Tektronix RM35A / 535A (Tek 535) Oscilloscope Restoration

 

Yet another one …lol…

From

53273006149__E8B755B6-4E1D-47C2-9029-795A876C92A0

To

final4

Yes, I rescued this guy from going to landfill. For 20$, I brought this as a donor for my main project back in October 2017, the 549 Restoration

I used 2 to 3 6DJ8s from this scope for restoring my 549, and this was sitting in the rack since then.  It was way too dirty and dusty, I never bothered to even touch it. Since I had some free time after the 576 curve tracer project I started looking in to it on a bored evening. I was surprised to see what I found inside, Bugle Boys !!!

All over the scope, almost 90% of the 6DJ8 tubes were bugle boys !!!

 

 

 

 

 

 

 

 

 

I am glad that no tube scavenger found this. That kinda inspired me to restore this fella.

Please refer to the 549 Restoration for detailed steps and instructions on typical restoration process.

Refer Tek Wiki  http://w140.com/tekwiki/wiki/535 for more Reference details.

So here is the condition I  got it in, I marked some interesting components.

 

 

 

 

 

 

 

 

 

 

lots of dust, Bumble Bee Caps,  Vintage Oil impregnated Paper caps at HV section. Someone had already replaced a lot of bumblebees in the power supply section, but still there were a lot remaining in rest all parts of the scope.

Calibration sticker still on over all the calibration Pots.

First step is to clean it inside out, remove all knobs and clean, remove tubes and clean too (without wiping the marking off). Here is the condition after cleaning

 

 

 

 

 

 

 

 

 

Looks much cleaner and new !!

Now proceeding to electrical troubleshooting. Again Please refer to the 549 Restoration for detailed steps and instructions on typical restoration process.

CAUTION:

This instrument contains potentially deadly voltages, up to 10,000V for the CRT supply. Regulayed DC output is 500V. It can deliver lethal DC and AC Voltages at substantial current levels to RIP you in one shot. There are better ways to die if you are up to that.

DO NOT ATTEMPT TO REPAIR/RESTORE ONE OF THESE UNLESS YOU ARE QUALIFIED AND KNOWS WHAT YOU ARE DOING AND HAVE THE RIGHT TOOLS, PROBES AND FOLLOWS ALL SAFETY PRECAUTIONS.

EVEN IF YOU ARE QUALIFIED, PLEASE DO TAKE GOOD CARE AND ATTENTION WHILE WORKING ON THIS.

 

Oscilloscope moved near to my lab bench on a strong table.

repair

First step was to test the Caps. Definitely electrolytic caps are bad,Here you can see the -150V filter caps showing dead leak.

repair2

Next one was to confirm the bumble bees cap. They are definitely leaky, but still wanted to confirm. So here it is.

{ Heathkit IT-11 used for electrolytic as it has got a discharge resistor inside, and EICO 950B for testing the bumblebee. To explain more, after testing the cap with High Voltage, you need to discharge it. in EICO 950B, its done using the POT (Wire Wound) which is a bit weak to handle high currents an electrolytic can offer during discharge. In Heathkit IT11 there is a dedicated discharge mode and power resistor to handle this}

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There are bumblebees and oil impregnated Paper caps all over, hidden under tubes, mounted on the timing switch for both Timebase A and B. Here is the oil caps

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HV Caps are under the Rectifier Tubes. You need to remove the HV Rectifier tubes to access them.

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Here is one oil impregnated Paper capacitor from HV Section.

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So natural process of replacing all caps has to be done.

Tip: Refer electrical parts list in service manual and mark each PTM (paper) type cap in the list, and trace it on the circuit, and then locate them physically on the oscilloscope and replace them one by one.

Here is what I did, for example in CRT section.  Get capacitor number from parts list, mark it in schematic, locate it physically, mark once it done. I repeated the same for every section of the schematic.

rep-ref2.jpg

So Here it is after replacing all Electrolytic and Bumble Bee/Oil Caps. Sorry for the bad job with electrolytic  caps. Had no choice.

