Marconi TF144H/4S
 

I'm working on a TF144H/4S at the moment. Nearly put my back out carrying it to the table!

Re: Marconi Collectors
 

Some reading suggests that the Marconi TF144H/4S is a bit delicate with regards to a thermocouple which protects the meter movement.

I ought to really read into testing everything out and finding where this thermocouple is and checking it.

Re: Marconi Collectors
 

The thermocouple is used to generate a small DC drive to the level meter by sampling the output RF. Unfortunately, it's a rather fragile component and a stock fault is that drifting-high resistors in the series regulator circuit result in the HT supply going way above the designed 250V with the output of the 5673 power oscillator rising accordingly with bad consequences for the thermocouple.

Re: Marconi Collectors
 

I wonder if I should remove it from the circuit whilst checking the HT supply out. In fact I need to establish if the thermocouple is working ok also.

Re: Marconi Collectors
 

I've got the thermocouple out to take a look, but even under a magnifying glass it's very difficult for me to tell. I don't want to put a meter on it just yet just in case.

However, I wonder if with RF diodes if a replacement can be made.

Re: Marconi Collectors
 

I bottled out of testing the thermo couple, i could'nt find an answer as to how to test.

Andy.

Re: Marconi Collectors
 

Thankfully, some googling and going through drawings of my other sets suggests that the TF144H/4S is the only model sporting the thermocouple driven meter.

Re: Marconi Collectors
 

If fault finding theTF144H/4S, I would defiantly disconnect the thermal couple. Only reconnect when you have monitored the output for some time and are happy the amplitude is stable. A forum member kindly made me a square law detector to try as a replacement. Unfortunately it was not entirely successful when I fitted, but results with the detector on the test bench were perfect, my particular generator suffered with rubber sleeving that became conductive, there is a thread on here somewhere.

Re: Marconi Collectors
 

Thermocouple removed shouldn't offer any adverse affects to the set should it?

It had been working, as I did have an output, then it stopped giving an output. I don't recall the meter ever going full scale, so here's to hoping.

Re: Akai 1720L solid state
 

Yes I would like a copy please but not sure how to get it from you .Thanks very much

Re: Marconi Collectors
 

Just starting to look at the HT section. The EL81 has imploded! So that's not going to help.

I will have to look through my valves and see if I have one.

This could be a thread split as this TF144H/4S could well need a thread of its own.

Re: Marconi Collectors
 

I don't think a missing thermocouple will cause a problem. The feedback should control the output. One of the faults I had was that series regulator, which overheated the glass melted and it gassed. (This weakness also mentioned by Turret Slug)

Re: Marconi Collectors
 

Yep, I wonder what caused it? I'd need to do some checking before I put another EL81 series regulator in only for it to happen again.

Re: Marconi Collectors
 

I think I may still have one spare thermocouple for MI gear if yours has gone OC Richard.

Re: Marconi TF144H/4S
 

I changed R208 and R209 in the regulator circuit which were around 30% high. I also changed the plate rectifier for a modern one and fitted a series resister on its output. I wondered at the time if the valve was on its way out anyway.

Re: Marconi TF144H/4S
 

One of the tests and setting up procedures in the Marconi manual tells you to feed 2 volts into the output socket to calibrate the output meter. DON'T!!!!!

I did and it blew the thermocouple. There's a thread on here about it somewhere.

Very good generator when working properly.

Jim

Re: Marconi TF144H/4S
 

I picked up an EL81 today at the MK rally and Sean kindly gave me a thermocouple. So before putting the new valve in I'd better do some other checks.

Re: Marconi TF144H/4S
 

I ended up changing 50% of the resistors and all the capacitors in the HT stabiliser circuit.
The valve holders seem to be poor quality.

The heater voltage stabiliser circuit derives its reference voltage from the HT stabilised output.

I found the 6.5V output was drifting upwards slowly.

Re: Marconi TF144H/4S
 

When you power up without the Thermocouple shunt. Make sure the amplitude is stable on EVERY frequency range. In my case several of the coil packs had internal breakdowns.

Re: Marconi Collectors
 

I had the same problem with the EL81. Resistor drift is a common problem on these instruments.

https://www.vintage-radio.net/forum/...803#post347803

Edward