
The 55V transformer tapping is the most important of the secondary windings. The "Set AC" procedure merely alters the transformer primary slightly, so that a valve's grid always gets a peak 55V -ve pulse. As my "very simplistic" diagram shows - CT160 Valve Testers test amplifying valves with AC 50Hz 10mS half-wave "Pulses". Ie. if human's eyes were quick enough - we would see the meter's needle flickering 50 times a second. Any mention in the AVO Manual of RMS AC voltages or DC voltages are merely for calibration purposes. However, AVO VCM's & CT160's can, when accurately calibrated, get pretty close to a pure DC derived Gm "Curve" and indicated desired mA/V.
If you are struggling to come to terms with the CT160's circuitry & calibration - PM me your address & I'll send you a DC standardised pentode(6AU5 or 6AQ5), plus its Gm graph, for just postage cost only. As I still have a couple remaining from a Gm graphing Forum project I ran about 3 years back. There must be about 20 valves floating about there amongst Forum participants. Maybe one or two are down in the SW of England.
Whatever you are doing with your CT160, for God's sake don't "Whang" the meter's needle. Also remember - "Coarse" & "Fine" Ia pots DO NOT control Ia(they are for "Bridge" balancing), only the Vg pot RV2 controls Ia under "Test" conditions. Both replacement meters & RV2's have been known to cost £200 plus each - rocking-horse manure !

Regards, David

[HBWOODY]

Is it ok to accept a voltage of 45v across RV3 when the panel needle has been successfully centred.

[Station X]

Which test are you doing?

[HBWOODY]

Section 4 - checking the set~ indication