Oscilloscoe Failed

[nevetslab]

I went to resume chasing an intermittent signal on a client’s Aguilar DB751 this morning. Powered up the bench, powered up the amp, and had it fed with 1/3 Oct 50Hz Pink Noise (from a B & K 1027 Sine-Random Generator thru a 1617 Filter). This particular format signal yields wildly fluctuating 50Hz Sine Wave, having a min/max range of a bit over 20dB, where the 1/3 oct filter strips the wide bandwidth pink noise down to a sine wave signal. This is what that test signal looks like on the functioning Tek 7633 scope (in Storage Mode):



I went to look at the signal on the scope, and found no image at all. Not even a baseline. I switched the vertical plug-ins to GND, which always yields a steady baseline, but nothing there. Pressed the Beam Find, and again found no image. Tektronix 7633 Mainframe, 7A18 vertical plugins, 7B53A time base. Time base working fine, triggering properly as tally lights indicated.

RATS. I don’t have time for scope maintenance right now. What are my options. I have a Tek TM502 Mainframe next to the scope, which could accept one of my SC504 80MHz dual channel scope plugins, but those are at home. That removes being able to use that Mainframe for my TM500 plugins that are often in use. There’s not really enough bench space for my TM503 mainframe, as the TM502 just barely sits on the edge of the bench.

I had to resort to dismantling the stack of gear atop the Amber 3501a Audio Analyzer to place my Tek 326 Battery-Powered scope on top of it, removed the Krohn-Hite Filter that I haven’t been using (with the Burst Pink Noise source), so put that scope on top of the Amber. Handle is sort of in the way, stuck out straight. Inputs to the scope are on the left side….no very convenient, but will have to do for now.



It’s been years since I’ve had to go tearing into either 7633 scope mainframes. I have a second one in storage, but don’t recall the status of it. But, that’s at least spare parts. I had been seeing minor trace LF modulation….Power Supply issues being indicated on this one that now has no CRT trace visible.

Sigh…….I’ve always kept quality reliable instruments on the bench so I don’t have to think about it. At least I have back-up scopes, however inconvenient to what I’m used to. I guess there’s never a good time for a major piece of daily-dirt test gear to fail on ya. At least I have the service manual for it., and spare parts from the other mainframe, though that’s a 60 mile round trip and renting a car to go fetch it. I don’t take my car on the freeway, knowing I’m on borrowed time with a timing belt that hasn’t yet been replaced.



While the camera image only showed a segment of the variable-amplitude 50Hz sine wave, in those two close-ups of the 326 scope, the analyzer's meter below do show a portion of the magnitude fluctuation. The analyzer meter's ballistics are slow, not showing that character that you see on the top scope image taken some time ago on the 7633 in storage mode. I briefly watched the Generator's meter, set for 0.1S Meter Averaging Time, showing well over 10dB Fluctuation of the min-max range of that envelope.

I hope I can get this Tek 7633 scope restored soon. Too many scope functions on it that I use a lot,such as X-Y and Storage that I can't do on that little scope, or the SC504 that I'll bring in tomorrow. Where to put that for the time being? I hate instrument failures! No time scheduled for that imposition.

[xtian]

It must have taken you an hour to assemble this very nice post with photos...

[nevetslab]

More like 20-25 minutes to figure where to park the little Tek 326 scope. I knew I had some photos of that often-used 50Hz Pink-Noise/wide variable-amplitude Sine Wave signal. Comes in real handy getting amps to bark at low frequency where current limiters often misbehave. Tomorrow when I come in, I'll take down that stack of gear sitting atop the Tek 326 and park my Tek TM503 mainframe that has an SC504 80MHz scope and DMM in it, so I can leave that TM502 on the right side of the failed scope....for now. Didn't take but a few minutes to take the photos...more time to process them though. So yeah, time is fleeting when writing up a post.

Fixing the scope is another matter. I knew the day was coming from seeing the periodic tell-tale power supply waggle on the waveform, but it's not been obvious. I just hope it's not a serious failure. I really miss having the Tek 7834 4-plug-in storage scope. That was a loaded tek scope cart package that I had at a friend's keyboard shop in Encino, and he's disappeared with all of the gear I had there. Plus a great collection of tools. Somebody I DO want to find again!



