Marconi TF2438 Universal Counter/Timer - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

mhennessy · 5th Jul 2016, 10:46 pm

Ah - that makes more sense...

My barely-legible diagrams don't have any designations for the pass transistors, but looking at the better copies, I see that they are labelled TR1, 2 and 3. Which is confusing, but perhaps valid because they are not on the PCB...

Anyway, give it a really good visual inspection (I use a x4 or x7 loupe for this sort of thing), and look for bad connections or shorts. And then try replacing TR2 (and perhaps TR1) - it doesn't need to be an exact replacement. Transistors can fail intermittently.

At this stage, I'm still not sure if it's being overloaded, or if the PSU is being wimpy. And I'm not sure how TR2 can cause the symptoms, unless there is an intermittent short between +5 and +12. But keep at it!

thomass · 6th Jul 2016, 6:33 pm

I've just been testing Mark's solution as per post #4 again and these are the results

Voltage rails with no external bench supply connected across D21

5.1v = 4.0
-5.1v = -4.0
12v = 4.0
-12v = -4.0

Voltage rails with an external bench supply connected across D21 and the Marconi powered from the mains.

5.1v = 5.0
-5.1v = -4.98
12v = 4.99
-12v = -5.03

Voltages across D21

When only powered from an external bench supply
4.85v

When powered from an external bench supply and the mains
6.31v

I've also been looking for dry joints and shorts with my 10x magnifier but I don't think I can see the wood for the trees at the moment. I haven't got any suitable transistors to replace TR2 / TR1 in stock, some have been ordered so I will have another look in a few days.

Dickie · 6th Jul 2016, 7:34 pm

So it looks as though the +/- 5V regulators are working fine, and something is dragging down the +12V line. As the +12V tracks the +5V to within a few mV my vote goes to Mark's suggestion of a short between them. Have you checked for that?
Alternatively but less likely, I suppose, is a short across R127 or an o/c R128, which would produce the same effect.

thomass · 6th Jul 2016, 10:42 pm

I've checked for shorts but can't find any. R127 & R128 check out ok

Thomas

Dickie · 7th Jul 2016, 7:52 am

Sorry to labour the point, but have you actually measured the resistance between the +5 and +12 rails?
The +12V doesn't actually connect to much of the circuit, only the input buffers and their trigger level pots, the 733 wideband amp, and the reference oscillator. This latter is suspect, as it gets both a +5V and +12V feed. It would be easy to check by removing it.

thomass · 7th Jul 2016, 5:17 pm

I've measured between the supply rails, the results are.

between +5 and +12 3.2K
between +5 and -12v 3.6k
between -5 and -12v 3.4k
between -5 and +12v 3.0k

R127 & R128 check out ok.

Thomas

thomass · 7th Jul 2016, 7:12 pm

I also removed the 10MHZ ref oscillator but this made no difference to the rails. I also temporarily disconnected the +5V pass transistor but the 12V still low.

Dickie · 7th Jul 2016, 9:52 pm

Curious. The finger is beginning to point to TR1 or its connections.

thomass · 9th Jul 2016, 11:11 am

It is rather, I will try swapping TR1 once the replacements arrive.

thomass · 9th Jul 2016, 3:01 pm

The replacement transistors arrived so I thought I would change TR1 but now the stupid thing has popped one of the primary fuses.

I haven't got any so I will need to get some before I can continue.

Dickie · 9th Jul 2016, 6:09 pm

According to the schematic it has both L and N input fuses so assuming only one has popped you could replace it with a larger one and not reduce the level of over-current protection. The safety will (arguably) be slightly compromised.

thomass · 9th Jul 2016, 7:12 pm

I replaced the fuse with a slightly higher rated one as that's all I had. I have now replaced both TR1 & TR2 but it's still the same.

I can't replicate what I did the other day when it sprang into life, so I think I'm going to abandon this and not spend anymore time on it.

Thanks for everyone's help much appreciated.

Thomas

Dickie · 9th Jul 2016, 7:58 pm

Well yes sometimes its best to put it down. Maybe on some wet sunday afternoon in the winter you'll attack it with renewed enthusiasm!

thomass · 10th Jul 2016, 11:01 pm

Well I don't want to speak too soon but I think it's cured. I was measuring the voltage at R112 & D21 which was 12v / 6.2v respectively then it suddenly dropped down.

I thought I would check D30 again, which I‘ve changed it and soldered one leg directly to the track which goes to pin 3 on IC20 and IC23. It now works every time I power on and all the supply rails are correct.

I will put it all back together and see if full functionality has been restored.

mhennessy · 10th Jul 2016, 11:26 pm

So was it intermittently leaking? Any way of checking it?

