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.


Heathkit IM-21 AC VTVM Repair

This is the first piece in a larger lot that I decided to tackle.  It’s the simple cousin of the Heathkit IT-18, which also measured DC voltages & resistance.

I beat my head against this for far too long until I realized that the calibration pot would sway wildly out of spec depending on the position of the contact – from 40Ω to hundreds of ohms.  I replaced it with a 50Ω pot and it was able to be calibrated to mostly within spec.  I’ve never found these early, cheap VOMs to be particularly accurate, but they’re fine for basic work.

Heathkit T-3 Restoration


This is not my first signal tracer, but it’s the first one I’ve gone to this much trouble on.  These have been getting pricey ever since musicians discovered they make cute little practice amps, so whenever I see one at a good price, I’ll snag it.

When I got it, it ‘worked’.  The indicator indicates, and sound played as expected.  It had a bit of hum and noise – not unreasonable, but definitely room for improvement.  At first, I thought I’d just replace any parts that were out of spec, but given how this is constructed, I decided a ground up re-build would be a better approach.

Here’s the before pic – the component count isn’t so high that it’s difficult to trace out in it’s condition, but it’s not exactly easy on the eyes.

Unbuilt Heathkits are getting fantastically expensive, but I figured if I did a complete tear-down, and ordered all new caps & resistors, I’d essentially get some resemblance of the new kit experience.  So off I went ordering parts, and stripping the chassis.


Everything showed up as ordered, but I didn’t pay attention to the physical size of the resistors.  Everything that showed up was 1/2 W as ordered, but they’re suspiciously small – even though they’re probably ‘correct’, they look out of place. They look like 1/8 W, they’re absolutely tiny. 

All of these really are 1/2 W resistorsThe one’s in question are:
47Ω – on the output
10kΩ – in the power supply
470Ω – on a cathode
There’s also the 1W 1kΩ in the power supply (the one that was looking toasty in the original build). It’s bigger, but it still just doesn’t feel right.  I’m going to go 2W on all these, somewhat for piece of mind, and somewhat for looks.


The solid-core for the wire used throughout measures .025  which puts it at 22 AWG. The OD w/ insulation is .056.  I have a bunch of 24 AWG solid core PVC jacketed hookup wire in various colors, and some cloth covered 20AWG from New Old Sounds.  The PVC wire has a slightly smaller OD than the original stuff, whereas the cloth covered stuff is on the thick side; slightly thicker than the transformer leads.  24 AWG is really fine for all of this, and I may use the cloth covered stuff for leads that pass through the deck.  The highest current is probably the heaters.  Each tube is 150 mA, and 24 AWG is good for 3.5 A, so we should be fine.

Color Coding

Most, if not all Heath gear of this era uses the same grey, solid-core wire.  I’d like to impart some color coding, and a quick search unearthed this:

Ground = Black
Filaments = Brown
B+ = Red
Control Grids = Green
Plates = Blue
AC Line = Grey


I was able to strip out almost everything, with the exception of the transformers and a few of their connections.  There’s really no slack on the transformer leads, so I didn’t want to risk it.

Here it is as disassembled as it was going to get – note I already started on re-stuffing the electrolytic capacitor.

Speaking of capacitors – this one was a bear to empty out.  I think it tested OK, but I was trying to make it future proof.  In my experience, really dried out, dead electrolytics seem to come apart rather easily.  When I opened this one up, it was still a damp, packed, impervious mess.  Picking, poking, & drilling did nothing, nor did soaking in water.  Boiling did the trick – a poor mans double-boiler with a sauce can kept me from exposing my cookware to the nasty innards.



Tube compliment:
12C8 –
12SH7 –
12A6 –
1629 –


I generally followed the instructions, making a few modifications to component placement, particularly where smaller replacement components allowed for a neater layout.

Started with the twisted heater wiring


Power & fuse wiring

The face-plate was installed after getting a polish.  It’s not perfect, but it’s better.  The screw-heads & jack hardware also got hit with a wire wheel – It’s easy, and I think makes a big difference.

