UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 5th Jan 2016, 7:21 am   #1
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Marconi TF144H CT452A RF Generator

I wondered if any one out there has experienced similar problems.

I found that the lower 3 frequency ranges were just slightly unstable in amplitude. The upper 3 ranges suffering poor amplitude stabilization, to the point where it took out the thermalcouple shunt, any one got a spare? The middle bands seemed very stable, until whilst fault finding one of these took the HT supply down.

On inspection of the turret I have found numerous insulation breakdowns on nearly all the coil packs, due to the sleeving covering the enameled copper wire becoming conductive. I am currently re sleeving all the coil packs with silicon sleeving. If you megger the old sleeving you can see it has become conductive.

The condition of the rest of the genny appears very good, it looks like it's been well looked after.
vintage_8bit is offline  
Old 28th Jan 2016, 7:37 am   #2
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

Following a rebuild of the turret the 1st 8 bands are all stable. I still have varying amplitude on the 4 upper frequency bands when the frequency is changed. When injecting a D.C. supply of around 1.4V on the output side of R107, 1M ohm I can vary the output from zero to way over the max of 2V RMS with just a few mV change of the DC supply. I am now looking around the ALC monitor / rectifier circuit. I have removed the open circuit thermocouple shunt for now.
vintage_8bit is offline  
Old 28th Jan 2016, 9:54 am   #3
G4XWDJim
Octode
 
G4XWDJim's Avatar
 
Join Date: Oct 2008
Location: Littlehampton, West Sussex, UK.
Posts: 1,465
Default Re: Marconi TF144H CT452A RF Generator

I was interested to hear about the insulation breakdowns on nearly all the coil packs. Presumably that's the short lengths of pink rubber sleeving. The pot cores commonly give insulation problems.

Quite some time ago Colomor were selling spare thermocouples for this sig gen but in another thread on here about a year ago others expressed their difficulty in locating spares so I think you'll find that the thermocouple is 'unobtanium' unless you can find someone scrapping one.

One of the setting up procedures in the manual advises feeding just over 2 volts into the output socket to calibrate the meter. This is known to damage the thermocouple.

In the earlier thread I mentioned people were discussing designing a work around for the dud thermocouple. Whether it ever came to anything I don't know but it might be worth searching this forum.

Jim
G4XWDJim is offline  
Old 28th Jan 2016, 10:46 am   #4
turretslug
Dekatron
 
turretslug's Avatar
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
Default Re: Marconi TF144H CT452A RF Generator

In the way of things, there are probably a few thousand of those thermocouples still in sealed packaging in landfill or the proverbial disused mineshaft...

The thermocouple problem is famously not helped by the tendency for the PSU series regulator to go faulty and give high output voltage and I don't suppose this insidious problem helps with insulation breakdown, either. Whilst valves were/are wonderful and capable things that achieved 20th century miracles, I've always cast a jaundiced eye over valved series regulators- the high headroom needed for effective operation under all circumstances is not only wasteful and heat-generating but can be catastrophic in the event of several fault modes. One that is notorious in the TF144H is the tendency of the resistors in the regulator's feedback potentiometer to go high in value- the one most likely to be affected is the highest value one in the upper section, the effect being to drive the EL81 towards saturation.

If you've got one of these fine instruments, it's worth paying attention to this point before the thermocouple goes pop or the insulation goes sizzle- fortunately, the PSU circuitry is about the most easily accessible part of this instrument.
turretslug is offline  
Old 28th Jan 2016, 8:49 pm   #5
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

Thank you for your replies.

I think the rubber sleeving was 3 different colours it covered the leadouts from the coils to the terminals and the mica caps. I have a spare turret and that had a different colour rubber sleeving but still had insulation problems.
I saw the thermocouple alternative problem discussions thanks, and coudent see a fix there. I think I would have to be really confident before I replaced the thermocouple. Isnt it strange that 2V's DC can damage the device yet its ok with 2V's RMS? I won't try that anyway. I was wondering if it could be protected with a string of germanium diodes. There is a protection circuit connected but its AC coupled so if you have any DC its ineffective.

Now that series regulator! I have allready had the experience. The glass melted and it slowly imploded. I found a couple of high value resistors here, and yes, that valve gets hot.

While we are here, I also had a problem a while ago with poor regulation on the heaters. This was due to an open circuit diode in the plate / selenium? rectifier. Fixed with a modern bridge rectifier and a dropper resistor.

