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 Post subject: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 12:10 am 
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Joined: May Mon 13, 2013 8:58 pm
Posts: 12
Hello everyone,


Where my Hallicrafters s40-B is almost done I'm turning my attempts over to my transmitter which is the aforementioned Heathkit DX-35 in the subject line. I do have a few questions that I hope people can answer as my search abilities are horrible in this area.

I would like to up, for starters, the filtering in the power supply. On the schematic it has two 20 uf 450 volt caps in series feeding a 5u4gb rectifier tube which amounts to 10 uf 900 volts to the tube. Now I would like to put two 80 uf 450 volt caps in series still using the 15 k 10 watt ohm resistors giving the filtering up to 40 uf 900 volts. 40 uf is the maximum amount of filtering that the tube can take according to the my RCA tube manual. Is this a bad idea or should I not max the rectifier tube out and just use two 40 uf 450 volt caps to bring it up to 20 uf 900 volts of filtering? I say this since I would prefer to do this as when I build guitar tube amps I usually have the power section at its maximum when dealing with a tube rectifier as I hate under filtered power supplies.

I would also like to eliminate the 8 pin socket which normally connects the vfo to the unit. I plan on getting a VF1 and when I do I want to have an external power supply for that. Plus I want to use that space after I put a plate over the hole for putting in a fuse as the unit was never designed with one.

In the audio portion of the circuit I also want to put in a 1 meg pot in place of the 1 meg resistor so I can vary how the microphone would sound should I use a different mic on the unit. I'm not sure if that is just being picky or good planning. Thoughts on this anyone?

The person who owned this before me added two switches in the circuit. As I see this one of them definitely looks like a power switch, the other might be a standby switch as I've heard that the wafer switches that control power get burnt up over time. Does that mean that the center tap of the power transformer should be grounded where it won't be connected to the wafer switch?

Lastly, their are two sets of 6.3 volt filament taps. One is ground lifted the other has one end grounded to the chassis whilst the other is going over to the filament pins. On the filament set that has one grounded can I put a 100 ohm resistor in as I'm wondering about hum in the filament circuit.

Thank you in advance;

Dennis


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 1:31 am 
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Dennis,

In general it is safe to increase the capacitance on the output side of a choke input supply (which the DX-35 has) HOWEVER just having a choke on the input side doesn't always mean the supply will operate as a choke input supply and until a critical amount of current is drawn it acts like a capacitor input supply so you would need to analyze your supply to make sure that it is operating as a choke input supply under all conditions from standby up until the final begins to load down the supply. Otherwise it begins to act like a capacitor input supply and because this series of Heathkit transmitters has a known marginal power transformer you don't want to do anything that will increase the load on it. Increased capacitance, if it is not behaving properly as a choke input filter, will cause increased peak current and greater transformer heating. The filtering in the DX-35 is sufficient so I wouldn't risk increasing the filter capacitor size.

Along those lines it is a very good idea that you are supplying the VF1 from another power supply. It will be more stable and will keep the DX-35 transformer running a bit cooler. From your post I take it that you are very concerned about hum so make sure that you have a very well filtered supply feeding the VF1 because it only takes an exceedingly small amount of ripple to hum modulate an oscillator.

A fuse is a good idea. Heathkit started by copying Johnson using a fuse plug with fuses in each side but they may have dropped that from the line. I generally just add a fuse under the chassis using an inline holder for gear that didn't come with one or used one of the dual fuse plugs. A properly sized fuse won't "nuisance blow" so if it goes open the case usually needs to be opened anyway and I prefer this to modifying the gear.

Do NOT try to modify the filament circuit. I am not quite following your plan for the 100 ohm resistor in the "traditional" filament circuit. The other circuit has the filament floating at a DC voltage above ground because otherwise the cathode to filament breakdown limit is exceeded in the 12BY7 tube due to the creative power supply configuration of the DX-35. It is similar to what is done in a CRT circuit where the filament is floated at a high DC voltage to avoid breakdown between cathode and heater.

Making the 1 meg resistor variable will have some impact upon microphone response and depending upon its intended load impedance it may have a major impact on output level. It is a safe experiment and you may like the results.

Good luck with your project!

Rodger WQ9E


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 2:08 am 
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Joined: May Mon 13, 2013 8:58 pm
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Hello,

Thank you for the fast reply. I appreciate it. I plan on getting my replacement caps from Hayseed Hamfest as I did that on my Hallicrafters and that worked out quite well.


Dennis


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 2:29 am 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1226
Location: Roslyn,Pa 19001
+1
Had a DX-40 that the pt caught fire while it was on stby. Burned up the wire harness and got the osc section pretty toasty. I was on the fence abt fixing and ended up fitting it w/a pt from a HW-16 parts set. Converted to a voltage doubler the achieve a 600v b+. Rewired the osc and buffer in parallel so I could use a single filament winding. Worked out ok had it working but has a chirp issue that needs to be sorted.
Definitely add a fuse!

