Telequipment S51A working correctly but dim - UK Vintage Radio Repair and Restoration Discussion Forum

samt_32 · 2nd Sep 2013, 8:19 pm

Hi.

I have a Telequipment S51A oscilloscope that works but when I was using it the brightness reduced by about half or more. I was adjusting the trig level and that is when the brightness reduced. The scope did not make any funny sounds or smoke when this happened. It still functions but now is not very bright. I have the service manual for it and was wondering where to start to try and fix this problem. I do not have a high voltage meter or probe and I am wondering how I can safely discharge the scope and test it for problems.

glowinganode · 2nd Sep 2013, 10:45 pm

Hi, welcome to the forum.
I seem to recall the S51 uses a voltage doubler for the EHT supply.
Chances are one of the rectifiers has gone leaky, reducing the EHT voltage.
Did an increase of width accompany the loss of brightness?
Rob.

samt_32 · 3rd Sep 2013, 12:56 am

I am not sure. By width do you mean that from the left to right on the screen the beam is longer then it was before? I have not tried turning it on in over a year in hope that the high voltage will reduce.

Nickthedentist · 3rd Sep 2013, 4:40 pm

Quote:

Originally Posted by samt_32

I have not tried turning it on in over a year in hope that the high voltage will reduce.

I would have thought any residual charge would be gone in a matter of minutes or hours at most

dragonser · 3rd Sep 2013, 4:50 pm

Hi,
as far as I know it is television tubes ( crt's ) that can hold their charge for a long time. if you are worried I would make up a probe with a medium value resistor so you can discharge to the chassis any of the caps you may be worried about.
I have a Telequipment S51 to look at myself. the HT fuse seems to have blown. I'm going to fit a fuse holder, new fuse and then check things carefully.
I'm considering changing the smoothing caps, as I intend to keep this scope.
Does anyone know if the ones available from audio cap.net might fit ?
regards Peter B

AC/HL · 3rd Sep 2013, 6:01 pm

Look here for recommended suppliers

https://www.vintage-radio.net/forum/...ad.php?t=55701

samt_32 · 3rd Sep 2013, 7:02 pm

Quote:

Originally Posted by dragonser

Hi,
as far as I know it is television tubes ( crt's ) that can hold their charge for a long time. if you are worried I would make up a probe with a medium value resistor so you can discharge to the chassis any of the caps you may be worried about.

By medium value resistor do you mean around 1M? Also what wattage should the resistor be to avoid the high voltage jumping the resistor? I have some 1/2 Watt light blue colored resistors that are 2M and up.

MotorBikeLes · 3rd Sep 2013, 8:38 pm

@Dragonser, I may have some NOS caps (some mint, some "pulls" from incomplete chassis) if you let me know which ones. Photo or description plus values needed. You can PM me if you wish.
Les.

dragonser · 4th Sep 2013, 8:57 am

Hi, after thinking about it I would suggest a value of about 47k to discharge the caps ( of course if the equipment has been left a while there may not be any charge left ) But I am all for being very careful where high voltages are concerned ! a high wattage resistor ( 1 watt or above ) with a high voltage rating should be ok.
regards Peter B

samt_32 · 4th Sep 2013, 7:50 pm

How would I know if a resistor is high voltage or not?

samt_32 · 9th Sep 2013, 12:04 am

Ok. I built this:

The total discharge time is approximately:

LV (TV and monitor power supplies, SMPSs, electronic flash units) - up to 1000 uF, 400 V. Discharge time of 1 second per 100 uF of capacitance (5RC with R = 2 K ohms).

HV (microwave oven HV capacitors) - up to 5,000 V, 2 uF. Discharge time of 0.5 second per 1 uF of capacitance (5RC with R = 100 K ohms)

EHV (CRT second anodes) - up to 50,000 V, 2 nF. Discharge time of 0.01 second per 1 nF of capacitance (5RC with R = 1 M ohm). Note: discharge time is so short that flash of LED may not be noticed.
Adjust the component values for your particular application.

(Probe)
<-------+
In 1 |
/
R1 \ 2 K 25 W (LV) Unmarked diodes are 1N400X (where X is 1-7)
/ 100 K 25 W (HV) or other general purpose silicon rectifiers.
\ 1 M 10 W (EHV)
| Resistors must be rated for maximum expected
+---------+--------+ voltage. For non-continuous use, the wattage
__|__ __|__ | can be lower.
_\_/_ D1 _/_\_ D5 /
| | \ R2
__|__ __|__ / 100 ohms
_\_/_ D2 _/_\_ D6 |
| | +-----------+
__|__ __|__ __|__ __|__ Any general purpose LED type
_\_/_ D3 _/_\_ D7 _\_/_ LED1 _/_\_ LED2 without an internal resistor.
| | | + | -
__|__ __|__ +-----------+ Use different colors to
_\_/_ D4 _/_\_ D8 | indicate polarity if desired.
In 2 | | |
>-------+---------+--------+
(GND Clip)

I used 15 1/2 Watt resistors in series (10 10K and 5 1K) and got to a resistance of 103K. Would this work?