Pains me every time I think about it. I was apprenticing under him a couple days a week doing the grunt work on keyboards for him, learning the ropes so to speak. It was enough to let me know I didn't really want to do keyboard work. Takes WAY too much space and once disemboweled, too many parts and fragile. Need the kind of space it takes to fold parachutes. Tim Conniff. He was the last person I thought would ever burn me, if that's what happened. His wife was Extremely ill, and he was in a really hard spot trying to contend with that and keeping creditors at bay. Love to have my shop assets back, but I'd just like to know what happened to him and why. Doubt I'll ever seen this gear and tools again, though I trust somebody is getting excellent use out of it.

[tedmich]

Timothy Conniff, engineer/owner at Synthoffenders in Encino eh?

Maybe the company name was a tipoff!??

Sorry you had a friend do you like that Steve, and I hope you can bring your Scope back to life!

[nosaj]

If you want to contact him (408) 272-1896 (951) 529-6639 listed on radaris.
nosaj

[drewl]

Had to fix the horizontal amp on my Tek 465B at work last week.
All the other scopes at work are dead or damaged from the move in June or from our last move six years ago.
I think we have some other good scopes packed away somewhere, just need to find the container they're in.

[nevetslab]

If you want to contact him (408) 272-1896 (951) 529-6639 listed on radaris.
nosaj

Hey, excellent. Last time I attempted to contact him, he didn't acknowledge it was him or knowing me. Hopefully I can get past that. I had found him listed in LinkedIn but hadn't succeeded in getting thru to him. I still have a good friend who knew him, as Tim taught both him and his dad servicing synthesizers.

Yesterday, looking thru my service manual for the 7633 and the 7623A, I did find the 7623A uses the same power supply module and circuits to power all of it's circuitry, and I have THAT here at home. It didn't power up properly last time I tried to run it, but at least it produced a trace. On the Tek scopes, the -50VDC regulator in their system is the reference for all the other regulators. And I see the 130VDC regulator in use in all of the higher voltage circuits that run the CRT. Hopefully it's just issues in the power supply. As I recall, Tek made use of CapCan Electrolytics in that section, though each one is a single value and not multiple sections on each can. The layout does show a pattern for three sections, but the schematic doesn't indicate that as multiple. It's old enough to warrant replacing all of those caps while I'm in there.

[GoldtoneWorkshop]

Makes me think about recapping my 2246. It runs great, but I know it needs a thorough service. Might have to switch to a different scope for a while and take the time to tear into that project.

[nevetslab]

Tomorrow I'll drag in my R7623A Scope, as it's power supply unit is the same as what's in the 7633 scope that has failed. Tek's service manuals from the 70's and 80's were extremely well written, and have assisted me in keeping these fine instruments running all these decades. I'll also get to see the physical details on the CapCans they used in the power supply, so I can get the lead spacing/diameter for ordering the replacement caps. More to follow, as the surgery will be documented.

[nevetslab]

This morning, I brought in my somewhat newer Tektronix R7623A Storage Scope, loaded with the same 7B53A Time base, a 7A18 Dual Vertical Amp and a 7A22 Differential Vertical amp plugins.

After working on my 2021 Tax Prep files, I finally removed the top cover of the scope mainframe, oriented myself with the location of the power supply assembly as well as one of the connectors that would yield the main voltages from this power supply module

-50V Reference Supply reads -50.01VDC
-15V reads -12.5VDC
+5V reads +5.00VDC
+15V reads +15.04VDC
+50VDC reads +49.99VDC
+130VDC reads +128.86VDC

The supply caps and their unregulated voltage on them:

C806 250uF/150V reads +129.53VDC
C808 1800uF/75V reads -63.67VDC
C809 1800uF/75V reads +63.56VDC
C811 14,000uF/25V reads -16.63VDC
C813 & C814 both 9600uF/30V, and are in parallel, reads +22.74VDC

C811 and C813/C814 are fed from a full wave bridge, center of the caps at Chassis Gnd. The schematic in the service manual indicates the unreg voltage on C811 and C813/C814 should be reading -18V & +18V, and their regulators should yield -15VDC and +15VDC. This R7623A has issues of it's own, yet to be figured out, but.....I brought it in because it has the same power supply assembly as is used in my 7633 scope that failed.

On the positive side of things, while the CRT display isn't yet functions properly, it IS yielding a trace, and the time base is changing the rep rate of what IS being shown on the CRT, so at least there's hope there. I have NO CRT functions at all on the 7633 at this point.