When running normally, it's reverse-biased and plays no part in normal operation. It's only there to put a non-zero voltage at the reference point at start-up, otherwise the op-amps all think "well, I've got 0V at both inputs, so I'm happy". So before D21 is able to provide a high-quality reference, D30 supplies a "rough and ready" reference which is less than D21's voltage, but enough to start the process. In theory, it's 4.3-0.6V, approx...

I'll keep my fingers crossed, but do remember that I've seen those other misdemeanours whilst the PSU was working correctly. Either way, here's hoping

Well done for the progress so far!

thomass · 11th Jul 2016, 1:47 pm

I'm not sure, it did check out ok. There was defiantly an intermittent connection somewhere because if I moved it then it stopped working.

I think I have fixed the issue of reading high by re-soldering a lose connection of the flexible cable which goes to the switch board. Taking of which does anybody know where I can get a replacement flexible cable from?

Fingers crossed.

Dickie · 11th Jul 2016, 5:24 pm

I'd be surprised if you can find the exact part as 19-ways is a bit of an oddie. But you could build one from two 10-way cables. Farnell stock a few, e.g.
http://uk.farnell.com/te-connectivit...y-3/dp/1310330

thomass · 11th Jul 2016, 6:45 pm

Thanks, I’ll have a look. I soldered the 10MHz oscillator back in having removed it while troubleshooting which is running correctly, having checked it with another frequency counter.

So it’s up to its old tricks, where it either displays zeros or the count is high, approx. double the frequency. Would I be correct in thinking the problem lies with the switch board or control board somewhere.

Dickie · 11th Jul 2016, 8:56 pm

Based on those symptoms, the only answer is "probably". If you haven't done so already, reflow all the through-hole rivets everywhere, they are very prone to fracture due to thermal stresses. Also worth doing is to clean all the pushbutton switches. Do you have a 'scope to aid faultfinding?

thomass · 11th Jul 2016, 9:17 pm

OK I'll give it a going over. Yes I do have a scope, an old Telequipment D65.

5th Jul 2016, 10:46 pm	#21
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Marconi TF2438 Universal Counter/Timer Ah - that makes more sense... My barely-legible diagrams don't have any designations for the pass transistors, but looking at the better copies, I see that they are labelled TR1, 2 and 3. Which is confusing, but perhaps valid because they are not on the PCB... Anyway, give it a really good visual inspection (I use a x4 or x7 loupe for this sort of thing), and look for bad connections or shorts. And then try replacing TR2 (and perhaps TR1) - it doesn't need to be an exact replacement. Transistors can fail intermittently. At this stage, I'm still not sure if it's being overloaded, or if the PSU is being wimpy. And I'm not sure how TR2 can cause the symptoms, unless there is an intermittent short between +5 and +12. But keep at it!

6th Jul 2016, 6:33 pm	#22
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer I've just been testing Mark's solution as per post #4 again and these are the results Voltage rails with no external bench supply connected across D21 5.1v = 4.0 -5.1v = -4.0 12v = 4.0 -12v = -4.0 Voltage rails with an external bench supply connected across D21 and the Marconi powered from the mains. 5.1v = 5.0 -5.1v = -4.98 12v = 4.99 -12v = -5.03 Voltages across D21 When only powered from an external bench supply 4.85v When powered from an external bench supply and the mains 6.31v I've also been looking for dry joints and shorts with my 10x magnifier but I don't think I can see the wood for the trees at the moment. I haven't got any suitable transistors to replace TR2 / TR1 in stock, some have been ordered so I will have another look in a few days.

6th Jul 2016, 7:34 pm	#23
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2438 Universal Counter/Timer So it looks as though the +/- 5V regulators are working fine, and something is dragging down the +12V line. As the +12V tracks the +5V to within a few mV my vote goes to Mark's suggestion of a short between them. Have you checked for that? Alternatively but less likely, I suppose, is a short across R127 or an o/c R128, which would produce the same effect. __________________ Regards, Richard, BVWS member

6th Jul 2016, 10:42 pm	#24
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer I've checked for shorts but can't find any. R127 & R128 check out ok Thomas

7th Jul 2016, 7:52 am	#25
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2438 Universal Counter/Timer Sorry to labour the point, but have you actually measured the resistance between the +5 and +12 rails? The +12V doesn't actually connect to much of the circuit, only the input buffers and their trigger level pots, the 733 wideband amp, and the reference oscillator. This latter is suspect, as it gets both a +5V and +12V feed. It would be easy to check by removing it. __________________ Regards, Richard, BVWS member Last edited by Dickie; 7th Jul 2016 at 7:58 am.

