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Marconi TF 1101 sig gen

 
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Marconi TF 1101 sig gen

Post by Refugee » Tue Apr 07, 2015 6:00 pm

I have acquired one of these for a very reasonable price plus more liquid tax than the asking price to collect it from Rugby.
It was part of an estate dispersal that was offered on UKVRR.

The carry handle is a bit of a token as it is a bit heavy to be portable.
There is a bag containing some old capacitors so perhaps the works will need little more than inspection before it is ready to go :thumb

Has anyone got one of these they have restored?
The full manual was easy to find on the web so no need for a data request for this one.
It is in pretty good nick with just a couple of minor scratches on the sides.
It has the classic Marconi styling of the day with the picture frame style front panel and the pendulum dial that was such a feature of these instruments.
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Re: Marconi TF 1101 sig gen

Post by Refugee » Mon Apr 13, 2015 10:31 pm

I have lifted the covers off today to reveal a nice clean chassis.
First I spotted that there is a cordless drive on the fine frequency control. When I turned it around I found that the next section of the drive does indeed have a cord.
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It has a very nice chassis layout with easy access to all the components and I have even spotted military arrow logos on some of the large capacitors.
It has a lovely 4 gang variable capacitor.
All the valves look like they have got good getters and it looks like it has been powered up at some time perhaps when the two capacitors were changed.
These old capacitors look like they have come from the oscillator section as there are two mustard capacitors in there and also there is an electrolytic capacitor C15 that does not look original. It is in the level stabilizer circuit.

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So what if any work needs to be done before I show it some power?
Time for a close visual inspection in conjunction with the service information.
The first issue I spotted was a "zit" on C24 output transformer snubber and the the series resistor on C23 is high on the other half of the output transformer primary. They are Dubillier paper types and are very likely to fail leaky.
There is another large blue paper capacitor like the ones that have already been changed in position C20 output stage phase splitter. If leaky it will drive the single triode phase splitter on so that the output will become distorted at high level due to the valve turning hard on on the peaks of one side of the signal.
The previous owner should have looked at this one as well as the two in the oscillator. Now I will have to have a look for another mustard to match the other two already replaced ones.
There was a spot of wax on the cabinet floor and this was traced to the cathode resistor in the output stage. It does not look like it has been very hot and the transformer measures all right on both halves of the primary.
There are two very large grid coupling capacitors that are the most likely culprit for this and they will drill out for restuffing without issue. These are C21 and C22.

I have also spotted tin whiskers growing on the chassis and they really do show up on camera too. Zoom it up and have a look.
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A born again teenager with acne just like a real one
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But it is not an AF117!
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It was only just hot enough to drip off
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Re: Marconi TF 1101 sig gen

Post by Refugee » Fri Apr 17, 2015 2:45 am

First light today.
Yes the red light did come on after a quick run up on the Variac.
The regulated power supply came up spot on but sadly dropped by about 8 volts during soak test.
A paper capacitor looks highly suspect and will be sorted out next session.
Also in the power supply something very interesting indeed lit up :cch
It is a 6CD6.
To me it looks like a double cathoded beam tetrode with a common anode. It is internally connected to look like a single beam tetrode at the base pins. Perhaps I will have to start a new thread for it with loads of photos. I questioned it being the correct valve but the number is correct. It was difficult to get photos due to the top cap connector being well jammed on and there is no way I am going to risk trying to take it off to get the valve out. I would have to unlace the wiring to get the anode wire free. It is staying firmly plugged into the chassis.
I have got one hell of a stable instrument here even with a bit of drift on the HT.
Nixie tube counter time for the dial calibration check so more nice things to light up with this one ttt:
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Re: Marconi TF 1101 sig gen

Post by sideband » Fri Apr 17, 2015 8:02 am

Interesting about the 6CD6. I do know that the 50CD6 is a line output valve used in some KB TV's and thought that the 6CD6 would be the same valve with a different heater. Certainly the 50CD6 can be replaced by a PL36 with some base rewiring.

I suppose it's possible that two valves by different manufacturers could have similar coding but be totally different. Some of the audiophool websites mention a 6CD6 being used as an audio output valve and they also mention 50CD6 so......? :aab

Very nice piece of kit by the way :thumb . Built like a tank, laced wiring, easy to work on (mostly)...a proper job!

 
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Re: Marconi TF 1101 sig gen

Post by Briancuff » Fri Apr 17, 2015 10:05 am

IIRC, the 6CD6 is used as the line output valve in the Marconi MkIII Image Orthicon camera from the early 1950s (my favourite camera). Remember the original Grandstand opening titles?

