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 Post subject: Reading Heathkit Switch Diagrams
PostPosted: Feb Wed 21, 2018 7:27 pm 
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Can anyone offer a lesson on reading the rotary switch diagrams in their schematics? I have difficulty figuring out what is connected to what in the various positions, the diagrams seem very vague. Contacts are of varying "lengths" (does that tab make contact or not?), its not always clear which way the switch turns (some have arrows) , and how far the contacts move with each "click". Adding additional confusion, sometimes there is a connection from one side of the wafer to the other marked with some very small symbol. Surely it is easy if you know how. I wonder why they used these diagrams as opposed to the more understandable and standard technique for showing switches. Thanks for any help.


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Wed 21, 2018 7:51 pm 
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Unfortunately, schematics were not drawn such that understanding switch contacts is easy or sometimes even possible. But, first start with the notes on the side of the schematic diagram that tells you in which position the switch is drawn. A person then needs to understand the purpose of the switch. This can be helped by looking at the block diagram of the way the radio works and the manual discussion of the circuit description. Then a person can stare at the switch as it moves from position to position to see what it actually looks like and where the wires go. Finally assemble all of the above into an understanding of what the schematic is attempting to show.

Believe me trying to know if the drawing shows the back or front of a wafer, which wafer in the radio is which, and which position the drawing shows and which direction is the next position is not easy. That said, it is useful to dig in and try and figure it all out when the radio does not work right and indications are that the switch is involved.

Norm

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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Wed 21, 2018 8:12 pm 
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Heathkit actually does better than most and generally label front and rear on the switch wafers but that isn't always the case.

Usually the contacts will be one of two lengths with the longer one being the common wiper contact to the inner diameter of the rotating segment while the shorter contacts only make contact with the outer areas of the rotating segment. In complex switches the long common may also be used as a simple contact when the rotating center portion is broken into two or more insulated segments.

One hint to help you follow what is going on when rotating one of these complex switches is carefully sketch the rotating segment on a piece of transparency (clear plastic) material and then you can accurately rotate it and determine what parts are making contact at each position of rotation.

Note that the labeled contact numbers often don't reference the number of switch positions or a specific starting or stopping position but simply the number of contacts on a switch and you will have to figure out the starting and ending portions from the information on the schematic. Most specify the controls are either in a fully clockwise or CCW position but others specify specific control settings are depicted by the schematic.

Something else that often throws people is you will often see both switch contacts and/or wafer segments that are shorted through so that the front and back half of a switch wafer have common contacts and sometimes these are depicted in unusual ways on schematics. I think it might have been a Geloso receiver I was working on but one of the European models depicted a single wafer with parts of the front and back connected as consisting of a front, rear, and middle "sandwich" which threw me until I studied the schematic and realized what they were showing.

Rodger WQ9E


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Thu 22, 2018 6:28 pm 
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As always, thanks for taking time to post the replies. I was hoping that I was missing something but it doesn't sound like I was - its just difficult and probably best done with the actual switch in front of you. I like the idea of tracing the switch to plastic so you can rotate it on top of the diagram to simulate rotating the switch.

I guess I shouldn't feel too bad about the difficulty here because the newer radios use microprocessors to do the equivalent and those are totally impenetrable unless you have all the development tools and code which no one has. At least with switches you can figure it out, at least in theory.


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Fri 23, 2018 7:29 pm 
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It is sure nice to hear that I am not the only one!!! It usually takes me hours of study and experimentation with an ohm meter to figure out some of these. All this time I thought I was just slow! :roll:

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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Sat 24, 2018 2:42 am 
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This is a great topic! In all other respects I have no problem understanding schematics, but rotary switches are a complete mystery to me. I am almost never sure what the schematics for rotary switches are supposed to mean. It could all be made so simple if they would only depict them by an arrow coming out of central point, and a set of points on a circular arc, indicating a contact for each switch position. Instead, there is commonly a confusion of sliders, and unclear depictions of the actual switch geometry. All that is needed is a clear and unambiguous indication of what connects to what at each position of the switch. But only rarely is that made clear.

Chris


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Sat 24, 2018 4:45 pm 
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Greetings to Chris and the Forum:

Chris writes:

Quote:
It could all be made so simple if they would only depict them by an arrow coming out of central point, and a set of points on a circular arc, indicating a contact for each switch position.


