Dim oscilloscope screen

[David Hess]

Here V(G1) is higher than V(K). I believe it's because the labels on the PCB is misleading. They probably use K(G1) to indicate the pin could be either K or G1 in different models. And in my case, I should read the labels inside the parentheis...

The schematic shows the same thing.  I am still puzzling out which voltages were on which CRT elements.

[yugu]

Thanks guys. As a beginner, it's much easier for me to isolate the tube out and measure it directly. My theory is that if the voltage bias of the tube is correct, then it should be working properly. Otherwise, there's something wrong with the tube itself. I'm not convinced that I need to measure the low voltage circuit to infer the problem indirectly.

If I understand it correctly, the brightness can be controlled by adjusting either V(H1, H2) or V(K, G1). Since the tube is working fine except the brightness, the voltage bias that could go wrong would be these two.

However, looking at my measurement, I didn't see anything wrong with these two voltages.

+ When I turn the intensity knob from min to max, V(K, G1) dropped from 68v to 42v, and the maximum brightness happens at about 60v. So the the dimness could not be because of this voltage is too high or too low.

+ The voltage of the filament is 6.7v. though a bit higher that the 6.3v in the spec, it looks good enough.

The other voltages also look good, V(K, P1) ~= 550v, V(K, G2) ~= 2.1kv.

I also measured the voltage change of V(K, G1), V(K, P1) and V(K, G2) when I turned the intensity knob. Only V(K, G1) changed and the other two were stable.

My conclusion is, the tube is bad. Let me know if I made any stupid error in the above reasoning.

I am going to do more experiments on the tube, e.g. increase the filament voltage, zap it, etc.

[yugu]

An update: I zapped the tube with high voltage for a few minutes and voila, the screen is much brighter now! 

[tautech]

An update: I zapped the tube with high voltage for a few minutes and voila, the screen is much brighter now! 

Well done.
Did you follow the steps in KE5FX's CRT restore page?
Care to accurately describe your procedure?

[yugu]

Sure. The procedure is very simple (I got it from a friend who fixed lots of televisions):
+ Find a high voltage igniter
+ Connect K and G1 to the igniter
+ Zap it

In my case, I just connect the tub to my gas stove igniter, leave it there with the igniter turned on for two minutes. After that the brightness improved magically. I zapped it for another 10 minutes to see if it would improve more, but found no noticeable improvement.

In summary, it's a simple trick, takes little effort to try, and is quite safe (the energy released from the gas stove igniter is tiny). Anyone who has a tube with similar issue may consider give it a shot. Do it at your own risk, though 

[David Hess]

So the CRT is suffering from interface resistance and/or cathode depletion.  The former is caused by operating it with the intensity blanked (they made special vacuum tubes designed to operate in cutoff to prevent interface resistance) and the later by long operating hours.  I think cathode depletion causes double peaking but interface resistance should just make the trace dim and maybe cause unblanking problems.

I find it difficult to believe that someone used an analog oscilloscope this new for long enough to cause these problems.

[VK5RC]

@yugu Thanks for sharing, hope it goes well! I have some old gear that the tubes are getting a bit soft so it is always nice to have "plan B" confirmed.

[adamf]

Hi, and sorry for reopening this old thread; I thought someone could help me, as I have the exact same problem as the OP.

I have a Hitachi V-552 oscilloscope, and the trace is very dim -almost completely gone. The intensity knob behaves in the exact way as described by the OP: you can't see the trace until you reach 70%, then the intensity goes up until 90%, and then begins do dim again up to 100%.

I read the whole thread, and even though I'm not as good with electronic as the OP, I don't think the problem is with the CRT; at least in my case. The reason is this that at the beginning the problem when on and off; as if a component was failing or loose, rather than a CRT getting old for overuse. I also bought the oscilloscope on Ebay for a similar price, and as I said, the trace was alright when it arrived. After a couple of weeks, I began experiencing the problem, but not continuously; it would be dim some times, specially when turning it on, and then after an hour or so, it would suddenly fix itself; sometimes failing again after a while, and so forth. It behaved like that for a few months, and then it stopped; it stayed dim all the time.

I have the schematics, and checked all the components from the 'Intensity' knob and the 'CRT Bias' pot up to the HV circuit; all the voltages and components seem to be fine. I checked the whole area because the PCB looks quite dark in that part of the circuit, as burnt from excessive heat. I also checked the temperatures, and the highest comes from a transistor at 70 degrees centigrade; but the voltages in and out of it seem to be fine. Also took it out and the beta seems also seems to be within specs. I can't check the HV, as I don't have a probe; so now I'm stuck, I don't know where else to look.

Anyway, I would appreciate any help or suggestions. Thanks...

