Trouble with -15V Power Supply Regulator on Tektronix R7623A Scope

[nevetslab]

YUP. Uneasy is the word. My latest discovery with this mainframe is....the internal triggering system doesn't appear to be working. That's something I've always relied on with their scopes. I just spent some time swapping plug-ins from the dead 7633 scope...first swapped the 7B53A Time Base plugin, with the same problem. Then swapped the 7A18 Dual Trace Plug-in from the 7633, and the same problem there too. I restored the plug-ins, then installed a right angle T to feed the input signal over to the Ext Trigger input and that got me triggered. I tired connecting signal to the other plugin...7A22, and it too doesn't yield internal triggering. There's a Trigger Interface board that sits between the middle and right plug-in connectors of the Interface board. I had removed the other board that hid the power supply filter caps on the interface board, so hadn't disturbed this little Trigger board.

I never did put this mainframe into service when I bought it, so I don't know it's history. I remember it working, but never looked at everything. I use X-Y mode on these scopes, via the 7B53A....can't remember if that trigger mode is involved in that., I don't need is a scope that won't yield internal triggering. So, another task at hand. I did look to see what that IC was on the Trigger Select board parked on the interface PCB. It's a Tektronix IC. I hope THAT's not the cause of this triggering problem. They use a total of three of them in the scope.

[nevetslab]

This morning, I opened up the R7623A Mainframe, removed the three plug-ins, then removed the cover plates attached to the small Vertical Interface Board and the Trigger Select Board, both mounted to the Plugin Interface PCB assy. I removed the Trigger Select PCB, as that's my suspect on this lack of internal Trigger. It has to pass thru this board on the way to the Horizontal Plugin connector. A sample of that trigger signal goes to the Output Signals PCB on the Rear Panel. Signal Output BNC comes from this board. I get NO signal thru that connector, and while I found slight burn evidence on a 69.8 ohm resistor from the +15V supply to feed the pair of NPN xstrs that pass that Trigger Select signal on out, the resistor still measures 69.8 ohms. Only evidence of burn I've seen thus far.

The Trigger Channel Switch IC...Tekronix-design/fabricated IC P/N 155-0022-00 is used on this board, and two more in the scope...one on the Vertical Interface board next door on this Plugin Interface PCB, and the other one on the Horizontal Amplifier PCB. All of the semis', resistors, caps on this board measure ok, apart from this custom Tek IC.

It (and the Trigger Select PCB) is used on 7603's, 7613, 7623, 7633, 7623A, 7623B scopes for sure. I have a second 7633 scope like the one on my bench in storage, but I don't take my car out on the freeway these days, living on borrowed time with a timing belt that hasn't been changed yet.

So, for the time being, I'm using Ext Trigger on the Time Base to get stable triggering. Scope still running healthy.

I've brought in my R5031 Dual Beam Storage Scope that I had posted a thread on Feb 2021 when I replaced some lamps in the Fiber Optic Readout Display next to the CRT to give the vertical and horizontal switch settings. The scope died on my shortly after I had posted that thread:

https://music-electronics-forum.com/...adout-assembly

Now to dig into that one to see what's failed.

[nevetslab]

After getting assistance from some of the members of tekscopes@groups.IO in chasing the absence of Internal Trigger via Tek's Trigger Selector PCB that has one of their custom Trigger Channel Switch IC's, I finally found the fault dragging down the differential output voltage that feeds both the Trigger signal to the Time Base Plug-in as well as to the Output Signals PCB assy to drive the Vertical Signal BNC Output, I have disconnected a cable to that Output Signals PCB assy. With it disconnected, I now have a working scope to either temporarily replace my 7633 Storage Scope that presently has NO Trace on the CRT, or it becomes the new scope on the bench. Problem is bench space....my 7633 is a vertical format, half the width of this R7623A. My bench isn't deep enough to make it work without adding something to sit this rack mount scope onto. But, at least I've now a working replacement.

To fix the Output Signals PCB assy, getting to that PCB is major disassembly of the Power Supply Rectiver/Filter Cap assy, which I had to do in early September to replace the one part that couldn't be removed without complete dissection. Wish I had known there was this fault on the PCB when I had it apart. I can't think the last time I've needed that output on any of my scopes, so I can no doubt do without it for now. Job for laterman.

I guess I'll have to park my Sound Tech 3100B Audio Generator as the pedestal to sit this scope onto. That elevates the scope above the 5KVA Isolation Xfmr that is in the way on this bench. I don't use that generator that often, but real handy when I work on high output power amps, where you need a small burst signal output to see max output and watch the power supply sag from that high current output. Can't run 9kW power amps at steady-state, but can under short bursts. Storage Scope comes in real handy in capturing the output waveform.

So, this has been a learning experience....and assuming I'm NOT dealing with failure in the HV CRT circuits, I'm more acclimated to try and find what failed in the 7633 storage scope, which will finally get extracted from the pile of gear on the right side of the bench.

Onward..........

[nevetslab]

Yesterday I pulled the right side of my Test Bench apart to extract the Tektronix 7633 with the Dead CRT and moved the Sound Technology 3100B Audio Generator into place as the pedestal to stack the Rack Mount R7623A Storage Scope on top of it. Had to put a short 3/4" thick board under the rear feet of the 3100B to clear the height of the Amber 3501a Audio Analyzer so the R7623A scope would sit above that instrument. Then parked the HP 3580A Spectrum Analyzer on top of the scope, set it all the way to the left to have just enough room for the Tek TM502 Mainframe for my TM500 series plug-ins that had been next to the 7633.

I hope the large Tilt Bail/handle of the Sound Tech 3100B is robust enough to support the weight of the scope, spectrum analyzer and TM502 package. Tempting fate here.



The waveform shown on the R7623A scope is 500Hz Sine-Random signal from the Bruel & Kjaer, set for 100Hz Random Noise Modulation, with the scope in triggered single-sweep Storage capture of that random waveform. So, I'm back to normal..almost. I found out today that the CRT Graticule Lamps are burned out on this scope. Took that display assy apart from the front to find not only are the three T 1-3/4 6.3V/200mA long life incandescent bulbs burned out, but there's now no voltage present on that small shaped PCB to which the tiny lamps are soldered onto. Other end of that cable is inaccessible at the rear end of the Storage PCB assy that sits next to the CRT assy in the scope chassis. I also found yesterday after rearranging this side of bench, that the + Gate and + Sawtooth Waveforms are not working, which tells me the Output Signals PCB assy at the back end of the Power Supply Rectifier/Filter chassis has more than just mating circuit that normally passes the Trigger Signal out to the Vertical Signal BNC on the rear panel of that scope.

I also found the two BNC cables that are connected to the back side of my Hameg HM8042 Curve Tracer chassis had the remaining X-Axis Label fall off one of those two cables that would connect to the left plug-in of the scope, setting the instrument in X-Y mode to yield a Curve Tracer display. So, I have to identify those cables again.

[Tom Phillips]

Congratulations for the revised test bench set up.

... Tempting fate here...

Let's all pray that the California earthquake demon keeps resting.

...So, I'm back to normal..almost...

Ha. You are far above normal. I have recently seen web postings by self described amp builder/repair gurus whose bench sports little more than a hand held DVM and a soldering iron.

Cheers,
Tom