Telequipment S51A oscilloscope - UK Vintage Radio Repair and Restoration Discussion Forum

cinema1 · 25th Mar 2017, 3:55 pm

Hi,
info on s51 oscilloscope, cramping on the right side of the sceen, would this fault be in the sawtooth sweep generator or the eht.
kind regards.
cinema1.

Al (astral highway) · 25th Mar 2017, 4:20 pm

Hello,

Best to post pic?!

ms660 · 25th Mar 2017, 4:24 pm

In another section.

Lawrence.

Mod edit: Thread moved.

WME_bill · 27th Mar 2017, 10:34 am

Sounds like half the X Amplifier is failing. Alternatively, as you don't say what the "cramping" effect is, if on one range only, try the timebase sweep capacitor. If all ranges, the timebase generator valve is limiting - old afge, or wrong voltages. Get you meter out and check all the voltages, with the TB not operating. Very unilkely to be EHT, as that would affect both X and Y sweeps, across the whole screen width. wme_bill

Al (astral highway) · 27th Mar 2017, 3:20 pm

Hello again - seriously, post a pic! It takes moments to grab a pic and upload
to here and can tell people way more than a narrative description!!

cinema1 · 27th Mar 2017, 7:40 pm

Hi,

Here are the pics as requested, would be very grateful for any help on the fault.

WME_bill · 29th Mar 2017, 9:51 am

S51 non linear Time Base scan.
You say you have checked the valves, . Have you measured the voltages around the TB valve (V6a pentode), and the X amplifier (the pair V7). Keep the TB off, by Stability fully counter clockwise.
If fault appears on all TB ranges, check the common capacitor C37. Does the TB Variable speed make any difference.
To identify if fault is in the X amplifier, have you tried altering the X Gain to minimum and then see what effect is of the X Shift. Is it always the RH end of the trace, or only the right hand side of screen.
Try feeding the same voltage into the Y and X amplifiers with gains set the same and Time Base off of course. Should give you a straight diagonal line. If it bends on the right side of the screen, then fault in X amplifier. Easiest with a AC voltage, but you could use a variable DC voltage, and plot the results on paper.
Though checking the voltages should have identified that already and much more easily.
Substitute one of the other Double Triodes ECC88 for the X amplifier. I see that later versions of the S51 have a transistor instead of a resistor R46 in the tail of the X amplifier. Has that failed? - voltage checks will show that up immediately.
The manual is good, and the circuit shows most of the voltages. Expect within say 10%.
wme_bill

cinema1 · 29th Mar 2017, 4:41 pm

WME BiLL,
I took your advice to use my meter and to check the valves voltages,
valve 7 was the culprit. The two anodes 7b, 7a have 39k resisters 7b was 180v 7a 47v after changeing the two 39k resisters 7a--7b 280v and the cathodes 32v the cramping of the trace as gone everything is ok
kind regards .
Eric
cinema1

25th Mar 2017, 3:55 pm	#1
cinema1 Tetrode Join Date: Jun 2008 Location: Preston, Lancashire, UK. Posts: 50	Telequipment S51A oscilloscope Hi, info on s51 oscilloscope, cramping on the right side of the sceen, would this fault be in the sawtooth sweep generator or the eht. kind regards. cinema1.

25th Mar 2017, 4:20 pm	#2
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Telequipment S51A oscilloscope Hello, Best to post pic?! __________________ Al

25th Mar 2017, 4:24 pm	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Telequipment S51A oscilloscope In another section. Lawrence. Mod edit: Thread moved.

27th Mar 2017, 10:34 am	#4
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Telequipment S51A oscilloscope Sounds like half the X Amplifier is failing. Alternatively, as you don't say what the "cramping" effect is, if on one range only, try the timebase sweep capacitor. If all ranges, the timebase generator valve is limiting - old afge, or wrong voltages. Get you meter out and check all the voltages, with the TB not operating. Very unilkely to be EHT, as that would affect both X and Y sweeps, across the whole screen width. wme_bill

27th Mar 2017, 3:20 pm	#5
Al (astral highway) Dekatron Join Date: Nov 2006 Location: London, UK. Posts: 3,496	Re: Telequipment S51A oscilloscope Hello again - seriously, post a pic! It takes moments to grab a pic and upload to here and can tell people way more than a narrative description!! __________________ Al

27th Mar 2017, 7:40 pm	#6
cinema1 Tetrode Join Date: Jun 2008 Location: Preston, Lancashire, UK. Posts: 50	Re: Telequipment S51A oscilloscope Hi, Here are the pics as requested, would be very grateful for any help on the fault. Attached Thumbnails

29th Mar 2017, 9:51 am	#7
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Telequipment S51A oscilloscope S51 non linear Time Base scan. You say you have checked the valves, . Have you measured the voltages around the TB valve (V6a pentode), and the X amplifier (the pair V7). Keep the TB off, by Stability fully counter clockwise. If fault appears on all TB ranges, check the common capacitor C37. Does the TB Variable speed make any difference. To identify if fault is in the X amplifier, have you tried altering the X Gain to minimum and then see what effect is of the X Shift. Is it always the RH end of the trace, or only the right hand side of screen. Try feeding the same voltage into the Y and X amplifiers with gains set the same and Time Base off of course. Should give you a straight diagonal line. If it bends on the right side of the screen, then fault in X amplifier. Easiest with a AC voltage, but you could use a variable DC voltage, and plot the results on paper. Though checking the voltages should have identified that already and much more easily. Substitute one of the other Double Triodes ECC88 for the X amplifier. I see that later versions of the S51 have a transistor instead of a resistor R46 in the tail of the X amplifier. Has that failed? - voltage checks will show that up immediately. The manual is good, and the circuit shows most of the voltages. Expect within say 10%. wme_bill Last edited by WME_bill; 29th Mar 2017 at 10:09 am.

29th Mar 2017, 4:41 pm	#8
cinema1 Tetrode Join Date: Jun 2008 Location: Preston, Lancashire, UK. Posts: 50	Re: Telequipment S51A oscilloscope WME BiLL, I took your advice to use my meter and to check the valves voltages, valve 7 was the culprit. The two anodes 7b, 7a have 39k resisters 7b was 180v 7a 47v after changeing the two 39k resisters 7a--7b 280v and the cathodes 32v the cramping of the trace as gone everything is ok kind regards . Eric cinema1