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Old 29th Oct 2019, 6:53 pm   #61
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Default Re: Marconi TF144H/4S

Quote:
Originally Posted by vintage_8bit View Post
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.
Your broken down coils, are they the coils in the tuning turret?
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Old 29th Oct 2019, 7:47 pm   #62
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Default Re: Marconi TF144H/4S

Hi again. The coils were ok. It was the rubber insulated sleeving on the individual coil packs, in the turret. The sleeving had become conductive. It was sticky to the touch and an oily substance was oozing from it. Where the sleeving touched another terminal or wire it caused a breakdown. You could here the crackling sometimes. You may be ok as I posted about this previously and no one else reported the same fault. I spent many hours rectifying that. I had the same sleeving elsewhere on the chassis as well. Hope yours is ok. Colin
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Old 29th Oct 2019, 9:35 pm   #63
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Default Re: Marconi TF144H/4S

Hello, thanks for the information. I didn't get any gooey stuff on any of the wire or component lead sleeving apart from, I noticed a gooey sleeving on an OC71, but that's a sleeving over the actual component body and not any leads.
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Old 29th Oct 2019, 9:39 pm   #64
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Default Re: Marconi TF144H/4S

Ok, so I've brought it up into life.

I have an output, a sensible one by measuring the frequency so far only on band A between 10 and 12k.

I set up a pair of volt meters to monitor the 410 volts from F201 into V201 and the 250v out from pin 3. I did not get the expected readings, so have powered off for now whilst I have a think, although measured RF out was sensible.
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Old 29th Oct 2019, 9:52 pm   #65
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Default Re: Marconi TF144H/4S

I've got the right readings, but had my DMMs on AC instead of DC which was a bit silly of me.

So, HT input to the series regulator is 385V which is a touch lower than expect and HT regulated out is 267V which is a touch higher than expected but not high enough to cause issues I don't think.

So far all of these tests are with no thermocouple.

I may try again with an AVO8 as that's what's specified, the OPV is likely higher on my DMM's. I think I might have an Avo somewhere.......
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Old 29th Oct 2019, 11:26 pm   #66
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Default Re: Marconi TF144H/4S

After perhaps an hour or so I've got 260V and 390V, so it's changing a little but not by much. It'll probably more than likely do, but I'll see if I can hone in on it.
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Old 29th Oct 2019, 11:47 pm   #67
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Default Re: Marconi TF144H/4S

Bear in mind that input mains voltage changes will result in raw HT changes of about 410/240 times though, and mains can wander around a bit. Also, a raised output from the regulater over nominal will result in slightly raised HT current, thus a bit of a drop on the input to the regulater. Both the HT and LT set pots could have gone a little scratchy with age, too, so a little rocking either side of the current position might be necessary, and having changed resistors, the new voltage-setting ones either side of the pots may be closer tolerance than the old ones anyway.

So far, so good- at least nothing is smoke-inducingly out!
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Old 30th Oct 2019, 8:49 am   #68
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Default Re: Marconi TF144H/4S

Yes I'll take a look at the pots. Although they're old carbon pots I really didn't want to have to touch them if I didn't have to. What I ought to do is look through the manual and see what the setting up of them is as like you say the new resistors may mean some adjustment is needed anyway.

After that, it could well be a good and safe time to install the thermocouple.

It ran for hours last night and behaved. I tested it on all ranges too.
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Old 31st Oct 2019, 7:13 am   #69
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Default Re: Marconi TF144H/4S

"It ran for hours last night and behaved. I tested it on all ranges too" That's good to hear.

Andy.
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Old 2nd Nov 2019, 10:04 pm   #70
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Default Re: Marconi TF144H/4S

So tonight I've adjusted the HT out via RV201 to 250V. The HT-in is fluctuating a little at around 405V.

Next job will be to check the attenuated output levels to see how close they are to the set voltages.

Other than that, a bit of a bright flash from the valves for a second or so on power up. I've heard of the phenomenon of 'Mullard flash', but I don't know if these are Mullard or not.
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Old 3rd Nov 2019, 7:14 pm   #71
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Default Re: Marconi TF144H/4S

So checking the output amplitudes measured when adjusting the attenuators shows that the levels are a little higher than what they should be when the RMS value is measured into a 50ohm load.

