Marconi TF2700 Bridge

[acorn]

Has any one refurbished the Marconi TF200 bridge, this also brings up the question are there any stock faults to look out for.

I also note that it uses an 0C200 and GET102s. Are there any modern equivalents for these transistors, all help would be much appreciated.

Acorn

[WME_bill]

Have you missed out a digit from the type number? It does not seem immediately familiar. Perhaps a picture would clarify the matter.
I suspect you intended TF2700, as the other Marconi bridges - TF1313 and TF868 are valve.
The 1962 version of TF2700 used Get102 and OC200. The 1974 version uses ACY17 and BCY72 respectively. The circuit of the amplifier putput Vt4 and Vt5 was changed completely and uses a single BCY71 to drive the meter. Nothing particularly critical, so alternative types could be used. If you are short of the germanium, I can find a few on an old circuit board I was contemplating dumping recently. The Silicon much easier.
If this is the correct model, and you want the circuit, I can scan it.
Stock faults. It seems a reliable model. Clean and lubricate the switch contacts and the balance pots. I use white spirit with a little vaseline dissolved in it. Suggested years ago by Marconi or Tectronix, I forget which. Long before the days of Electrolube and Detoxit with their strong marketing.
wme_bill.

[acorn]

Bill

Yes my mistake its the TF2700 which uses the GET102 and the 0C200 could you suggest any modern PNP types that I could replace them with, I have the manual here.

Acorn

[Herald1360]

AC126 is quite close to GET102- GET102 has 30V, 100min gain and 0.75MHz ft

OC200 is silicon pnp, so just about any Si PNP should do, though there may or may not be issues with finding one with low enough gain and ft to be stable. Start with something like BC557 perhaps.

http://alltransistors.com/

is a useful resource for this sort of problem.

[acorn]

Many thanks for you help

PS, can you still obtain the 9 volt batteries.

Acorn

[Herald1360]

Batteries- depends on what size and how much you want to pay.

Had a poke- looks like it's a PP9 so yes and they're about £4 or so.

Cheaper alternative is to buy a 3*2 6 cell AA holder and stuff it inside the case of a dead PP9. Capacity is only about half but for occasional use / demonstration that's no problem. I suspect the TF2700 takes rather less dc than most big trannies anyway.

[turretslug]

You can still get PP9 batteries, though stockists are more rarified now- there are lots of folk here who run Hacker/Roberts etc. portable radios that use them, so someone will know of a good source. (Purists, avert your eyes at this point). I fitted my TF2700 with a PP3 connector lead to use with one of the widely-available 6x AA cell holders and, as it sips gently at the power (something I do like about early, straightforward transistor stuff) they last a long time.

Lovely little piece of kit from a stalwart and much-missed British name, it's something of a cliche that probably gets used here quite a lot but they really don't make 'em like that any more.

Edit: As Herald says! And apologies for my foot-in-mouth quoting "Marconi" as a British name, you-know-what-I-mean, a British company....

Someone else with a very European surname

[short wave]

Hi Acorn some info here on the later version (1977 as per post #2) that may help you or others.
link http://www.bartola.co.uk/valves/2014...an-lcr-bridge/
regards S-W

[acorn]

Once again, many thanks for your help on this matter.

Acorn

[Ferguson 2000]

Hi,

I have used one for many years and it is an excellent instrument. PP9 batteries last a long time, so long as you remember to turn it off after use.

Best wishes, Geoffrey.

[jez_145]

Just had a look in some local shops for PP9 batteries. Wilkos seem to be the best bet at 3.49. Homebase 6.99 and Maplin a staggering 9.99. They're still handy for the Marconi TF2700 and Roberts portable radios. As Geoffrey states, they do last a very long time in the TF2700 and it's hardly worth making an alternative power supply for this unit.

Jerry

[G4_Pete]

HI,
One trap I fell into with my TF2700 was to put 9 Volt battery clips on the battery leads and then run the unit on a small power supply. Now wind on a year or so when I had forgotton that extending the leads with battery clips reverses the red and black wires and hence the "normal" red/black polarity. Fortunatly I had the current limiter on the PSU set low so although the TF2700 sunk about 300 Ma it survived.

Pete