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7904/7834 repair questions
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I have 3 7904's one is missing the boards one the calibrator side. One fires up, gets me a readout, but no trace, the 3rd one mostly works, the right side vertical shows a readout, but no trace.
the 7834 started up with no trace any where, no response to controls, but did show a readout. After spraying the 7834 main frames with a little deoxit, and on the plug-in pins, I get trace now on both channels, position knobs respond, readout responds to timing and voltage changes, storage seems to function. However they are ALL a bit intermittent. pots are all scratchy in operation sp forth. So before I start going through the PS's much, what's the best way to clean?? The pots looked sealed, do they need to be opened up or just replaced? Would the simple green bath from from Stan G be ok on these scopes? or is it down to very careful alcohol, toothbrush, and lots of deoxit? Also the mostly working 7904 has the P11 crt, so I would think i should be able to get at least 1 good working 7904, and the 7834 really acts like it's mostly a cleaning issue. So any tips from those experienced in resurrecting these would be very very much appreciated!! Also one 7904A, has a not that says "works, 15V has a short" so haven't even looked into that one yet. Thanks in advance, I'm not a complete dummy with electronic repair, but I know when it's time to ask for help. 73 Don Frasher AD7LL |
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All I can give you is an opinion.
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The 7904 is a good scope, and is my main scope (I do have others). OK, opinion time: I'm assuming physical condition is good on everything. scope without calibrator boards.? I assume it doesn't fire up at all.? Don't know at the moment how necessary they are.? That it's missing boards suggests that there's another fault and it was used as a parts donor.? I'd check the PS voltages as much as possible and consider using it as a parts donor (still). readout but no trace.? That says that the H and V amplifiers are working,?? It does suggest that possibly the unblanking signal (or the H signal from the sweep, OR the switching circuitry) is possibly bad.? I'd try with a known working 7B plugin (or just use a 7A plugin).? Another possibility is that the A or B intensity is bad (or switches thereof....) You've tried switching to the B slots? The third one that has no trace suggests that the vertical switching may be at fault, or the signal path from the right side plugin is faulty. If the readouts are where they are supposed to be, then the H and V frame amplifiers are likely fine. If a signal is missing, and the plugin is good, then the path from the slot to the mainframe amplifier (which includes switching) likely has a problem. For the P11 phosphor, note that it is designed for photographic use, exposing blue sensitive film (orthochromatic) with a very blue short persistence light.?? It will do not so well with low sweep rates.? The standard P31 phosphor is a good compromise between persistence, burn resistance, and light output.? You may consider either swapping CRTs or making the frame with the P31 phosphor the main one. Most pots get scratchy with age, a good quality control lube could be recommended along with exercise. I suspect that the 7834 is a cleaning exercise. On the 7904A, isolate the supply, do resistance readings, and then isolating each board, check for shorts.? The usual suspects are the tantalum gum drop capacitors that were underrated for voltage (and Tek didn't know any better at the time, it's what they were told). Those would be my suggestions. Harvey On 1/7/2022 5:36 PM, don frasher via groups.io wrote:
I have 3 7904's one is missing the boards one the calibrator side. One fires up, gets me a readout, but no trace, the 3rd one mostly works, the right side vertical shows a readout, but no trace. |
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Don,
Harvey is right about cleaning and checking things. The pots may have small screws on the rear holding them together. Loosen the screws enough to allow a gap to expose the element to cleaning. No need to totally remove the screws. A blade will pry the sections apart enough to get cleaner in it. Hold the control back together with one hand while turning the shaft with the other to wipe the element(s). Once the element(s) are done cleaning, tighten the screws back and oil the shaft(s). You will have to remove the controls that have the screws to do this. I have done this enough times with dirty controls. Clean the IC sockets, transistor sockets and the coax "pins". Move the parts in and out some when cleaning. Dirty contacts will cause a lot of problems. Clean the switches and exercise them. Cleaning the ribbon connectors is a good idea. Some here have had dirty contacts in these and cleaning and moving them a couple of times solved problems. I know cleaning all the parts will take time. This will help and could make things start working again. If you decide not to use the P11 crt, let me know. I will get it from you. The 15V supply with a short Harvey is right about. You can use Nichicon ULD 47mfd 25V types to replace many of the decoupling caps. Raising the value will be fine. Most of the 1mfd types can be replaced with a film type of 50/63V or 25V tantalums. Raising the value of some can be raised if it is a decoupling type (readout board). I think with some effort, you will get a working unit out of the three. Mark |
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One thing to note is to very carefully read the sections in the manual about how to clean switches and the like.? Not everything can be cleaned the way you'd think, especially the cam switches in the plugins.
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Harvey On 1/7/2022 10:52 PM, Mark Vincent wrote:
Don, |
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Don, bonjour monsieur
7904 is a great scope, my workhorse for 7000 plug-ins. 0/ please DO NOT use any cleaning on plug-ins attenuators before you read the precautions and proceedures, eg 7A29 has elastomeric membrane contacts and polysolfone PCB attenuators. 1/ In PSU, check voltages and ripple, may need recapping, 2/ A frequent source of problems and intermittents are the single row "harmonica" flat cable connectors that mate to pins on the PCBs, after decades the plastic becomes brittle and crumbles, so the contacts springs loose tension. 3/The square Bourns "mod posts" can be cleaned by just spraying the shaft and sides, oftentimes enough cleaner leaks inside to fix scratchy control without disassembly. 4/Finally the main backplane (and perhaps other boards) have axial tantalum caps which sometimes fail open r shorted. 5/ Exchange of CRT will require complete recalibration, so have the service manual and requiert equipment on hand. Note the (rare) TEK reccomended calibration plug-in, 067-0587-02 Signal standardizer. I wouldn't swap CRT if it can be avoided. Take all safety precautions in handling a CRT! DANGER OF IMPLOSION! hope that this is of interest Bon courage et Bon chance Jon |
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Jean-Paul,
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One comment. The 7904/7904A has a 500 MHz bandwidth. The 7834 has a 400 MHz bandwidth. The 067-0587-02 standardizer has a 1 GHz bandwidth, and the 067-0587-01 standardizer has a 500 MHz bandwidth and may be used in both the 7904 and 7834, and it is somewhat less rare and expensive than the 067-0587-02 standardizer. DaveD On Jan 8, 2022, at 04:37, Jean-Paul <jonpaul@...> wrote: |
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One additional thought: The 7904A is newer and easier to access stuff to fix. The upper and lower chassis hinge apart. I would hunt down the 15v short and fix that one first and see how the unit works. As a newer unit, it might be in better overall condition. Also, check if it has the boxer fan on the back or if it has the older tek custom fan inside. The boxer fan units were the very latest production. I have one of those and it looks brand new all over.
Reinhard Metz |
Jean-Paul
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