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Old 13th Oct 2019, 10:11 pm   #41
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Default Re: Marconi TF144H/4S

Were there different revisions to this model and drawings? As looking at V102 between pin 7 and 8 I've got a completely different resistor, 270k, yet the drawings are showing 180k.
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Old 14th Oct 2019, 10:59 pm   #42
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Default Re: Marconi TF144H/4S

In the two images attached...

10 ohm resistor connected to V101 pin 6. The only resistor on the drawing to pin 6 is R185 which is 100 ohm. The capacitor is removed, as you can see.

There are the two 10 ohm resistors above which connect to the turret section.

Next, 270K ohm resistor (measures higher as out of tolerance). From the drawing this could be R112 which should be 180K, connected between pin 7 and 8 of V102

So I'm tempted to replace with what's there (the out of tolerance ones) rather than what the drawing suggests!

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Old 16th Oct 2019, 8:46 am   #43
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Default Re: Marconi TF144H/4S

Dug it out, re R187 post #26 is a 4K7 2%, here's some pics of the other section you were interested in, they're not the best, light is V bad here at the mo, artificial light doesn't help.

It's a bit hard to see which section is which from your pics, and not sure where V101 is without getting the manual out. The valve in pic 4 below is the second valve from the right, it has a 100r on the bottom pins and two 10r's going to the pins on top, next pin on the right, from that which the cap with the yellow sleeve it attached. these next two pins are joined and have the two 10r's in parallel going to them.

The valve on the extreme right, pic 5 dark pic has two R's connected, a 180k and 4k7.

Andy.
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Old 16th Oct 2019, 8:51 pm   #44
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Default Re: Marconi TF144H/4S

Thanks for looking.

Ok, yours has had some work done to it as those resistors are more modern than the unit is. So someone followed the drawings with your one.

Mine appears original as it has the old carbon resistors in it, but why there are some different values still boggles me.
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Old 17th Oct 2019, 6:40 pm   #45
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Default Re: Marconi TF144H/4S

I guess unless there's any known revisions, then I'll just replace the resistors in question with what's actually on the drawing.
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Old 18th Oct 2019, 8:00 am   #46
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Default Re: Marconi TF144H/4S

Yes, sorry, mine has had extensive work but someone's done a decent job using 2% resistors. Unless it's a critical component value you should be fine, valve DC operating parameters are ish anyway.

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Old 23rd Oct 2019, 5:10 pm   #47
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Default Re: Marconi TF144H/4S

From what I can see, I've replaced every carbon resistor now. Most were incredibly out of tolerance by quite some margin.

I'll just need to make a few more checks before I power up. Capacitors haven't been replaced, but at least the HT supply issue should be put well to bed with the new resistors. And there's a new EL81 to go in too since the last one melted.
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Old 23rd Oct 2019, 11:08 pm   #48
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Default Re: Marconi TF144H/4S

An example of silly high resistors, one in particular was R224 which is a 22k resistor - it measured 50k. It's a 1W resistor, but is about the size of a 3W resistor, which it may have been if that's what they had to hand at the time.

The small black resistors (carbon according to the drawings) have all been spot on.
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Old 24th Oct 2019, 7:55 am   #49
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Default Re: Marconi TF144H/4S

"From what I can see, I've replaced every carbon resistor now." Sounds like fun, did you just cut them out or try and unwind the leads? It'd be why mine has a lot of resistors replaced, good luck with the power up.

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Old 25th Oct 2019, 3:08 pm   #50
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Default Re: Marconi TF144H/4S

Quote:
Sounds like fun, did you just cut them out or try and unwind the leads? It'd be why mine has a lot of resistors replaced, good luck with the power up.
I don't understand what you mean about cutting out and unwinding. They're soldered into place as one would usually expect to see.

I should add the only ones which haven't been replaced are the resistors with odd values in increments of 0.03, 0.25 ohm which are R122 through to R145, as I can't see that they're available.

Also as of yet, as the turret has been removed and put in a safe place, those resistors haven't been touched, as I'd not considered them due to only focussing on the power supply and then whatever else was in view at the time.
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Old 25th Oct 2019, 6:36 pm   #51
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Default Re: Marconi TF144H/4S

There are two common techniques for replacing components - one is to cut the leads close to the components and wire the new parts to the stubs, the other is to desolder and unwrap the component leads from the tags.
The first is quicker and easier, especially if there is a large number of parts to replace or if they are not very accessible.
The second is often favoured where original appearance is preferred, but has a higher risk of damaging the tags or other components.

I think that's what Andy was referring to.
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Old 25th Oct 2019, 8:48 pm   #52
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Default Re: Marconi TF144H/4S

Can't really say I had much issue, there was a little wrap around some of the tags, but nothing which was hard work really. Once the solder was off it was quite alright to do.
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Old 26th Oct 2019, 7:46 am   #53
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Default Re: Marconi TF144H/4S

Yep, that's what I referring to Dave, some things I've come across have the wire really tightly wrapped around the tags several times and pinched, which are a nightmare to unsolder and unwrap often causing damage as you say.

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Old 26th Oct 2019, 9:37 am   #54
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Default Re: Marconi TF144H/4S

I see what you mean now. There was some little wrap and a few optimistic wrapping, but use of braid and lots of flux helped get the solder off to bend the component leads back for removal.

Power up still pending.
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Old 27th Oct 2019, 10:37 pm   #55
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Default Re: Marconi TF144H/4S

The coarse attenuator and fine attenuator must be be hidden inside something as I can't see the circuit itself, but I don't want to disturb any screened cans unless I really have to.
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Old 28th Oct 2019, 6:52 am   #56
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Default Re: Marconi TF144H/4S

They're the other side of the box with the turret and associated circuits I presume meaning a lot of disassembly.

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Old 28th Oct 2019, 6:03 pm   #57
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Default Re: Marconi TF144H/4S

I assume you haven't had to replace yours and your unit runs well? All being well, power on should happen soon-ish. I just want to re check my work first.
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Old 28th Oct 2019, 8:49 pm   #58
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Default Re: Marconi TF144H/4S

I was hoping to avoid this, but some of the turret resistors are high too.

The turret is such a beautiful work of art, I really don't want to touch it as it won't look very pretty after I've been near it.
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Old 29th Oct 2019, 7:43 am   #59
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Default Re: Marconi TF144H/4S

"I assume you haven't had to replace yours and your unit runs well" I got as far as checking the PSU, getting an OP where there was non and then stopped. Mine has OP on the bottom sig out , memory not good here, got no OP on the meter after attenuators.

I stopped until I knew more and could get more info re the thermo-couple and have a better idea of what I'm doing.

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Old 29th Oct 2019, 6:41 pm   #60
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Default Re: Marconi TF144H/4S

Ok, so, I'll leave the turret for now, since the main issues were the power supply and what it can do to the thermocouple.

I'm beginning to 'instrument' up the Marconi and if all goes to plan will power on tonight. I'll gently bring it up via the variac. I'll probably have quite a few meters on it at once monitoring LT and HT voltage and a frequency counter. But I should also monitor the output amplitude too.
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