Terry N3GTE


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 3:36 am 
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Joined: Mar Thu 05, 2015 2:31 am
Posts: 36
100% agree with Roger. That first cap needs to be about what was original in value. The other side of the choke and further down the line can be beefier caps. And also agree leave the heater circuit alone. You will have enough fun dealing with ripple and stray hum if using a high Z mic in that radio.

The 35 is another of the crazy control carrier scheme from Heath. There are a number of mods out there (go to AMfone) that address this.

I made a lot of changes to my dx60, similar TX. Once updated they work quite well with excellent audio.

Cheers and good luck.
Peter


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 4:08 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 3099
Location: Lincoln City, OR
Greetings to Peter and the Forum:

As Rodger points out, the DX-35 (if unmodified) has a single section choke input filter. There is no input filter capacitor.

I also agree with everyone here that the power transformer is fragile and care should be taken before considering any modifications to the power supply.

One modification that Dennis might consider is replacing the 5U4 rectifier with a 5R4 if space in the case permits. The 5R4 draws only 2 amps of filament current as opposed to the 5U4's 3 amp load. The 5R4 has a higher PIV rating than the 5U4. The other difference is that the 5R4 is good for slightly less current than the 5U4, but its maximum current rating is well in excess of what the plate supply in the DX-35 can provide, so it really is worthwhile to think about the swap. I made this change in a Johnson Adventurer that I gave to my brother for Christmas. Similar small transmitter except CW only. Worked out well.

Good Luck,

_________________
Jim T.
KB6GM


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 5:37 am 
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Jim,

Excellent point on the 5R4, it is a good choice and that small drop in load can be the difference between a living or dead power transformer in the DX-35.

Rodger WQ9E


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Fri 02, 2018 3:21 pm 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1226
Location: Roslyn,Pa 19001
You may find that there'a a bit more of a voltage drop across the 5R4 vs 5U4 which the transformer will like too.

Terry N3GTE


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Sat 03, 2018 2:44 am 
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Joined: May Mon 13, 2013 8:58 pm
Posts: 12
Hello all,


Thank you so much to everyone who had posted a reply to my questions, lots of food for thought here. I appreciate that a lot and I will be doing the 5r4 tube swap but I have one final question about the Power Transformer. I plan on putting an actual standby switch in, although to to be up front the previous owner looks like he did that, but one thought occurred to me. I'd like to put the center tap that is normally wired to the wafer switch and ground it. Does anyone think that that is a bad idea at all?


Thanks in Advance. :-)

Dennis


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Sat 03, 2018 6:36 am 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR
Greetings to Dennis and the Forum:

Re your last question:

I don't think I understand what you are asking. If you ground the center tap permanently, then you defeat the standby function. If you are talking about lifting it from the rotary switch and moving it to a toggle switch, then that is essentially the same function, just a mechanical move from one physical location to another. If your intent is to reduce wear and tear on the rotary switch, that would do it. Otherwise, I can't really see any advantage.

It would allow you to place the transmitter in either the AM or CW mode without having any plate voltage applied, so that throwing a single switch would enable transmitter output. However, plugging a key into the key jack would accomplish the same thing. The transmitter is silent with the key up, so you could operate break-in. For AM, a key equipped with a shorting lever would act as the standby switch.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Sun 04, 2018 2:14 am 
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Joined: May Mon 13, 2013 8:58 pm
Posts: 12
Hello,


Yes, just to be clear, my goal is just to move it away from the rotary switch and onto a separate switch. I have read that arcing can happen when the switch gets well worn over the years of use. There at least if something happens then the only thing that would get ruined would be the mechanical switch and that is easy to replace. I'm not sure how easy it would be to locate antique rotary switches for this transmitter, unless Mouser has something similar that can be used. If I get around to it I'll put up a copy of the original schematic and then put up one which shows the changes I'd like to make so it may benefit others and put a more concrete idea to others. Maybe this weekend, not sure. :-)

Dennis


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Sun 04, 2018 2:32 am 
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Joined: Jun Sun 19, 2011 2:31 pm
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Those rotary switches can be problematic and I think using them for transmit/standby switching puts more wear on them than the typical switch. My Geloso 212 and 222 transmitters also use a wafer on the function switch to switch the antenna between the transmitter and the receiver.

Sometimes these rotary switches can be saved by using the compromised contacts to switch a relay instead of directly switching the plate supply since the relay doesn't draw much current and slightly compromised contacts will still work to activate the relay. The same can be done with gear (i.e. some of the Hallicrafters receivers and Heathkit transceivers) that use a now hard to find SPST switch married to the back of a rotary switch as the main power switch. If these switches work at all they can be used to control a solid state relay to do the actual power switching.

And for a treasured piece of gear you might want to add this relay circuit as a proactive measure to save the switch.

Rodger WQ9E


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 Post subject: Re: Heathkit DX-35 restoration questions
PostPosted: Feb Sun 25, 2018 8:50 pm 
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Joined: May Mon 13, 2013 8:58 pm
Posts: 12
Hello,

Here is the modified schematic that I was mentioning about. Sorry it took me so long to do so as I've been battling with an old computer and scanner that did not want to cooperate.


Attachments:
dx35.jpeg
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