Tag Archives: Simpson

Simpson 476 Mirrorscope: Success!

Posted on by -- Last edited on November 20, 2016
2

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.

img_7070

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!

img_7073

 

Simpson 476 Mirrorscope – continued adventures

Posted on by -- Last edited on November 19, 2016
1

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:
img_7003

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:
screen-shot-2016-11-19-at-4-40-37-pm

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.

img_7061

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:

img_7064-1

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.

img_7066

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

 

Simpson 476 Mirrorscope

Posted on by -- Last edited on September 25, 2016
Reply

Up next on the bench, a Simpson 476 Mirrorscope.

img_6849

This example belongs to Benton Bainbridge, an NYC video artist.  He’s got two, and this one needs the most help, so I figured I’d get it on my bench and get it working again.  That’s the hope, anyway.  As can be seen from the photo, the CRT points upward, and the operator views it through a 45° mirror, that flips up as the top of the chassis.  This yields a benchtop oscilloscope at an unheard of depth of only 8″.
It turns out, the manual for this beast is no where to be found.  I’ve reached out to Dave at Arktek manuals to see if he can dig up something.  I’ve gotten hard-to-find manuals from him before, so hopefully he can work some magic.  I also reached out to Steven Johnson, who’s got a great page and manuals for sale.  The Simpson 480 Genscope is approximately one of these mirrorscopes with a signal generator on either end.  I can’t even begin to imagine how unweildly that is to work on, but at least there’s a manual available.

I met Benton at his place, and we started comparing the “good one” and the “bad one”:

img_6824

Sweet DJ rig, yo

The problem seems to be in the vertical amplifier section, made up of (2) 6K6 twin triodes. One the good unit, the signal to the deflection plates varies between 170 – 240v while on the bad unit, they’re pegged at 350v, which appears to be full anode voltage.  Tubes were tested, and swapped, to no avail.  I brought my Heathkit TT-1, which Benton got a kick out of – it’s such a fun instrument to operate.

There were a few ground connections that appear to have been severed, but reconnecting them didn’t solve the issue.  There are a number of resistors that are running hotter, so much so that they’re discolored; however when measured cold, they appear to still be in tolerance.

There are a few differences between the 480 and the 476:

  • The 5Z4 rectifier is absent in the power supply.  This appears to power only the oscillators, so no surprise there.  The power supply consists of 2 6X4s, one for the CRT HV, at about 900v, and one for the rest of the circuits, at about 350v.
  • There’s an extra 12AU7 not present in the schematics for the 480.  I suspect this might be a pre-amp for the vertical input.  The 476 has more rages than the 480: 4 steps from .5v to 500v as opposed to ‘low / high’ on the 480.  Alternately, it’s a part of the sweep / trigger circuit?  I know that this is scope is capable of ‘synchronized’ operation, but I can’t for the life of me see how this thing triggers by looking at the schematic.
  • Some other component value differences.

Here’s the relevant part of the 480 schematic.  simpson-480

Benton offered that I should take both, to have  good one to compare with.  I declined, only wanting to bring two additional cubic feet of oscilloscope into my apartment, but I may take him up on it if I can’t get this figured out.  On old gear like this, I immediately suspect old caps in the power supply, but since the horizontal sections seems to be working (ish), that doesn’t screen out as the culprit.  I think I’ll just start checking / changing out caps in the PS for good measure, as I familiarize myself with this unit.

The other thing I’m definitely going to do is grab my boss’s FLIR One to scope out any really hot (or suspiciously cold) components.

But before I do anything else, I gotta order some more caps, my HV Electrolytic stock is almost depleted.  I kind of like running out of parts, it means I’m keeping busy.