Tek 7904 scope

If this is why it was cheap, the seller would have had to have known that it was going to fail.... See if it "comes back" after having been off for a while. Perhaps this is a known condition of the unit. Shouldn't be all that tough to fix... IIRC it uses a switching invertor to generate an internal 50v supply which all of the lower voltage supplies feed from - Sounds like the regulation of the invertor is not working and it is getting shut down. I've seen other 7904s which exhibited "ticking" like this, so it may be a common problem. I recall that the service manual has a pretty good description of the power supply - if you don't already have it, you can get it in PDF form from: http://bama.edebris.com/manuals/tek/7904 [If you can get it - the mirror has been spotty lately - if you can't, I do have a copy of it, and we can work out some way to get it to you - iirc it's pretty big.] I had my own positive experience with a dead Ebay scope lately... I bought a dead HP54201D (300Mhz DSO) in the hopes of getting parts for mine which has been experiencing problems (and HP doesn't provide service information) - It was in pretty rough shape, power-switch broken, case banged up, screen-burn, no probes or state pods, and listed as "immediately blows it's line fuse" - but I took a chance on it and picked it up in the hope that the analog board would be in better shape than mine. Arrived a couple days ago - inside looked to be in pretty good shape, and I immediately noticed one of the primary line side filter capacitors was "humped up" at the top of the can - so I removed it and found that it had exploded/leaked ... cleaned up the board and investigated a bit more and found a shorted diode in the primary line recitifier bridge - replaced both components, and the scope works perfectly! (Much better than my other one). It also has newer firmware than mine, so I simply swapped all boards into the nicer looking chassis... Next step is to use the good/working scope as a reference to track down the triggering problem in the other one. (Schemtics would sure be helpful!) Dave -- dave06a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.classiccmp.org/dunfield/index.html