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 Post subject: Heathkit HO-13
PostPosted: Aug Mon 23, 2021 6:39 pm 
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Location: Wood River, Ill.
I picked up this example of a HO-13, recently, as the price was about right, and I figured that I could use it with either IF coil, be it the 455 kC or the high frequency coil. I have several pieces of Heathkit gear that use the high frequency IF, but could also use it with some of my old equipment with the 455 kC IF if it came that way, as well. I debated even posting anything on this one, but since I could not find much about these on here, I thought I would post something.
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I ordered a full re-cap kit from Hayseed Hamfest with the resistors, and installed the new capacitors before I powered it up for the first time. (I didn't want to risk damaging the power transformer.) Once completed, I was getting a good trace, and was able to input a signal from my signal generator and see the pip on the screen. As it turned out, it had the high frequency coil installed. I could see a 3 mC signal pip on the screen and move it around as I tuned the SG.
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Going thru the tube voltage measurements, many of the pin voltages were off in the RF & IF stages. When I say off, I mean way off! Digging into this further, and comparing with the schematic, it looks as though a previous owner had tried to make a few modifications. :o

Un noticed by me, when I started this, :oops: I found that another coil had been added to the back side.
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I should have spotted this right off, that it did not belong there. :oops: These were supposed to be wired up for either the 455 kC or the higher frequency IF receivers. Since I found the high frequency coil installed, where it should be, and the appropriate capacitors for use with the Heathkit SB-100 or SB-300 receivers (3300 kC or there abouts) I assumed that it was wired for these radios. What I have since found, is that someone has attempted to wire it for both high and low IF frequencies, as it appears to have both sets of components in it. Not properly wired for either one.

I pulled the extra coil mounted on the back side, and hoped that it might be the missing low frequency IF coil (455), but it did not have any marking on it. The slug is frozen in place, and it measures out at about 105 µh. The 455 coil is supposed to be 305 µh (if my memory is about right.

I decided to continue to attempt to change it back over to the stock HO-13 with the high frequency coil and appropriate capacitors to work with my SB series of receivers. So I am in the pain staking process of trying to remove the parts that do not belong, and make sure that the ones that do, are there.

OK, even though I should have known better than to trust the "last guy" that worked on it, I didn't. (Lesson #1) Another lesson: Don't order a cap kit until fully inspecting the equipment before hand. (Lesson #2) I know these things are fairly limited in their use, but it does make good eye candy. I am in it too far now to scrap it out. This may take a while. :shock:

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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Tue 24, 2021 2:38 pm 
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Nice post Chris. I bought both the HO-10 and 13 a year or so ago. they are nice little units and do provide useful information when running your BA stations.

I modified mine so they set vertically on the shelf, and installed fans to help cool them internally.

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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Tue 24, 2021 6:36 pm 
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WOW! Mike those look great! I wonder why Heath didn't build them like that to begin with. I like them both!

I have both the HO-10 & HO-13, and will be reworking the HO-10 after finishing up on the HO-13. Hopefully I will not be finding similar issues with it, as I have with the 13. I am making progress, though, having removed most of the parts that do not belong, and making sure the ones that do belong are installed.

Thanks for sharing! :D

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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Wed 25, 2021 1:55 pm 
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I am fighting the fight of trying to set the bias. As of last night, I was still finding components in the circuitry that do not belong. I am attempting to set the bias, per the book, for a voltage of -9 VDC. This is what is called for when using an IF of 3395 kC. In the process, I am finding all kinds of resistors in the circuitry that do not belong. I think a PO was attempting to set it up to use with multiple IF frequencies, or one that was not on the chart. I am slowly finding these extra components and removing them to return to the stock condition so it will operate at 3395 kC IF.

