EICO 950B Resistance - Capacitance - Comparator bridge DC Voltage problem

Thread Starter

ticassse

Joined Dec 11, 2017
11
Hi all,

I would really appreciate if someone can shed me some light on this. I have acquired an old EICO 950B (schematic attached) recently, it had the original caps in it so I re capped it and I also changed a few resistors (R3, R8, R10).

The problem I have is the voltage across C9 is way too high, it's supposed to be 200V but the first time I did turn up it C9 just blowed. So I replaced it again and then I turn it up with a variac to check the voltage across it.

Here's the result when the variac is +/- 30Vac I got +/- 50Vdc across C9 (wich look fine) but when the variac is 40Vac the voltage across C9 goes up to 130Vdc and with the variac at 50Vac the voltage across C9 goes to 180Vdc.

Did someone have an idea why this voltage on C9 is going that high?

Thanks
Al.
 

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KeithWalker

Joined Jul 10, 2017
3,096
Hi all,

I would really appreciate if someone can shed me some light on this. I have acquired an old EICO 950B (schematic attached) recently, it had the original caps in it so I re capped it and I also changed a few resistors (R3, R8, R10).

The problem I have is the voltage across C9 is way too high, it's supposed to be 200V but the first time I did turn up it C9 just blowed. So I replaced it again and then I turn it up with a variac to check the voltage across it.

Here's the result when the variac is +/- 30Vac I got +/- 50Vdc across C9 (wich look fine) but when the variac is 40Vac the voltage across C9 goes up to 130Vdc and with the variac at 50Vac the voltage across C9 goes to 180Vdc.

Did someone have an idea why this voltage on C9 is going that high?

Thanks
Al.
The shunt regulator tube V2 is not doing it's job. Replace it and check the values of R3 and R10.
 

AlbertHall

Joined Jun 4, 2014
12,346
Set P3 to minimum and set the variac to some value that keeps the 200V at a safe level. What voltages do you get on the 200V and -520V rails?
 

LesJones

Joined Jan 8, 2017
4,190
Make sure that nothing is pulling the negative end of C1 to near ground potential. (If C1 was inserted the wrong way round that could be the cause. As you have a variac on the input turn it up until you have about +100 volts on C9 +. Then measure the voltafe on C1 - I would expect about -260 volts at that point. It could also be R8 is too high in value or R7 or P3 are too low in value.

Les.
 

Thread Starter

ticassse

Joined Dec 11, 2017
11
Set P3 to minimum and set the variac to some value that keeps the 200V at a safe level. What voltages do you get on the 200V and -520V rails?
Thanks for the reply, note that I'm kind of new to electronics (only a few months) so I'm not sure if this what you were asking for.
So I did put the variac to +/- 60Vac which gave me near 200Vdc on C9, 150Vdc on C1 and 290Vac on the third tap (the 560Vac)
Al.
 

Thread Starter

ticassse

Joined Dec 11, 2017
11
Make sure that nothing is pulling the negative end of C1 to near ground potential. (If C1 was inserted the wrong way round that could be the cause. As you have a variac on the input turn it up until you have about +100 volts on C9 +. Then measure the voltafe on C1 - I would expect about -260 volts at that point. It could also be R8 is too high in value or R7 or P3 are too low in value.

Les.
I checked and C1 is connected correctly, kind of strange if you look at my previous post I had C9 at around 200V but C1 was 150V
 

AlbertHall

Joined Jun 4, 2014
12,346
Thanks for the reply, note that I'm kind of new to electronics (only a few months) so I'm not sure if this what you were asking for.
So I did put the variac to +/- 60Vac which gave me near 200Vdc on C9, 150Vdc on C1 and 290Vac on the third tap (the 560Vac)
Al.
Was this with P3 set to minimum?

With the power off, and give it a while for the caps to discharge, then measure the resistance across C1 and C9 (still with P3 at minimum).
 

Thread Starter

ticassse

Joined Dec 11, 2017
11
Was this with P3 set to minimum?

With the power off, and give it a while for the caps to discharge, then measure the resistance across C1 and C9 (still with P3 at minimum).
Yes it was with P3 at min. The resistance across C1 start at 1.7M then drop to 97K and the resistance across C9 is 67.5K
 

LesJones

Joined Jan 8, 2017
4,190
I suspect that C1's leakage current increases rapidly above a certain voltage. Do you still have the original C1 ? If so try re fitting it. (I think this is Albert's line of reasoning.)
Edit. I have only just noticed that Albert asked for basically the same test in post #4 as I asked for in post #5

Les.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
12,346
Yes. I was wondering if leaving it switched on at safe voltages might reform C1. Check if C1 voltage rises and C9 voltage falls with time.
Refitting the original might be worth a try as well.
 

Thread Starter

ticassse

Joined Dec 11, 2017
11
I suspect that C1's leakage current increases rapidly above a certain voltage. Do you still have the original C1 ? If so try re fitting it. (I think this is Albert's line of reasoning.)
Edit. I have only just noticed that Albert asked for basically the same test in post #4 as I asked for in post #5

Les.
Unfortunately I don't have the original one anymore and it was the only new one I had
 

Thread Starter

ticassse

Joined Dec 11, 2017
11
Yes. I was wondering if leaving it switched on at safe voltages might reform C1. Check if C1 voltage rises and C9 voltage falls with time.
Refitting the original might be worth a try as well.
Ha, good Idea I'll try that tomorrow for now it`s time for a beer ;-)

Thanks
Al.

It would be funny if reforming a capacitor would solve a problem on a tool that can be use to reform capacitors -;)
 

LesJones

Joined Jan 8, 2017
4,190
One other thing you could try is to lift one end of R7. With this load removed from the negative rail I would expect to see all the voltage from the rectified AC to appear across C1. If this does not happen then C1 must be leaky.

Les.
 

Ylli

Joined Nov 13, 2015
1,088
May be back to a defective indicator tube (1629). The data sheet says the typical target current is 3 mA @ 200 volts. As the circuit sits, it looks to me like the magic eye target current needs to be 2.05 mA to give 200 volts across C8. If it is not drawing enough current, then the voltage across C8 will go up.

Either that or C1 is leaky.
 

Thread Starter

ticassse

Joined Dec 11, 2017
11
Ha, good Idea I'll try that tomorrow for now it`s time for a beer ;-)

Thanks
Al.

It would be funny if reforming a capacitor would solve a problem on a tool that can be use to reform capacitors -;)
So It looks like it's was that, I'm currently running it on the variac and slowly increasing the voltage. The voltage across C9 decreases slowly and the C1 voltage is increasing.
.
The variac is now at 85Vac, yesterday at this level C9 would have busted. For the next few days, I will run it from the variac to see if C1 reacts normally.

Thanks Albert for that suggestion and thanks to all for the help.
Al.
 
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