 
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Re: Marconi TF 1101 sig gen

Post by Refugee » Fri Apr 17, 2015 1:11 pm

Yes indeed there is a series heater version 50CD6 and lots of audiofile photos on the web. My 6CD6 is a 6.3 volt one and has its own heater winding in order to tame the heater cathode volts as it is on series regulator duty in the Marconi.
There is a couple of less critical capacitors to restuff and then it is the home run to a lovely sig gen for use in my workshop.

 
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Re: Marconi TF 1101 sig gen

Post by Refugee » Mon Apr 20, 2015 12:33 pm

Replacing the capacitor in the regulator produced an excellent result with the 6CD6 regulating very strictly indeed.

Yesterday evening I started up one of my frequency counters as an alternative to the scope and calibrated oscillator arrangement described in the manual.
All the ranges are reading low by about 2%. The figure quoted in the manual is 1.5%.
The only way I can see for this happening is by the capacitor between the two halves of the bridge having less leakage than before.
What do others think?
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Re: Marconi TF 1101 sig gen

Post by Terrykc » Wed Apr 22, 2015 10:38 am

Refugee wrote:... The only way I can see for this happening is by the capacitor between the two halves of the bridge having less leakage than before ...

!!!

I would hope that any capacitor (other than an electrolytic) would have zero leakage! A capacitor with less than zero leakage might seem desirable but it is practically and theoretically impossible to achieve!

Is your 2% error consistent throughout each band or does it vary from end to end? If so, have you considered that it might be a mechanical problem whereby the pointer is slightly offset from its datum mark - like a radio where the pointer position has moved on the cord?

If the problem is electronic, a low reading suggests slight loss of capacitance somewhere - considering that the error is only 0.5% out of spec, it won't be much!

Another thing to consider is how accurate is your DFM? And how do you check its calibration?

 
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Re: Marconi TF 1101 sig gen

Post by Refugee » Wed Apr 22, 2015 11:32 am

I checked the DFM against another DFM and it is spot on.
I have looked at the internal photos of the oscillator section and the resistors are in two banks. One bank is factory fitted and they are 2% carbon film and the other are 5% composition and are the ones that are selected during calibration. I will check them to see if they have drifted high as that would make it go slightly low. On one band it does come to a mechanical stop just before the top end of the band is reached.
Loss of capacitance would make the frequency go too high and not too low.
I feel that it is worth a check before I call it a day and put the covers on. I will also check with my other DFM too.

 
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Re: Marconi TF 1101 sig gen

Post by Terrykc » Wed Apr 22, 2015 5:43 pm

Refugee wrote:I checked the DFM against another DFM and it is spot on ...

So, if both DFMs are in error by the same amount ...

Its a pity that DSO has removed a very useful reference source for checking
calibration - the 15.625kHz line timebase and the 4.43MHz colour subcarrier, both locked together and specified to a very high degree of accuracy ...

Locally generated signals from set top boxes, etc. cannot be expected to be as accurate!

 
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Re: Marconi TF 1101 sig gen

Post by Cathovisor » Wed Apr 22, 2015 7:35 pm

There is of course the 198KHz carrier from Droitwich which is still a frequency standard: you'd need a 198KHz crystal filter to remove the audio, which is what the off-air frequency references seemed to do (well, my 200 KHz one does, put it that way).

 
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Re: Marconi TF 1101 sig gen

Post by Refugee » Fri Apr 24, 2015 1:58 am

I am pretty sure that the oscillator section is indeed stable enough to do a beat frequency with radio4 for a full counter check and then I will sort out some modern resistors to keep it nice and stable with an accurate dial and it looks like a fairly easy job too.
Will be onto it tomorrow if nothing else happens.
We have moved all the chat about the counter to its own thread started in 2012 as that is the best place for it.
http://forum.radios-tv.co.uk/viewtopic.php?f=15&t=3439

 
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Re: Marconi TF 1101 sig gen

Post by johntheboffin » Fri Apr 24, 2015 3:25 pm

I have a couple of TF1101s in my collection of old test equipment - see https://sites.google.com/site/marconiin ... oscillator.

They are excellent oscillators. I do remember having to service them when I was an apprentoid at MI. A frequent problem was getting the harmonic distortion in specification at the very low frequency end of the range (20 Hz). This depended critically on how well the two output valves were matched and sometimes it wasn't possible to find a matching pair. So we would "force" a match by sticking one valve on the Avo VCR and deliberately over-running it (quite severely!) for a few seconds until it had changed sufficiently to match the other one.

Quite improper, of course, but it got the instrument past the QA final test!

TF1101s run quite hot and several of the techies there used to keep a bag of peanuts nice and warm on top of one!

John

 
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Re: Marconi TF 1101 sig gen

Post by Refugee » Fri Apr 24, 2015 4:37 pm

In mine the electrolytic capacitors show dates nov/dec 1962 and the serial number ends /094 indicating that it was built in 1964. It has the light grey front panel.


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