Maintenance on Tektronix R5031 Dual Beam Storage Scope

[nevetslab]

Having a break in Backline Gear repairs here in the shop, and still needing to extract my Tek 7633 Storage Scope from my bench set-up to tackle the absence of the CRT system working, I brought one of my Tek R5031 Scopes back in from home. Feb 2021, I had it over at the temporary shop where CenterStaging's Rental Inventory got moved to, and I didn't tear apart my bench here on Winona Ave, so I brought this scope in from my storage lockers. I needed the storage function for aid in looking at a Vox Tremolo modulation, but I hadn't had this powered up since early 2000 at BGW Systems.

I had powered it up, found there was some lamps out in the fiber optic readout next to the CRT screen (they used this elaborate character generator method on this scope as well as on the 576 Curve Tracer that many have acquired for use on vacuum tubes characteristics). I found the replacement parts, and pulled that assembly out, figured how to repair it and got that task done. I had the scope back together and used if for a few days...then it quit working.

So, I have it again on the bench, digging into it, and thus far, have gotten past the Pass Xstrs that often fail....using TO-66 xstrs and a couple TO-3 Germanium Pass Xstrs that I found yesterday and am somewhat mystified in finding them. PNP devices..Tek P/N 155-0165. They measured fine in Diode Test Mode, but I found a reading in one direction reverse-biased between emitter and collector. That seems odd.

Meanwhile, turned the instrument over, and removed the cover to expose the Rectifier/Capacitor PCB assy. Documented the color code/wire connector harness assy, just in case. Beautifully harnessed, with all but a few connectors broken out of the harness so it's easy to restore the connectors...except for those few extra connector wires from a separate harness.

Took photos of everything per normal practice in servicing these instruments.







Now I'm finally able to see what I have to work with, and with Tektronix surperb documentation in the service manual on this instrument, which details the wire colors of that power supply harness i unplugged from this Rectifier/Capacitor board, plus my notes not having found their wiring chart yet, I'll be able to put the repaired assembly back into the instrument. Date Codes on these cap-cans ranges from 6921 to 7032...so over 50 yrs old. Interesting device on the PCB. A 5000 hour timer. There's also one on the Storage Board.

Beautifully designed, engineered and built instruments. Gold was more affordable in those days. Look inside any vintage Hewlett-Packard or Tektronix instruments of that era, and you find the PCB's all gold plated top/bottom, connectors all gold. Harnesses beautifully crafted. I don't think they were conceiving these would still be in use 40 and 50 years later, but they DO last that long and hold their accuracy quite well. They took the time to create well-conceived operators and service manuals. The manual for this instrument is loaded with well-produced photos to aid in the service routines.

I checked the four filter caps on this board...first just checking to see what the charging characteristics looked like with a DMM to see if they charge up in a predictable fashion. Then I applied a 20V P-P 5kHz Square Wave that I use for checking ESR...if I go thru the full procedure to calculate what it is. Usually I'm just looking for gross behavior, as I found on the Tek 7623A's 14,000uF buss cap, which only loaded that waveform down to 11V P-P instead of around 100mV to 150mV P-P as it should typically. All four caps...one being a dual-section (100uF/250V, 700uF/75V) all yielded typical low P-P readings in the 100-150mV range with that waveform, so I think I'll just take the time with each cap and charge them up on a power supply in stages thru a resistor and check to see what the change in charging current looks like. And look at them on the General Radio 1617 Capacitance Bridge, which has Leakage Current provisions. These ARE old enough to want to replace them, but, mechanically, it would be a challenge to get it done and remain mechanically stable like the original parts they bought from Sprague. Can't buy these parts any longer.

This is a low-bandwidth instrument. Each channel has very high sensitivity....10uV thru 10V-Div, max bandwidth of 1MHz. Differential inputs, also has provisions for their P6021 AC Current Probe, providing 1mA thru 1A/Div. Largest of the CRT's in their instrument line until the recent all-digital scopes of today's array of instruments. I used one like this from 1976 onward...bought a Wavetek 185 5-decade log Sweep Function Generator, and with a log converter and that Log-sweep generator, it gave me a nice Frequency Response Analyzer from sub-audio thru 1MHz (with HF rolloff from the BW limitation). Display in XY Storage Mode..

