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Author Topic: Reviving a Heathkit HA-14  (Read 2924 times)
N0MKC
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Posts: 87




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« on: November 01, 2017, 10:28:45 AM »

A few weeks ago, I came across a "barn find" Heathkit HA-14 "Kompact Kilowatt" linear & matching HP-24 power supply.  Once I'd blown off the strata of barn dust, I found it to be unmodified and in exceptionally good condition; thus, I want to revive it.

I've found parts & a manual/schematic for it; if the transformer checks out okay this weekend, I'll be ordering stuff in the next week or so. (I'll be hitting the hardware store for components to build a "dim bulb" tester this evening...)

Are there any "gotcha!" things I should be aware of?  I know this is a repackaged SB-200 for the most part, but with some changes - the tube filaments are in series here instead of parallel as in the SB-200, for example. 

I'll be replacing the electrolytic and molded paper caps as a matter of course; however, there are 9 caps listed in the assembly manual as "resin" - what are these, and should I replace them?  They are mostly used in the tuned circuits for each band, and I expect the values here may be somewhat critical - they are:
115 pF
200 pF
230 pF
290 pF
310 pF
360 pF

How about the "doorknob" caps (Heathkit refers to them in the manual as "tubular") - keep or replace?  I'm leaving the ceramic disc ones alone unless I find individual ones which need to be replaced; resistors will also be checked, and out-of-spec ones replaced as well.

Also, while rebuilding the power supply (using a Harbach Electronics kit), I intend on fitting Y2 safety caps to the 110 line - does anyone have any suggestions for values?  (I'll likely start with 0.01 pF caps & see how it goes unless I find recommendations for another value in this application.)

As always, the goal is to get this working again without letting out any of the magic smoke... (When the magic smoke escapes, electronic stuff quits working!)
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N3AJB
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Posts: 39




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« Reply #1 on: November 01, 2017, 01:07:25 PM »

Hi,

I have run several HA 14s over the years and currently am using one with a Kenwood TS 480.  The pair make a great compact KW (input) station. 

With regard to the capacitors in the tuned input circuit, just check (and adjust if necessary) the SWR for each band.  As long as the SWRs are close to 1:1, there is no need to replace the caps.

I would replace the 33 ohm grid resistors with 5 watt instead of the  stock 1 watt resistors.  Check the grid bypass caps to make sure they are ok.  It's probably easier just to replace them.

I also rebuilt the power supply with bigger filter caps than the stock 125uf.  I also added a step start circuit to the power supply.  If you can find the room, add a relay (controlled by the on/off switch) to turn on the power supply.

The "caveat" is to realize the HP24 is not as robust as the power supply in the SB 200. The plate transformer in the HA 14 is about 2/3 the size as the transformer in the SB 200.   I built a separate power supply around the transformer from an SB200 and routinely peak to 600-700w output, with about 60-70 watts.  When I use the HP24, I limit my output to 400-450w max. 

Also, put a muffin fan over the tubes drawing the heat out of the amp since there is no fan internal in the amp.  Mine always runs cool regardless of the output power.  I never have burned the paint above the finals.

Otherwise, it a great little amplifier and often overlooked by the ham community.  It's the fastest QSY amplifier there is. Since the load caps are all fixed, all that is needed is the peaking of the output power with the tune capacitor.

One of the downsides is that it is not possible to monitor either the plate or the grid current.  As long as I don't overdrive it, I've never had any trouble with this little gem.

Jon
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W1QJ
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« Reply #2 on: November 01, 2017, 01:39:00 PM »

The grid grounding capacitors and the caps in the tuned input circuits are most likely MICA and they most likely will be just fine.  They are very stable and "time" should not be an enemy to them like paper type electrolytics.  SO yes, I would replace the very few electrolytic caps that are in there.  Mica's I would not bother with not ceramic disc bypass caps.  They should be fine.  As far as resistors, the only important resistors to look at would be the grid resistors which are 1 watt carbon 33 ohms.  AT BEST I would change them (if they are way off from 33 ohms) with 2 watt NOS carbon ones.  I would not suggest going beyond 2 watts with these as the do act as fuses in certain situations and you will want them to remain "sacrificial".  The other resistors you will want to check is the series HV metering resistors.  If they are off tolerance you will want to replace them with newer type units that hold tolerance much better.  You will want to check the band switch and make sure t hasn't been arced and is NG.  The power supply could use upgrading with new filter caps and bleeder resistors.  A good cleaning is in order also.  Stop back and keep us informed on your progress, if you need any special parts let me know I may have what you need.  Lou
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N0MKC
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Posts: 87




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« Reply #3 on: November 01, 2017, 02:39:28 PM »

Thanks for the replies!  Here's some more detail...

My plan is to rebuild the HP-24 power supply using the Harbach PM-HP24 kit - just about everything will be new except for the case & transformer.  The filter caps in the kit are 220 uF (original are 125 uF).  I'll be adding a soft-start module as well (Harbach SS-100-120).

I found replacement HV cable at the local NAPA Auto Parts store - they have a roll of 7mm Belden wire-core spark plug wire in back, and 3 feet cost me a bit over $2.  It fits the HV connectors perfectly!

I'll be taking my time on the amp itself, checking everything; as I mentioned, it's in near-new condition under the dust - no dents, discoloration, scratches, etc. except two 3/8 inch holes drilled in the top of the case.  I have no history on it, so checking everything seems to be the wisest course of action; I'll clean, replace, and/or lubricate as needed.

