Gunson's Testune - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

WaveyDipole · 4th Nov 2014, 10:51 pm

Glad to help but sorry its bad (well not so good perhaps) news. So the resistor has been running hot enough to burn off the paint, but it survived!

I also have a picture of the track side with the function switch removed if would help determine where to reconnect the burned tracks. Just having a quick look at my board, in order to char that resistor, excessive current must have passed through one of the two presets marked PR8 and PR9. Any sign of a charred track or spot on one of these? One end of the resistor connects to a 5.6v zenner diode to the left of PR9. Might this have broken down under excessive voltage or current? Any damage to the tracks under the function switch or the switch wipers?

Looking at the 10A fuse, it looks like as you said, that the switch would need to have been set to 'Current'. The +ve wire links to the center track of the function switch which would then connect to the track leading to the fuse. From there it goes across the shunt to the -ve wire, thus passing most of the current through. However some is taken via a track leading down to PR7 (marked AMPS) and via a 24ohm resistor from where it gets taken via another set of contacts on the function switch to other parts of the circuit (note that the switch is actually positioned slightly off the horizontal to acheive this) and one end of the meter coil. The other end is linked via other parts of the circuit. That 24ohm resistor (R14 I think) might also be worth checking as well as whether you have continuity across the meter movement coil itself.

Beyond that, I guess a circuit diagram would help.

PS: I have spare 5.6v 500mw zenners should you need one (due to a minimum quantity requirement on the order).

GMB · 5th Nov 2014, 10:22 am

Oh dear, it's terminal.

I took the switch disc off to see what was going on underneath having traced the stressed tracking to it.

Well it seems that the extra thermal insulation of having the switch disc above the tracks was enough to allow them to be vapourised. Three of the tracks that make up the switch are just gone!

The really amazing thing is that although the power flowed through the contacts, the contacts themselves are completely fine. It seems to have been all about thermal inertia.

It's a pity as I could really have done with this working. I don't like to take indoor test gear up to the field to work on the tractor.

WaveyDipole · 5th Nov 2014, 2:57 pm

Sorry to hear that!

The contacts appear to be made of a different material to the tracks so I guess they probably have different thermal characteristics. Doesn't help though and its a shame it can't be easily repaired.

threeseven · 5th Nov 2014, 3:58 pm

Just in case it's of help, there is one on ebay now with a best offer available on it.

GMB · 15th Nov 2014, 10:37 am

I have ended up buying another as it looks like a useful kind of thing.
Now I have the other version of it (older?) as pictured above https://www.vintage-radio.net/forum/...03&postcount=4

Not sure how it wotks though as it has more connections and no instructions.

hysteresis · 17th Nov 2014, 12:38 am

Quote:

Originally Posted by GMB

Not sure how it wotks though as it has more connections and no instructions.

The "Instructions For Use" are available here:
http://www.gunson.co.uk/items/pdf/Te...structions.pdf

GMB · 17th Nov 2014, 10:21 am

Thanks for that link. It seems it is the same model number in spite of being utterly different. Looks like I now have a working one!

4th Nov 2014, 10:51 pm	#21
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Gunson's Testune Glad to help but sorry its bad (well not so good perhaps) news. So the resistor has been running hot enough to burn off the paint, but it survived! I also have a picture of the track side with the function switch removed if would help determine where to reconnect the burned tracks. Just having a quick look at my board, in order to char that resistor, excessive current must have passed through one of the two presets marked PR8 and PR9. Any sign of a charred track or spot on one of these? One end of the resistor connects to a 5.6v zenner diode to the left of PR9. Might this have broken down under excessive voltage or current? Any damage to the tracks under the function switch or the switch wipers? Looking at the 10A fuse, it looks like as you said, that the switch would need to have been set to 'Current'. The +ve wire links to the center track of the function switch which would then connect to the track leading to the fuse. From there it goes across the shunt to the -ve wire, thus passing most of the current through. However some is taken via a track leading down to PR7 (marked AMPS) and via a 24ohm resistor from where it gets taken via another set of contacts on the function switch to other parts of the circuit (note that the switch is actually positioned slightly off the horizontal to acheive this) and one end of the meter coil. The other end is linked via other parts of the circuit. That 24ohm resistor (R14 I think) might also be worth checking as well as whether you have continuity across the meter movement coil itself. Beyond that, I guess a circuit diagram would help. PS: I have spare 5.6v 500mw zenners should you need one (due to a minimum quantity requirement on the order). Last edited by WaveyDipole; 4th Nov 2014 at 11:07 pm.

5th Nov 2014, 10:22 am	#22
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Gunson's Testune Oh dear, it's terminal. I took the switch disc off to see what was going on underneath having traced the stressed tracking to it. Well it seems that the extra thermal insulation of having the switch disc above the tracks was enough to allow them to be vapourised. Three of the tracks that make up the switch are just gone! The really amazing thing is that although the power flowed through the contacts, the contacts themselves are completely fine. It seems to have been all about thermal inertia. It's a pity as I could really have done with this working. I don't like to take indoor test gear up to the field to work on the tractor.

5th Nov 2014, 2:57 pm	#23
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Gunson's Testune Sorry to hear that! The contacts appear to be made of a different material to the tracks so I guess they probably have different thermal characteristics. Doesn't help though and its a shame it can't be easily repaired.

5th Nov 2014, 3:58 pm	#24
threeseven Octode Join Date: May 2010 Location: Worcestershire, UK. Posts: 1,880	Re: Gunson's Testune Just in case it's of help, there is one on ebay now with a best offer available on it. __________________ BVWS member

15th Nov 2014, 10:37 am	#25
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Gunson's Testune I have ended up buying another as it looks like a useful kind of thing. Now I have the other version of it (older?) as pictured above https://www.vintage-radio.net/forum/...03&postcount=4 Not sure how it wotks though as it has more connections and no instructions.

17th Nov 2014, 10:21 am	#27
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Gunson's Testune Thanks for that link. It seems it is the same model number in spite of being utterly different. Looks like I now have a working one!