At 12:31 PM 2/18/2007 -0500, you wrote: Well, I have said device in my hands now and have been playing with it, trying to see if all the bits work. I haven't played with the curve tracer yet, but I have the manuals for all the plug-ins downloaded, also three or four slightly different manuals for the scope itself. I'm having a lot of fun working with it. It plays pretty nice. I've been running through the checkout process in the manual are so far there's only two concerns: 1. The display swims, more like twitches, from left to right, rapidly and constantly, somewhat less than one minor division, meaning about half a millimeter at a rough guess. It does it on any/all channels on both the 7A26 vertical amp and the 7A18 amp. I don't have another timebase to check out, but when I switch to the curve tracer instead of my 7B53A, the twitch in the on-screen readouts for controls settings stops. Of course, with the curve tracer, you can't sync up, but the readouts clue you that it is/is not dancing around. I tried swapping the timebase into the A slot, instead of the B slot (the 7CT1N wants to be in an inside slot) but the twitch is still there. 2. The Ch1 input on my second vertical amp seems to attenuate the signal by a tenth. The 4v calibrator at 1khz or DC reads 0.4v, set at 1v/div. The other input on the same plug-in reads 4v. Set to 0.5 v/div, the bad input reads about 1.2v. Switching the calibrator to 0.4v and the bad input to .2v/div gives similar results. I'm working my way through the manuals, learning as I go. [Toasts] Here's a toast to your new bride who has everything a girl could want in her life, except for good taste in men! --... ...-- -.. . -. ----. --.- --.- -... tpeters at nospam.mixcom.com (remove "nospam") N9QQB (amateur radio) "HEY YOU" (loud shouting) WEB ADDRESS http//www.mixweb.com/tpeters 43? 7' 17.2" N by 88? 6' 28.9" W, Elevation 815', Grid Square EN53wc WAN/LAN/Telcom Analyst, Tech Writer, MCP, CCNA, Registered Linux User 385531

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

Thats probably good news - nothing harder to fix than a problem which comes and goes ... If it firmly busted, it shouldn't be too hard to find. Yes, tants tend to die of old age ... Easy to find and fix. Depending on how much "ommph" the power supply has, bad tants can often be found by visual (or olfactory) inspection (If inspecting with power-on, wear safty glasses - I've had more than one cap blow up in my face). If not, tracing through the supply path should quickly take you to the problem area. "Ticking" often indicates a switcher trying to come up into to great a load (like a shorted cap) - Have you tried pulling all the modules to see if the DC supplies come up, if they do, it could be a bad module (also likely to be a bad cap). I'm assuming the internal circuitry of the mainframe will be enough load to let the supply come up normally - IIRC it will come up under no load, but I could be mistaken (leaky memory bits). Since it hasn't recovered, he probably didn't know it was about to expire - likely it has been unused for a good long time, he powered it up long enough to say "it works!" - and you powered it up long enough for some wheezing component to pack it in. All ya gotta do is find out what failed - power supply problems are usually pretty straightforward to diagnose. Any equipment this age can die at any moment, so even if it made it past 90 days or whatever, you still might need to fix it sooned than you would have liked. If I got a good price on it, and it's a piece I wanted, I'd probably just fix it - but thats your call. As Tony mentioned - giving the PS a good overhaul could also help with the twitchy traces you observed when it was working. Well... my expectations were low as it was advertised to be DOA - As the seller said - he "packed it carefully to make sure that it would arrive broken". I just got lucky and it was an easy fix - yours could be too! Best of luck with it. 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

On Feb 28, 2007, at 12:42 PM, Tom Peters wrote: This very much depends on the switcher. Generally speaking, all but the very oldest switching power supplies are fine with little or no load. I'm very sorry (and surprised) to hear that your new scope went boom. Those 7000-series boxes are generally indestructible. I hope you're able to resurrect it. -Dave -- Dave McGuire Port Charlotte, FL

What I really need is someone that's done troubleshooting on a 7904 power supply before, that can offer a few suggestions. Like, if I do locate a bad cap, what should I replace it with? The DigiKey catalog is vast. -T At 11:44 PM 2/28/2007 +0000, you wrote: ----- 273. [Philosophy] "What experience and history teach is this--that people and government never have learned anything from history, or acted on principles deduced from it." --Hegel --... ...-- -.. . -. ----. --.- --.- -... tpeters at nospam.mixcom.com (remove "nospam") N9QQB (amateur radio) "HEY YOU" (loud shouting) WEB: http://www.mixweb.com/tpeters 43? 7' 17.2" N by 88? 6' 28.9" W, Elevation 815', Grid Square EN53wc WAN/LAN/Telcom Analyst, Tech Writer, MCP, CCNA, Registered Linux User 385531

I was going to suggest PSU problems for the jittery display.... I don't haev the scheamtics for the 7904, but I saw them once (in the lab where I used the 7904). IIRC the manaul was entitled 'electrical parts lists and diagrams'. I do remember the PSU was a fairly complex switcher ;-( -tony