Simpson 476 Mirrorscope: Success!

Finally got a trace!  Turns out that the open cathode resistor I replaced that I thought was supposed to be a 2.7k was actually supposed to be a 39k, so after rectifying that (with a 33k), and replacing the incorrect bias resistor, it lit up like a charm.


After that, I put a sine in the X & Y, and fiddled with the bias controls until I got maximum linearity.  The sweep & trigger is a little wonky, so I think I’ll run some tests with an external sweep generator to dial the amp sections in first, before I tackle the sweep.  Besides, Benton (who’s scope this is) mostly only uses X/Y mode, so that’s more important.

There is some burn-in visible on the CRT, I may poke around eBay to see if there’s a replacement that’s available and inexpensive.  The filter caps in the power supply are shockingly good, but I may replace them anyway, at least the paper ones.

After a congratulatory sip of Bourbon (4 Roses, one of my faves), I realized that all the troubles in this scope stemmed from bad 39kΩ resistors. I wonder if there was a bad batch, or they were all just under spec’d?  In any event, I’m going to replace them all, and probably up the rating to 2W, just to be sure.  I may also replace the 68k resistors that have drifted as well.

Here it is triggering on a 100kHz square wave, not bad!



Simpson 476 Mirrorscope – continued adventures

So I took Benton up on his offer of giving me the 2nd working scope, and it’s been super helpful in having a unit to compare to.  It’s hard to diagnose this thing without real schematics – the ones from the 480 are different enough to be more of a source of confusion.

Before I got the second unit, I took the face place off in order to get better access to the components.  Unfortunately the face plate is what holds a lot of the unit together, so it’s in a very fragile state right now.  This thing was not made for easy service.  I have it on a plywood square that lets me move it around and spin it more easily.img_7016

There’s some alligator clips to keep parts of the circuit grounded that were otherwise grounded through the face plate.

I also grabbed the FLIR One from the office and took some temperature measurements.  Here are the two hottest components:

A resistor in the vertical bias sectionimg_7007

A resistor in the power supply:

Both ALARMINGLY hot.  (pardon the mixed units, switched to Celsius 1/2 way through).  The FLIR is a really useful tool – the only issue is that at these close ranges, the visual and thermal layers don’t quite line up due to the physical spacing of the cameras.

I started checking values and taking measurements and noticed a few things:

Unlike in the 480, the 2nd section plate resistors are different between the horizontal and vertical sections.  Horizontal are the same 68k, while the vertical is 39k.  All have drifted high, and in some cases out of tolerance. These are 1W resistors, according to the manual.

R109, 2.7k 1/2W, that connects the cathode to ground, was open.  I replaced it with the closest thing I had, a 2.2k.  After that, I could make tiny adjustments in the vertical position, but the beam shape went to shit.

Shortly after that, Benton dropped off the second unit.  I removed it from it’s case, and started taking measurements.


For starters, those resistors don’t get nearly as hot, so I started to get more confident that the problem was in the vertical biasing section (that bit with the pots hanging off the back)

Finally, I noticed the smoking gun:


That piggybacked resistor pair?  That’s R98, called out as a 39k 1W in the schematic, but that there’s a 270Ω in parallel with a 330Ω, for a total of 150Ω !!.  But wait, It gets weirder.  That same resistor in the ‘working’ unit, is color coded as a 39k, but measures at 260k!  W.T.F!?.  When I place a closer value, 33k in the bad unit, I start to get a little more range, but the beam is still fuzzy.  When I replace it with the 260k resistor, I get tons of range, but still a fuzzy beam.

Meanwhile, on the horizontal side of the bad unit, that 39k resistor (R97) has been replaced with 3 50k in parallel, for 16k.


Time to order a bunch of 1W resistors, I guess.