Due to what I'm measuring, I'm assuming the output EMF written on the dial is the RMS value (rather than peak to peak)?

For example, when the Course Attn set to 0.64 to 2V range, fine attn set to 2V, I measure 6.9V peak to peak/2.5V RMS

200 to 640mV range, set at 640mV measures 725mV RMS.

So those two examples are a little high, but not troublesome.
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Old 3rd Nov 2019, 9:39 pm   #72
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Default Re: Marconi TF144H/4S

Ahhh, without the ouput meter operating at the moment I have completely forgot about the set carrier control, because of course I can't set it against the meter at the moment!

Hmmmm, that will change the amplitude of course.
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Old 4th Nov 2019, 7:46 pm   #73
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Default Re: Marconi TF144H/4S

I'm still too chicken to connect the thermocouple up!

If the RF output voltage was a bit high, how high would be high enough to damage the thermocouple?

As the output is EMF I'm not so sure if I should be connecting this to a 50ohm load when making my measurements. I'm just hoping I'm confusing myself rather than the attenuators needing work doing to them.

The course attenuator, just forget working on at at all as the resistors are values such as 142.5 and 96.25 ohms. They're metal film, or at least the specification says they are, so here's to hoping as they would probably be ok - as I can't seem to find those values for sale, there's a surprise.

Also, the manual suggests that if one dissassembles the course attenuator, then woe betide - as you're unlikely to get it back together as it has a myriad of springs and magical things which were probably assembled as a trade in its own right.

Fine attentuator, there's probably more chance of replacement parts and doing the actual job.
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Old 4th Nov 2019, 8:57 pm   #74
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Default Re: Marconi TF144H/4S

Back the set carrier off to zero, confirm next to nil output and connect the thermocouple - then use the set carrier control as it was designed - there's little chance of damaging it that way.
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Old 4th Nov 2019, 10:14 pm   #75
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Default Re: Marconi TF144H/4S

Do you expect the output to be measured pk-pk or RMS? I can't find that information in the manual.
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Old 5th Nov 2019, 12:53 am   #76
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Default Re: Marconi TF144H/4S

To be honest I'd be expecting this to be pk-pk, but I can certainly get something closer when making RMS measurements, with set carrier fully clockwise.

The set is certainly working alright, just the output levels are possibly high. Possibly poor attentuation?

However, I am wondering what affect if any, running the set without the thermocouple is making.

Resistors previously in question - R112, going from 180K to 270K could make some difference but probably not this difference. R185 going to 10ohm from 100 ohm, I haven't worked that one out yet.

I wonder what you measure on yours?
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Old 5th Nov 2019, 9:31 am   #77
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Default Re: Marconi TF144H/4S

"I wonder what you measure on yours?" Got a virus/chest infection at present, been in bed for three days, I'll dig it out when I'm on the mend and do some measurements.

A.
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Old 5th Nov 2019, 9:58 am   #78
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Default Re: Marconi TF144H/4S

P-P or RMS - Interesting question.

The thermocouple will not respond quickly enough for a P-P measurement, so I would think RMS is more likely
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Old 5th Nov 2019, 5:00 pm   #79
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Default Re: Marconi TF144H/4S

Hmmm, here's a few areas which could be of interest...
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Old 5th Nov 2019, 7:44 pm   #80
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Default Re: Marconi TF144H/4S

TF144H Thermocouple.
It surely will read RMS as the meter movement measures the voltage generated across the thermocouple produced by the heating effect of the rf output voltage.
In fact thinking about it, I suppose it is current that is measured by the meter, as all low impedance. Still RMS. Or power?
What views from the experts here.
The ALC and other circuits use diode rectifiers which will approximate to peak readings. Other Marconi generators like the TF995, TF801, TF1066 & TF2015 all use diode rectifiers, so will show peak output.

I have a 1 m/a meter here which had a failed thermocouple across it (some 50 years ago). I made it into current meter, but it was difficult to use as the movement was quite undamped and oscillated wildly for a time.
Obviously the thermocouple acted as a low resistance shunt across the meter coil and damped it nicely. Rather like the universal shunt on our Avos. (Whereas I note that the GEC/SEI Super Selectest meters use meter coil damping, so allowing a less sensitive meter- 50ua instead of 37.5ua of the Avo.)
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Last edited by WME_bill; 5th Nov 2019 at 7:54 pm.
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