 

 

 

 

 

 

 

 

 

Remember again, don’t forget the hidden/hard to spot bumble bees under the ceramic strips and on Timing switches.  You need to follow the electrical parts list to locate all, visual inspection will not reveal all. They are hard to reach and replace. all the best 🙂  Example below.

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HV Section also needs replacement caps. Here it is. I used HV Ceramic Caps

 

 

 

 

 

 

 

 

Here is the bag full of “bees” I have replaced

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Now to the next critical component, Tubes. Used Tube Tester to test all Tubes. I typically follow the component list to mark tested tubes and replacements. Eg. Below.

 

rep-ref1

 

Here is the bad boy list of Tubes.

1X 6080 – on power supply

1x 12BA – on power supply

-both Series Regulators

1 X 6Au5 – for HV Transformer – All were electrically leaky. Less than few 100Ks between electrodes and 6AU5 was dead short –  due to bad caps at HV.

1 X 12AT7 was physically leaky – aka Air inside. replaced that too.

I used few Russian 6N1P (6Н1П) tubes for missing 6DJ8s.

WARNING: These tubes have~ half gM of a 6DJ8 (7500 Vs 12500-15000 for 6DJ8) and consumes almost double heater current than 6DJ8 (~600mA vs ~365 for 6DJ8) . Do not consider it as a straight replacement. I used to them in sections where the tubes are used as switch driven all the way up from cut-off to saturation.

Here is the bucket of bad Tubes,

IMG_0863

Since I am anyways in, Decided to remove and clean and reinstall the CRT too.

WARNING : BE CAREFUL. DO NOT REMOVE CRT UNLESS YOU KNOW THE RISKS INVOLVED. CRT  can IMPLODE if not handled properly  and can cause SERIOUS INJURIES.

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I usually check the CRT quickly for heater glow before installing it back, Tube Heater tested good, no smoke or leak. You can see the Anode coating in spiral and the tube is transparent.

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Now the most painful part, the Ceramic Strip is broken in Time Base B section. Both sides of the strip is cracked. I had ordered used a equivalent from ebay, and decided to replace it.

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Had to replace the entire strip by lifting all components from each side and removing the ceramic strip and replacing it and re-soldering every component back. Remember, there are components on TOP, wiring connecting different slots of the strip, and connections coming from wires (Power, Signals) and leads to tube sockets. Sometimes each notch can have up to 5 leads soldered in it.

 

 

 

 

 

 

 

 

and here is the final result after replacing both ceramic strips. The section in red block is with new ceramic strips

Repair22

Now time for a good round of visual inspection. for shorted leads, components or cables. Do that at least thrice over each area to ensure that  no leads are short circuited or touching each other inappropriately 😉

Next step is to power up, with Variac. As always monitor all rails to ensure there is no unexpected behavior in power circuit .

Here is the Rig for that.

 

 

 

 

 

 

 

 

 

 

Power up was smooth, however the time delay relay clicking was not very nice sound. It was almost felt like a lot of arcing. Decided to open and clean the relay contacts.

Open the Time base B Door to access the nuts holding the relay cover. and pull the cover off  from the other side after removing nuts. Relay contacts are accessible for cleaning.

 

 

 

 

 

 

 

 

 

Wondering what this is all relay and delay about ?

Here is a video explaining the same 

 

 

 

 

More close up of the Delay tube – from a 547 Though

 

 

 

 

 

After cleaning the arcing vanished, and here is the first ray of light.

Firsttrace1

Im not surprised that I don’t have horizontal trace. But V is working, as you can see the vertical sweep. Since this is the 7th or 8th oscilloscope I am restoring I know what the problem is !! – The Switch Contacts.

Had to clean and lubricate all rotary switches, that’s all needed to bring the H Trace back as I know rest all components prone to failure are tested and confirmed working prior to power on.

Here is the video of how exactly it happened,

 

 

 

 

 

NOTE: The “Preset Adjust” Pot on front panel may need adjusting to get the trigger going.

final1

and Here is the beauty, fully restored and fully functional.

 

 

 

 

 

 

 

 

 

final4

 

Back to Home Page for More Crazy Stuff

== THE END ==

 

 

 

 

 

 

 

 

 

 

One thought on “Tektronix RM35A / 535A (Tek 535) Oscilloscope Restoration

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