[SOLVED] Tektronix SC502 Alt and Chop mode fault

[BlownUpCapacitor]

Hi, I have an SC502 that I broke while trying to fix it.

I was initially trying to remove the mode select cam to clean it because it was full of gunk, but I ended up bending the golden contacts and had to disassemble the entire scope to fix it.

It appears that disassembling the scope and fixing the gold fingers has introduced a new problem. Alt and Chop modes on my SC502 no longer work properly.

Alt has a fault where the trace times are not 50% split, but rather split randomly. Video here: https://youtu.be/vQjHu17QHKY The switching waveform is what you'd expect. Sometimes it's high, sometimes it's low. The effect is not very noticeable at higher sweep speeds.

Chop mode is more strange though. There seems to be ringing? Some kind of distortion in the chop signal. The oscillation signal by itself seems fine. I'll include some images.

Not quite sure what happened. I've ruled out the cam switches. Maybe I damaged the chips with ESD while disassembling it? I work in my bedroom so it is possible.

[BlownUpCapacitor]

AHH DAMMIT. I SHORTED U360 WITH THE GROUNDING SLEVE AT THE END OF MY OSCILLOSCOPE PROBE TIP WHILE I WAS PROBING AROUND. THE WHOLE ERROR CHANGED NOW.

I guess I'll go purchase some new 7472 chips. These seem are though as the ones on digikey are SMD, and the ones on Ebay are expensive. Good bye money!

New symptoms: CH1 mode works just fine and dandy. CH2 mode works well, but the trace gets dimmer when in that mode. Alt mode shows only CH1 with a dim trace, same with chop and CH1-CH2.

[BlownUpCapacitor]

Well the 7472 being cooked revealed that U355 is just fine and dandy. I think U360 was faulty the whole time. But it had to be a rare chip eh? Anyway I ordered (I hope) an equivalent IC, the SN54H72. The 7472 and the 5472 share the same datasheet. I think the 5472 is just the military spec version of the 7472.

I ordered a golden flat pack 5472 from ebay thinking that it would last longer if it was gold. But now that I think more about it... I don't think the flat pack will fit in the socket...What was I thinking!? I'll probably end up having to spend more money on an adapter board for flat pack to dip-14.

The only benefit I see from this is that I'm now busy and have an actual task.

Also the 7472 on digikey is not a 7472. They dont have any 7472s.

Edit: AHHHHHHHH THE PINOUT OF THE FLAT PACK ISN'T THE SAME.

[BlownUpCapacitor]

Did I discover a manual error in the schematics?

The schematics say that the channel 1 position of the mode select should be 1100 for the last 4. The first one on the left is just power and is ignored in this part of the schematic.

However, when I look at my scope, it is 1001.

1 signifies a closed contact and a 0 is open.

Channel 2 mode position shows 1010 in the schematics, but also see 1010.

Is this a schematic error? Or is something very very wrong?

The rest of the positions follow the schematic fine. It's only the CH1 position that is off.

My scope also seems to have a low serial number, but SW300 is not marked as having been modified at higher serial numbers.

Channel 1 and 2 modes work fine. When channel 1 is on though the beam is brighter than usual. I can tell because I can see the sweep reset at full intensity. This was not an issue before. This only happens with ch1. CH2 seems to work fine except that ch1 position control has a mild effect on ch2's.

Alt and chop are very broken, likely because of that faulty 7472 chip. It'll arrive tomorrow so I can check then. CH1-CH2 works perfectly.

[BlownUpCapacitor]

I did some troubleshooting and found that the trigger signal wasn't being amplified well. Q350 looked like it could be causing some problems. So I desoldered it and lo and behold, it has a big ass crack in it!

Will a 2N3904 transistor be a good enough replacement? the original is 2N3565

Also the new 7472 IC arrived and it's back to the random switching

[BlownUpCapacitor]

Update: It's fixed! Q350 was the issue the whole time! Replaced it with 2N3904 and it works perfectly now!