The Tektronix again, 7603

I posted this in SER and figured maybe someone here would like to take a sh ot at it :

Sorry, I just like this scope. In my (excessive) spare time I have been pok ing at it again. I was getting a really weird display and I found it to be related to the readout circuit. I can live without that. I DO have a spare readout board though, so after getting the thing to actually work normally, I will look into that. For now it is mostly disconnected.

Now, vertical is OK. Horizonal is compressed and will only deflect on exter nal or line input, it will not trigger even on auto or P-P auto. That may b e related to the problem, it may not. If you remember, originally it had no trace. This was before I goofed by accidentally switching two plugs on the Z axis board which dumped 130 volts on the 5 volt line.

This would not be worth persuing had I not a parts donor. I have a 7613 whi ch had some sort of a short. Looking at

You can see that I obviously wiped out U510 which is of course pure unobtai nium. I changed it from the 7613. Originally I tried using the whole H amp board but it didn't work so I put the original back in with the good IC. I figure it's good because the donor had a short, this IC was not the short s o there is no reason to believe it is bad.

There was a mishap along the way and it wound up frying R558. After replaci ng R558 the sweep was even more compressed ! A look at the print might expl ain it. I don't have the unit in front of me at the moment but what is goin g on here is that CR530 and 533 are mostly reverse biased. Replacing R558 s imply reverse biased them more.

Since originally there was no sweep and at the time I did not switch it to line input I do not know if it was compressed at that time or not. That's w hy I do not know if the two problems are related or one is what I created. I hate it when that happens, my blind ass is getting more useless by the ye ar.

I have two horizontal plugins which display identical symptoms so I am ruli ng that out temporarily. I know they use the 5 volt line but only one of th em was in the unit at the time of the 130 volt incident.

In the manual (available at bama) the description of the operation of U510 is that when the XY shutdown goes high the input of the chip switches to th e input from the readout board. The readout board is putting out a tri leve l waveform at about 60 Hz to the XY shutdown, but not quite at the line fre quency. It is also putting out square waves to the vertical amp which was c ausing a dual trace when it was not desired. It is obviously whacked out bu t it seems to me it should work with those inputs (XY/RO) disconnected. Ind eed it did straighten out the vertical sweep. In fact the horizontal is act ually closer to working now but there is not enough gain and it is clipped (truncated) at the sides.

I did highly suspect U510 at one point but I have pretty much dismissed tha t. I also considered that maybe there was a source not getting to the plugi n. The output therefrom is terminated with 50 ohm resistors, so there is no t much of a signal there. When I disconnect P501 I get quite a bit more amp litude but looking at the print for the 7B50 I see that it is pretty much a current source so that is to be expected. I do notice though, that when di sconnected the signal is riding on a negative voltage.

I have checked pretty much every semiconductor on the H amp board and they all seem OK. There is a slight DC imbalance, and I am not quite sure what t o make of it. Since this thing is a differential amp a fault on one side ca n give misleading readings so it is hard to tell exactly where the fault li es.

The centering and centering range controls have very little effect. I could see how R525 would have limited or no effect due to something wrong with U

The voltages at the deflection plates are low, though not specified they ar e much lower than one would expect owing to the fact that they used a +130 volt source for them. The trace is also expectedly blurry.

All the sources are there and on the money according to Hoyle. Where is all this positive voltage that is obviously supposed to be there ?

Any help is welcome except sending me to Yahoo. Sorry, Yahoo left a bad tas te in my transistors a long time ago and I simply won't go there.

Thanks in advance.