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 Post subject: Hallicrafters HT-44 -- PS-150-120
PostPosted: Oct Sun 29, 2017 1:54 am 
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I have been getting my HT-44 ready to align--however, I am wondering what voltages are going to be okay for the transmitter. The power supply (new caps,resistors adjusted, CL40 inrush) has pretty high voltages. With a 5 K Ohm load, the HV is 628 VDC. Tube data for the 6DQ5 shows 990 VDC plate supply max, so I am not concerned with the HV. Without a load it is 750 VDC. The radio calls for 575 VDC.

The LV is a potential problem. The power supply and radio specs call for 250 VDC. However, the tube pin chart shows 285 VDC at a few of the tubes, so I know the transmitter probably had about 285 plus VDC at the LV line when the chart was made. My supply puts out 300 VDC with a load of 2.5 K and 370 VDC at 10 K.

My mains is over 125 VAC. Should I add a 150--250 Ohm resistor into the LV circuit after the transformer and before the diodes? I am tempted to do this just to make it easier on the tubes.

Norm

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Oct Sun 29, 2017 3:31 am 
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So, playing around with the transmitter hooked up, it looks like a 200 Ohm resistor brings the LV down to 297 VDC in standby and it drops to 250 VDC in transmit. This seems pretty much in line with pin voltages in the manual. I am using a 25 watt resistor after the chokes because that is what I have.

With my Kill A Watt in the circuit, the power supply draws 2.81 amps at transmit--which is pretty close to the 3 amp slow blow fuse. This isn't after I have fully explored the top end for the transmitter so it doesn't leave much head room. I wonder if I should bump the fuse up to a 4 amp.

Norm

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Oct Sun 29, 2017 4:41 am 
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The manual calls for matched 6DQ5 final amplifier tubes. Mine don't quite match--37 ma compared to 49 ma. I have ordered a set of matched tubes (who knows if they will match). How important is it that I don't go any further with the unmatched tubes. They both read strong in the tester with no difference, but in the radio, one draws less at resting current.

Norm

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Oct Sun 29, 2017 3:42 pm 
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Norm,

Even when tubes are matched they don't stay matched as tubes age differently. And most matching is done at idle or some other low current while matching is more important as the tube develops rated input.

The HT-44 is a pure parallel circuit (not push/pull) so the only problem with mismatched tubes is the stronger tube will hog more of the current generally leading to shorter life.

A few rigs have been built with individual tube adjustments to set idling current but this isn't common. The best setup I have seen is in the Kenwood TL-911 amplifier (five 6LQ6 tubes in parallel). Its plate current meter can be set to read individual tube current and there are individual adjustments for each tube so that they can be matched both at idle and at full rated input. It also has a very nice electronic overload protection circuit. It is too bad more sweep tube rigs weren't built with the same attention to detail as done by Trio/Kenwood with this nice compact amplifier.

Rodger WQ9E


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Oct Mon 30, 2017 9:31 pm 
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Thanks, Rodger. I am continuing on with the transmitter. A couple things I have noticed:
1.) The meter for RF output seems to swing a little more than I like. I am not sure what to expect there.
2.) Several times when the RF level is full CCW, I still have the meter showing significant power out when I switch to MOX in the CW position. Switching back to Standby and then MOX again clears the condition.
3.) A previous owner installed an antenna relay, which is fine. I will need to make sure the installation matches the manual.
4.) I am not a big fan of the neutralization procedure and do wish the transmitter had a PA plate current meter.
5.) In the lower PA compartment with the relay, the final tuning coil seems to have melted in the past. I have photos. Any thoughts on what I should do with this situation?

Note: I would suggest that anyone acquiring one of these first change out the 10 Ohm resistor in the HV power supply circuit between the test points. Mine was 1.5 Ohms high and it does greatly affect setting the bias.

Norm


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Oct Mon 30, 2017 10:15 pm 
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Norm Johnson wrote:
Thanks, Rodger. I am continuing on with the transmitter. A couple things I have noticed:
1.) The meter for RF output seems to swing a little more than I like. I am not sure what to expect there.
2.) Several times when the RF level is full CCW, I still have the meter showing significant power out when I switch to MOX in the CW position. Switching back to Standby and then MOX again clears the condition.
3.) A previous owner installed an antenna relay, which is fine. I will need to make sure the installation matches the manual.
4.) I am not a big fan of the neutralization procedure and do wish the transmitter had a PA plate current meter.
5.) In the lower PA compartment with the relay, the final tuning coil seems to have melted in the past. I have photos. Any thoughts on what I should do with this situation?