AC/HL · 9th Sep 2013, 12:46 am

That doesn't seem to make sense, did it look OK before you sent it?
If it's a file, can you send it as an attachment, or provide a link to it.

samt_32 · 9th Sep 2013, 1:40 am

Sorry about that, I used the quick reply. Here is the link to what I am talking about: http://www.repairfaq.org/sam/captest.htm. About halfway down the page.

Tim · 9th Sep 2013, 6:09 pm

I replaced the broken(literally) HT stick rectifier in one of these with about 5 BY133's in series with 1 meg resistors in between. It worked, but as I no longer own it I cannot vouch how reliable my repoge was.

Repoge Def. Somewhere between a repair and a bodge, but reasonably competent in terms of engineering implementation or physical neatness. (Coloq Pullin household 2013) Not to be confused with a Brepair, which although it acheives the same result , is less skilled in implementation.

samt_32 · 9th Sep 2013, 10:15 pm

I have managed to discharge all of the caps in the scope and remove the board that has the high voltage rectifiers on it. How would I test them to see if they are broken? I tried my multimeter on diode mode and it did not get a reading however it only can test up to 1 volt. Arrg! A red thin wire has come disconnected from I assume the transformer. At least I think that it is from the transformer because I had to unscrew it to get the board out. The wire runs runs to the front of the scope and I cant see where it is connected to. I also tested this wire with my meter connected to ground to see if I could figure out what it is connected to but I do not get a reading. Does any one know where this wire is suppose to be connected?

samt_32 · 10th Sep 2013, 12:44 am

Correction, I found out where the red wire goes! It goes to the front of the scope about 1.5 inches from the front and stops. There are actually 2 red wires that do not connect to any thing?

Are they there for a similar model of scope that would have used them? I do not know. Also I measured the 3 resistors on the HV board and I think that 2 out of the 3 are out of spec. The white resistor measures 1.8M (R20) the one behind it measures 689K (R21) and the one behind it measures 326K (R13). I am not sure what R13 is suppose to be because the schematic says 270K and the parts list says 330K. What is correct? Also the resistor that connects to the focus control (R27) measures 1.1M instead of the listed 1.8M is this a problem to?

2nd Sep 2013, 8:19 pm	#1
samt_32 Retired Dormant Member Join Date: Sep 2013 Location: Calgary Alberta Canada Posts: 13	Telequipment S51A working correctly but dim Hi. I have a Telequipment S51A oscilloscope that works but when I was using it the brightness reduced by about half or more. I was adjusting the trig level and that is when the brightness reduced. The scope did not make any funny sounds or smoke when this happened. It still functions but now is not very bright. I have the service manual for it and was wondering where to start to try and fix this problem. I do not have a high voltage meter or probe and I am wondering how I can safely discharge the scope and test it for problems.

2nd Sep 2013, 10:45 pm	#2
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: Telequipment S51A working correctly but dim Hi, welcome to the forum. I seem to recall the S51 uses a voltage doubler for the EHT supply. Chances are one of the rectifiers has gone leaky, reducing the EHT voltage. Did an increase of width accompany the loss of brightness? Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

3rd Sep 2013, 12:56 am	#3
samt_32 Retired Dormant Member Join Date: Sep 2013 Location: Calgary Alberta Canada Posts: 13	Re: Telequipment S51A working correctly but dim I am not sure. By width do you mean that from the left to right on the screen the beam is longer then it was before? I have not tried turning it on in over a year in hope that the high voltage will reduce.

3rd Sep 2013, 4:50 pm	#5
dragonser Heptode Join Date: Aug 2009 Location: Carshalton, Surrey, UK. Posts: 735	Re: Telequipment S51A working correctly but dim Hi, as far as I know it is television tubes ( crt's ) that can hold their charge for a long time. if you are worried I would make up a probe with a medium value resistor so you can discharge to the chassis any of the caps you may be worried about. I have a Telequipment S51 to look at myself. the HT fuse seems to have blown. I'm going to fit a fuse holder, new fuse and then check things carefully. I'm considering changing the smoothing caps, as I intend to keep this scope. Does anyone know if the ones available from audio cap.net might fit ? regards Peter B

3rd Sep 2013, 6:01 pm	#6
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Telequipment S51A working correctly but dim Look here for recommended suppliers https://www.vintage-radio.net/forum/...ad.php?t=55701

3rd Sep 2013, 8:38 pm	#8
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Telequipment S51A working correctly but dim @Dragonser, I may have some NOS caps (some mint, some "pulls" from incomplete chassis) if you let me know which ones. Photo or description plus values needed. You can PM me if you wish. Les.

4th Sep 2013, 8:57 am	#9
dragonser Heptode Join Date: Aug 2009 Location: Carshalton, Surrey, UK. Posts: 735	Re: Telequipment S51A working correctly but dim Hi, after thinking about it I would suggest a value of about 47k to discharge the caps ( of course if the equipment has been left a while there may not be any charge left ) But I am all for being very careful where high voltages are concerned ! a high wattage resistor ( 1 watt or above ) with a high voltage rating should be ok. regards Peter B

4th Sep 2013, 7:50 pm	#10
samt_32 Retired Dormant Member Join Date: Sep 2013 Location: Calgary Alberta Canada Posts: 13	Re: Telequipment S51A working correctly but dim How would I know if a resistor is high voltage or not?

