开云体育

Another possible project in my 7K organizing and cleanup program is making a regular 7613 out of a rack version R7613. I've had this R unit for years. It works fine, but is a real pig, space-wise, and awkward. A rack style can be handy sometimes, depending on the situation, but I think overall I'd prefer to have it in the standard bench scope format. I have a carcass of a 7403N, which was similar to 7603 but lesser performance, and shot anyway. I've been pondering using it as the platform to transplant the guts into from the R7613, but am not sure if it's totally possible or practical. I think it should be OK electrically, but maybe not mechanically.

Does anyone know if this is possible without too much grief? I don't know if certain boards and such are different for R-form, or even if the standard 7603/7403 cabinet is the right length for the storage versions. I'd appreciate any info/experience/wisdom on this matter.

Ed

On 11/24/24 12:30, ed breya wrote:

I'd appreciate any info/experience/wisdom on this matter.

I've taken apart about 6 7xxx scopes since starting my national labs junk business in 2022, and think a little about how they go back together as I do them. It's a lot of time to nondestructively, swap frames, but probably the cables are long enough for either way and could be done without custom cables.

I don't recommend it, since by using adjustable shelves hanging over your bench, rack cases can be usable and just fine and have a stack of two scopes vertically with room to get probes from both to a DUT on the bench below. Adjustable shelves are not super easy to get, but they come from retailers doing remodels for free sometimes. You can see examples at most big chain grocery stores, with vertical posts with rectangular holes in them that steels shelves plug into on one or both sides.

The 7603 has a bigger crt, you might find the supports for the crt might not transfer over if part of the frame, and the front panel will definitely be different.

The smaller face of the 7613 CRT is accommodated by additional hardware that bolts to the main front casting. The 7603 uses the same front casting as the 7613.; however, the Tekwiki manual for the 7403 does not show the part number for the casting. I suspect that it is the same raw part but missing the screw holes necessary to mount the CRT hardware.

I brought back from the farm my 7603 to see the physical items and layout involved, and also found I have a bunch of saved parts from junking a couple 7603s long ago. The R7613 transplant looks like it would have some big mechanical issues, especially in the length of the CRT shield and its mounting. I'm thinking now of trying to make another 7603 from the 7403N carcass, which should be fairly easy, IF there are enough 7603 parts still good. If not, then at least I'll have many of the items common between the 7603 and 7613 worked out, and will be able to see how much more is needed for the complete conversion.

Ed

I have success finally on the conversion. I did start with trying to make another 7603 from the salvaged parts, but too many things were wrong with various items, so I decided to try the R7613 to 7613 switch after all. It turned out to be quite easy structurally, except that I had to give up the original fan cooled power supply unit (which is essential in rack-mount I think), so the end result is convection cooled only. It is possible to mount a small DC fan onto the internal structure to get some circulation and flow on the pass transistor heat sink assembly, where most of the dissipation is from. I'm doing some burn-in testing and assessment of temperature rise, then I'll decide if I need to add some fan help. This unit will never be used toward the higher specified temperature (up to 50 C) extreme, so I'm OK with reasonable lab/garage conditions. The real 7613 and my 7633 are fan cooled, and the covers are solid. With this one, I'm using the 7603 type perforated covers for regular convection cooling of the cabinet, and I'll probably modify the cover that goes on the back over the heat sink assembly to get easier flow. It will run hotter, comparatively, but I think it will be OK, and quieter too.

Mechanically, I had to drill only two holes in metalwork - one in the handle spine for a clamp to hold the CRT's Alden connector, and one in the 7603 style cross bracket that supports the CRT socket end so I could flip it around to fit the longer assembly. I used a salvaged 7603 transformer/rectifier section (with the optional signals out back end) that was good, and the low voltage regulator assembly from the R7613.

I found out quickly that the hardest, most tedious part is rewiring all the interconnecting cables. On the attempt at making a 7603 first, it was a PITA to hunt through my limited stock of Tek harmonica/ribbon cables. Somewhere I have lots more but couldn't find the missing box. Even with plenty of pieces, it takes a lot of work to sort out the lengths, conductor counts, and plastic housing color codes if you want them as original. When I decided to do the R7613 teardown I made some notes on certain parts of the wiring, but I should have done more and thorough work. I went through extra grief to keep it as modular as possible, and managed to take the whole CRT side out as a unit to minimize its local rewiring. This involved cutting and bending the rack chassis apart in certain spots, which also made it a one-way trip for this project - no going back to the original if I should fail the conversion.

So here was the left side guts assembly with cables hanging every which way, and the rest of the sections sort of separated, and sort of connected to each other, but removed from the chassis, and a lot of mystery cable ends that had to be pulled and noted for re-hooks, or would need to be figured out. I kept the pieces together as much as possible so I could gradually move the cables into the new scope chassis. The left side CRT module slid right into the top chassis section, and there are plenty of extra holes to accommodate the different mountings needed around the CRT and front control panel, so it was a breeze to install it. I made the mistake of keeping a dubious salvaged 7603 main interface board in place rather than transplanting the (known to have been working) one from the R7613, thinking it would save some time. I paid the price later. After that it was mostly the tedium of figuring out where all the cables went at each end, and whether they were long enough to reach, and gradually putting the remaining big pieces and their cabling into the chassis. I had to build some new cables from various pieces on hand, and tried to keep the color coding original, but couldn't get them all.