Note: I would suggest that anyone acquiring one of these first change out the 10 Ohm resistor in the HV power supply circuit between the test points. Mine was 1.5 Ohms high and it does greatly affect setting the bias.

Norm


1. When you say swing do you mean it isn't holding steady when you have it keyed in CW?
2. Put a regular RF output meter on it and see if the rig is actually producing output or it is the metering circuit hanging up. With the key removed in CW then the key jack is effectively closed so it will transmit a steady carrier when put in the MOX position but if the output is significant with the RF level at minimum then you may have instability somewhere in the exciter chain. The RF level control at minimum should short the output from the balanced modulator.
4. I keep a meter plugged into the power supply test points but be cautious because this meter is at FULL high voltage above ground and the current capability is significant. The meter should read 1 volt for every 100 mils drawn from the HV supply.
5. It looks like that coil really got hot at some point. Mis-tuning or a final that took off can cause very high recirculating tank current so maybe a rig fault caused it. I have seen those insulators "rot" many times but never melted except for small areas because of soldering taps nearby without heat sinking. Google "radio coil insulator strip repair" and you will find some methods for repairing these coils.

Rodger WQ9E


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Oct Mon 30, 2017 10:44 pm 
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Thanks, Rodger. Yes, I do use an external wattmeter which does hold steady. I will check the metering circuit. It resides in the rear compartment on a terminal strip. I may need to rig up a VTVM I can dedicate to this purpose while I work on the transmitter. Seeing the dip on an analog meter is a lot easier than a DMM. I will investigate the PA coil situation. Making some slotted strips wouldn't be too difficult. The existing one can probably be saved, cleaned up, and straightened.

Norm

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Sat 04, 2017 9:36 pm 
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The HT-44 that I had, used an external plate current meter in a plastic tupperware container for voltage protection. The meter was plugged into the two test points in the power supply.
In addition, the previous owner had converted the SSB generation to using crystals rather than the original phasing method.

Marc


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Sat 04, 2017 10:14 pm 
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Looks to me like someone poured glue all over the coil when the stock styrene formers broke apart.

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Sun 05, 2017 5:07 pm 
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I am in the process of creating a new coil for the PA output. The old coil was wound on a 1.25" dia form from #18 wire. I have a maple dowel that I will modify for the form and some plastic that I will make the form strips out of. Fortunately the old coil is intact enough to provide all the parameters. The plan is to use the dowel to hold the notched plastic strips in the right places while I wind the coil. The plastic strips will then be fastened to the wire and the coil slipped off the dowel. There are strategic dents in the coil wires at the takeoff attachment points. These will probably be created during the winding process using a punch and notches in the dowel. The dowel will need a slot that can be compressed to reduce the diameter when it is time to slide off the coil. This slot will hold one plastic piece that will be the mounting strip as well as slotted to hold the seperation of the turns. I am planning on using silvered copper wire for the coil. The silver tarnish conducts RF whereas other tarnishes have reduced conductivity. At least that is what I think I have learned while studying how this will work.

Norm

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Sun 05, 2017 7:10 pm 
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Glad to see you're on the down hill leg of the project, and are ready to attack the tank construction.

Here are some pics of what I did with a tank coil some years ago.

Obviously these type of tanks are no longer available and considerable work is needed to re-construct them or build new ones. Since they use spacers the fact of whether or not they have the enamel intact is of little consequence. Good luck with your new tank coil project.

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Sun 05, 2017 10:26 pm 
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That's great, Mike! I really love the way you did that. I have something very similar in mind, but will use 1/8" plex as the center support much like the original and some pieces of 1/8" plex for the secondary spacers as well.

Norm


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Sun 05, 2017 11:52 pm 
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Beautiful work Mike!

Rodger WQ9E


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Mon 06, 2017 12:05 am 
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Thanks guys.

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Nov Sun 19, 2017 11:32 pm 
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Last week everything came that I needed to rewind the output coil--a 1.25" dowel and a spool of silver coated copper wire. I wasn't sure the wire would be correct for this project but it turned out to be a bit stiffer than bare tinned copper--which worked well. Plus it looks really cool with the silver coating. I had some 1/8" plex which would be the permanent form. The slots for the wire were done with a bandsaw. A hack saw would also be the right dimension cut for the #18 wire. I left the dowel full length--which gave me something to hold onto and clamp to the bench top. The slot for the plex was made oversize and shims were used. This allowed me to later pull the shims and reduce the diameter so the coil could slide off. The coil has "indents" at either side of the solder connections so that the solder joint maintains a spark gap to the adjoining coil wires--or when the solder tap falls inside the coil diameter, it is made to the "indent." These have to be created as the coil is wound and I used a wooden chopstick to push the wire into the recess I had shaped in the dowel. The coil was about 17 turns by 1.25" dia. by 1.75" long with 4 soldered taps.