7th Jul 2016, 5:17 pm	#26
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer I've measured between the supply rails, the results are. between +5 and +12 3.2K between +5 and -12v 3.6k between -5 and -12v 3.4k between -5 and +12v 3.0k R127 & R128 check out ok. Thomas

7th Jul 2016, 7:12 pm	#27
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer I also removed the 10MHZ ref oscillator but this made no difference to the rails. I also temporarily disconnected the +5V pass transistor but the 12V still low.

7th Jul 2016, 9:52 pm	#28
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2438 Universal Counter/Timer Curious. The finger is beginning to point to TR1 or its connections. __________________ Regards, Richard, BVWS member

9th Jul 2016, 11:11 am	#29
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer It is rather, I will try swapping TR1 once the replacements arrive.

9th Jul 2016, 3:01 pm	#30
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer The replacement transistors arrived so I thought I would change TR1 but now the stupid thing has popped one of the primary fuses. I haven't got any so I will need to get some before I can continue.

9th Jul 2016, 6:09 pm	#31
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2438 Universal Counter/Timer According to the schematic it has both L and N input fuses so assuming only one has popped you could replace it with a larger one and not reduce the level of over-current protection. The safety will (arguably) be slightly compromised. __________________ Regards, Richard, BVWS member

9th Jul 2016, 7:12 pm	#32
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer I replaced the fuse with a slightly higher rated one as that's all I had. I have now replaced both TR1 & TR2 but it's still the same. I can't replicate what I did the other day when it sprang into life, so I think I'm going to abandon this and not spend anymore time on it. Thanks for everyone's help much appreciated. Thomas

9th Jul 2016, 7:58 pm	#33
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2438 Universal Counter/Timer Well yes sometimes its best to put it down. Maybe on some wet sunday afternoon in the winter you'll attack it with renewed enthusiasm! __________________ Regards, Richard, BVWS member

10th Jul 2016, 11:01 pm	#34
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer Well I don't want to speak too soon but I think it's cured. I was measuring the voltage at R112 & D21 which was 12v / 6.2v respectively then it suddenly dropped down. I thought I would check D30 again, which I‘ve changed it and soldered one leg directly to the track which goes to pin 3 on IC20 and IC23. It now works every time I power on and all the supply rails are correct. I will put it all back together and see if full functionality has been restored.

10th Jul 2016, 11:26 pm	#35
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Marconi TF2438 Universal Counter/Timer So was it intermittently leaking? Any way of checking it? When running normally, it's reverse-biased and plays no part in normal operation. It's only there to put a non-zero voltage at the reference point at start-up, otherwise the op-amps all think "well, I've got 0V at both inputs, so I'm happy". So before D21 is able to provide a high-quality reference, D30 supplies a "rough and ready" reference which is less than D21's voltage, but enough to start the process. In theory, it's 4.3-0.6V, approx... I'll keep my fingers crossed, but do remember that I've seen those other misdemeanours whilst the PSU was working correctly. Either way, here's hoping Well done for the progress so far!

11th Jul 2016, 1:47 pm	#36
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer I'm not sure, it did check out ok. There was defiantly an intermittent connection somewhere because if I moved it then it stopped working. I think I have fixed the issue of reading high by re-soldering a lose connection of the flexible cable which goes to the switch board. Taking of which does anybody know where I can get a replacement flexible cable from? Fingers crossed.

11th Jul 2016, 5:24 pm	#37
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2438 Universal Counter/Timer I'd be surprised if you can find the exact part as 19-ways is a bit of an oddie. But you could build one from two 10-way cables. Farnell stock a few, e.g. http://uk.farnell.com/te-connectivit...y-3/dp/1310330 __________________ Regards, Richard, BVWS member

11th Jul 2016, 6:45 pm	#38
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer Thanks, I’ll have a look. I soldered the 10MHz oscillator back in having removed it while troubleshooting which is running correctly, having checked it with another frequency counter. So it’s up to its old tricks, where it either displays zeros or the count is high, approx. double the frequency. Would I be correct in thinking the problem lies with the switch board or control board somewhere.

11th Jul 2016, 8:56 pm	#39
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2438 Universal Counter/Timer Based on those symptoms, the only answer is "probably". If you haven't done so already, reflow all the through-hole rivets everywhere, they are very prone to fracture due to thermal stresses. Also worth doing is to clean all the pushbutton switches. Do you have a 'scope to aid faultfinding? __________________ Regards, Richard, BVWS member

11th Jul 2016, 9:17 pm	#40
thomass Pentode Join Date: Jun 2014 Location: Amersham, Buckinghamshire, UK. Posts: 147	Re: Marconi TF2438 Universal Counter/Timer OK I'll give it a going over. Yes I do have a scope, an old Telequipment D65.