UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio 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.

Reply
 
Thread Tools
Old 11th May 2019, 11:28 pm   #21
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,051
Default Re: Marconi TF2370 fault fixing

Not much time today but the 100KHz is OK so it's the complicated 36.4MHz oscillator that is the culprit. The scope images match the repair guide and I followed the realignment instructions that centres the control voltage and it quite happily runs at 36.4MHz with a fixed voltage but then runs faster as soon as the feedback connection is restored.

I will have another run through tomorrow to try to understand whats going on by breaking the loop, varying the control voltage and monitoring the frequency and phase comparator output. It's using another double balanced mixer this time with the 4 diodes on an IC. It converts the edge of the 100KHz clock to a pulse then adds ringing at approximately 36.4MHz and phase locks to this. I guess the ringing might be off frequency?

The 36.4MHz oscillator, is only used for the counter and tracking generator and it could have been wrong for a while. There is still the original fault of low amplitude.
PJL is offline   Reply With Quote
Old 13th May 2019, 12:09 pm   #22
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,051
Default Re: Marconi TF2370 fault fixing

I followed the alignment instructions again and this time adjusted L3 as it was about 40Hz out. After reconnecting the feedback the frequency went low and not high as before. So, I decided to adjust the lock offset when the feedback was connected and found there was a range where it locked so I have left it set to the centre of this range. I would guess the mixer is not quite right but as it works I am going to leave it alone.

Now back to the level issue!
PJL is offline   Reply With Quote
Old 13th May 2019, 12:25 pm   #23
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 11,831
Default Re: Marconi TF2370 fault fixing

That is pretty much the right way to adjust a PLL.

With the loop opened, the phase detector of a good system will bang against one end stop or the other for a phase/frequency detector, or give a sinewave at the unlocked error frequency for a sampling type phase detector (the sinewave may be small if it is high with respect to the loop filter frequency range.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 13th May 2019, 11:30 pm   #24
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,051
Default Re: Marconi TF2370 fault fixing

Now I am in trouble! I am looking at the 2nd local oscillator buffers (AA7 page 213) that drives the mixer (AZ1 on AC1) that takes off 36.4MHz to make the 200MHz signal for the counter/tracking generator. This is the signal that appears to be low.

The circuit is attached and what I am seeing is the output marked AZ2 on AB9 , also a balanced mixer, is almost twice the amplitude. I can see that the common base TR8 is going to attenuate the signal but I would have thought that any loss would be recovered by TR9 but I guess it would all depend on the loading? I also note that R20 is 270 ohms but it has a mark by it and I wonder if that means adjust to obtain required output level?

So based on this it would seem there is a problem with the TR8/TR9 buffer stages or the loading from the AZ1 balance mixer. I need to search through the manual to see if there is any mention of the amplitude of the AZ2/AB9 output.
Attached Thumbnails
Click image for larger version

Name:	AA7 buffer.jpg
Views:	34
Size:	38.2 KB
ID:	183047  

Last edited by PJL; 13th May 2019 at 11:56 pm.
PJL is offline   Reply With Quote
Old 15th May 2019, 10:15 pm   #25
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,051
Default Re: Marconi TF2370 fault fixing

I have bought a 1.5GHz low loss probe and a 440MHz high speed log detector. The detector is coming from China but I can still do the sub-150MHz checks in the meantime. Amazing what you can get from ebay for 17 total. At least I won't need to do the dBm conversions then!

Last edited by PJL; 15th May 2019 at 10:24 pm.
PJL is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 11:37 pm.


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 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.