So, hopefully, I can restore what failed and get this instrument back up and working.

Onward.....

[Tom Phillips]

YES! Everywhere the glint of gold.
There is a nice YouTube post at https://www.youtube.com/watch?v=7weZ0TNRcuw that shows the Tektronix PCB process circa 1969. It's a 30 minute program that appears that it was original released on film. The fabrications of the physical PCB begins at 10:25. The gold plating process begins at 17:40. It turns out that the gold also served as an etching mask at one point in the fab process.

[nevetslab]

YES! Everywhere the glint of gold.
There is a nice YouTube post at https://www.youtube.com/watch?v=7weZ0TNRcuw that shows the Tektronix PCB process circa 1969. It's a 30 minute program that appears that it was original released on film. The fabrications of the physical PCB begins at 10:25. The gold plating process begins at 17:40. It turns out that the gold also served as an etching mask at one point in the fab process.

Cool! I'll watch that tonight after the Dodgers/Padres game.

Well, that sure brought back memories of working on the light table in the layout process. I worked at 2:1, not needing the higher resolution, and allowed larger PCB's to be laid out in a reasonable space. We didn't have the photographic facilities for the film steps...made use of Century Graphics a few blocks from us to produce the 1:1 reductions, then sent the artwork out for prototyping, and eventually production boards once all the revisions were done.

Interesting to see the steps and, by today's standards, antiquated gear used for that generation boards...but all the same basic steps. Interesting that Tektronix kept so many of the processes in-house. This R5031 is just a marvel of engineering and mechanics used in producing such a cool and reliable instrument that has lasted decades.

Interesting to find two separate 5000 hour timers installed on the PCB's...one on the power supply rectifier/filter cap board, and one on the Storage board.

Enjoyed the video!

[nevetslab]

I fetched my General Radio 1617 Capacitance Bridge to the bench, as it has a good Bias Voltage/Leakage Facility on it. I measured the capacitance and DF on the five capacitance sections on the PCB. One is a dual cap-can....700uF/75V and 100uF/250V. The 700uF section measured fine for both capacitance and leakage, but it's partner...100uF/250V has high leakage. There's nominally 240V on that cap. Watching the leakage current rapidly increase as I nudged the bias voltage up from 200V to the nominal 240V indicated its not happy at that voltage potential.



I also found the 3500uF/25V cap, that normally sits at 18.5VDC, it too is on the verge of being unhappy if that voltage is above that potential. Leakage current on it rapidly increases above 20V, so there's two cans on this PCB assy that need to be replaced. That's tomorrow's challenge....find some suitable parts that I can retrofit into the assembly. Luckily I have around 58 mm of height below the PCB to work with. I should be able to find something within the length and diameter to fit both patterns.

Onward.......

[nevetslab]

I finally received the last of the three replacement electrolytic filter caps to replace the two capcan's.....a dual (C730A/B---700uF/75V and 100uF/250V) and a single (C755--3500uF/25V). I had selected three parts...a 100uF/250V, a 680uF/80V and a 3300uF/35V....choosing the highest ripple-current rating at 120Hz and -20deg-105deg C temp rating. After those three arrived, I found I made a mistake in ordering the 100uF cap with just a 250V rating, as the range in operation exceeded 250V. So I selected the best 100uF/350V part that was available, again highest 120Hz ripple current rating. That part came in today.

Last week, when the caps came in, I measured them on the General Radio 1617A Capacitance Bridge, selecting the lowest leakage rating/lowest ESR rating. The larger/higher voltage 100uF cap (100uF/350V) came in this morning, so I checked the two of those out, selecting the best of the two of those.

Removing the original capcans, as careful as I was, I still managed to loose partial top side foil pattern of the ground tabs, which is totally frustrating to have happen. Made do with the what was left,and using some nylon #6 flat washers to space the new 7.5mm L/S parts (16 and 18mm dia bodies), got the new parts installed and circuit pattern restored. Added additional solder onto the 0.025" square pins' solder pads, just to sure I had good connections.

The updated board is ready to get installed into the chassis now.





