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So, crap like that happens every so often - I haven't boycotted Bourns over that - their quality is usually excellent - and it would be nice if the manufacturer would 'fess up, without dancing around an issue which is plain as day for anyone to see.

Did that pot have a short trace of silver between the end of the resistor to the terminals?

How about a condensate drain valve? Do you have that to let water out the
bottom of the
compressor shell/tank? A small refrigeration compressor from a water
cooler might be a good size
for venturi vacuum generating.

From: "petertech99h via Groups.Io" <petertech99h@...>
Sent: Jul 28, 2018 11:02 PM
To: [email protected]
Subject: Re: [TekScopes] Slightly OT - Bourns trimmer pot failure

Thank god it was not the quality enclosed Bourns pots - I use them all the time!

Replace the pot and hopefully you'll be back up and running! I don't trust open frame pots over 1M!

Cheers!

Pete


On Saturday, July 28, 2018, 6:32:24 PM EDT, Michael A. Terrell <mike.terrell@...> wrote:

Did that pot have a short trace of silver between the end of the resistor to the terminals? It can oxidize, and open. I've seen a lot of that type that failed, but they were mostly other brands. I don't like open frame pots, but they are cheap so a lot of companies used them in non-critical applications.


Michael A. Terrell

From: Adrian <Adrian@...>
Sent: Jul 28, 2018 4:43 PM
To: [email protected]
Subject: [TekScopes] Slightly OT - Bourns trimmer pot failure

I finally succeeded in breathing life? back into my Tek 634 display today but, as it turned out, only briefly.

Thanks to google, an expired ebay link and a really helpful chap in the US of A I got my hands on a spare A4 HV Multiplier board and also tracked down a failed transistor and diode on the A6 interface board and all came back to life. I ran it for about an hour keeping an eye on the current drawn by the HV board and everything was fine.
The contrast was a bit horrid and the display seemed slightly dimmer than I remembered but as the HV board was a NOS replacement I figured it would need setting up.

I ran through the procedure which worked fine up to the point of setting the cut-off level to make the background raster just invisible. This is a? bias applied to Grid (2?) of the CRT and is derived from a separate winding on the HV transformer with a 5M Bourns 3355 series trim pot as part of a divider network. These pots are a PCB vertical 'skeletal' type with a blue molded plastic thumbwheel type adjuster.

At first it responded as I expected but then the screen suddenly started getting very bright and the current to the HV board started climbing so I switched of sharpish! Investigation revealed the trimmer was open end-to-end and neither end was connected to the wiper. I opened it up - with extreme prejudice - and cannot see any evidence of over-heating or flash-over and the resistive track seems intact and continuous but is just no longer connected to the solder tag stake at either end.

On that basis I will find something to replace it and try again, but for reassurance I'm just wondering if these trimmers were used widely by Tek and if so if anyone has seen this sort of (mechanical?) failure mode before?

Thanks,
Adrian

From: 搁别苍é别 <k6fsb.1@...>
Sent: Jul 28, 2018 9:27 PM
To: [email protected]
Subject: Re: [TekScopes] Desoldering Iron vacuum

For a filter I used two small pill bottles one nested inside the other,
hose from de-soldering head in large one with a? tube from the top to
near the bottom, Cotton balls fill the smaller, which has holes in the
side near the bottom, the Vacuum from its top to the vacuum source. used
O rings to seal bottles, and Silicone rubber to seal the metal
connecting tubes that go to the outside. Use small bottles as feasible
because the large ones take longer to build vacuum. debris fills large
one and cotton keeps the "air" cleaner for the venturi.
works great, BTW it is an old antique Weller ds100, which has a glass
tube with fiberglass ball in the head to collect the most of the hot
stuff. I have been making my own tips from copper and brass on my small
lathe...length and mass changes temperature.
I mounted the Air compressor ( 150 psi Harbour Freight ) in the garage
about 75 feet away and ran 3/8 air line from garage through the attic
(PE) to my work area so noise is elsewhere, also allow me compressed air
in the "shop".....little 1/2 inch hole is easy to fix when/ if i ever move.
I like the idea of the insulin or B12 bottle it is even smaller! now to
get an empty....hmmm I know someone who uses....Thanks for the idea Michael!
搁别苍é别