Unfortunately, it isn't that simple. There are switches that connect several contacts together, more as the switch is rotated. There are switches with a notch in the rotating segment, so that all the contacts are connected together EXCEPT the one where the notch is. ...And, there are infinite variations on these schemes.

I agree with Rodger.... the Heathkit diagrams are actually among the easiest to read. Another approach is used by Hewlett-Packard in many of their diagrams.... they draw two columns of dots representing the switch contacts and then show a slider configuration between the two columns that accurately describes the interconnection scheme. This takes a bit of getting used to at first, but is actually the best and most understandable way to do it.

Unfortunately, as many have pointed out here on ARF from time to time, many old radio schematics were drawn by (we are guessing here) architectural draftsmen who have no understanding of what they are drawing and just want it all to fit on the page. I have seen plenty of older drawings on Nostalgia Air and other places that give me heartburn.... what were they thinking? Was there a contest with a prize for the greatest degree of obfuscation that could be incorporated into a schematic?

Compared to these, Heathkit schematics are a breeze.

Regards,

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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Sat 24, 2018 6:18 pm 
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This is a great topic but what I was really looking for was an education on how best to read the diagrams. No fault being laid on Heathkit, I restore their rigs because of the fine documentation available. I was hoping that I was missing something in how to read them that would make it easier. As was pointed out, these switches can be quite complex. As one relatively simple example the meter switch on the SB-401 connects both sides of the meter to various points in the circuit simultaneously; similar to the way one would move probes around. I am not sure that this complexity couldn't be boiled down to simpler diagrams but that wasn't what I was after. Thanks for all the great posts! I shall return to looking at the switch and diagrams to figure them out. Typically I want to know how some function works like, in my case, how grid current is measured. I drew the circuit without the switch (substituting opens or shorts for the switch) when "grid" is selected. That removes the switch complexity and allows me to focus on the problem circuit. Still haven't found the missing grid current but the search goes on. Next step is to study tube fundamentals a bit more, I was trained after tubes had become uninteresting (at least to academics) so I am probably not understanding something correctly.


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Sat 24, 2018 7:03 pm 
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The final grid circuit in the SB-401 is pretty straightforward with R33 (4.7K) acting as the shunt resistor for the meter.

Does the final draw normal idling current when in transmit mode? If not figure out the cause of this before going forward.

The most likely cause of lack of grid drive is a failure on a single band from either a hetrodyne crystal or band switch issue. Try more than one band if you haven't already.

The second most likely reason for lack of grid current in the SB-401 is lack of sufficient drive to the final and the 6CL6 is a good suspect because this class A driver tube operates at a very high temperature and it is important that only the original equipment black shield be used unless you want to upgrade to a IERC style heat conducting shield which is better. A standard tube shield will kill this tube in short order and it is also critical that the shield is well grounded because the tube will easily break into oscillation.

A 6CL6 that tests OK in a tube tester may very well fail to work properly at RF so don't count on the tube being OK just because it passes a check in a tube tester.

If you have a scope available it is excellent for finding problems in an exciter chain.

Rodger WQ9E


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Sat 24, 2018 11:49 pm 
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Jthorusen wrote:
Greetings to Chris and the Forum:

Chris writes:

Quote:
It could all be made so simple if they would only depict them by an arrow coming out of central point, and a set of points on a circular arc, indicating a contact for each switch position.


Unfortunately, it isn't that simple. There are switches that connect several contacts together, more as the switch is rotated. There are switches with a notch in the rotating segment, so that all the contacts are connected together EXCEPT the one where the notch is. ...And, there are infinite variations on these schemes.

Yes, you are right. I was being a bit too simplistic in what I said. What I really had in mind was that even when the switch is very simple, schematics often seem to make it obscure. No wonder then that more complicated switches are very hard to follow in many schematics.

Chris


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Sun 25, 2018 1:39 am 
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Rodger,

Thanks for the tips on the grid current issue. As soon as get a few minutes I'll follow the bread crumbs. I need to work on a couple other issues first so I can get back to where I was a week or so ago. Back one large step, forward a small step...

I turned on my trusty RS DMM from a hundred years ago and it was dead, so that's one thing but there are others as well. And no, it wasn't something simple, I checked.


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Feb Wed 28, 2018 11:37 pm 
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Relative to the rid current issue.