Getting to the problem; the bias voltage is measured at the test point shown in red, below.
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This is supposed to run anywhere between -.5 to -10 VDC, as I recall. For the IF frequency of 3395 kC, it should be adjusted to -9 VDC. I am not getting anything close to this value. It has been measuring about 1/10th this value, with a VTVM. I immediately went to the voltage divider, outlined in red, looking for problems here. I did find some out of tolerance resistors, here, as well as a few extras thrown in just to confuse me more. :shock: The extras were removed and wired back to stock. There are other resistors that are not marked with values in this schematic that are specific to the 3395 IF. These were either verified or replaced with the correct values. Now I am getting no readings at the test point.
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This circuit goes to the Grid of V2B, which is the sweep oscillator. The voltages on this tube were checked, and I am seeing 100 VDC or a little more on the plate, zero on the cathode, and nothing on the grid. I believe the two issues are directly related. All of the components for the V2B are checking out as good and within tolerance, but I am not seeing anything close to a -10 VDC on the grid.

Going back to the voltage divider. Unless I am missing something, the bias voltage is derived from the 1500 VDC circuit. I am measuring 1500 VDC or more at the input to the divider, and around 500 volts or more at the junction of the two 12 meg resistors, but nothing at the test point. The resistors have been replaced, except for the two 12 meg resistors. (All I have in my stock are 12 meg, 1/4 watt) The PIP Center control checks out, and the Seep width control looks to have been replaced. I have not checked it out for proper value, yet. The 12 meg resistors are measuring high, at around 14-15 meg. I would have just replaced them, but don't have any available right now.

There is also a diode, D5, and the only description that is that it is a voltage variable capacitor. I don't know if it is good or bad. It measures out at around 3k ohms in either direction. I suspect this isn't good, but really don't know, and have nothing to go by. If it is some sort of variable capacitor, it looks like it shunts across the L2 coil and C28 capacitor. Any ideas as to what this is, or where I could find a new one? But, this said, when I take it out of circuit, it has no effect.

This is where I left it in frustration, last night. It was getting late, and I was not trusting myself around the high voltage, so decided to throw in the towel for the night. I am hoping to get a little better perspective on things tonight. So the story goes on...

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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Fri 27, 2021 10:38 pm 
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Today I did a little experimentation. I am still fighting the setting of the bias for the 3395 kC IF. The voltage at the test point, discussed above, is supposed to be adjustable from -.1 to -20 VDC. Up until today's experimentation, the best I have been able to find at that point is a little over -.1. I have now come to the conclusion that diode D5 is the culprit. I believe that it is bad, because I measure approx 3k ohms across it in both directions. The Heathkit part number for D5 is #56-23 and is described as a voltage variable capacitor diode. Doing a search on Heathkit cross reference pages yields very little. I was able to find one site (Colorado Radio Collectors) that does have a listing for 56-23, and it is shown to be a 1N954, 40 v germanium diode.

This is where the experimentation starts. I tacked in a 1N4148 diode, mostly because I have so many of them. When I use this diode, I can get a pip on the screen at 3395 kC from my signal generator. Now I am getting a voltage at the test point that I can measure and it is close to the -9 volts the book is asking for. The grid voltage on pin 9 of V2B is not quite the -10 volts listed, but is running around -8 to -9 volts, much closer than the less than -.1 volts I had been seeing.

My question now is; what is a Heathkit 56-23 and what can I substitute in it's place? I do have other diodes, even some 1N34A and 1N60P germanium on hand. I have not tried any of them, because I don't have many. Does it sound as the germanium diodes might work?

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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Fri 27, 2021 11:17 pm 
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D5 is a varactor diode. Varactor diodes are widely used within many RF designs. They provide a method of varying the capacitance within a circuit by the application of a control voltage.
See more info: https://en.wikipedia.org/wiki/Varicap

Further, according to my info, Heathkit part number 56-23 maps to varactor diode 1N954.
Spec sheet is here: https://datasheetspdf.com/pdf/943747/KN ... OR/1N954/1
OR here
https://www.datasheets360.com/part/deta ... 264974466/

1N954 further maps to a NTE614; see: http://www.nteinc.com/specs/600to699/pdf/nte610.pdf

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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Fri 27, 2021 11:45 pm 
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Location: Montreal, Quebec
D5 is a varactor.