Norm


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Dec Sat 02, 2017 7:59 am 
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Today I got back to testing the transmitter. Initially it was very unstable on the bands above 40 meters. I went through the PA components and checked them all--especially the neutralizing capacitor. Everything checked out. The RF Level pot had drifted more than double its spec so I replaced it.

For those not familiar with the HT 44, the PA plate current is measured at the power supply using a voltmeter connected across a 10 ohm resistor in the HV line. The bias for the pair of 6DQ5's is set between 90 and 100 ma or .9 to 1 volt. I used my Fluke DMM to measure the voltage because it is my best protected instrument and it is in a 900 volt DC line. However, seeing the plate current dip was not possible. I ended up just finding a stable setting for the neutralizing capacitor and then tweaking it for a little more output. At 100 watts on 40 meters, I run 350 ma. The 6DQ5's are rated to run on average 285 ma and max 1000 ma. The way I have it set now, the transmitter puts out 115 watts on 80 meters, 105 watts on 40 meters, 75 watts on 20 meters, 95 watts on 15 meters, and 35 watts on 28.5 MHz.

It is supposed to put out 100 to 130 watts CW. The manual doesn't specify what to expect on different bands. My next step is to get out my old analog 1500 v VTVM and see if I can find the plate current dip so that I can fine tune the neutralization and get a little more output on the higher bands.

Norm

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Dec Sat 02, 2017 1:00 pm 
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WOW I'm Impressed!! GREAT work Norm!


For the neutralization, you might be better off with a Simpson or Triplett VOM hanging off the B+ line.

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Last edited by Mikeinkcmo on Dec Sat 02, 2017 4:11 pm, edited 1 time in total.

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Dec Sat 02, 2017 2:37 pm 
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Norm, Mike, color me awed!

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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Dec Sat 02, 2017 4:23 pm 
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Norm,

Beautiful work!

Your output on 10 meters is definitely low and could be the result of low grid drive on 10 meters; check this with a scope if you have one. 12BY7 tubes are known for going flat on higher frequencies as they get old and it is possible the problem could be the final tubes themselves.

Check the connections to and values for C104 and C105 since this is the only loading capacitance for 10 meters.

The HT-44 reflects Hallicrafters desire to market the transmitter as being simple to operate and this accounts for both the missing metering functions and fixed loading and both of these were unfortunate choices; the fixed loading in particular. Transmitter output is highly dependent upon the load and since the "tune" is the only variable component in the pi net for each band you end up tuning to compensate not only for the desired resonance but also to compensate for imperfect load capacitance on the output side of the tank with the result that output and efficiency often suffer. In general too much loading capacitance (aka "light" loading) will result in inability to develop normal plate current even with excessive drive with resultant low output and increased distortion while insufficient load capacity (heavy loading) results in excessive plate current even at far less than optimal drive resulting in low output, horrible efficiency, and very hot finals.

Rodger WQ9E


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 Post subject: Re: Hallicrafters HT-44 -- PS-150-120
PostPosted: Dec Sat 02, 2017 7:15 pm 
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I think I solved a few more problems. The driver tube was bad (I probably finish killing it trying to get the neutralization right) so that got changed resolving some fading output. I was able to find the neutralization spot using my DMM and a little more tweaking. So I don't have to risk another meter by connecting it to 900 volts. I have a stack (2) of DMM's that I have fried by connecting them to PA HV lines. The Fluke is the only meter I now trust with high voltage.

My bigger problem was that the RF Level was flaky. At one point, when I turned on the transmitter first thing this morning, there was no output--almost like the carrier wasn't functioning. The plate current never climbed above bias. Eventually I was able to get the PA started and then as it warmed up, the transmitter worked as well as last night. This had me suspect a capacitor in the carrier area--like it had to heat up by leaking to open a short or something. I also suspected the switch contacts might be intermittent--again needing to start a path through the carbon before working. So I changed all the paper caps. The carrier cathode caps were an assorted lot of mica molds and oil filled that someone had tacked to the terminals. Those really caught my eye. I also cleaned all the switch contacts with Q-tips and contact cleaner. They were pretty dirty.

Now the transmitter puts out well over 100 watts on all the bands except 28.5 MHz, which is 55 watts. Another noticeable difference is that the RF Level only needs to be turned up 1/3 to get good output compared to almost full clockwise before. I may just live with the lower 10 meter output for now and move forward with touching up the remaining alignment items.

Norm

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