Face-plate installation

B+ Wiring

Coupling capacitors & plate wiring

Shielded cable

Shielded cable – termination detail

Upper Deck

Wiring up the eye tube socket

Final Assembly

I’m pretty proud of how this came out.  There was one guy razzing me on the Vintage Test Equipment group about how some of the runs were too long & unsupported, which would cause variations in capacitance when they vibrate, or there was a possibility of exposed leads shorting if the unit was dropped.  I’m dismissing those as non-issues; I’m not going to sweat variations in picofarads on a device that tops out at a few kilohertz, and any drop hard enough to cause these components to come into contact would have much larger consequences anyway.  It may be difficult to tell from these photos, but leads that look close to touching are either at the same point in the circuit, or are a few cm away from each other.  These construction methods aren’t appropriate for high frequency circuits, but for audio, it’s just fine.


With the final assembly complete, all that’s left to do is power this up, slowly on a Variac.  It came to life with little fanfare – no creaks, pops, crackles, or smoke.  Once I saw the glow of the eye tube, but didn’t hear anything, I was convinced I’d screwed something up – I expected at least a little noise.  I tried playing back some music through the 1/4″ just for kicks, and was pleasantly surprised to hear clean audio reproduced.  It’s sounds as good as can be expected for this tiny speaker, but it’s quiet and clean. I haven’t tested the wattmeter function – I may at some point, even though I’m confident it’ll never get used.  The paint on the outer case isn’t in great shape, but I don’t have the means to strip it down, so that will be a future project – perhaps doing a whole batch in one go.


Heathkit H89A – Continued boot failures

So I’ve tried just about everything, and the consensus is that I’ve just got bad disks.  A bunch of people on the SEBHC Board has been super helpful, a few even offering to send me boot-disks.  I took one gentleman (Ken) up on his offer, so hopefully that’ll get me up and running.
Lee suggested some lines on the floppy connector to probe, which I’ll get to tomorrow.  May even sick the 7D01 on this…
This manual has me convinced that the BIOS is correct. It’s a newer one to go along with the floppy controller card, which just isn’t referenced in any of the printed documentation I have.
Ken confirmed my understanding that the floppy drive is a fairly dumb device, and it’s the controller that determines sector firmness, track count, & density.  He’s even shared wiring instructions to get 3.5″ drives.  I’m eager to try some newer drives and disks once I get up and running.

Heathkit H89A – floppy config & attempted boot

I’ve tried every disk in every drive with what I thought was the working configuration, to no avail.  Every failure is the same: hitting ‘b’ to boot, getting ~5 seconds of disk activity, and failing with ‘? Boot Error’.  Heads appear to be in impeccable condition, and the disks look clean.  Here’s a closeup of the heads on the hard sector drive:

Floppy drive head closeup

And here’s the collection of disks I’m working with:

img_6437 img_6439

SW501,4 controls which card to boot from:
0 = H88-1 Card at P506/P512 (the right most slot), I/O Port 174
1 = Z89-37 Card at P504/P510 (the left slot of the right bank, I/O Port 170

I’ve tried both; 0 boots to the internal, hard sector disk, 1 boots to the 1st drive in the H-37 enclosure, a 96tpi soft sector drive.

The internal drive is set to Unit Zero (or DS1) for use with the H88-1 card
Screen Shot 2016-08-14 at 3.45.36 PM

But wait, we can also use this drive as a soft-sector drive on the the Z89-37 controller.  Note that the drive settings are different:
Screen Shot 2016-08-14 at 3.45.44 PM

Yes, it’s backwards.  My predecessor was nice enough to not cut the links, but to simply bend the pins of the package out.  unfortunately, they’re super fragile, and a single attempted bend broke the pin, several times. I ended up using a portion of the original package and some wire.

Note that J4-J7 needs to be set correctly on the Z89-37 card.  It’s now set to J4, meaning the single drive ID’s as DS1 plugged into P3 (the top port on the card) is Drive 0.

As I was shuffling connectors and moving the board around, this happened (took me a few minutes of failed boots to spot):

I temporarily crammed it back in, and got to the H: prompt.  Still won’t boot to a disk.