On 2018-07-28 05:03 PM, Michael A. Terrell wrote:

I've seen an inline filter with a cotton ball in a glass sleeve so you could see when it needed changed. I've save a bunch of my empty insulin bottles, to see if I can cut the bottom off to use like this.

Somewhere I have a box of spares from a Pace desoldering system from a vocational school. Spare tips, fuses and other items that were supplied to each new student. If they quit the course, it went into a box for spares. The instructor was thinning his herd when I stopped by to visit and they followed me home.


Michael A. Terrell

From: John Griessen <john@...>
Sent: Jul 28, 2018 9:16 PM
To: "[email protected]" <[email protected]>
Subject: Re: [TekScopes] Desoldering Iron vacuum

On 07/28/2018 07:56 PM, Michael A. Terrell wrote:

I would put the compressor in a foam lined box

That could have fire safety problems. If it is too silent, and set to run via a vacuum switch,
either continuous sucking, or a failed switch could let it run continuously and heat up
inside the insulated (sound AND heat) box.

I've seen an inline filter with a cotton ball in a glass sleeve so you could see when it needed changed. I've save a bunch of my empty insulin bottles, to see if I can cut the bottom off to use like this.

Somewhere I have a box of spares from a Pace desoldering system from a vocational school. Spare tips, fuses and other items that were supplied to each new student. If they quit the course, it went into a box for spares. The instructor was thinning his herd when I stopped by to visit and they followed me home.


Michael A. Terrell


-----Original Message-----

From: John Griessen <john@...>
Sent: Jul 28, 2018 7:30 PM
To: [email protected]
Subject: Re: [TekScopes] Desoldering Iron vacuum

On 07/28/2018 06:15 PM, Ed Breya via Groups.Io wrote:

Now, of course, using a vacuum supply changes the way the desoldering unit operates, so it would need re-plumbing to eliminate the venturi and make the solenoid valve work for vacuum.

Another thing to consider is the smoky output of desoldering. I recently repaired a Hakko 470, that has
an on-demand vacuum pump, and found the smoke had gummed up the rubber flap reed valves of the pump.
It's a good idea to have some kind of receiver container up stream of the solenoid you use, since it
would be in line to get rosin smoke. Maybe a pipe fitting container with wads of TP in it?

From: "Ed Breya via Groups.Io" <edbreya@...>

Yes Michael, a window AC compressor would work too, but is kind of big for the application. I've found that mini-frig compressors are the handiest size for small dedicated P/V applications.

BTW most AC compressors are scroll type, with really good vacuum, compared to the common swashplate-piston refrigeration ones. Scrolls are the preferred kind for many DIY medium high vacuum uses, but still not as good as a two-stage vane pump - the workhorse of roughing,

Ed

From: John Griessen <john@...>
Sent: Jul 28, 2018 7:30 PM
To: [email protected]
Subject: Re: [TekScopes] Desoldering Iron vacuum

On 07/28/2018 06:15 PM, Ed Breya via Groups.Io wrote:

Now, of course, using a vacuum supply changes the way the desoldering unit operates, so it would need re-plumbing to eliminate the venturi and make the solenoid valve work for vacuum.

Another thing to consider is the smoky output of desoldering. I recently repaired a Hakko 470, that has
an on-demand vacuum pump, and found the smoke had gummed up the rubber flap reed valves of the pump.
It's a good idea to have some kind of receiver container up stream of the solenoid you use, since it
would be in line to get rosin smoke. Maybe a pipe fitting container with wads of TP in it?

