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KG7RS
03-09-2011, 03:14 PM
Hi Folks,
Looking for some solid advice from those more initiated than myself. I recently picked up a nice Hallicrafters HT-44 SSB transmitter with matching PS-150-120 power supply. Both are completely original and untouched including all 6 electrolytics and 5 semiconductor diodes in the power supply. Everything functions normally but I did note the transmitter goes into transmit by itself once in awhile.

1. When I was changing antenna cables on the back, I got "bit" by what is probably leaking AC line bypass capacitors on the primary of the power transformer. I'm confident that replacing those will cure that problem, but I'm also wondering if this condition may be the cause of the transmitter going into transmit by itself. I haven't studied the schematic in depth but if the PTT circuit is expecting a clean ground when PTT is enabled and there's leakage voltage present...any thoughts?

2. Although the original electrolytic filter caps appear perfectly serviceable, I'd like to replace them all with good 'ol Sprague Atom TVA-series. As much as I'd like to keep it all original, this rig will see regular use and it just seems like its asking for trouble not to replace them.

3. Finally, There are are 4 1N3487 diodes configured in a bridge rectifier circuit on the secondary of the HV supply section. This type appears to be long gone. Best I can find is a recommendation for an NTE506 or BY127MGP as substitutes. I can't even locate a spec sheet for the original 1N3487 to determine ratings. Is it best to just keep the original rectifiers in service or do I run the risk of ruining the power transformer if one/more should short? I have concerns with using NTE parts in general. The 5th diode is for a low voltage/current bias supply which can easily be replaced by a common 1N4005 silicon diode.

I appreciate all thoughts. Thank you.

73, John, KG7RS

WB2WIK
03-09-2011, 04:32 PM
Hi Folks,
Looking for some solid advice from those more initiated than myself. I recently picked up a nice Hallicrafters HT-44 SSB transmitter with matching PS-150-120 power supply. Both are completely original and untouched including all 6 electrolytics and 5 semiconductor diodes in the power supply. Everything functions normally but I did note the transmitter goes into transmit by itself once in awhile.

1. When I was changing antenna cables on the back, I got "bit" by what is probably leaking AC line bypass capacitors on the primary of the power transformer. I'm confident that replacing those will cure that problem, but I'm also wondering if this condition may be the cause of the transmitter going into transmit by itself. I haven't studied the schematic in depth but if the PTT circuit is expecting a clean ground when PTT is enabled and there's leakage voltage present...any thoughts?

I doubt that's the problem. The PTT circuit expects a return to chassis ground to complete a circuit. If the chassis was floating at 1000V, that's still a ground return for the circuit.


2. Although the original electrolytic filter caps appear perfectly serviceable, I'd like to replace them all with good 'ol Sprague Atom TVA-series. As much as I'd like to keep it all original, this rig will see regular use and it just seems like its asking for trouble not to replace them.

I'd let it cook for a few days, powered on (no load). If it passes that test, I can't imagine how changing the capacitors would be an improvement; but then it shouldn't hurt to change them if you want to. I have all original electrolytics in Collins gear built in 1956, never replaced -- still fine.


3. Finally, There are are 4 1N3487 diodes configured in a bridge rectifier circuit on the secondary of the HV supply section. This type appears to be long gone. Best I can find is a recommendation for an NTE506 or BY127MGP as substitutes. I can't even locate a spec sheet for the original 1N3487 to determine ratings.

It's in the JEDEC book, I just looked it up. This is an old specialty rectifier with two stacked silicon dice inside. It's rated 1200 PIV, so if you replace these you should use two 600V (or higher) rectifiers in series to replace each 1N3487. There is absolutely nothing special about it at all except that each one is really like two diodes in series. A pair of 1N4005s, 4006s or 4007s in series to replace each one should work fine. The 1N3487 had a very low surge current rating, so I guess in the Hallicrafters design it must have been well surge limited by something.


Is it best to just keep the original rectifiers in service or do I run the risk of ruining the power transformer if one/more should short?

Why would that happen? That's what fuses are for. Normally if a rectifier shorted, you'd blow a fuse instantly, protecting the transformer. Make sure you have the correct value fuse installed.


I have concerns with using NTE parts in general.

I kind of do, too: NTE is not a manufacturer of anything. They're a distributor who buys stuff in sufficient volumes to have their "brand" labeled on parts. You can do that also, if you buy thousands of parts. You never really know who made any NTE part, so you just hope for the best. I remember this company from almost 40 years ago when it was started by a couple of kids in a barn in Bloomfield, NJ as "New Tone Electronics." They've come a long way.