I wonder if this is noteworthy:
Every boot attempt seemed to take the same amount of time to fail (3-4 seconds), regardless of which configuration or even if a disk was present.  When I jump J4 on the Z89-37 (what I believe to be the correct config), an attempted boot without a disk tries indefinitely and never fails.



Heathkit H89A, First Light

Progress continues.  I tested the regulators and they were fine.  I ultimately traced the problem to C513.  It was shorted, and has been replaced by a comparable ceramic cap.

After bringing her up on a variac, I’m greeted with an H: prompt!

I ran a memory test, which is one of the routines in the monitor ROM.  A full test would take over 2 days, so I just let it run for a bit after quitting.


377377 is 64K in octal.

There’s a routine you can enter manually that prints characters to the screen:


Finally, there’s a routine to run a drive speed test.  Using a tweaker to adjust to 200:


Unfortunately, I can’t get it to boot off any medium I have, either hard sectored on the built-in drive, or soft sectored using the external H77 drive.  Not sure what to try next…


Heathkit H89A computer, first repair attempts

I’m just jumping right in –

I acquired the following from Ray (WA1FFT) in NJ.
  • Heathkit 89A
  • Complete set of spare boards from H89A (different mb)
  • H17 Dual hard sectored 5 1/4” floppy drive
  • H77 Dual soft sectored 5 1/4” floppy drive
  • Box of hard & soft sectored disks.
I was really just after the H17 to go along with my H8, but $30 for all that? No brainer.
Ray warned me that two caps popped on the main board,  and indeed C508 & C515, 2.2u tantalum caps, failed violently.  We’ve got to see what other damage they did; tantalum capacitors are like the suicide bombers of the electronics world.
They were replaced with 10u caps I had in inventory (close enough):
I found Herb Johnson’s 2016 adventures in Heathkit H89 rebuilding while surfing around, and his page is a great reference.  As I got a spare set of boards as well, I figured I’d first just swap them out.  No go.  The boards are slightly different, and that difference includes the power supply connectors.  According to Lee Hart, the key’d power supply molex connector indicates a late-model H89A.  That would be the 2549 board
I’m assuming that also took out U585, a 79M12 and U565, (only have a heath part), as there’s no -12V & +12V on the output of those regulators.  I also have a spare, earlier model board to compare with (and possibly steal parts from if I’m impatient), but I’m noticing more and more inconsistencies between the two boards, and with the documentation.  Here’s where it gets (more) annoying:
U515 is a 71M12C, your run-of-the-mill -12V regulator.  OK, terrific.  Same on the old board as well, and consistent in all docs I have.
U567 only has a heath part#, 442-674.  It’s in parallel with U568, they both feed separate +12V supplies.  Here’s where it gets squirrelly:
  • In the docs, it’s called out as part 442-663
  • The pinout labeling is different (common pin center, whereas board image & earlier board call common pin as the left one)

Here’s the old board (85-2208-1) incorrectly marked pin-outs, but check out that sexy ceramic ram chip:

Here’s the documentation for the newer board, but the pin-outs are still mis-marked:

Here’s what I’m left to deduce, and can confirm after some metering:
  • The earlier board silkscreen and artwork in my manual (for the later board) are both wrong for U567 & U568
  • U515, a negative regulator, indeed has it’s common (gnd) on the left; it’s input and output are swapped from positive regulators.
  • U567 & U568 are almost certainly 7812s (+12V regulators), as they are on the earlier board.  They are only silkscreened correctly on my newer board.  The new board artwork illustration, and the old board silkscreen are both incorrect.
OK, well U515 seems to be working well out-of-circuit, but there’s almost no resistance between ground and it’s output in the circuit.  Suspecting C509 & 511 next.  Lifted both, no dice. I’ve got to be careful with the desoldering gun, the traces are easy to lift.
So what else is connected to the -12V line?  Besides the 2 -5V regulators, and the card bus, I’d imagine it’d be the built-in UART used to communicate with the terminal board, since for whatever reason, it’s full blown RS-232 between the two.  Yep, sure enough there’s a quad EIA-232 driver. Nope, there’s still a short with that removed.
So it’s the -5V regulators?  Nope.  Forgot about C513.  Yep, it’s shorted.  It’s always the last place you look…
Next up – replace the regulators and attempt to power up again.