UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 20th Jul 2004, 9:59 pm   #1
Radio_Dave
Nonode
 
Radio_Dave's Avatar
 
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
Posts: 2,543
Default AVO 2 panel valve tester servicing

Hi,

I thinking about giving my AVO 2 panel valve tester a bit of a clean up and service.

The roller switches on the valve holder are very dirty and a bit too stiff. I want to take it apart and give it a good clean and maybe paint in the numbers. Unfortunately I have this horrible vision of springs and bits flying out all over the place and never being able to put it back together again!

Has anybody ever stripped one of these switches before? Is there anything to worry about?

There is also a metal diode in the meter base, it seems to be working fine, at the moment, but should it be replaced if so with what?

Many thanks

David
Radio_Dave is offline  
Old 21st Jul 2004, 8:51 pm   #2
Chris_C
Hexode
 
Chris_C's Avatar
 
Join Date: Jan 2003
Location: Darlington (DL3) North East U.K.
Posts: 394
Default Re: AVO 2 panel valve tester servicing

Hi David

I have a couple of AVO valve testers and have stripped and cleaned the roller switches with no problems. They are very simple indeed. There are no springs or bits to fly out. I found the grease AVO had applied originally had well dried out just leaving the clay filler .

Yes it takes some digital gymnastics but is really straight forward .

If I can give you any more tips, please let me know.

Cheers
__________________
Chris C G8TJR
Chris_C is offline  
Old 15th Sep 2004, 12:33 pm   #3
Chris_C
Hexode
 
Chris_C's Avatar
 
Join Date: Jan 2003
Location: Darlington (DL3) North East U.K.
Posts: 394
Default Re: AVO 2 panel valve tester servicing

Hi Robin

I have all of the circuits for the 2 panel tester.

Please let me have an email address via PM, if you like, and let me know what you need.

Cheers
__________________
Chris C G8TJR
Chris_C is offline  
Old 15th Oct 2004, 5:36 pm   #4
RobinBirch
Hexode
 
RobinBirch's Avatar
 
Join Date: Apr 2003
Location: Cotswolds, UK.
Posts: 465
Default Re: AVO 2 panel valve tester servicing

Finally started to work on this and going from Chris's circuit diagram I have a dead diode. This is the one in the line from the set zero pot to the mA/V pot. I was going to replace it with a 1N4004 or something similar but before I do has anyone got any suggestions?

Robin
RobinBirch is offline  
Old 15th Oct 2004, 8:25 pm   #5
Leon_Crampin
Guest
 
Posts: n/a
Default Re: AVO 2 panel valve tester servicing

The selenium rectifier in the 2 panel tester can be replaced directly with a Si device, as it simply provides the backoff supply as adjusted by " set zero " . I have done this without problems, but the substitution makes " set zero " a bit more critical to adjust. No doubt a series resistor could be added.

Leon.
 
Old 17th Oct 2004, 9:46 am   #6
jim_beacon
Retired Dormant Member
 
jim_beacon's Avatar
 
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
Default Re: AVO 2 panel valve tester servicing

The forward volt drop of a single selenium element is very low, less than 0.1V, BUT the PIV is also low, so for anything more than about 20V, you will find a number of elements in series. The easiest way to estimate the forward drop, is to count the number of elements (easy if it is finned, there will be an element between each fin, and usually one each end), if it is a low current device, look at the stack, and count the lines in the side - this is where the elements meet.

The forward resistance is a different matter - all rectifiers exhibit some forward resistance. In the case of silicon, this is very low, but selenium exhibits significant forward resistance, and so the forward drop you see in a circuit is related to the current drawn. In old TV sets, this could result in a drop of 30 to 50 V across the rectifier. It will be difficult to estimate this without taking measurements from a working unit.

Jim

PS I have seen selenium used in alarm circuits in old charger systems - the rectifier was used to rectify a sample of the ripple current (transformer coupled), and the resulting DC used to hold off a relay (via a germanium transistor), to give a low current alarm. The rectifiers failed, and the chargers had to be scrapped, as nothing else had a low enough forward volts drop to work!
jim_beacon is offline  
Old 17th Oct 2004, 10:37 am   #7
Leon_Crampin
Guest
 
Posts: n/a
Default Re: AVO 2 panel valve tester servicing

Good advice above - does anyone have a Tektronix curve tracer? It would be interesting to look at the V-I curves of a few of these rectifiers.

In the Avo tester, if I remember correctly, the rectifier has a series resistor, and it a type RM0. I would suggest starting with about 47 Ohms in series with the rectifier, judging from my experiences with these devices in radios. The " trade off " is ease of adjustment of the " set zero " control, versus adjustment range. For some valve types, a good deal of backing off bias is needed. although I have never run out of adjustment.

Let us know how you get on with this.

Leon.
 
Old 17th Oct 2004, 1:42 pm   #8
jim_beacon
Retired Dormant Member
 
jim_beacon's Avatar
 
Join Date: Mar 2003
Location: Bishop's Waltham, Hants, UK.
Posts: 939
Default Re: AVO 2 panel valve tester servicing

If it is a fin cooled device, it is probably a RM0, which has a forward restince of about 100 Ohms (from the Brimar book - it drops 5V at 50mA, on the linear portion of its curve). If the device looks like a large diode, with one end painted red, then it is probably a M1, which has a forward resistance of 1200 Ohms.

As these devices are used in a nulling bridge circuit, and have a 50 Ohm series resistance, the forward resistance of the device is important.

Jim.
jim_beacon is offline  
Old 17th Oct 2004, 3:10 pm   #9
Leon_Crampin
Guest
 
Posts: n/a
Default Re: AVO 2 panel valve tester servicing

Jim, I wouldn't strictly describe the backing off circuit in the Avo tester as a bridge. When the valve under test is passing anode current, the component of this current passing through the meter circuit (only) is backed off, using the " set zero " control. This then enables the change in anode current for a 1V grid voltage change (produced by moving the key switch to " mA/V " ) to be read directly on the meter. The tester thus gives a direct readout of gm when correctly nulled.

I don't think the forward resistance of the rectifier actually matters, as long as a null can be achieved. Selenium rectifiers were not exactly well specified in this parameter!

The rectifier is an RM0.

Regards, Leon.
 
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:29 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.