I haven't yet extracted that instrument from the stack of gear on the bench. I figured I'd start by educating myself with a similar instrument of the same family (7613, 7623, 7633 is this scope family), and the 7623A came along later to replace the 7623. I think they retired the 7633 while the 7623A remained, before retiring the 7000 series scopes for the 11,000 series scopes.

I unplugged the three plugins to see what changed on the unregulated supplies. C811 increased to -17.5VDC and C813/C814 increased to +23.5VDC. So, there's still loads of circuits left connected to this power supply. I did stop to check the bridge rectifier that feeds C811 and C813/C814, and it measured ok. I was wondering if perhaps one of the diodes in that bridge rectifier was open. I hope that 14,000uF/25V buss cap isn't going bad on this supply.

And I've yet extract the power supply module from the 7633 mainframe....have more reading to do before getting there, and, I expect to tear down the stack of gear that has the failed scope at the base so I can open it up and have a look at what it's power supply readings are. There are, of course, much higher power supply systems in the instrument that I'm NOT looking at at this point. The -50VDC Regulator is the system supply reference that runs them all, and the +130V supply is also a reference to the higher supplies that I haven't gotten to yet. Crawling first before attempting to waddle on my knees, before I try to walk, so to speak.

One difference between the two scopes....the 7633 has a fixed attached power cord, while this 7623A that we're looking at open has an IEC-320 Cord connector. I haven't gotten far enough thru the manuals to find if that's a major stumbling block. The thought here is making use of this later power supply module to swap out that in the 7633.

The date codes of these large computer-grade buss caps that you can sort of see under the main supply PCB (look like Mepco/Philips. and NOT using #10-32 screws/insulated bushings, but instead using multiple pins and have space below the PCB and the caps, but are of that computer-grade quality. When I saw that, it sort of eliminated that thought of just ordering new supply caps and doing a retrofit. That thought just got more difficult, if that's part of the solution.

I haven't yet taken the documents to the copier to scan schematics to provide in this procedure.

Onward.

[Tom Phillips]

...After working on my 2021 Tax Prep files...

I hope that's a typo.

[loudthud]

Generally there is an unregulated Voltage that goes to a switchmode power supply for the High Voltage. Cathode Voltage (-2KV or so) generated directly and anode Voltage (+20KV) (Don't measure) comes from a Voltage multiplier brick. Tek liked to use a Triplet VOM with a 6KV range to measure/adjust the CRT Cathode Voltage. Don't worry about/adjust the HV if it's close, concentrate on getting a trace on the screen first. The storage system needs some moderate Voltages, probably derived from the +130 along with Focus Voltage. Again these shouldn't affect the screen unless Storage is turned on.

[nevetslab]

I hope that's a typo.

I had filed for an extension in April....due in October

[nevetslab]

Generally there is an unregulated Voltage that goes to a switchmode power supply for the High Voltage. Cathode Voltage (-2KV or so) generated directly and anode Voltage (+20KV) (Don't measure) comes from a Voltage multiplier brick. Tek liked to use a Triplet VOM with a 6KV range to measure/adjust the CRT Cathode Voltage. Don't worry about/adjust the HV if it's close, concentrate on getting a trace on the screen first. The storage system needs some moderate Voltages, probably derived from the +130 along with Focus Voltage. Again these shouldn't affect the screen unless Storage is turned on.

I saw that HF switch on the primary of the HV system for the CRT. I'm not there yet, as far as this R7623A mainframe goes. I opened this up to see what the Power Supply assy looks like in the flesh, and to see if this might be a viable replacement power supply assy that I could swap out in the 7633. Only thing that concerns me with this one is the +/- 15V regulator system, as the input filter voltages are out of balance. I don't yet know what's causing that without further digging.

I know I have a high voltage probe...a large red-handled Simpson probe, if memory serves. And a Tek P6009 X100 High Voltage Scope Probe (limit to 1500V). Not there just yet.

[loudthud]

Symptom: No trace or readout visible on the screen.

Possible causes:

Beam is being deflected off screen. Turn readout off, check DC Voltages at deflection plates with no plugins installed.

No HV.

Z axis problem. There is a z-axis amplifier where unblanking pulses from the horizontal plugin, the readout and intensity control all come together. This is fed to a circuit called the DC restorer before it connects to the grid of the CRT. It can be hard to find problems in the DC restorer, but you have a good one to compare against.