Now the two capcans I removed....both measured ok, but I was seeing the leakage current was on a rising knee at the applied voltage in circuit that I had dialed in on the GR Capacitance Bridge looking at ripple current. I don't know what the actual load current is on these three caps. So, I don't yet know if what I've chosen to replace is going to restore operation of this scope. For all I know, the problem is in the odd Pre-Regulator circuit Tektronix designed on the primary circuit of the power transformer. The huge heat sink on the rear panel is allocated to a single TO-3 power xfmr that is there to take care of the inrush current (when an SCR runs briefly) and then handles keeping the peak AC voltage to minimum ahead of the voltage regulators. Hope THAT area isn't where the problem is.

I do have a Simpson 10kV 1000:1 Voltage Divider probe, not having the special Triplett 630 VOM that has the 4100VDC setting that's called out in Tek's service manual, for checking the HV circuits for the CRT. That probe is, of course, NOT here in the shop but tucked away in storage. Rats.

The four pass xstrs on that heat sink are TO-66 metal cans, but are NOT coupled to that large heat sink. Years ago, I had the one on the 50V regulator fail on the same model, but not this particular instrument. All of the power supply semi's have measured ok, as far as DMM Diode-Test mode goes, but haven't had any on the curve tracer.

So, I'll attempt powering this up later today, perhaps.

[nevetslab]

I installed the power supply PCB assembly back into the chassis, following my wiring chart and got it restored correctly with all harness wiring. When I first plugged it into the AC mains, I got no current draw, so unplugged the power cable, and opened up the Fuse Holder assembly (a large assembly typical of what Tektronix would do). Found the 1.5A fuse (AC Mains Fuse, type 312 Fast Blow) was open, so replaced it. All I had were Type 313 (Slo Blow 1.5A), so installed that and tried again. I briefly got the CRT graticule lamps on, while seeing 500W being drawn by the scope. Within a second, the fuse blew, and this time, there's the smell of something having burnt. I didn't see any trail of smoke come out, but definitely NOT a good sign.

R5031 PS Circuit Description-1-2.pdf

R5031 Service-PS Sch.pdf

[nevetslab]

Today, I moved the scope back up to the bench, opened up the top cover, removed the protective cover for the HV/LV Power Supply Regulator board to see if there was anything obvious there. I also released the rear heat sink that's on a pivot-axle that swings out from the rear panel, exposing the small Pre-Regulator mounted to a heat transfer block coupling to the large cast heat sink. Unplugged the Pre Regulator assy for inspection. I found the resistor R707 6.8 ohm 1/2W Carbon Comp resistor burnt, measuring around 25 ohms, and it broke apart during removal. The SCR Q707 is called out as a 1.6A 400V TO-5 part, RTD0440T. I didn't find any part fitting that number when I googled it, so changed search to 1.6A 400V TO-5 SCR and came up with a family of parts in this TO-5 package, 2n2329 being the highest voltage (400V), and 2N2322 being the lowest voltage (25V). Data sheet for this:

2N2329_1.6A-400V TO5_SCR.pdf

I only get reading between what would be the base and emitter terminals on this existing SCR when I unplugged it. I haven't tried to set up a test on this part to see if it functions as an SCR thus far.

I removed what was left of the 6.8 ohm 1/2W resistor, and replace that with three series-connected 2.2 ohm 1W MO1S Metal Oxide resistors in a PI mechanical configuration to fit the pattern. All the other parts on that assembly measured ok.

Removed the blown 1.5A SB Fuse, replaced it with an 8A SB fuse, and connected the scope thru my 2A Magnetic Circuit Breaker (trips at 2.7A). I plugged the scope's power cord into that Circuit Breaker thru my 20A variac/Vahalla 2101 Power Analyzer, held my breath and pulled the scope's power switch out (linkage to switch internal toggle switch on and off), and saw over 500W of power being drawn as I immediately switched it off, not even waiting for the breaker to trip.

The smell I was catching was off of that damaged 6.8 ohm R707.

I found Allied Electronics had stock on this replacement SCR. Now writing up a thread on this trouble-shooting of the Tek R5031 Dual Beam Storage Scope to post on TekScopes@groups.io to see if there's any response on this maintenance exercise.