9th Sep 2013, 12:04 am	#11
samt_32 Retired Dormant Member Join Date: Sep 2013 Location: Calgary Alberta Canada Posts: 13	Re: Telequipment S51A working correctly but dim Ok. I built this: The total discharge time is approximately: LV (TV and monitor power supplies, SMPSs, electronic flash units) - up to 1000 uF, 400 V. Discharge time of 1 second per 100 uF of capacitance (5RC with R = 2 K ohms). HV (microwave oven HV capacitors) - up to 5,000 V, 2 uF. Discharge time of 0.5 second per 1 uF of capacitance (5RC with R = 100 K ohms) EHV (CRT second anodes) - up to 50,000 V, 2 nF. Discharge time of 0.01 second per 1 nF of capacitance (5RC with R = 1 M ohm). Note: discharge time is so short that flash of LED may not be noticed. Adjust the component values for your particular application. (Probe) <-------+ In 1 \| / R1 \ 2 K 25 W (LV) Unmarked diodes are 1N400X (where X is 1-7) / 100 K 25 W (HV) or other general purpose silicon rectifiers. \ 1 M 10 W (EHV) \| Resistors must be rated for maximum expected +---------+--------+ voltage. For non-continuous use, the wattage __\|__ __\|__ \| can be lower. _\_/_ D1 _/_\_ D5 / \| \| \ R2 __\|__ __\|__ / 100 ohms _\_/_ D2 _/_\_ D6 \| \| \| +-----------+ __\|__ __\|__ __\|__ __\|__ Any general purpose LED type _\_/_ D3 _/_\_ D7 _\_/_ LED1 _/_\_ LED2 without an internal resistor. \| \| \| + \| - __\|__ __\|__ +-----------+ Use different colors to _\_/_ D4 _/_\_ D8 \| indicate polarity if desired. In 2 \| \| \| >-------+---------+--------+ (GND Clip) I used 15 1/2 Watt resistors in series (10 10K and 5 1K) and got to a resistance of 103K. Would this work?

9th Sep 2013, 12:46 am	#12
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Telequipment S51A working correctly but dim That doesn't seem to make sense, did it look OK before you sent it? If it's a file, can you send it as an attachment, or provide a link to it.

9th Sep 2013, 1:40 am	#13
samt_32 Retired Dormant Member Join Date: Sep 2013 Location: Calgary Alberta Canada Posts: 13	Re: Telequipment S51A working correctly but dim Sorry about that, I used the quick reply. Here is the link to what I am talking about: http://www.repairfaq.org/sam/captest.htm. About halfway down the page.

9th Sep 2013, 6:09 pm	#14
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Telequipment S51A working correctly but dim I replaced the broken(literally) HT stick rectifier in one of these with about 5 BY133's in series with 1 meg resistors in between. It worked, but as I no longer own it I cannot vouch how reliable my repoge was. Repoge Def. Somewhere between a repair and a bodge, but reasonably competent in terms of engineering implementation or physical neatness. (Coloq Pullin household 2013) Not to be confused with a Brepair, which although it acheives the same result , is less skilled in implementation. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly." Last edited by Tim; 9th Sep 2013 at 6:15 pm.

9th Sep 2013, 10:15 pm	#15
samt_32 Retired Dormant Member Join Date: Sep 2013 Location: Calgary Alberta Canada Posts: 13	Re: Telequipment S51A working correctly but dim I have managed to discharge all of the caps in the scope and remove the board that has the high voltage rectifiers on it. How would I test them to see if they are broken? I tried my multimeter on diode mode and it did not get a reading however it only can test up to 1 volt. Arrg! A red thin wire has come disconnected from I assume the transformer. At least I think that it is from the transformer because I had to unscrew it to get the board out. The wire runs runs to the front of the scope and I cant see where it is connected to. I also tested this wire with my meter connected to ground to see if I could figure out what it is connected to but I do not get a reading. Does any one know where this wire is suppose to be connected? Last edited by samt_32; 9th Sep 2013 at 10:44 pm.

10th Sep 2013, 12:44 am	#16
samt_32 Retired Dormant Member Join Date: Sep 2013 Location: Calgary Alberta Canada Posts: 13	Re: Telequipment S51A working correctly but dim Correction, I found out where the red wire goes! It goes to the front of the scope about 1.5 inches from the front and stops. There are actually 2 red wires that do not connect to any thing? Are they there for a similar model of scope that would have used them? I do not know. Also I measured the 3 resistors on the HV board and I think that 2 out of the 3 are out of spec. The white resistor measures 1.8M (R20) the one behind it measures 689K (R21) and the one behind it measures 326K (R13). I am not sure what R13 is suppose to be because the schematic says 270K and the parts list says 330K. What is correct? Also the resistor that connects to the focus control (R27) measures 1.1M instead of the listed 1.8M is this a problem to?