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Team,

I have over the past few months acquired 5 7844 scopes, none of which worked. Good thing it was just the cost of picking them up. I have repaired 3 of them so far, one of them still has the power supply on the bench, one was the parts source. On the latest project a R7844, after replacing caps in the power supply, replacing beam 1 vertical amp twice, original was very noisy, first replacement gain was very compressed on the positive side. Working very well now, calibration done. Discovered one last problem. Intensified sweep using a 7B92A. Beam 1 works perfectly, beam 2 the delayed intensified sweep is very dim . Intensity control does control it but at max you can just discern on the screen. Reducing the separation until it corresponds with the main trace it works perfectly.
Now for the weird part if I use the older 7B92, intensified Alt trace, it works perfectly

Thoughts?

Mark
W7HPW

Real Radios Glow in the Dark

Mark,

The s/n of the piece will make a difference. If below 60000, it is likely the tunnel diodes on page <3> are bad. I have had this problem before. These going bad will cause your problem from what I saw. I did replace the leaky 0367 transistors in the ones I have that tested leaky, diode, from C to E with KSP10BU. The leakage did cause triggering and other problems in mine. Resistors R479 and R939 should be 1W. I have found a transistor open that is on the lower left of a page which I do not remember that is run almost at its voltage rating. I did use the above number as a replacement. This transistor is part of a board schematic.

Mark

Mark

Thanks for the reply

The one I have is serial #B141271 It’s the R7844 I have another 7844 serial B142533 with exactly the same problem but on both beams, The one with out this problem is B121284.

If I could trouble you Page 3? The rest of these references on what board?

Thanks again

Mark

Mark,

The s/n I meant is for the 7B92As. The page <3> is for the plugin.

Mark

Mark,

Also, it is possible the still gold stiff pins need added solder to the boards. A bit of solder on each side.

Mark

Mark,

Thanks for the clarification, I have 7 7B92A's serial numbers ranging from 64600 to 99661 all of which the exhibit the same behavior of having a very dim or non-existence alt. delaying sweep but only when the separation control is set to offset the alt sweep and only when routed to Beam 2. All of the 7B92A's work correctly on beam 1 and in the other main frames that I have them in. 7904 (2) 7934 and another 7844.
The commonalty Beam 2 its doesn't mater whether its coming from the A time base or the B time base

I see the tunnel diode that you are referring to now on the main trigger board CR114,126,172 and the delayed trigger board CR314,322,372,376 and I also see that the are only used on Serial #69999 and below, #70000 and above doesn’t have them. Right?

I am glad That I only have one with tunnel diodes they cost more that gold.

Your thoughts?

Mark
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Mark,

If all the plugins do the same thing in some scopes, the scopes are at fault. It sounds like they work properly when in other scopes. I do have mine set with the 'level' turned to 'runs after delay time' on the time bases. See if your 10 turn control is at zero and set the contrast control to what you want. Turning the 10 turn pot will make part of the trace brighter as you turn. I assume you have the brightness set to max on the time base to barely see the trace. See if the separation control is set so traces are far enough apart.

Since they work in one 7844, swap the boards behind the working one with another that does not work to see if the problem goes away. If so, it could be a contact problem with the sockets where the sockets are, dirty contacts on the solid state devices that are in sockets on the board or a part such as a resistor is open. The solder on the sockets may be cracked. Resoldering is a PM.

The tunnel diodes you mentioned are the correct ones.

Mark

Mark,

What you just said is exactly as you describe, well not exactly, When the separation control if fully CCW the behavior is as you describe. When you try to separate the traces the alt trace dims so much even at max brightness that if you didn't know it was on you would miss it completely.

Thanks again for your suggestions!

Mark


?? ----------------------------------.
? /? Don't take life so seriously, __/
?/?????? it's not permanent.????? /
?\_________.---------------------'
???? _ |/
??? oo\'
?? (__)\?????? _
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????????? c__; c__;? '-..'>.__

Mark,

See if swapping the horiz. interface boards make a difference. If not, the logic board is next. Swap ICs coming off of Y, Z, AA and AB and the two after the transistor array ICs. An IC could be bad. I will assume you cleaned the switches. A dirty contact on an IC in its socket could be a problem. These are in the scope.

Mark

Mark,

I will check these things a report back.

Thanks for suggestions!

Mark

?? ----------------------------------.
? /? Don't take life so seriously, __/
?/?????? it's not permanent.????? /
?\_________.---------------------'
???? _ |/
??? oo\'
?? (__)\?????? _
???? \? \??? .'? `.
????? \? \? /????? \
?????? \? '"??????? \
??????? \????? (? )? \
???????? '-| )--| :.? \
?????????? | |? | | \? '.
????????? c__; c__;? '-..'>.__