There is also a fair bit of psychology involved too. In the case the of the Keithley here, and most of my video repairs, I go into it secretly hoping it's going to be some elusive electronics fault that I can track down like an episode of Columbo. And that probably subconsciously influences how I do things.
As soon as I noticed that values tended to "come good" after switching ranges when Dave started playing with it, I suspected the switches.
That plug is not "funny". It is a European 220V plug.
As an owner of a ~30 years old amplifier (the first stereo my parents bought when I was a kid that my mother gave to my grandmother and then inherited back after my grandmother passed), I am quite well acquainted with the effects of crusty old switches since I periodically have to cycle though most of the amplifier's switches when it starts popping or dropping out. As soon as I noticed that values tended to "come good" after switching ranges when Dave started playing with it, I suspected the switches.
I really should remember to try contact cleaner next time I open it for a tune-up, that could spare me a few minutes of frustration every couple of days. Now that I have a scope, I really should poke around and see if I can fix the gain mismatch between channels too.
What IS funny is that unit has 220V plug and set for 110V ?
What IS funny is that unit has 220V plug and set for 110V ?
Hey Dave,
You may want to check to bottom side of the PCB.
The gang of switches may have hairline solder cracks around some of the pins. Especially after years of use, and two Australians banging on the thing
Maybe reheat the connections to be safe.
I also didn't see you check the bottom of the PCB for blown lands, the thing did get a 30KV pulse that blew a resistor.
-Paul
You may want to check to bottom side of the PCB.
The old gear is fun, I have plenty of it myself.
Just a word of caution regarding contact cleaner and switches. (post are a different story)
The insulating material on some switches will soak up the contact cleaner, that material will swell up and cause the rivits holding the contacts to become loose.
The old gear is fun, I have plenty of it myself.
Just a word of caution regarding contact cleaner and switches. (post are a different story)
The insulating material on some switches will soak up the contact cleaner, that material will swell up and cause the rivits holding the contacts to become loose.Yeah, there is that possibility too, depending on what sort of material is used for the insulation and the specific nature of the contact cleaner, much like how spraying fine petroleum-based lubricants on rubber is usually a bad idea since the lubricant will often either dissolve the rubber or cause it to dry out.
One of the switches is a fancy four positions linear selector driven by a flat open-frame mechanical transmission cable which connects to a rotary knob on the front. Instead of bringing weak input signals to the front, they bring the mechanical input all the way to the rear to keep the signal path as short as possible.
How about fixing the Tektronix scopes that you got in a previous mailbag?
Those scopes are notoriously horrible to fix. Uses a ton of custom parts. My spidy sense tells me it would just end up like the DSA repair.
Restoring one of these scopes is very labor intensive, but well worth it.Quote
That's why I'm not doing it, I don't have the time. Murphy will ensue that they will have all sorts of issues making any repair repair attempt even longer. It will turn into the DSA repair all over again, I know it.QuoteThe only real challenge I saw with your scopes is the missing covers, rear panels, and hardware. All of those parts are out there (I had to replace a number of broken/missing structural/mechanical parts on mine), but I suspect replacing all those missing parts on your scopes would cost somewhere in the neighborhood of $100 USD per scope.
You also have to try and get them in Oz.
How about fixing the Tektronix scopes that you got in a previous mailbag?
Those scopes are notoriously horrible to fix. Uses a ton of custom parts. My spidy sense tells me it would just end up like the DSA repair.
Nope, sorry Dave. Those Tektronix 24x5 scopes would be far easier to repair than the DSA. I'm active in the tekscopes community and I own one of these scopes. The two most common problem areas are the aluminum electrolytics in the PSU failing due to age and the SMD aluminum electrolytics on some of the later processor boards can leak and do minor board damage. Another somewhat common problem area are the versions of these scopes with use a Dallas battery backed memory module. When those fail, the scope looses its cal data. All of these issues can be overcome without that great of difficulty. While these scopes can have failed hybrid modules, those faults are not very common, and those parts can also be found with some asking around. Restoring one of these scopes is very labor intensive, but well worth it. The only real challenge I saw with your scopes is the missing covers, rear panels, and hardware. All of those parts are out there (I had to replace a number of broken/missing structural/mechanical parts on mine), but I suspect replacing all those missing parts on your scopes would cost somewhere in the neighborhood of $100 USD per scope.
As for the switches in the Keithley 177, those are a problem area in any gear of this vintage. 1980s Fluke and Simpson meters which use these type of push button (slide contact) switches tend to have the same sort of issues. Schadow (ITT) sold millions of these style of switches and they were used everywhere. Even my early 1980s vintage Tektronix 2213 scopes used a single push on/push off Schadow switch for the power button, which after 30+ years, tend to stick, and had to be serviced. What complicated matters with the 2213s, is they trimmed away the top connection pins on the switch and insulated it with a slide-on plastic cover. The cover cannot be removed to allow the switch to be cleaned without first desoldering it from the board, which involves a lot of disassembly.
Speaking of, I need to renew my stock of contact cleaner/lube. Is there a US version of that EML - Contact Cleaner Lubricant under a different name? Doesn't appear to be readily available here in the US.
Restoring one of these scopes is very labor intensive, but well worth it.
That's why I'm not doing it, I don't have the time. Murphy will ensue that they will have all sorts of issues making any repair repair attempt even longer. It will turn into the DSA repair all over again, I know it.QuoteThe only real challenge I saw with your scopes is the missing covers, rear panels, and hardware. All of those parts are out there (I had to replace a number of broken/missing structural/mechanical parts on mine), but I suspect replacing all those missing parts on your scopes would cost somewhere in the neighborhood of $100 USD per scope.
You also have to try and get them in Oz.
The switches and relays used in this type of equipment can be several different types, but all have some factors common. Low current contacts (aka "signal" / "dry" / "logic level") have either brass, alloy, silver or gold plating on the contact points. Most are silver. If the visible parts of the contact are gray or black, the silver is oxidized. Silver Oxide is an insulator. Certain contact cleaners will clean these well. The "grease" you may find is not there primarily as a mechanical lubricant, but to keep oxygen away from the silver. After cleaning with a spray liquid cleaner, the "grease" is washed away, ensuring future contact problems. To make a lasting repair, use an electric grade of silicone grease on both the moving and stationary contacts. If there are still issues, disassemble the contacts, clean them (IPA works) and look at them under well lit magnification. If it has the silver or gold worn away by mechanical friction from use, your only option is to replace the switch or relay (or even pot). This is usually impossible. Another alternative is to add and wire-in relays or semiconductor switches.
Don't use any cleaner which is flammable, because these may harm / swell the plastics holding the contacts.
Restoring one of these scopes is very labor intensive, but well worth it.
That's why I'm not doing it, I don't have the time. Murphy will ensue that they will have all sorts of issues making any repair repair attempt even longer. It will turn into the DSA repair all over again, I know it.