From: Rick Boswell <frboswell@...>
Sent: Jul 28, 2018 7:23 PM
To: [email protected]
Subject: Re: [TekScopes] 495P A54 Memory Board Troubleshooting: continued

Harvey,

After spending some additional time with the service manual. I see there is a way to cause the microcomputer to cycle through the full range of memory addresses on a repetitive basis. This is done by relocating a jumper on the processor board. Every time the options switch is addressed I should see the tri-state buffer enable line go low and the option switch setting appear on the output of the tri-state buffer/data bus (high for SW4 OPEN, low for SW4 CLOSED). This will repeat approximately every 154ms (if I am reading correctly), making verification much easier than if it is a one-time event with each power up. This should also isolate the problem to either the U3045 decoder or to the tri-state buffer. IOW, no enable to the buffer = decoder problem, incorrect option switch setting on the tri-state output = tri-state buffer problem. Getting closer! (I think!)

PS: Dave, Sorry about the re-editing. Necessary to correct some key typos. Now that I know this causes others problems, I'll try to get it right before posting!

From: "Ed Breya via Groups.Io" <edbreya@...>
Sent: Jul 28, 2018 7:15 PM
To: [email protected]
Subject: Re: [TekScopes] Desoldering Iron vacuum

Rather than having to provide compressed air for a venturi, it's quite easy (and much quieter and efficient) to use an old refrigeration compressor to provide vacuum - they are very good at it. Using one for vacuum is simpler and safer than dealing with possibly very high output pressure (200-400 PSI) if something goes wrong. A hermetic R-12 unit can be run for quite some time with the normal amount of oil in the case, before needing to recirculate or refill, depending on your plumbing arrangement. You can run the high pressure output through an oil recovery unit, or vent it into an oil trap. As long as it's freely vented, the pressure can't build up too high.

The inlet vacuum side can go to a receiver that can be made from an old refrigerant tank, or almost anything (don't use glass unless it's safely contained, implosion-wise) that will hold vacuum and is big enough. You may not even need an external receiver, since the hermetic case is always the low side, and its volume (minus the motor, pump, and oil) may be sufficient. Once the vacuum level gets up there, the air flow through the compressor will be almost nothing, so hardly any oil will be drawn. For continuous running like this, it's good to allow a little leak from the outlet side to keep some oil moving. A vacuum switch can make it automatic so the compressor doesn't need to run all the time.

Stefan wrote that the oil he encountered was hygroscopic, so it was likely an R-134a unit. The old R-12s and R-22s used hydrocarbon oil, so would not have this problem. These will become harder to find as time goes by, so save 'em if ya got 'em and expect to ever need one. You can use an R134a unit for air/vacuum service by draining the PPO oil and refilling with regular hydrocarbon oil.

Now, of course, using a vacuum supply changes the way the desoldering unit operates, so it would need re-plumbing to eliminate the venturi and make the solenoid valve work for vacuum. Usually, reversing its pipe connections will do. Even better is to use a true vacuum-rated valve. If not available, the best regular pneumatic valve option is a low-pressure (say 30 PSI or less), large port type. Experimenting and design mods can make it work even better.

As an example, I built a vacuum unit as part of my OGWS ("old guy work station" for electronics and detailed work - lots of magnifications, lighting, and built-ins) project. I have lots of frig compressors, but I used a nice small industrial diaphragm pump since I had it on hand, and it easily fit the cabinet (a frig one would give much higher vacuum though). A vacuum switch controls its operation cycle, and a remote foot switch will work the solenoid. The receiver is a one-pound propane bottle (emptied of course), tapped for NPT. I used a big low pressure solenoid valve with about a 3/16" D seat, and modified spring. The piping is as big as could fit in the cabinet, with a very short trip from the receiver to the valve to the bulkhead fitting on the panel, where it will go directly to the yet to be built desoldering iron/head via largish PVC or silicone tubing. I have other "regular" desoldering stations etc, but this one is to be a super duper one.