Heathkit TT-1 troubles

While checking a 6EJ7 for my IM-21, I noticed a whiff of smoke coming from my trusty TT-1.

Per a suggestion of someone in the Vintage Test Equipment group, I turned to my trusty TU-75 and finally used it’s series bulb feature.  An incandescent bulb in series acts as a current limiter, which both protects the Device Under Test, and gives a visual indication of when the DUT is trying to draw more current than you’d like. Sure enough, when the tube was connected using the ‘normal / disconnect’ switch, the lamp glows, and I see the voltage drop appreciably.

Side note: I finally got around to measuring and labeling the lamps in my TU-75.  According to the wattmeter, the largest lamp is 500W!  I seem to remember an even larger one that unfortunately got broken due to shoddy packing when I bought this 4 – 5 years ago.





What led me to the problem was this line adjust rheostat starting to smoulder.

See me. Spin me. Smell me.

It was suggested that this is not the source of the issue, rather an indication of something upstream drawing too much current.  Hopefully not an underlying issue with the transformer. With 40 taps across a half dozen windings, It’s basically unobtanium.




I didn’t recall ever having an issue with this tester in the past, and sure, things age, but what’s different about this test than countless one’s I’ve done before.  Well, for one thing, I don’t ever recall testing a tube with a 600mA before.  I decided to try testing something with a fixed voltage filament instead.  A 12AT6 with it’s 6.3V filament did not yield the same excess current draw, and tested fine under this setup.

This Page has a great document on refurbishing the TT-1.  It also has the last published tube data addendum.

Here’s another in-depth document on the TT-1.  It references Kent’s document as well, and pages 12 – 15 describe the theory behind the ballast capacitor. The reactance is inversely proportional to the capacitance, so as the capacitance goes up, it’s equivalent resistance goes down.   In my case, they’re all measuring a bit high:


stated value

measured value



3.6 uF

5 uF



3.6 uF

5 uF



7.1 uF

11 uF


C9 + 10 + 11

14.3 uf

21 uF


The sum is what’s important for this range – looking at the switch, it appears that the capacitors are paralleled sequentially through the last three filament settings (the current settings).  Is a 47% overage enough to account for this error?  That would mean at 600mA, it’s theoretically allowing ~900mA: Not great, but I’m surprised it’s enough to cause this problem.  There’s nothing else in this circuit – does this mean I should suspect a short somewhere in this tap?  It’s the top-most tap of the filament winding, #29. 

I’m going to try a lower current filament and see if I end up with the same issue, as well as some larger, higher current tubes.

Testing a 6L6 with it’s 6.3v filament at 900mA gets me a little bit of a dip in voltage down to 105V when testing with the dim bulb, but nothing like I was getting with the 600mA setting. With the bulb bypassed, it seems to work fine, and nothing is getting warm or stinky.
Testing a 6HZ6 with it’s 450mA gets me a dip down to 80V.  With the bulb bypassed, I get a good test, but started to get just a hint of fresh roasted components, so I killed it before I started to see smoke.
Went back to test a 6AW8 which calls for a 600mA, this time with a current meter on the common line on the ballast capacitor.  With the bulb, the line voltage dipped down to 70V & the filament is only able to draw ~400mA.  With the bulb bypassed, the filament drew ~700mA. I didn’t leave it on this setting long enough to summon the stank.
With the same tube & settings, switching to the 6.3V tap doesn’t cause the same brown-out.

My working theory is that there’s a short somewhere in the part of the winding that feeds the current filament settings, but that does not affect the other taps.  Not being able to use the current settings actually isn’t too big of a problem, as the tubes that call for it also have a published filament voltage.


Simpson 476 Mirrorscope

Up next on the bench, a Simpson 476 Mirrorscope.


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”:


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.

Tek 453


This was actually the first scope I got as “an Adult”.  I think I paid about $100 for it about 6 years ago.  Since then, it’s been my daily use scope, since it fits atop a rolling art-cart under my bench.

Everything works great, except the B sweep only triggers on ‘starts after delay’, not ‘triggerable after delay’, which means there’s something fishy towards the front end of the B trigger circuit.

Great write-up of 454 triggering diagnosis here. Punch-line: it was the tunnel-diode.

Also suspected Nuvistor v633, type 8393.  Substitution with a claimed-good tube didn’t change the issue, so I’m suspecting it less, but unfortunately I can’t locate the test parameters for the the 8393.
Found it for the 6cw4 (same pin-out?)
Tube    Plate  Bias   Fil.    Meter   Sig.     Selectors                 R.P.       Remarks
6CW4  C        7L     6.3   32        4         03050-0010-706    1430      Tr;X4.

Boom – An 8393 is a 7586 w/ a 13.5v filament.  So, for the TT-1:
Tube    Plate  Bias   Fil.    Meter   Sig.     Selectors                 R.P.       Remarks
8393  O        7L     12   36        4         03050-0010-706    1140      Tr;X4.

UPDATE 5/3: Everything around that Nuvistor seems OK, but nothing is coming out of it, with either the original or replacement.  They both test dead.  So either:
1) I’m testing them wrong, and it’s something else in the circuit, or
2) I have two bad 8393s.
replacements en route, 3 for $45.  Damn these things are getting expensive.

I also realized that this Nuvistor is a part of the signal chain when in XY mode, which I could never get working.

UPDATE 5/8: Replacing v633 Nuvistor fixed the X/Y mode, but ‘B’ is still not triggering on it’s own, so there’s still some drama in the triggering circuit.