How to troubleshoot a faulty 7B50 timebase


Thierry Delaitre
 

Hello All

I have a 7704A with 2 x timebase: 7B53 and 7B50

The 7B50 has developed a fault suddenly and the screen is now all lighting as green and cannot see the trace anymore when using this timebase

The led for on the 7B50 still appears to sync with the input signal but the screen of the scope is all lighting green when using that timebase

Swapping the timebases also keeps the fault with the 7B50 which shows this is not a problem with the 7704A chassis itself

I am wondering how to start troubleshooting the 7B50 and whether that fault is a known issue?

Many thanks

Thierry


Harvey White
 

Does the beam finder bring anything on screen?

It sounds to me as if the sweep is deflected off screen.  One suggestion would be to follow the ramp through the plugin. Another is to check the horizontal positioning control to see if it provides a voltage.  If the 7B50 has an XY mode, do you get the same problem there?

Harvey

On 3/6/2021 4:44 AM, Thierry Delaitre via groups.io wrote:
Hello All

I have a 7704A with 2 x timebase: 7B53 and 7B50

The 7B50 has developed a fault suddenly and the screen is now all lighting as green and cannot see the trace anymore when using this timebase

The led for on the 7B50 still appears to sync with the input signal but the screen of the scope is all lighting green when using that timebase

Swapping the timebases also keeps the fault with the 7B50 which shows this is not a problem with the 7704A chassis itself

I am wondering how to start troubleshooting the 7B50 and whether that fault is a known issue?

Many thanks

Thierry





Thierry Delaitre
 

When using beam finder, I can see a single vertical bar of 3cm height on the two thirds of the right hand side of the screen

The 7B50 does not have XY mode.

I tried to put the timebase into an input slot and I can also see horizontal bar when using beam finder

The 7B50 appears to work again when the plugin was cold. The beam then disappears on the 7B50 timebase after 15/20 seconds or so after the scope is switched-on

Thanks

Thierry


 

Could you upload pictures of the malfunction ("all lighting green") and the effect of BEAM FIND ("vertical or horizontal bar")?


Harvey White
 

Replies Interleaved....

On 3/6/2021 5:21 PM, Thierry Delaitre via groups.io wrote:
When using beam finder, I can see a single vertical bar of 3cm height on the two thirds of the right hand side of the screen
Which points back at the plugin, saying that the trace is deflected all the way to the right.  Normal location for an inactive trace is the left side of the screen.

The 7B50 does not have XY mode.
It would have been nice to feed in a signal and bypass the sweep generator.

I tried to put the timebase into an input slot and I can also see horizontal bar when using beam finder
Not sure that you should be seeing a horizontal bar, but that's more or less moot for now.

The 7B50 appears to work again when the plugin was cold. The beam then disappears on the 7B50 timebase after 15/20 seconds or so after the scope is switched-on
Ok, what I suspect is that one of two things can be going on.

1) the sweep is still active, but the driver transistor that couples to the horizontal amplifier is open/leaky, or you have a capacitor that is making that transistor behave like that, thermally related.  Upsets the bias on that output stage and I think it will be turning off.

suggestions:

    if the sweep pickoff is in the plugin, then look at that.  If not, then use a separate scope to see what is hung up.  A can of freeze spray may reverse the symptoms long enough for you to figure out what part is bad.

2) the sweep itself is not working, however, the triggered light indicates (to me) that the sweep is working, since I think it's frequently driven by the sweep gate itself.

suggestion:  remove side panel from scope, remove cover from plugin, insert into far right slot, and use another scope to see what's going on.


Now, given that you do have a 7000 series scope (you'll like this part), instead of a separate scope:

put your working sweep plugin in slot #3, put the scope on chopped horizontal mode, and use the working vertical channel and sweep to debug the faulty one.

Harvey




Thanks

Thierry





Thierry Delaitre
 

thank you.

I can find a sweep at the base of the Q784 (output driver) and nothing after.

I have correct voltages for the line voltages -15V and 50V

Other voltages are as follows (with a time dev of 1ms):
- Collector of Q774: 5.6V instead of 6.41V
- Collector of Q764: 6.6V instead of 6.42V
- Emitter of Q794: 11.5 instead of 7.22V
- Emitter of Q784: 6V instead of 7.33V
- 13.1V across C799

I've also tried to disconnect C799 just in case if it was leaky but it is still the same

I've also checked the resistors (in-circuit) around Q774 and Q784/Q794 and none have exceeded their values.

Does it mean that one of the output driver Q784 and/or Q794 is leaky?

Should I replace them with a 2N4122 or are there any other equivalents that are easier to source?

Thanks

Thierry


Harvey White
 

Q784 and Q794 form an emitter coupled differential amplifier. Put a signal in on the base of Q784 and you should see a signal on the collector of Q784 and the inverted signal on Q794.

It looks as if Q794 B-E junction is open.

It would be interesting to see what the collectors of Q794 and Q784 are doing.  I'd think (offhand) that Q794's collector would be very high (close to -15 volts), and likely Q784's  got too much current flowing through it.

I"d be suspicious of Q794, and I'd want to check Q784 as well.

Not sure of the replacement transistor, but if you can find a decent replacement for cheap, I'd be tempted to replace both.  I'd certainly think that swapping the two ought to make the bad readings symmetric.  VBE on any transistor turned on should be 0.6 volts or so, and Q794 seems to be, oh, 5 volts too high.

Harvey

On 3/8/2021 4:43 PM, Thierry Delaitre via groups.io wrote:
thank you.

I can find a sweep at the base of the Q784 (output driver) and nothing after.

I have correct voltages for the line voltages -15V and 50V

Other voltages are as follows (with a time dev of 1ms):
- Collector of Q774: 5.6V instead of 6.41V
- Collector of Q764: 6.6V instead of 6.42V
- Emitter of Q794: 11.5 instead of 7.22V
- Emitter of Q784: 6V instead of 7.33V
- 13.1V across C799

I've also tried to disconnect C799 just in case if it was leaky but it is still the same

I've also checked the resistors (in-circuit) around Q774 and Q784/Q794 and none have exceeded their values.

Does it mean that one of the output driver Q784 and/or Q794 is leaky?

Should I replace them with a 2N4122 or are there any other equivalents that are easier to source?

Thanks

Thierry





Thierry Delaitre
 

Thank you

Testing Q794 on a transistor tester was showing Hfe as 4 whilst Q784 was showing a Hfe of nearly 200.

I sourced some 2N4122 and replaced both of them and the 7B50 is back to life :-)

Many thanks and really appreciated your help and from the forum

Whilst I was troubleshooting the 7B50. I remembered I had a 7B92 which is also faulty but that one never worked since I had it. I'll open a separate thread for it

Thierry


Harvey White
 

Glad you got it fixed.

Harvey

On 3/10/2021 1:19 PM, Thierry Delaitre via groups.io wrote:
Thank you

Testing Q794 on a transistor tester was showing Hfe as 4 whilst Q784 was showing a Hfe of nearly 200.

I sourced some 2N4122 and replaced both of them and the 7B50 is back to life :-)

Many thanks and really appreciated your help and from the forum

Whilst I was troubleshooting the 7B50. I remembered I had a 7B92 which is also faulty but that one never worked since I had it. I'll open a separate thread for it

Thierry