Colin
vintage_8bit is offline  
Old 30th Jan 2016, 8:29 am   #6
trh01uk
Octode
 
trh01uk's Avatar
 
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,648
Default Re: Marconi TF144H CT452A RF Generator

This issue of "conductive" insulation potentially has wider implications for all kinds of vintage electronics. The "pink rubber" sleeving that I have in mind was widely used in all kinds of equipment for a long time. I remember as a young engineer in the 1970s at Pye Telecom being taught how to put this stuff on to cable terminations.

Has anyone recovered a sample of the faulty insulation and done some tests? One question is whether the problem is due to surface contamination or due to the absorption into the material of contaminants that make it conductive?

If its surface contamination it may well wash off. Maybe with the help of an ultrasonic bath. If the material has broken down and changed its chemical structure then the problem is doubtless incurable - other than replacing the sleeving with new.

I suggest its quite important to establish what is going on here, because its possibly another of those severe/incurable problems - like the "tin whiskers" in AF117 transistors.


Richard
trh01uk is offline  
Old 30th Jan 2016, 9:05 am   #7
trh01uk
Octode
 
trh01uk's Avatar
 
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,648
Default Re: Marconi TF144H CT452A RF Generator

And regarding the thermocouple and the damage from feeding in 2V DC......I had a look at the manual's technical description of the circuit.

In para 3.12 it states that the limiting circuit MR103 and MR104 limits the voltage applied to the thermocouple to 6.5V p-p, which is equivalent to 2.29V rms.

If the damage level is only a few hundred mV above the test voltage of 2.0V, then I suspect that people who have blown their thermocouples have been fairly casual about getting the 2.0V right. On the average DC power supply (e.g. a Farnell LT30-2) its quite difficult to set the output to 2.0V accurately, with a meter that reads 0 - 30V. There is very little room for error here, and great care - using a DVM or accurate AVO - is required to ensure the 2.0V is pretty close to the right value before connecting to the geny.

I suspect the best answer for anyone who wants to use this geny as useful bit of test gear is to abandon the thermocouple circuit altogether, and fit a modern rms detector IC, such as the Analog Devices AD8361, which operates to a couple of GHz, and is highly linear. But I haven't tried it myself as yet.

Richard
trh01uk is offline  
Old 30th Jan 2016, 2:22 pm   #8
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

Thank you Richard.

I have only rekindled my interest in valve equipment in the last year with an early Murphy B40/a, wired with a thick rubber isulation. A latter B40/A in PVC or similar, and 3 Vidor portables. And now of course the Marconi genny where the turret was wired with a combination of : Pink, Red & Black rubber sleeving.

In my limited experience I have seen rubber insulation in 2 stages of deterioration. The early B40 insulation had fossilised, going hard dry and brittle. The sleeving on the genny had in places a brown oily deposite on it. You could scrape it on to a screwdriver easily. I assume the final stage is the fossilisation?

I have seen rubber detiriate like this in stages before, thinking of a washing machine door seal for example.

Going back to the problem of the poor output regulation I have found that some, or all (not lifted all of them yet) of the .1uF caps in the ALC and protection circuis are leaky. They are plastic encased. The parts list says they are paper. Will let you all know result when changed.

That AD8361 seems worth pursuing. I love origionality but I need the genny to be reliable as the restorations are building up.

Colin
vintage_8bit is offline  
Old 2nd Feb 2016, 11:06 am   #9
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

Fault reminder. Varying amplitude when adjusting frequancy on ranges I, J, K, & L only.

Whilst changing the leaky .1 uF capacitors I have found that MR102 Is leaky. This is may be more significant than the caps. The nearest replacement I have is a HP2835. The original was a HP2800. I had no luck finding the datasheet for the 2835 to see if that would be a suitable replacement.
vintage_8bit is offline  
Old 2nd Feb 2016, 7:34 pm   #10
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
Default Re: Marconi TF144H CT452A RF Generator

HP2800 and HP2835.
I attach data sheet covering these, dated 1990/91. The HP2835 has a lower breakdown voltage. But in the TF144H circuit as MR102, it is operating below that limit voltage. The earlier TF144H/1 uses the Hughes HD1870 diodes in that position, which are rated at 15V. So I think you could use them. wme_Bill
Attached Files
File Type: pdf HP2800-HP2835 diodes-wm.pdf (1.24 MB, 162 views)
WME_bill is online now  
Old 4th Feb 2016, 7:20 am   #11
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

Thank you for the data sheet and advice. I have now fitted the diodes. That tag board is not in an easy place to work on! Hopefully power up tonight!
vintage_8bit is offline  
Old 4th Feb 2016, 10:22 am   #12
CambridgeWorks
Nonode
 