The time finally came to fire it up, and the screen lit up just fine, but there was some kind of vertical failure showing compression so the traces would hardly move up and down, and the readout was miniaturized. After some connector checking and more study and measuring I couldn't find anything to blame, so figured it was time to change that main interface assembly. Sure enough, putting the original one in as I should have fixed it right up. Then everything seemed to work except the right vertical readout was gibberish. I went on a wild goose chase with all sorts of grief trying to figure out what happened, then finally found I had made a mistake on one of the cables I made for the readout board - two wires were swapped.

Now it all seems to work, and will need checkout and some adjustments. The focus seems kind of crappy, but actually better than it was when it was in the rack chassis. I'll probably be starting a new thread on some of the issues that I find.

Ed

Fascinating story Ed. It sounds like your hard work paid off. I did this kind of swap once with a 502, but it was very simple compared to yours. I basically just removed the front panel, turned the scope on its side, and bolted it back together with the RM502 panel in place.

Definitely fascinating, Ed!
It would be great to see some picture of the process, should you have some; or, at least, the final product. If not, most of us would have to use our own imagination, which would make it even more fascinating...
Titi

With the standard 7603 type vented plastic cover over the heat sink, I estimate the case of the hottest TO-3 transistor (center top) reaches about 75 deg C rise over ambient, or about 115 C with 40 C maximum ambient. This appears to be the pass Q for the -15V supply. Assuming none of the six transistors dissipate more than 30W, this indicates maximum junction temp about 160 C, for high ambient normal operation with some nominal load - in this case a 7A18, 7A26, and 7B53A installed. At reasonable lab temp it should be around 125 C.

I picked the common 2N3055 as a representative example, with Max Tj 200 C and Rth J-C of 1.52 C/W, and it looks like there's a decent margin for normal conditions. Without some additional cooling, it will be toasty, but not too bad. Under fault conditions, a lot can change, but assuming the supplies use some degree of foldback current limiting, and the raw supplies collapse somewhat more with excessive loading, the dissipation in the Qs may be not too bad either. I'll be studying it and measuring a few things to see how it looks.

Ed

BTW I wasn't planning on any pictures. I didn't think to take any shots of the teardown or other intermediate steps. You would hardly notice any internal difference from the real thing except for the rear CRT bracket flipped around, and ugly wiring in some spots where I used or made cables that were only just long enough to reach, and I took some shortcuts in the lead routing and dress. Externally, the back looks like a 7603 with the "signals out" option.

I will be putting up some pictures though, related to an issue that this unit has in common with the 7633. Some of you may remember my 7633 problem from a year or two back, where the high voltage was mysteriously crashing after some warmup time. The apparent cause I discovered was interference between the CRT neck mounting hardware and a high voltage lead going into the CRT socket. I cut a notch in the plastic socket cover to relieve the interference and the problem never came back. In fact, I brought out and opened up the 7633 to use as a reference for rewiring this 7613 (it's mostly identical), and it reminded me to check. Sure enough, same issue, where the hot lead is pinched against a steel screw head that's grounded. It hasn't shown any problem, but this time I'll nip it in the bud.

Ed

OK, the CRT socket wiring interference issue is fixed, and while I had everything opened up I did some fan cooling experiments.

I managed to fit a 3" diameter low (<5/8") profile DC fan in there between the main guts and the low voltage regulator board. I tried one that's thinner yet, but it was much noisier and less pressure (not enough blade) and didn't achieve enough flow to make much difference. This thicker one does the job. These are both 12V, but I set up to run at reduced voltage around 5-7V. I picked off the raw supply feeding the 5V regulator as the source, which is nearly 12V at current conditions. I ended up with a big 5.1V Zener 1N5338B to take it down to about 7V, where the noise is definitely audible, but not too bad, and the decent flow seems to give over 20 deg C improvement in temperature rise, so the cooling is now quite good even though the fan is quite far removed from the load.

I tried to maximize the clearance on the vacuum side of the fan for best compression inlet efficiency. The outlet blows against the LV regulator board and just makes a lot of turbulence in the whole back section, and enough air manages to get around the edges to the other side to help cool the inner heat sink surface, and out through various openings into the outside surface to help there too. It doesn't feel like a lot of flow outside, but it's way more than the convection alone. I can feel warm air coming out of the rear-facing plastic cover vents up to about an inch away. I could make it even cooler and quieter by squeezing some small duct work in there, but it's not worth the extra grief. It's effective enough even though most of the outlet air goes right around the fan back to the inlet.

I'm calling this conversion project done and satisfactory in operation. Apparently fully functional, not too hot, and not too noisy.

Ed