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Author Topic: Reviving a Heathkit HA-14  (Read 7517 times)

Posts: 103

« Reply #15 on: November 02, 2017, 12:48:41 PM »

I have rebuilt a few 70's vintage stereo receivers and always used the 100 watt dim bulb method any time I had replaced components on the amplifier boards.

Never thought about it with a HF amplifier.

Let me know how it works and what wattage bulb you used, if you don't mind.

My only aim is to check out the transformer this time around.  I think I'll start with a 40-45 watt bulb; the power supply won't be connected to the amp, so if all is good it won't be drawing much current.  If that goes well, then I'll move on to checking the transformer secondaries for open windings, than on to check no-load voltages (I've already verified my meter & leads are good to 2Kv, and the HV precautions will be rigorously observed; working at a power plant reinforces my caution. At least I shouldn't need arc-flash protection... Smiley )

If the transformer is okay, the next step will be the Harbach power supply rebuild kit.  If not, then I'll have to come up with a "Plan B", which likely will be to put it all back on the shelf while I look for another transformer, and attack another project (I've a list that seems to get longer each month)...  The soft-start may be a later addition, or perhaps not.

I expect that this will be my winter project; there's no hurry to get it done, and I want to go over it very thoroughly.  Also - anyone have any input on what value to choose for the Y2 safety caps?
« Last Edit: November 02, 2017, 12:51:45 PM by N0MKC » Logged

Posts: 3160

« Reply #16 on: November 03, 2017, 09:46:29 AM »

Heathkit of the Month : HA-14
by Bob Eckweiler, AF6C

W6OVP in Vancouver, WA has a nice HA-14 web page with photos.

Dave Larson, KK4WW recorded a YouTube video

Posts: 103

« Reply #17 on: November 05, 2017, 02:11:12 PM »

Well, the HP-24 transformer has passed the smoke test, and no breakers tripped.

Since I plan on rebuilding this anyway, I removed the HV & bias leads from the existing circuitry prior to taking resistance measurements and powering it up, thus lessing the chance that something would go "ZZZAP!" and damage the transformer when voltage hit the primaries for the first time quite a while.

The first power-up was done with a 25 watt bulb in the dim-bulb tester; this resulted in a dim orange glow. A few minutes in this state produced no increase in current draw, no bad sounds / smells; after few minutes plugged in at full voltage were uneventful, I took the below measurements.

both primary windings (in parallel; I didn't disconnect & test them separately) = 0.5 ohm
bias secondary = 14.2 ohms
filament secondary = 0.1 ohm
HV secondary = 30.6 ohms

no-load secondary winding voltages (@ 123 Vac into the primary):
bias = 140 Vac
filament = 13.8 Vac
HV = 895 Vac

(All measurements were taken with my Fluke 77-III meter; nevertheless, I regard the <1 ohm measurements as primarily demonstrating "really-low-resistance" continuity rather than near-absolute values...)

Progress is being made, albeit slowly...  The PS rebuild kit order goes out tonight!

(Yes, I double-checked the Fluke capabilities before starting, and it's only rated to 1000 Vac - still works, and only needed the single HV measurement ...  Glad l double-checked nevertheless.)
« Last Edit: November 05, 2017, 02:14:53 PM by N0MKC » Logged

Posts: 4710

« Reply #18 on: November 05, 2017, 02:36:54 PM »

Unless the filament transformer is way overrated it provides a fine step start.

The HA-14 does not have a filament transformer. There's just the one big combined plate-filament-and-bias transformer in the HP-24 power supply. To make it even more of a sporting course, the 572B filaments in the HA-14 are in series rather than parallel, with one end of the string grounded and the other end connected to 12 volts.


Posts: 3160

« Reply #19 on: November 11, 2017, 06:28:46 AM »

Quote from: N0MKC
... anyone have any input on what value to choose for the Y2 safety caps?
0.01 uF safety capacitor should be adequate.

Y2 version : Metalized polyester film (non-inductive type).
Flame-retardant plastic case / epoxy resin sealed (yellow common).
Tinned copper leads typically 20mm long.
Ideal for "line-to-ground" line filter applications where high AC voltage exists.
Also idea as "antenna-coupling" capacitor.

X1/Y2 version : Typically a ceramic dielectric in disc shape with epoxy resin coating (blue common).
Radial leads can be up to 27mm long.
Ideal for both "across-the-line" and "line bypass" purposes where high steady AC voltage exists.

« Last Edit: November 11, 2017, 06:37:10 AM by W9GB » Logged

Posts: 184

« Reply #20 on: November 14, 2017, 04:38:20 PM »

KOP:  You're right however, while the cap bank is being charged the tubes are coming on slow...both the HV and possibly the filaments.  (I used to have two of these amps but forget how they're powered)

572Bs and 3-500z tubes are designed as "instant on" and as I understand it, don't like step start."
Depends on the car , some Chevys have their batteries in the trunk.

Posts: 103

« Reply #21 on: November 17, 2017, 12:23:41 PM »

Well, I got the Harbach power supply kit installed without any problems; the biggest nuisance was unbolting the transformer to tilt it out of the way so I could drill mounting holes for the new  power supply board.

I did discover something which may have contributed to the amp being relegated to a shelf in the barn, though...  One of the jumpers used to wire the power supply for 110 volts was attached to a solder lug by a cold solder blob with no mechanical connection at all!  It got nudged while I was working on the project and came completely off one of the lugs it was supposed to be attached to; I expect that this may have caused some problems, likely intermittent ones at that.

Needless to say, all solder joints in the power supply are now good, with secure mechanical connections where appropriate.  The filament and bias voltages are where they should be; checking the HV output will need to wait until I get an HV probe for my multimeter.

Now to start in on the amp itself...  Judging from the power supply, I'm going to be redoing every solder joint it has, just to be sure.
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