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Old 8th Feb 2014, 6:47 pm   #1
16dibbins
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Default Marconi TF144H/4S - No Carrier output

I am having trouble getting any output from my TF144. It worked fine last time I used it but of course that was a few months back.
There was a smell of burning and I opened the RF box to find the 4.7K resistor R169 had burnt out. See attached circuit.

I have checked the HT and it is 250vDC and the LT is fine at 6.3 vDC. Looking at the circuit I thought the first suspect would be the 6U8 so I changed that and the 4.7K and again the 4.7K went up in a puff of smoke! I have also replace the 5763.

Checking the DC conditions with the 4.7K o/c and it indicates there is 225VDC on the side of the resistor that is not connected to the valve and for the life of me I can't see where this is coming from. It doesn't help that it is quite difficult to work on inside the RF box.

Has anyone come across this before and can provide some pointers? I have checked the Rs&Cs in the circuit and they check OK

Thanks
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Old 8th Feb 2014, 6:57 pm   #2
BobGreen
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Default Re: Marcon TF144H/4S - No Carrier output

It looks like R169 feeds the grid of the 6U8 pentode section pointing to a serious fault with that valve. I see no 5763 in this part of the circuit.
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Old 8th Feb 2014, 7:04 pm   #3
16dibbins
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Default Re: Marcon TF144H/4S - No Carrier output

Bob

Thanks for the reply. I have changed the 6U8 with a known good one so it hopefully shouldnt be that.

I have attached the full circuit as that may help pinpoint the problem
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Old 8th Feb 2014, 7:21 pm   #4
vinrads
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Default Re: Marcon TF144H/4S - No Carrier output

Hi Steve I am having problems with the same generator , check the thin wire that runs close to the trimmer caps in the coil pack the insulation falls off and could cause a s/c
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Old 8th Feb 2014, 7:23 pm   #5
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Default Re: Marcon TF144H/4S - No Carrier output

Probably one or more capacitors gone short or breaking down.

I suspect g1 in the pentode side of BOTH your 6U8s are now a bit ratty.

One reason I bought an AVO transistor tester is that the diode test goes up to 1000v and it can be used to test things other than diodes... reform capacitors etc etc. Sometimes you're looking for high voltage problems that don't show at multimeter ohms range voltages.

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Old 8th Feb 2014, 9:51 pm   #6
BobGreen
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Default Re: Marcon TF144H/4S - No Carrier output

There must be a solder bridge or stray wire etc applying many volts to the non-grid end of R169. I doubt if the replacement 6U8 is very happy now. Measure the volts at g1 with the valve removed. Is it 200V +? If so, where from??
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Old 9th Feb 2014, 6:32 pm   #7
Chris55000
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Default Re: Marconi TF144H/4S - No Carrier output

Hi!

This is a strange one!

I've studied the repair words, and certainly it *can't* be any form of internal s/c between electrodes of the ECF82 pentode section as both anode and g2 are high-value feeds!

However, pin 1 (triode anode) *is* directly connected to the + h.t. line and a short between pins 1 and 2 will cook up the valve,*but* there's no low-resistance paths in the pentode grid circuit either! One end goes up via R117/C151 in parallel and R117's 1 meg, and then there's a d.c. block C130 stopping any nastiness going that way, and going down, we've got (from chassis-earth) R101 + R192 + R102 + R103 + R171 + R116 then the grid pin 2, and that little lot tot up to 1.34M approx if I've read the circuit correctly!

Lift R169 at the end *remote* from the grid pin of the valve, measure the insulation between pin 1 and the free end, if this is clear, repeat between the point where R169 was connected to the A.L.C., etc., and the + h.t. line, if you're clear here, lift one end of each of the two HP2800 diodes to prevent damage to them, switch on the generator and measure the junction of R116/R117 volts-wise with respect to earth using the highest range on your dvm - even a 100M leak between here and the + h.t. line can put 2-3 V positive here, a point C.A. Quarrington made specially in Vol 3 of his Radio and Television Books!

Otherwise it's just a matter of tracing thro' patiently at it, remembering always that leaks or shorts of this type may be only apparent when the H.T. is on! Don't forget this generator has silicon H.T. rectifiers, so any leaks could be potentially worse *before* the valves warm up!

Chris Williams

PS! Sorry for being a bit long - I'm afraid it's a bit of a failing of mine when I'm explaining something!
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Old 10th Feb 2014, 1:25 am   #8
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Default Re: Marconi TF144H/4S - No Carrier output

Two off topic posts moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=103598
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Old 10th Feb 2014, 1:57 pm   #9
16dibbins
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Default Re: Marconi TF144H/4S - No Carrier output

Thanks all for the replies. I will have a look at this weekend and trace through carefully. It really is strange and I too have looked long and hard at the circuit to see where this could come from. When I find out the cause I will post what it was.

Thanks all again

Steve
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Old 15th Feb 2014, 3:01 pm   #10
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Default Re: Marconi TF144H/4S - No Carrier output

Just to provide an update. Got the Generator on the the bench and removed the coil pack so I could trace the problem a bit easier. What I found is that the diagram was not for the Generator that I had. Mine was a later model that was modified around the ALC tagstrip. My TF144 had a neon and a different component count mounted on tags. Did a bit of searching and found the correct diagram.

Well to cut a long story short the fault was caused by a one of the components piercing a HT link that connected some of the tags together and that put the 250vDC on to the 6U8 grid the 4.7k acting like a fuse. This was quite difficult to see as the components are within the RF box but was the cause of the problem. Replaced the 4.7k resistor and the TF144 lives again.

Again, thanks for all the pointers
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Old 19th Feb 2014, 8:48 pm   #11
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Default Re: Marconi TF144H/4S - No Carrier output

Hi!

Well Done Steve - it had to be a fairly obvious enigma of that sort!

Chris Williams
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