CambridgeWorks's Avatar
 
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,851
Default Re: Marconi TF144H CT452A RF Generator

Colin, I may have 2 thermocouples, I need to look in next day or two. Will let you know.
Rob
__________________
Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd
CambridgeWorks is offline  
Old 4th Feb 2016, 12:55 pm   #13
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

That would be great, many thanks. I would love to keep it original if I can. Colin
vintage_8bit is offline  
Old 4th Feb 2016, 3:13 pm   #14
karesz*
Heptode
 
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
Default Re: Marconi TF144H CT452A RF Generator

Hi Rob, all persones,
I have similar problem with my good old TF144G what is to restaurate: It needs an ThermoCouple, the valves I have all NOS from Marconi yet...
Can you -sombody- evetuel help me too please with an passable thermocouple? _of course I think it not free of costs...
Thank you very much!
Best greetings - Karl
karesz* is offline  
Old 5th Feb 2016, 2:58 pm   #15
CambridgeWorks
Nonode
 
CambridgeWorks's Avatar
 
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,851
Default Re: Marconi TF144H CT452A RF Generator

Hi Colin, attached the 2 pictures showing top and bottom. I am pretty certain they were from a TF144. A big heavy beast that was in very poor condition externally and was stripped down. The one with additional components gives 2 separate resistance readings as i would expect, that is in (co-ax) and dc out.
The one with the 2 yellow wires has one terminal o/c to the other 3, so is probably faulty? It was not connected in circuit, just attached within sig gen as a spare? Can post both to you, please pm me your address.
Hi karesz*, from above it looks as only one is ok. Sorry, I do not have any more.
Kind regards, rob
Attached Thumbnails
Click image for larger version

Name:	therm 1.jpg
Views:	224
Size:	113.0 KB
ID:	119568   Click image for larger version

Name:	therm 2.jpg
Views:	218
Size:	102.4 KB
ID:	119569  
__________________
Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd
CambridgeWorks is offline  
Old 5th Feb 2016, 4:47 pm   #16
karesz*
Heptode
 
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
Default Re: Marconi TF144H CT452A RF Generator

Hi Rob,
thanks for checking and answering me; I'm on the sourching for that...
Best regards!
Karl
karesz* is offline  
Old 6th Feb 2016, 12:40 pm   #17
vintage_8bit
Heptode
 
Join Date: Feb 2013
Location: Rainham, Kent, UK.
Posts: 523
Default Re: Marconi TF144H CT452A RF Generator

Karl. I will regularly using my generator so will always be looking for spares. Will let you know if I find anything.

To all.
After changing the .1 caps and MR 101 & 102 I still have varying amplitude on the 4 highest ranges. Looking at the output on R107 this dosent increase in sympathy with an increase in output. In fact It decreases at some frequencies when the output amplitude increases. I have disconected the modulation to C129 and C195 is disconected to the overvoltage clamp. Any ideas? Its a shame the turret is in the way when fault finding!
vintage_8bit is offline  
Old 6th Feb 2016, 2:38 pm   #18
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Marconi TF144H CT452A RF Generator

The reason for the thermocouple device is so that the level setting meter responds to the mean output power. A diode detector or one of the Analog Devices logarithmic detectors will tend to show peak power which will depend a lot on what modulation is applied.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 6th Feb 2016, 5:38 pm   #19
karesz*
Heptode
 
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
Default Re: Marconi TF144H CT452A RF Generator

Colin- thousand thanks in advance, be lucky with your renovated!
Best regards Karl
karesz* is offline  
Old 6th Feb 2016, 9:53 pm   #20
trh01uk
Octode
 
trh01uk's Avatar
 
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,648
Default Re: Marconi TF144H CT452A RF Generator

Quote:
Originally Posted by Radio Wrangler View Post
The reason for the thermocouple device is so that the level setting meter responds to the mean output power. A diode detector or one of the Analog Devices logarithmic detectors will tend to show peak power which will depend a lot on what modulation is applied.

David
David,

perhaps you can try and explain why they needed two separate systems to monitor the output voltage? They had the thermocouple which only drove the meter.

Then they had the diodes MR101 and MR102 in a sort of bridge arrangement, which is labelled "ALC Monitor", and which feeds back into the ALC amplifier.

Why not use the thermocouple output to feed into the ALC loop? Was it a matter of speed of response (thermocouples being extremely slow).

Richard
trh01uk is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:27 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.