AVO CT378 Attenuator fault - UK Vintage Radio Repair and Restoration Discussion Forum

mole42uk · 29th Apr 2016, 12:51 pm

My AVO CT378B works! It needs a bit of TLC but it's running right now on the variac set at about half the correct voltage. Nice sine wave on the 'scope, low level, but it looks good.

The attenuator doesn't work. Set to "FORCE" I get an output, but move either 'Coarse' or 'Fine' controls stops the output altogether. The manual has several pages about exchanging the attenuator card and a long paragraph about adjusting the attenuator, but I wondered, before I attempt to repair it, whether anyone had done a similar job and knew about the potential pitfalls!

Thanks!

ms660 · 29th Apr 2016, 1:04 pm

Not had attenuator problems but if you loose internal modulation check out C1 across the AF osc's lump of iron, it went S/C on the one I had.

Lawrence.

mole42uk · 3rd May 2016, 8:45 pm

Well, I took the plunge and removed the attenuator. It has two controls with carbon tracks, one of which shows evidence of being connected to something powerfully destructive. There's a hole in the carbon track.

Now, I moved the wiper away from the hole so that it now connects to sound carbon, but these tracks form a variable potential divider so there is always a resistance in line and a resistance to ground at any point on the dial.

Am I likely to achieve a working attenuator? On the other hand, has anyone dismantled a CT378 and has an attenuator going spare? Long shot eh?

Vintage Engr · 6th Jun 2016, 4:45 pm

I have one with the same problem. Have not used it for years because of the fault. If one turns up, like you I'll repair it!
You could modify it & use a standard attenuator, available in either 50 or 75 ohm versions. Or, set it to 'Force' & use an external attenuator, such as the HP 355C.
At least that would make it useable for the time being.

Dave.

WME_bill · 7th Jun 2016, 6:10 pm

Avo CT378 Family.
I have had a number of these Signal generators over the years: CT378, TF/M, AFM2. The suitcase design is most ingenious, but the panel attenuator can be the problem.
Once damaged by excess current, you are stuck. The MoD handbook of 1961 quotes part numbers for a replacement, but not unexpectedly, they are no longer available.
For a worn track, I have shifted the wiper position slightly. Also I have touched up the track by rubbing with a soft B pencil, same trick as with crackly volumn controls, but it does not last well.
You might be interested in the attached article from Wireless World 1957 which explains the idea.

Incidentally, I find I have a couple of leads with the Burndept plugs that fit the output, and the extra fixed attenuators,and the strange plug that fits on the other end of the fixed attenuators. If anyone is interested, send a message, and I will sort them out and see what I have.
wme_Bill. m0wpn

AC/HL · 10th Jul 2016, 12:29 am

One post moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=127938

29th Apr 2016, 12:51 pm	#1
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	AVO CT378 Attenuator fault My AVO CT378B works! It needs a bit of TLC but it's running right now on the variac set at about half the correct voltage. Nice sine wave on the 'scope, low level, but it looks good. The attenuator doesn't work. Set to "FORCE" I get an output, but move either 'Coarse' or 'Fine' controls stops the output altogether. The manual has several pages about exchanging the attenuator card and a long paragraph about adjusting the attenuator, but I wondered, before I attempt to repair it, whether anyone had done a similar job and knew about the potential pitfalls! Thanks! __________________ Richard Index: recursive loop: see recursive loop

29th Apr 2016, 1:04 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: AVO CT378 Attenuator fault Not had attenuator problems but if you loose internal modulation check out C1 across the AF osc's lump of iron, it went S/C on the one I had. Lawrence.

3rd May 2016, 8:45 pm	#3
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	Re: AVO CT378 Attenuator fault Well, I took the plunge and removed the attenuator. It has two controls with carbon tracks, one of which shows evidence of being connected to something powerfully destructive. There's a hole in the carbon track. Now, I moved the wiper away from the hole so that it now connects to sound carbon, but these tracks form a variable potential divider so there is always a resistance in line and a resistance to ground at any point on the dial. Am I likely to achieve a working attenuator? On the other hand, has anyone dismantled a CT378 and has an attenuator going spare? Long shot eh? __________________ Richard Index: recursive loop: see recursive loop

6th Jun 2016, 4:45 pm	#4
Vintage Engr Heptode Join Date: Jan 2008 Location: Shrewsbury, Shropshire, UK. Posts: 841	Re: AVO CT378 Attenuator fault I have one with the same problem. Have not used it for years because of the fault. If one turns up, like you I'll repair it! You could modify it & use a standard attenuator, available in either 50 or 75 ohm versions. Or, set it to 'Force' & use an external attenuator, such as the HP 355C. At least that would make it useable for the time being. Dave.

10th Jul 2016, 12:29 am	#6
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: AVO CT378 Attenuator fault One post moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=127938