Valvestate 8240 troubleshooting

[E biddy]

I'm trying to learn how to troubleshoot, so a friend of mine gave me his non-working Marshall Valvestate. I started out by tracing a signal. (400Hz) I have signal on the left side of C1, but not on the right. I figured an open capacitor, so I replaced it. Same thing, so I checked the voltages around the opamp (tlo72) I have the voltages listed, they seem very strange to me and I'm lost.

Any help greatly appreciated.

Eric

[The Dude]

Looks to me like your op amp is shorted. Normally, an op amp with a differential supply should only have voltages on pins 4&8 (there are exceptions). Sometimes an op amp will have pin 4 at ground and supply at pin 8. The voltages on the other pins will then be something between ground and supply. Not the case here.

[Jazz P Bass]

Generally speaking, you will not see the signal on the right side of C1.

That is the opamp input & is considered a 'virtual ground'.

I simply shoot for the output pins.

[Enzo]

Agree, if the op amp is powered, look at pin1 and see if there is signal there. You usually cannot read signal at the input pins.

Howver your voltage readings look like a bad op amp. Pull the op amp, and see that there is no longer any DC voltage on solder pads for pins 5,6,7.

[E biddy]

Thanks guys, I will pull it tonight.

Enzo, good to see another Lansing guy!

[Enzo]

Hi. My shop is in Lansing, but I am hiding out in downtown Mason. I can walk to the Bad Brewing Company.

[E biddy]

I live in DeWitt and work at MSU.

[Enzo]

Cool. I haul my recycles over to the MSU facility south of the tracks, and I love going to the surplus yard across the street from the recycle. And right up the road is the MSU dairy store, where I love the ice cream. And now we have that whole FRIB thing, plus the cyclotron lab, which has grown into a giant V'ger of a thing.

[E biddy]

A couple of my athletes have worked at the Cyclotron. I was always asking one of them questions about it that he didn't know, so he asked me if I wanted a tour. I figured he would get some physics grad student to show me the place. I go over there and Thomas Glasmacher, the director of the FRIB project, shows up to give me the tour!

[E biddy]

Thanks guys, that fixed it. I put in a socket for the op amp and tested it with a opa2604 because I didn't have an 072 available. Can I leave that in there, or do I need to get an 072.

[The Dude]

Nothing will be harmed by leaving it. If the amp sounds ok for you, not a problem.

[Enzo]

Does it work? Then leave it in and stop fixing it.



When I was a freshman physics major at MSU, my advisor was a professor whose office was in the cyclotron lab. At the time it was a very small building. I'd go over there to add or drop a class or whatever. A friend of mine had a log-in account number from somewhere, and we'd go to the cyclotron lab late in the evening. Classic "look like you know what you are doing, and no one will bother you". There was a computer terminal in the lab somewhere, and he'd log on, and we'd play Space War for hours. That game was a sort of crude two player Asteroids. The environment was perfect, the place was so...labby. test gear everywhere, racks of stuff with knobs and lights. Plumbing and cables all over. Rah rah. That tiny building is still there in the center of the huge facility that was built around it.

Geez, that was over 50 years ago.