It's across a tuned circuit which must be the oscillator in the panadaptor. It gets a sweep.voltage that makes the varactor cause the oscillator to tune across the band, then reset and start again. That ramp vo!tage goes through a pot so you can adjust how much the oscillator will tune. The other pot lets you adjust a fixed dc voltage, so you can set the center frequency.

A varactor will show a low forward voltage drop, just like any diode. Unless you put it in a circuit, no way you can tell it's a varactor by a meter.

A 1N954 is a varactor according to a web search. Whether Heathkit used a varactor or some diode would rep nd on the Heathkit part number, and what it really is.

Random diodes work as varactors if they are reverse biased, but the results vary. If you can't get enough capacitance range, you'll have to try another.

It would have been germanium because solid state devices were germanium at the time. It shoukdn't matter one bit that you replace it with a silicon diode.


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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Sat 28, 2021 12:14 am 
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Varactor tuning diodes aren't really so special. There is a very good article by Hans Summers online:
http://www.hanssummers.com/varicap/varicaporig.html

This article links to others in which the author explored using common silicon rectifiers such as the 1N4001 family, or LEDs as voltage variable capacitors.

The key point affecting the HO-13 project is that a diode used as a variable capacitor is (almost) always back-biased, that is turned off. The applied reverse voltage controls the capacitance. So the presence of the diode shouldn't affect the control voltage divider circuit, unless the diode is shorted !

<deleted comment about diode tuning of BFO in R-648>
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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Sat 28, 2021 2:48 am 
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And in 1974 in Ham Radio magazine Ken Robbins had an article about a 6M synthesizer, where he suggests trying diodes on hand. That's the srticle where he notices hum on the oscillator, finally realizing the diode he was using had a clear case, so thedesk lamp was modulating it.

But the master source is Sam Harris's article about parametric amplifiers in CQ, about 1958. The first practical paramp,it relied on the relatively new varactor.

And he tells the readership to get things going using an available diode. Then get a good varactor.

So theconcept was there almost from the start.

But varactors have reliable calacitance range. Which does matter in some applications, especially with large max to min capacitance, but also if you need high capacitance.


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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Sat 28, 2021 7:08 pm 
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Thank you for all the replies and information. Since my last posting, I did a little searching with google, and found the same thing you did, Pete. So I have a few NTE614 on their way. For the time being, I left the 1N4148 tacked into the circuit. I was able to go thru most of the alignment, and in the process found that the sweep width control was wired backwards from the way it should be. It had been replaced with a 5.5 meg pot, by the PO, and a 5.6 meg ohm resistor added to make up for the difference, replacing the 2.2 meg resistor that should be on the high side of the pot. It seems to work this way, and the pot was rewired per the book. I did a little looking for a 7.5 meg pot to put back into the circuit and make it right, but could not find a suitable fit. I guess a 7.5 meg pot is kind of unusual.

It has been fun trying to find what belongs and what doesn't but it has kept me occupied. We will see how the proper diode in the D5 position works out when it comes in. For now, I will leave it alone and move on to something else. (Probably doing a little reading up on all the information provided by you all.)
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Thanks again :D

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 Post subject: Re: Heathkit HO-13
PostPosted: Aug Sun 29, 2021 3:41 am 
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I could be mistaken, but as I recall, I built a TV kit that used varactors in the tuner. That has been many years ago but now some of it is coming back to me. Thanks for the articles on the subject.

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 Post subject: Re: Heathkit HO-13
PostPosted: Sep Mon 20, 2021 6:32 pm 
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I was able to find the exact match varactor diode for the Heath part number on Ebay. (I had also ordered the NTE diodes, mentioned above, as back-ups.) I installed the NOS diodes and proceeded with the alignment/adjustments per the assembly manual, and it aligned just like the book said it should.
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It was aligned for the 3395 kC IF as used in the Heathkit SB line, where I will be pairing it up with my SB-101, for now. I will need to modify the SB-101 to work with it, though. (As I recall the 101 that I have did not have the necessary mods for use with the Ham-Scan.
I am going to call this one complete, for now anyway, until I get it up and running with the transceiver.

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