Hallicrafters HT-37

Discussion in 'Amateur Radio Amplifiers' started by KD8BLR, Jan 1, 2011.

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  1. KD8BLR

    KD8BLR Ham Member QRZ Page

    I am very new too amature radio. I'v been licensed for about five years. Got tech. tkt. about three years ago, and general about two years ago. Failed two questions, do you have to have xmtr.on to send and do you have to have receiver on to liston. At seventy sixits not easy to know. 78years now!!!! Anyway this is the question-- I'm trying to repair a Hallicrafters HT-37. The first thing I did was to put the xmtr. on a variaic and turned the voltage up slowy. No smoke, no sparks, everything good. All filaments light up. The next thing I did was to replace the worn power cord, and then placed the xmtr. on the variaic again. No smoke or sparks. I diconnected the xmtr. from the variaic. turned on xmtr. and blew fuse. Replaced fuse, put the xmtr. back on the variac and turned up the votage slowy to 115v. No blown fuses, no smoke, no fires. being stupid and negligent I started to remove the metal shelds from the vacuum tubes, and got knocked on my rear end from making contact with the HV plates of the 6146. Wife thought I was trying to do myself in. This tells me that the high voltage side of the power transformer is working, right ?? wrong??? removed the xmtr. from the variac turned the power on and blew fuse. Im hoping that the troublr is not the power transformer. You would think the fuse would blow when you had the variac turned up to 115v-- right?? wrong??? When I first started I thought the 6.3 filment power supply could be leeking in the power supply transformer. I' hoping this is not true... Need all the help I could get 73 KD8BLR Martin
     
  2. AG3Y

    AG3Y Guest

    "Turning the variac up slowly" is a good way to re-form power supply capacitors that have been dormant for several years. But it doesn't always guarantee that the radio won't blow fuses when the power is suddenly applied to it.

    I would make sure that all the power supply filter caps are new. You cannot depend on caps that have been around for years, not developing some sort of leakage that will draw excess current from the line.

    You can get knocked on your keester with much less than the full voltage that the HV supply normally puts out! You can be glad you are still here to tell the story.

    My bet is with power supply filter caps. Good luck, and stay away from live circuits. Don't be touching tubes with power applied. But I guess you have learned that lesson, haven't you?

    Happy new year. 73, Jim
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Even if the HV is turned off, touching tubes can burn your fingers. Some get very hot, and are supposed to.:)

    The problem you're encountering could be a few different things, but Jim is likely right: If the rig has its original HV filter capacitors they probably need replacement.

    One thing you could try is this:

    With the rig off, cold, discharged, and unplugged from the AC power line, try disconnecting the HV leads from the transformer to its rectifiers, tape them up well to make sure they're insulated from everything around them, and then plug it in and turn on the power. If you have a short in the transformer, the fuse will probably still blow. If not, proceed to the next step.

    Randomly replacing parts without knowing if they're bad or not is usually a bad idea. Systematic troubleshooting, as if following a block diagram and isolating blocks, makes more sense.
     
  4. W3JN

    W3JN Ham Member QRZ Page

    Isn't it easier just to yoink the rectifier toobs?
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't have the HT-37 schematic handy; that might do it. If the tubes have anode caps, just yoinking those connections might do it also (even with the tubes still in place).:)
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The HT-37 uses a 5R4 as the high B+ rectifier and a 5V4 as the low B+ rectifier.

    Unfortunately, the power transformer in the HT-37 is the weakest link in the unit and having a blown transformer is fairly common. The most common failure is a short from one the 5.0 VAC rectifier filament winding and one of the other windings. This is due to the presence of the rectified B+ on the winding.

    You may be able to save the transformer by replacing both of the rectifier tubes with solid-state diodes AND completely disconnecting the filament windings from both rectifier tube sockets.

    I also would replace ALL of the electrolytic capacitors in the transmitter. Using a variac to slowly bring up the unit "may" reform the capacitors. However, even when reformed they are very likely to fail in a relatively short period of time. Unless the unit has been in regular use, there is an extremely good chance that the electrolytic capacitors are going to be bad. When the unit is in regular use the electrolytic capacitors stay "formed" and generally last for several decades. However, the HT-37 is now over 50 years old and even if the unit has been in regular use the electrolytic capacitors are going to be on their "last legs".

    The HT-37 is a good transmitter. There are most certainly better ones from the same era but the HT-37 is not bad. I had one back in the mid 1970s that I gave to an employee. I acquired another one about a year ago that I really need to get restored.

    Glen, K9STH
     
  7. NM5TF

    NM5TF Ham Member QRZ Page

    I would definitely suspect the 5R4 rectifier tube.....my HT-37 worked flawlessly for many years, then started blowing fuses....the 5R4 developed a short & blew fuses when power applied....they are available on E-Bay & other places...

    GL & 73 de Tom NM5TF
     
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    TF:

    Although I generally do not recommend replacing the tube type rectifiers with solid-state, there are exceptions and those include the Hallicrafters HT-37, HT-32 series, and the Collins 32V- series. All of those units have transformers that are notorious for shorting out and replacing the tube rectifiers and getting the rectified B+ off of the transformer definitely extends the life of the transformer.

    Glen, K9STH
     
  9. K2WH

    K2WH Premium Subscriber QRZ Page

    Go here:

    http://groups.yahoo.com/group/HallicraftersRadios/

    Join the group.

    BTW, Glen is correct. the HT-37 power transformers are notorious for failure. They were built back in the days when power line voltage was about 110vac. It is now closer to 120vac. The problem with the transformers is the 5v winding failing to the HV winding. However, WIK may be right also in that the caps need to be changed out.

    Anyway, join the linked group. Lots of Hallicrafters experts there.

    K2WH
     
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Actually, the HT-37 was designed for AC line voltages ranging from 105 VAC to 125 VAC. The "target" line voltage for most electric companies these days is 125 volts +/- 2 volts with the "+" side being very common. Even at the "olde tyme" standard of 117 volts, the transformers in the HT-37 and HT-32 series were failing. The insulation on the two 5 VAC filament windings are the problem. When the rectifier tubes are operating the full high voltage DC is also on these windings and the insulation eventually fails. The insulation can easily handle the 5 VAC, it is when the high voltages are applied that the problems start.

    If you don't have the manual, it is available for free from BAMA. Just go to

    http://bama.edebris.com/manuals/hallicra/ht37/

    and select the PDF version.

    Glen, K9STH
     
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