The band switch is bound up but not completely seized, since I can rock it back & forth a bit. I think it most likely that congealed grease & barn dust is the culprit.  The switch wafers all look good, with no signs of arcing, and nothing burnt.  Harbach does have replacement bandswitch wafers, in the event I do have to replace one.  The variable capacitor moves freely through its range, but is a bit stiff; the plates all look good, with no arc spots or misalignment.

Once I get it up, the plan is to use it with an SBE SB-34 I have here; that only puts out ~ 65 or so watts, so the HA-14 shouldn't be very stressed...  I'll be working on the T/R and ALC interfaces to get them to play nice together, once I have this going.

The barn sale I bought this from had a few other items I grabbed as well - in addition to the amp & PS, a Drake MN-2000 tuner, a 5-station remote antenna switch, and 3 vacuum variable capacitors were a part of my purchase; I offered the lowest price I could without offending my conscience, and it was accepted...  Essentially, the seller wanted the stuff gone, and wasn't too concerned about the price received; it was well worth the 90 minute drive to get there (besides, it was a nice day and pretty scenery)!

Tom
« Last Edit: November 01, 2017, 02:44:16 PM by N0MKC » Logged
KM1H
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Posts: 2598




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« Reply #4 on: November 01, 2017, 02:45:51 PM »

Quote
As far as resistors, the only important resistors to look at would be the grid resistors which are 1 watt carbon 33 ohms.  AT BEST I would change them (if they are way off from 33 ohms) with 2 watt NOS carbon ones.  I would not suggest going beyond 2 watts with these as the do act as fuses in certain situations and you will want them to remain "sacrificial".

Which is why you should not go past the 1W original rating and chance blowing the grid supply or something else. Would you use a 20A fuse when it calls for 10A? Or run your Corvette well past redline?

Dentron used 1/2W resistors as HV fuses, Ive pulled out many 2-5W versions installed by clueless hams and CBers....often the same person Roll Eyes
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KOP
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Posts: 245




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« Reply #5 on: November 01, 2017, 02:52:37 PM »

https://www.w8ji.com/metering_amplifier.htm

Tom addresses metering far better than I could .

https://www.w8ji.com/al811_important_modifications_changes.htm

So it's not an 811H, still applies.

And a personal note. If you decide to build your own power supply there is no reason not to ditch the 811's and optimize for the 572B's. As seen above directly grounding the grids does not mean you can't measure grid current .

just my 2 cents  
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October 02, 2017, 07:53:41 PM
K1ZJH
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Posts: 3326




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« Reply #6 on: November 01, 2017, 04:50:55 PM »

It uses a pair of 572 tubes and there is no advanced metering used in the design.  Not even a loading control, it used fixed loading.
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KM1H
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« Reply #7 on: November 01, 2017, 05:23:30 PM »

Quote
It uses a pair of 572 tubes and there is no advanced metering used in the design.  Not even a loading control, it used fixed loading.

Which assumed a near perfect match was presented by the antenna.

Not a good amp for mobile if you wanted to QSY with a Hustler or similar while driving. I had one for awhile including the DC supply but soon got away from mobile anyway except for some limited 6 and 2M SSB as a contest rover in the northern boonies of New England and Eastern Canada.
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K8AXW
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« Reply #8 on: November 01, 2017, 08:31:09 PM »

Step start for this amp is a waste of money.  The tubes are "instant on."  I've read where step start is actually detrimental to the tubes.  Research this before spending the bucks.
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KOP
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« Reply #9 on: November 01, 2017, 11:28:53 PM »

...the larger the filter total capacitance the greater the need for a step or soft start.
Step start has little to do with tubes and everything to do with HV supply inrush current.
The larger the total filter capacitance the more the rectifier is looking into a dead short.
This is what drives the necessity for a step start. It has little if anything to do with the tubes, instant on or otherwise.
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October 02, 2017, 07:53:41 PM
G3RZP
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« Reply #10 on: November 02, 2017, 03:41:14 AM »

Did Heath do a mobile PSU for this amplifier? I remember an article in 73, where the guy fitted an extra alternator and used 3 big transformers to get the 2kV HV supply.

The SCR299 approach of a towed generator could be easier.....
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K1ZJH
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« Reply #11 on: November 02, 2017, 06:09:05 AM »

Did Heath do a mobile PSU for this amplifier? I remember an article in 73, where the guy fitted an extra alternator and used 3 big transformers to get the 2kV HV supply.

The SCR299 approach of a towed generator could be easier.....

The mobile supply for the HA-14 was the HP-14.  It was transistorized. The little amp wasn't much
demanding than some of the more modern 12-volt 600 watt mobile amps.  All of which would
probably choke a car's stock electrical system.

I used one with my S-Line for several years when I was a teen, along with a homebrew supply.

Pete
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K8AXW
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« Reply #12 on: November 02, 2017, 07:33:43 AM »

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."
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KM1H
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« Reply #13 on: November 02, 2017, 09:36:29 AM »

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

OTOH if the On-Off switch is wimpy such as the SB-220 (not the 200) then a step start can protect it. I have replaced very very few in the past 30+ years and none in any of my other or customer brand amps.

Carl

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KM4AH
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« Reply #14 on: November 02, 2017, 11:27:53 AM »

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.
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