[tautech]

Welcome to the forum.

Sounds like you've checked all the usual causes. 
Have you inspected VERY carefully for any cracked or poor solder joints?
As you say there is often quite some heat in the CRT HV, particullary around the HV divider, this heat might have caused joint cracking or even cracked components with thermal cycling. Be sure to give some a gentle nudge to check they are still physically intact.

Can you provide a link to the schematic or Service manual?

[adamf]

Welcome to the forum.

Thanks for the prompt answer.

Be sure to give some a gentle nudge to check they are still physically intact.

I tried this many times, but never worked. For a short time before it completely failed, pushing some buttons of the front panel seemed to have some effect; but didn't last more than a couple of days.

Have you inspected VERY carefully for any cracked or poor solder joints?
As you say there is often quite some heat in the CRT HV, particullary around the HV divider, this heat might have caused joint cracking or even cracked components with thermal cycling.

I checked that part of the circuit, and even found one of the pin connections with cracked joints. But after soldering it, it continued failing; so I guess it had nothing to do with it. I might have missed it, so I'll see if I can take the PCB off again and redo it.

Can you provide a link to the schematic or Service manual?

Here is a link for a PDF copy of the service manual: http://skory.gylcomp.hu/alkatresz/hitachi_v-522-v-523-v-525_service_m.pdf
the HV circuit is on page 60 of the PDF (page 57 of the manual). Only the pages for the V-522 apply, as the manual combines various models.

[tautech]

In cases like these I replace many of the components that are under HV stress. Some are apparent, the hot areas that you have already described. Other components like the HV DC restore diode D1013, find datasheets for the listed Vf voltage and check these with a PSU and a DMM. HV caps do get tired too, in fact most of the CRT circuit is under some voltage and heat stress and just a few dollars will replace most components.
I'd check that all ather voltages the HV is referenced to are there in their correct values -8V, +8V, 75V etc.
I did have a zener go like D1020 in one CRO I had, so you need to carefully check all the active devices too.
You could try to add some hot air of freeze to see if you can localize where the problem lies.
Normally I would look hard at all high value and high wattage resistors R1014, R1017, R1024.

Patience and perserverence will pay off.

[Gyro]

You could try to add some hot air of freeze to see if you can localize where the problem lies.

Take it very gently with the freezer spray though - you don't want to get to the point of creating condensation in the HT section! 

[adamf]

Thanks again for the help.

I tested the -8V +8V and 75V, and they seem to be fine.

I also took out the PCB again and re-soldered everything in the HV circuit, the zone that looks overheated, connectors, around the 'CRT BIAS' pot, power supply, and everything else  that looked suspicious. I think the trace is a tiny bit brighter, but I'm not completely sure; it's hard to tell from memory without measuring it. In any case, nothing compared with the brightness it had when the scope arrived.

Next time I'll check the individual components you mentioned; probably need to take them out for testing; as within the circuit everything seems to be fine. Already took out for testing the 3 transistors in the burnt area (the H-AMP circuit); which includes the one which measures 70º C. All betas are well within specs.

Never tried to freeze or heat a circuit before; so I prefer to leave that option for last.

[adamf]

In cases like these I replace many of the components that are under HV stress. Some are apparent, the hot areas that you have already described. Other components like the HV DC restore diode D1013, find datasheets for the listed Vf voltage and check these with a PSU and a DMM. HV caps do get tired too, in fact most of the CRT circuit is under some voltage and heat stress and just a few dollars will replace most components.
I'd check that all ather voltages the HV is referenced to are there in their correct values -8V, +8V, 75V etc.
I did have a zener go like D1020 in one CRO I had, so you need to carefully check all the active devices too.
You could try to add some hot air of freeze to see if you can localize where the problem lies.
Normally I would look hard at all high value and high wattage resistors R1014, R1017, R1024.

Patience and perserverence will pay off.

Hi again,

Today I took out the components you suggested for testing, and all of them seem to be fine. The only component I have doubts is the D1013 rectifying diode. I couldn't test it with any of the testing tools I have, but I did as you said and used a PSU. It seems to rectify well from -25V to +25V; which is the maximum voltage of my PSU. It's the first time I test a HV diode, so I'm not too sure I did all the test necessary. In any case, I'm attaching to this post the measurements I took, and these are the only specs I could find:

Y10GA Silicon Rectifier (D1013)
V(RRM)(V) Rep.Pk.Rev. Voltage=6.0k
I(RM) Max.(A) Reverse Current=4.0u
@V(R) (V)(Test Condition)=6.0k
I(FSM) Max.(A) Pk.Fwd.Sur.Cur.=500m
V(FM) Max.(V) Forward Voltage=22
@I(FM) (A) (Test Condition)=5.0m
I(O) Max.(A) Output Current=5.0

On the other hand, I found something that's not right; though I don't think is the reason for the fault. On schematic #6 (page 55) states that R860 should be 1.2K; however, the resistor on the circuit is marked, and measures, 3.3K; also looks like an original component. I noticed it because the resistor next to it (R866) was a bit burnt, but after taking it out it seems to be fine (12.1K).

