FINE BALANCE POT
I've had a faulty TF2700 for years but only got around to troubleshooting it a week or two ago. The symptom was a failure to balance, the meter suddenly going hard over at about half rotation of the fine balance control on all resistance ranges - haven't got as far as L and C yet!
On checking the Fine Balance pot I find it's open circuit end-to-end. Opening it up and stripping out the wire-wound track - tricky but possible - I find that the track wire has two or three breaks in it on the edge away from the wiper. Looking at the grease in the body(it has green bits in it) it looks as though damp has wrought it worst on the pot.
Now where can one find a 110 ohm pot? I can fit a 100 ohm one but on the x1 range of the switch the error will be large though less so on other multiplier settings.
I see that others are aware of the weird value of this pot, has anyone a solution?
I seems a pity to scrap the instrument for the sake of a pot.
I'm aware that for L and C measurements all the electrolytics will need replacing but that is pointless at this stage if can't solve the fine balance pot problem.
Yes, I know I can buy an adequate modern digital LCR meter but the '2700 is a nice old comforting bit of kit that takes me back to my early days in the hobby of electronics
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I've had a faulty TF2700 for years but only got around to troubleshooting it a week or two ago. The symptom was a failure to balance, the meter suddenly going hard over at about half rotation of the fine balance control on all resistance ranges - haven't got as far as L and C yet!
On checking the Fine Balance pot I find it's open circuit end-to-end. Opening it up and stripping out the wire-wound track - tricky but possible - I find that the track wire has two or three breaks in it on the edge away from the wiper. Looking at the grease in the body(it has green bits in it) it looks as though damp has wrought it worst on the pot.
Now where can one find a 110 ohm pot? I can fit a 100 ohm one but on the x1 range of the switch the error will be large though less so on other multiplier settings.
I see that others are aware of the weird value of this pot, has anyone a solution?
I seems a pity to scrap the instrument for the sake of a pot.
I'm aware that for L and C measurements all the electrolytics will need replacing but that is pointless at this stage if can't solve the fine balance pot problem.
Yes, I know I can buy an adequate modern digital LCR meter but the '2700 is a nice old comforting bit of kit that takes me back to my early days in the hobby of electronics
.UPDATE ON FAILED BALANCE POT on TF2700
The failed fine balance pot in my TF2700 is a 110 Ohm wire-wound one made by Clarostat. Another 2700 that I own has a Colvern 110 Ohms component.
It looks as though the cause of the pot's going open circuit was that it had suffered from damp at some time before the bridge came into my possession as there were signs of green corrosion (verdigris?) on the track. I tried to solder the breaks but was worried that the former on which the track is wound would break up. The breaks were on the edge of the track away from the wiper, which makes me even more suspicious that damp was the culprit as that edge lies in the corner of the pot body away from any ventilation that would otherwise have caused it to evaporate.
I remembered from years ago that one could get a silver loaded 'paint' for repairing the car rear window demister elements that had been damaged - the surface type element not the ones embedded in the glass - and was lucky enough to buy a small syringe of it off Ebay for £1•16 included postage, it was sold as a PCB repair material.
I put the smallest blob that I could on each break and checked the track's continuity , sadly it was still open-circuit so I packed up for the evening, intending to try again the following day. To my relief when I checked the next morning the track read 112 Ohms end-to-end, obviously with hind-sight I'd needed to let the solvent evaporate. I reassembled the pot using 3 millimetre screws, washers and nuts in place of the rivets that originally held the track in place and checked that the wiper made contact throughout its travel with no sign of intermittent contact.
On fitting the pot back into the bridge I was gratified to find that, having replaced the ancient electrolytic capacitors (of course!), everything worked as Signor Marconi originally intended.
It is a tricky job getting the track out with wrecking it but with care and patience it can be done transforming what had become simply a door-stop into an accurate and versatile bit of test equipment and giving one a smug feeling of satisfaction!
Good luck if you decide to try the repair though if the pot has failed due to wear by the action of the wiper on the track I fear that my solution simply is not applicable.
I've now (end July 2014) put it on Ebay.
The failed fine balance pot in my TF2700 is a 110 Ohm wire-wound one made by Clarostat. Another 2700 that I own has a Colvern 110 Ohms component.
It looks as though the cause of the pot's going open circuit was that it had suffered from damp at some time before the bridge came into my possession as there were signs of green corrosion (verdigris?) on the track. I tried to solder the breaks but was worried that the former on which the track is wound would break up. The breaks were on the edge of the track away from the wiper, which makes me even more suspicious that damp was the culprit as that edge lies in the corner of the pot body away from any ventilation that would otherwise have caused it to evaporate.
I remembered from years ago that one could get a silver loaded 'paint' for repairing the car rear window demister elements that had been damaged - the surface type element not the ones embedded in the glass - and was lucky enough to buy a small syringe of it off Ebay for £1•16 included postage, it was sold as a PCB repair material.
I put the smallest blob that I could on each break and checked the track's continuity , sadly it was still open-circuit so I packed up for the evening, intending to try again the following day. To my relief when I checked the next morning the track read 112 Ohms end-to-end, obviously with hind-sight I'd needed to let the solvent evaporate. I reassembled the pot using 3 millimetre screws, washers and nuts in place of the rivets that originally held the track in place and checked that the wiper made contact throughout its travel with no sign of intermittent contact.
On fitting the pot back into the bridge I was gratified to find that, having replaced the ancient electrolytic capacitors (of course!), everything worked as Signor Marconi originally intended.
It is a tricky job getting the track out with wrecking it but with care and patience it can be done transforming what had become simply a door-stop into an accurate and versatile bit of test equipment and giving one a smug feeling of satisfaction!
Good luck if you decide to try the repair though if the pot has failed due to wear by the action of the wiper on the track I fear that my solution simply is not applicable.
I've now (end July 2014) put it on Ebay.
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