Friday, December 1, 2017

Repair Log: Tektronix SC-504 Oscilloscope

[Note:  My "Repair Log" blog posts contain my notes on equipment I've recently repaired.  Posted here in case someone else might find them useful.]

After repairing my Tektronix SC 502 oscilloscope module (see this blog post: SC-502), I noticed I also had an SC 504 oscilloscope module squirreled away in a dark corner of a closet shelf.

I had just finished with the '502 and I was still familiar with the disassembly/assembly procedure.  Why not tackle the '504?

So I plugged it into my TM503 chassis and powered it up.  The Power LED came on, but nothing else.

OK, so it's broken.  I knew it was.

The first thing I noticed after removing the covers was that one (of three) fuses on the A3 Trigger board was missing.  This was fuse F3488, a 0.3A slow-blow fuse that is in-line with 33.5VDC to the High Voltage power supply.

I installed a new fuse and reapplied power:  F3488 blew.

OK -- something is making the fuse blow.  Per the SC 504 Manual, F3488 protects transistor Q1380, which is part of the High Voltage supply's voltage regulator.

Poking around Q1380, Q1381, and Q1378, it was clear that the base-emitter and base-collector junctions of Q1380 no longer exhibited a (roughly) 0.7 volt drop, but were instead shorted and that this transistor would need to be replaced.

Digikey had the required  D44H11 transistor,  I ordered a few, they arrived, and I installed one.

The fuse still blew!
If fuse F3488 is blown, I would recommend a much simpler test than I describe  below in this post (which details my steps to discover what the problem was).

First, check Q1380 and replace it if it is no good.  And install a good fuse in F3488.

Then, before powering up the scope, disconnect the wire from T1475 pin 8 (to the HV Multiplier module) and check if the power-supply now comes up when you turn on the AC power. If the HV Multiplier module (Tek p/n 152-0634-00) is the problem (as it was in my unit), the power supply should now start working (without F3488 blowing) and you should measure 70 volts DC on the 70 volt power line.

(Note that the problem might not be the HV Multiplier module, but a short (or low impedance) from its output to ground.  This can be checked by disconnecting the HV connector to the CRT (with the wire from T1475 pin 8 still connected to the HV Module) and checking if the problem goes away, as I describe below.)
As an experiment to see why the "regulator" (which is actually an oscillator) was not oscillating, I attached a current-limited 33.5V bench supply (set to about 20V) to the J3490 side of F3488's fuse clip, and then connected a second bench supply, set to 0V, to pin 6 of T1475 -- this second supply essentially breaks the feedback path: I wanted to see if I could "start" the oscillator by increasing the voltage of this supply.  (Note:  I also needed to parallel this supply's output with 20 ohms because, otherwise, U1270 was forcing this point to be about 1.0 VDC.  The 20 ohm resistor creates enough of a voltage drop through R1370 to allow me to start increasing the voltage from below the turn-on point of Q1378.

As I increased the voltage at T1475.6 to about 0.6V, the regulator circuit started oscillating, but with a very loud audible squeal.  This oscillation stopped when the voltage went beyond about 1.07V, at which point Q1380 seemed to go into saturation.

OK, the HV supply's oscillator can be forced to oscillate, but when this occurs the +70V supply derived from this circuit only goes to up to about +30V, and there is that annoyingly loud squealing.

The squealing probably meant that an abnormal amount of current was flowing through the transformer's windings, causing its windings to vibrate and create an audible squeal.

Was the transformer overloaded?  What would happen if I disconnected T1475.8 from the High Voltage (HV) Multiplier module's input.

The squealing stopped!

Hmmm...maybe the HV Multiplier's load is the issue.  What happens if I leave the HV Multiplier connected to T1475.8, but now disconnect the multiplier's output from the CRT's anode?

The squeal is back!

So, the problem would seem to be the HV Multiplier itself.

Tektronix HV Multiplier, P/N 152-0634-00

I found a replacement HV Multiplier on eBay and purchased it.  When it arrived, I installed it, re-assembled the module, and then powered up the scope...

Success!  A trace!

OK -- that problem was fixed -- what else was wrong?

Channel 2's Vertical Gain knob shaft was broken and the knob spun freely.  This is an integrated knob/shaft combo (Tek P/N 366-1733-01), which I've highlighted in yellow, below:

Unfortunately, this part is difficult to find.  So, until I can find a replacement, I will be using the scope as a single-channel instrument (which was my cunning plan, anyway).

(By the way -- omniscient Google informs me that this Tek knob has an NSN number:  5355-01-186-1111.)

Another problem I had was that the focus shaft had had its tip broken off (to which a now-missing Focus knob attached), thus making the shaft too short.

I had a long length of 1/8" diameter clear plastic rod (available from Tap Plastics) and a spare 1/8" coupler.  So I clipped off a short piece of rod and used my extra coupler to extend the shaft:

(I also dug through my junkbox and found "good enough" replacement knobs for the missing Focus and the broken Intensity knobs).

Finally -- the Time Base knob's skirt had detached from the knob.  Superglue fixed this.

That's it!  Here's the SC 504 on the bench for testing with my FPGA SDR and ATU...


Instruction Manual PDFs (which include schematics) can be downloaded here:

Standard Caveat:

I might have made a mistake in my designs, equations, schematics, models, etc.  If anything looks confusing or wrong to you, please feel free to comment below or send me an email.

Also, I will note:

This information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


Jeff Olynuk said...

Thanks for the great info as my sc504 is suffering from the same problem as yours was. Before finding this blog I had gotten as far as replacing the shorted Q1380 only to find as you did that F3488 still blows. I would like to conduct the same experiment that you carried out but I'm not sure about the part where you "paralleled" the supply with a 20ohm resistor. Some clarification would be appreciated.

Jeff said...

Hi Jeff,

I placed the 20 ohm resistor across my lab supply's output. Otherwise (because of the way my lab supply's output circuitry works), the DC voltage that is normally at T1475 pin 2 will pull up the supply's output voltage, even if the supply is set to 0V -- the voltage at T1475 pin 2 is from the output of U1270 via R1370 (910 ohms). So the 20 ohm resistor acts a voltage-divider, with R1370, dividing down the output of U1270 to a level that won't turn on Q1380, allowing me to instead use my supply to slowly increase the voltage at T1475 pin 2 and see what happens.

But instead of doing this, I would first recommend a much simpler test. Disconnect the wire from T1475 pin 8 to the HV Multiplier and see if the circuit now powers up. If the HV Multiplier is the problem (as it was in my unit), the power supply should now start working (without F3488 blowing)and you should see 70 volts DC on the 70 volt power line.

- Jeff

Jeff Olynuk said...

Hello Jeff,

Thanks for your response and explanation about the resistor, I understand now. I did as you instructed and I was rewarded with yet another blown fuse. A quick check found that Q1380 had shorted again (good thing I ordered 10!). After replacing it with a fresh D44H11 and fuse the unit powered up successfully!!! There was even a trace on the screen which surprised me as I thought the crt wouldn't work without the 10kv supply.


Jeff said...

Hi Jeff,

Congratulations! (And I'm also surprised that there is a trace.)

By the way, I picked up my replacement HV Multiplier on eBay.
Search for its p/n: 152-0634-00.

- Jeff