I replaced the grid current shunt resistor because it had drifted up to about 5.4K which is out of spec but wouldn't have caused there to be no grid current. I also readjusted the 8.6MHz trap which seemed to go perfectly. I have good HET voltages on all bands. I tried again and now I have grid current. Yea for me. But only on 80M and 40M. I can pin the needle on those bands so I back it down to .5ma per the tune up procedure. Plate current looks good, output looks good at least on the scope, I didn't connect a power meter. However if I move to 20M all the above is true but the meter shows no grid curent. In other words I can get maybe 90W out on 20M but no grid current. The plate current is about 240ma or 250ma which I believe is about right for a pair of 6146s. Something is wrong. 15M and 10M also work (showing no grid current) but with lower drive and power out but I'm not too concerned with those bands.

It puzzles me that 20M appears to work almost "normal" yet I can see no grid current. The only switch in the measurement path is the meter switch and it works on other bands and is in the same position on 20M. So it seems as though there really is no grid current. Drive on the grids of the 6146 looks about the same as for 40M and 80M. Could this be caused by a bad driver tube?

I touched up the coils (HET, grid, plate) but they were pretty much peaked from the alignment I did previously.

Relative to the driver tube shield issue, mine looks original but is not "black". There are black tube shields but they are on other tubes and from what I can tell they are where they should be (I looked at some pictures from the web). Also the 6CL6 has a shield that "locks" into the socket and the black shields do not.

Thanks for any help.


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Mar Thu 01, 2018 12:18 am 
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I think Heathkit may have used an off-color dull finish shield on some but most seem to come with a black shield with horizontal ribs and that is the best Heathkit used although an IERC shield would have been better. It is the only shield that actually results in a cooler envelope, the other shields have very levels of effectiveness against excessively raising envelope temperature.

The same shield was used for the HW-100/101 transceivers and the SB-400/401 transmitters so many of the Heathkit parts units from this era will provide that black shield.

A bad driver tube is the most likely cause of your symptoms however the hetrodyne oscillator tube is also a suspect since low output here will reduce excitation to the driver tube. But I would say given the symptoms there is about an 80% probability that a good driver tube will solve the issue.

It doesn't take much drive for the class AB1 finals in the SB-102 but running full power in tune/CW mode pushes it slightly into the class AB2 region which causes some grid current to flow and this increases drive requirements. When operating normally it will easily develop grid current on all bands.

Rodger WQ9E


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Mar Fri 02, 2018 8:27 pm 
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cnpope wrote:
This is a great topic! In all other respects I have no problem understanding schematics, but rotary switches are a complete mystery to me. I am almost never sure what the schematics for rotary switches are supposed to mean. It could all be made so simple if they would only depict them by an arrow coming out of central point, and a set of points on a circular arc, indicating a contact for each switch position. Instead, there is commonly a confusion of sliders, and unclear depictions of the actual switch geometry. All that is needed is a clear and unambiguous indication of what connects to what at each position of the switch. But only rarely is that made clear.

Chris


I fully agree with all the above. This problem is not only typical to Heathkit but to allmost any U.S drawn electronic schematic I've seen. Ever tried to decipher the rotary switch function(s) in a SAMS Photofact ? If there is a drawing standard for rotary switches (and it looks there is) I've never seen it detailed or explained in the many thousands of electronic publications books and magazines I've read. Not a single article, nothing ! Looks like the best kept secret in electronics. And don't tell me it's self-explanatory because it isn't. I can easily read any schematic but I'm still confused each time I see a rotary switch. I usually need to redraw it after analyzing the circuit and if I already know what the switch is supposed to do. Strangely enough, rotary switches in European schematics are usually much easier to read/understand, but not allways...


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 Post subject: Re: Reading Heathkit Switch Diagrams
PostPosted: Mar Sat 03, 2018 2:21 am 
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Greetings to Tubologic and the Forum:

I guess it is what you grew up with. I don't find rotary switches in either Sams or Heathkit publications hard to figure out. Of course, the degree of difficulty rises with circuit complexity, and overall schematic layout can make a big difference.

I personally find most european schematics to be much more difficult to understand than U.S. ones. The schematics where every component is a rectangle with microscopic print inside that tells you the value of the resistor or capacitor or whatever are utterly useless. This is a european standard.... and one that needs to die a quick death... the quicker the better.

I guess that we will all have to muddle through the best way we can.... I do it by avoiding european electronics like the plague.... to each his own.

Regards,

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