Help judging corrosion in Marconi Instruments Universal Bridge TF2700 - Page 1
Forum section: electronics / repair

author: edwinsteiner
author_rank: Newbie
timestamp: July 11, 2019, 07:10:30 pm
content: Dear forum members,I'm new to restoring old equipment. I just got myself a nice Marconi Instruments Universal Bridge TF2700. Taking it apaaart (of course), I found some wires which look corroded (please see pictures).What would you advise? To me it looks like some corrosion has worked from the inside of the wires to the surface of the insulation. Should I replace such wires or should I leave them as long as they measure electrically OK? (I have yet to measure the ones in the picture. Could not yet get to may lab today.)Also some solder points have an orange crystal-like substance at the edges. Do you have any advice about that?best regardsEdwin

author: edpalmer42
author_rank: Super Contributor
timestamp: July 11, 2019, 07:47:39 pm
content: The insulation just looks to me like old cloth insulation rather than corrosion that's coming through. I would expect corrosion to have more green/blue color due to the copper conductor. If the resistor is in spec (100R +-0.1%) I'd leave it alone. Damaging that resistor would be bad. If you decide you're going to go for it, I'd recommend just removing the insulation entirely. On my TF2700, that's the only resistor of that style that has any insulation on the leads. The insulation doesn't look like yours. It just looks like normal plastic insulation. There's no obvious reason for it to be there.The orange substance is just rosin flux. It's totally inert and won't cause any problems. If you decide to remove it, use a Q-tip (cotton swab, cotton bud - whatever it's called where you are) dipped in isopropyl alcohol. A bit of scrubbing and it'll come right off.Ed

author: jimmc
author_rank: Frequent Contributor
timestamp: July 11, 2019, 07:51:33 pm
content: I think the orange crystals are just a residue of the rosin based flux used when the instrument connections were soldered during manufacture.It is non-corrosive and I would leave it alone. (The speck on the panel probably slashed when the tags were soldered.)I think that the component between the wires is R12, the reference for resistance ranges (100ohm 0.1% wirewound).The resistor would have bare leads which have been sleeved and it is the sleeving which has degraded, not any corrosion. Common enough in old kit.As there appears not to be any danger of a short even if the insulation completely fails, I would be tempted to leave it alone as changing it could lead to more problemsIf you do decide you want to change the sleeving take care not to stress the resistor either mechanically or thermally.One other thing, the TF2700 uses germanium transistors, these are easily damaged by excess heat when soldering.If you ever have to resolder them keep the leads long, use a thermal shunt on the lead being soldered and make the joint quickly.If you have three hands a small pair of pliers will work as a thermal shunt.Good luck with the it, I've had one for many years, it's a handy little device once you get used to using it.JimJust seen Ed posted while I was typing, good that we agree!

author: floobydust
author_rank: Super Contributor
timestamp: July 11, 2019, 07:52:11 pm
content: It's not corrosion, it is soldering flux residue from factory hand soldering.The ceramic resistor wires are old style wax-dipped woven cloth (cotton) insulation. Used before plastics took over. So the wire jacket always looks that. It melts and smokes when soldered to. I would just leave them alone, all antique stuff has that.The work will be cleaning oxide off the rotary switch contacts and wiper, silver tarnishes.

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2019, 08:02:59 pm
content: Do as little to it as possible, those TF2700 bridges are very well constructed and very useful instruments even today. Don't go randomly scrubbing stuff, you'll do more harm than good (the Polystyrene reference capacitor will be damaged by solvents for instance). Only touch the switch contacts if you are actually getting intermittent operation.PP9 batteries aren't that easy to find these days and the ones I've bought don't seem to last very long. It's pretty easy to replace with a 9V battery holder (6 x AA cells), there's plenty of space Luckily the plastic battery compartment is well separated from the rest of the instrument, but be careful to avoid the risk of leakage - wrap the battery holder in a plastic bag and check the cells for leakage occasionally. The power consumption is so low that the batteries will last a very long time (assuming that you remember to turn it off!).

author: edwinsteiner
author_rank: Newbie
timestamp: July 11, 2019, 08:06:07 pm
content: Quote from: floobydust on July 11, 2019, 07:52:11 pmThank you! Is regular contact cleaner (T6-Oszillin/DeoxIT) appropriate here or do the silver contacts require a different chemistry?If any contact cleaner is the right thing, I guess I should be careful not to spray it across any of the precision components. Do you apply it directly using a Q-tip, for example?Note: As Gyro has cautioned, I will anyway only attempt to service the contacts if they show any problems.