KG7RS
03-09-2011, 05:59 PM
WB2WIK - thanks for the response. If these bypass caps on the AC-in side of the transformer are removed entirely what negative consequences may I expect? It seemed to me the primary reason for them to be there was TVI-proofing - not really much of an issue nowadays. Easy enough to replace them, but perhaps I just need to remove them.

I need to study the PTT section of the schematic and determine where to look for the cause of the spontaneous transmit issue. The transmitter operates well otherwise - in fact I've already made dozens of QSO's with it. I'm also quite smitten with it so it's worth putting in the time and effort to correct any shortcomings.

I will take your advice and leave the power supply components alone. Makes good sense.

73, John, KG7RS

K9STH
03-09-2011, 06:13 PM
RS:

I suspect that the power supply still has the original 2-wire cord. To eliminate the voltage on the chassis, and for safety, you need to immediately replace the 2-wire cord with a 3-wire cord (green wire goes to the chassis).

Even when the AC line bypass capacitors are good, there will still be approximately 60 volts AC on the chassis. That is one reason why the "olde tyme" equipment had provision for a ground connection, to eliminate this voltage. With a 3-wire cord the chassis is connected to the AC mains ground and that will eliminate the "tickle" that you get from the unit.

Glen, K9STH

WB2WIK
03-09-2011, 06:54 PM
WB2WIK - thanks for the response. If these bypass caps on the AC-in side of the transformer are removed entirely what negative consequences may I expect? It seemed to me the primary reason for them to be there was TVI-proofing - not really much of an issue nowadays. Easy enough to replace them, but perhaps I just need to remove them.

You could just remove them, but you should always use a 3-wire power cord (replaced as Glen K9STH suggested).



I need to study the PTT section of the schematic and determine where to look for the cause of the spontaneous transmit issue. The transmitter operates well otherwise - in fact I've already made dozens of QSO's with it. I'm also quite smitten with it so it's worth putting in the time and effort to correct any shortcomings.

Might be something in the VOX circuit. I looked up the HT-44 and see it does provide for VOX operation, which of course would key the transmitter if something is causing it to trip.

KG7RS
03-09-2011, 07:11 PM
Thanks guys. Yup, still has the 2-wire cord. I didn't realize the AC potential would still be present on the chassis with good bypass caps. I will indeed replace the caps as well as install a grounded power cord.

VOX circuit...hummm....it does indeed have a VOX function now that I think of it. Hadn't tried it to see if it functions normally once enabled. I have a manual & schematic on order which should arrive later this week. Looking forward to finding the cause of this issue. Shouldn't be hard to find. KG7RS

WB2WIK
03-09-2011, 07:27 PM
The HT-44 manual with schematic are free for download from:

http://www.mods.dk/manual.php?brand=hallicrafters

KG7RS
03-09-2011, 08:03 PM
Thx WIK - The one on the BAMA site is incomplete and mostly unreadable. I can see that the relay coil is energized by a 1/2-section triode of a 12AT7 acting as a switch. Immediately before this is a diode section of another tube which rectifies VOX audio to bias this switch. I can't see how the manual PTT plays into this but the VOX circuit does indeed precede this "relay tube". Somehow this tube is becoming biased "on". A clear and complete schematic should answer all these questions. I'll try the site you recommended but should have an original manual delivered today or tomorrow. KG7RS

WB2WIK
03-09-2011, 09:22 PM
The manual on the mods.dk site is complete and very legible, give it a try if you want to save a day...

WA2CLX
03-22-2011, 04:51 PM
John,

If you want to maintain the 'look' on your units, while recapping, Tom N0JMY at Hayseed Hamfest (www.hayseedhamfest.com (http://www.hayseedhamfest.com)) made new custom electrolytics for me, and added in the other axial caps for a 'recap kit' for both the PS-150 and HT-44. Prices I was quoted were:

Description: Hallicrafters PS-150-120 re-cap kit Item #:
Unit price: $39.95

Description: Hallicrafters HT-44 re-cap kit Item #:
Unit price: $28.50

The PS-150-120 consists of:

1 - 2 X 33 uF @ 450V 1" twist-tab capacitor with metal wafer.

2 - 100 uF @ 450V axial lead capacitors

2 - 22 uF @ 450V axial lead capacitors.


The HT-44 kit contains:
1 - 2 x 33 uF @ 450V 105C 1" can capacitor

Polyester Caps (replacements for paper caps etc.)

4 - .01 uF @ 630V axial
3 - .1 uF @ 630V axial
1 - .22 uF @ 630V axial
2 - .47 uF @ 630V axial

Best regards, Phil WA2CLX

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