So, I'm stuck again... 

[tautech]

It seems to rectify well from -25V to +25V; which is the maximum voltage of my PSU.

That's wrong.
V(FM) Max.(V) Forward Voltage=22

This means the maximum voltage it will start conducting is 22V
However that is max Vf and it will likely be on at ~15V.

Normally this is well above the voltage a DMM diode test supplies but if it's conducting at low voltages and in either polarity it's most probably shorted.

[adamf]

It seems to rectify well from -25V to +25V; which is the maximum voltage of my PSU.

That's wrong.
V(FM) Max.(V) Forward Voltage=22

This means the maximum voltage it will start conducting is 22V
However that is max Vf and it will likely be on at ~15V.

Normally this is well above the voltage a DMM diode test supplies but if it's conducting at low voltages and in either polarity it's most probably shorted.

I got no current when I applied negative voltage. It only conducts when forward voltage is applied, though I could only test it up to -25V, which is a bit short considering the V(RRM) of 6KV.

I'll try to get a replacement on my local shop on Monday. Thanks for the help.

[tautech]

It seems to rectify well from -25V to +25V; which is the maximum voltage of my PSU.

That's wrong.
V(FM) Max.(V) Forward Voltage=22

This means the maximum voltage it will start conducting is 22V
However that is max Vf and it will likely be on at ~15V.

Normally this is well above the voltage a DMM diode test supplies but if it's conducting at low voltages and in either polarity it's most probably shorted.

I got no current when I applied negative voltage. It only conducts when forward voltage is applied, though I could only test it up to -25V. It should rectify up to 6KV, so my measurements are a bit short.

I'll try to get a replacement on my local shop on Monday. Thanks for the help.

Place a small load, say 1M ohm across the output of the diode, connect your DMM across the resistor and measure the voltage when it starts to conduct. It should be less than 22V and probably more than 10V.
These HV diodes can only handle a few mA so be careful with any tests. You only need to know that they conduct in one direction and what that voltage is, in this case less than 22V.

[adamf]

Place a small load, say 1M ohm across the output of the diode, connect your DMM across the resistor and measure the voltage when it starts to conduct. It should be less than 22V and probably more than 10V.
These HV diodes can only handle a few mA so be careful with any tests. You only need to know that they conduct in one direction and what that voltage is, in this case less than 22V.

It starts conducting at +5V. I didn't allow it to pass from 5mA, but anyway I put it back and everything is still the same as before.

The problem that I'm having now is finding a replacement for that particular diode. I asked in my local shop and searched on the internet for the Y10GA and can't find that component or an equivalent. Could you recommend me a replacement that would work?

[adamf]

I think I found a replacement that could work in a website called HVStuff. It doesn't have a reference, just says: 5mA 6kV 100nS High Voltage Diode HV Rectifier High Frequency.

The only difference I can see is the 20V Average Forward Voltage Drop insted of the 22V of the Y10GA. This are the specs on the website: http://hvstuff.com/5ma-6kv-100ns-high-voltage-diode-hv-rectifier-high-frequency

-Repetitive Peak Reverse Voltage    6kV
-Average Forward Current               5mA
-Maximum Surge Current                 0.5A
-Avg. Forward Voltage Drop             20V
-Peak Reverse Current                     2uA@25°C
                                                         5uA@100°C
-Reverse Recovery Time                   100nS
-Dimensions                                     3mm×8mm(0.12inch×0.32inches)
-Terminals                                         Axial Lead

Could this one do?

[tautech]

I think I found a replacement that could work in a website called HVStuff. It doesn't have a reference, just says: 5mA 6kV 100nS High Voltage Diode HV Rectifier High Frequency.

The only difference I can see is the 20V Average Forward Voltage Drop insted of the 22V of the Y10GA. This are the specs on the website: http://hvstuff.com/5ma-6kv-100ns-high-voltage-diode-hv-rectifier-high-frequency

-Repetitive Peak Reverse Voltage    6kV
-Average Forward Current               5mA
-Maximum Surge Current                 0.5A
-Avg. Forward Voltage Drop             20V
-Peak Reverse Current                     2uA@25°C
                                                         5uA@100°C
-Reverse Recovery Time                   100nS
-Dimensions                                     3mm×8mm(0.12inch×0.32inches)
-Terminals                                         Axial Lead

Could this one do?