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2019, 08:10:01 pm
content: P.S. Here's the manual if there isn't one still tucked into the slot at the back...http://bama.edebris.com/manuals/marconi/tf2700/

author: edwinsteiner
author_rank: Newbie
timestamp: July 11, 2019, 08:13:54 pm
content: Thanks. I'm glad to say that there is a nicely preserved original manual in the slot dedicated to this purpose. I love it!

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2019, 08:28:02 pm
content: Perfect.It's a very useful instrumentonce you learn how to drive it!. It will wring out measurements that modern component testers can really struggle with.

author: floobydust
author_rank: Super Contributor
timestamp: July 11, 2019, 08:33:41 pm
content: Quote from: edwinsteiner on July 11, 2019, 08:06:07 pmI don't recommend using a Q-tip, it just snags on the switch contacts and leaves fiber strands all over.In the old days (on these switches) I used a small contact cleaner (eraser) to scrub off the wiper oxide, and paper as fine sandpaper inside the contacts, then a lubricant after. Burt you can't get at things sometimes. I found cleaner sprays didn't work so great and just make a mess. Some have harsh solvents or mineral oil which I don't like on phenolic.I don't know which DeoxTI product is best here or how it performs.

author: bob91343
author_rank: Super Contributor
timestamp: July 11, 2019, 08:45:17 pm
content: I don't see any reason to mess with this. It looks good. The bits of rosin should be left alone; at most I'd use a pointed or sharp instrument to flake them away but again, it doesn't need to be fixed. It isn't broken.A good bridge is a wonderful tool. I have several and love them all.I used to own a Wayne Kerr 221B universal bridge and wish I had kept it. I currently have a GR 1650-A and a 1658. I also have my Q meters. The only bridge I have that I don't use is a Wayne Kerr 700B (?) VHF admittance bridge. I think it covers from 1 MHz to 1 GHz. It's fun to use but I have other tools I prefer. This one requires external source and detector but that's simple enough.

author: edwinsteiner
author_rank: Newbie
timestamp: July 11, 2019, 09:31:21 pm
content: Thanks to all who replied. It was a great experience to get so much considerate and knowledgeable advise!And no worries: I'll approach the venerable device with a "if it ain't broken, don't fix it" mindset. I was mainly worried that the strange-looking cable hinted at some ongoing corrosion problem which needed to be addressed but now I'm reassured.So far I only gave it a good outside cleaning and removed some, ahem, ... traces of a spider colony which had chosen to camp out in the poor thing.

author: edwinsteiner
author_rank: Newbie
timestamp: July 11, 2019, 09:43:25 pm
content: Dear @edpalmer42,As I understood you currently own a TF 2700: Is the "Sensitivity" knob on your device also *much* harder to turn than all the others? On my device, "Sensitivity" has so much friction that in the first second I thought it was jammed. It does not really need to be forced to turn it, but it is *very* tight and I'm wondering if that is normal or if, for example, the knob has not been mounted correctly.Edwin

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2019, 09:52:39 pm
content: It's pretty tight on mine too (not helped by it being a very small knob). A little sewing machine oil where the shaft goes into the bush helps a bit as does exercising it. Note that it is a carbon track potentiometer rather than wirewound like the others. Note also that it is a dual gang 2.5k / 25k one, so almost impossible to source a replacement.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 11, 2019, 11:35:29 pm
content: Yes, my sensitivity control is a bit tight, but not enough that I've felt the need to mess with it. If I was using it a lot, I'd probably get annoyed with it, but for occasional use it's okay.Ed

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2019, 09:56:48 am
content: Oh, btw, the most useful accessory I've made is a pair of croc clips soldered to the heads of a couple of 4mm plugs. On the lowest capacitance range you can actually see the capacitance change as you move the ends of the clips closer or further apart.Longer leads tend to swamp out low value measurements.P.S. The soldering is much stronger than it looks!

author: Andy Watson
author_rank: Super Contributor
timestamp: July 12, 2019, 11:28:32 am
content: Quote from: Gyro on July 12, 2019, 09:56:48 amTop idea. Looks like today's mini-project has been identified

author: edwinsteiner
author_rank: Newbie
timestamp: July 12, 2019, 05:35:11 pm
content: Quote from: Andy Watson on July 12, 2019, 11:28:32 amFunny, that was actually my first thought when I saw the placement of the connectors. The radial mounting of the clips as Gyro has done it is more useful than what I had in mind, though.

author: bob91343
author_rank: Super Contributor
timestamp: July 14, 2019, 02:02:19 am
content: It's likely that the sensitivity control will free up with some use. I wouldn't lubricate it (I know, too late). The high level of friction is probably due to some internal corrosion in one of the resistance elements.Just make it a point every day or so to exercise the control perhaps twice over its full rotation In a week or so I suspect it will be easier to turn.