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Author Topic: TS-930/TS-940 Power Supply "Crowbar" addition to protect final unit?  (Read 4628 times)
N6QWP
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« on: November 12, 2017, 07:12:35 AM »

While I have the rigs apart, would very much appreciate someone pointing out where a reliable "crowbar" circuit modification can be found to add to the 28 volt supply to protect the final unit(s) from catastrophic failure should a pass transistor fail.

Since the possibility of such a scenario is pretty prevalent, such a "fail safe" modification seems almost necessary.....given the difficulty and expense of repairing the ensuing results of such a overshoot of voltage in either of these rigs.

There have been several postings that refer to such a modification, but I have not been able to find one (with the corresponding circuit, parts and values) to proceed with installing such in both the 930 and the 940-whose supplies are still working properly.

Other than doing such a modification, one is left to either rolling the dice and hoping that such an incident does not occur....or changing the power supply to something more modern and less vulnerable.....or using an external supply.

(My apologies for raising this question a second time, but realized that I had asked at the end of another convoluted thread that was very long and that meandered all over the place.  Thought that it deserved it's own posting.  Seems important enough and of general interest to ask separately).  
« Last Edit: November 12, 2017, 07:34:23 AM by N6QWP » Logged
ZS5WC
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« Reply #1 on: November 13, 2017, 06:09:33 AM »

This Could Help:

http://www.resonantfractals.org/TS-940S/TS-940S-Repair.html

I would however rather crowbar the 40V supply just after the new fuse location and before the 28V pass transistors.

73 de William
ZS4L / ZS5WC
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N6QWP
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« Reply #2 on: November 13, 2017, 08:11:41 AM »

Thanx William!--That is obviously ONE of the mods available that I was looking for.....BUT, if you would change that circuit, where and what components would be moved, added or deleted Huh

IF you can see a weakness, can you BE SPECIFIC as to ANY changes that you would make to the mod.....OR to the original circuit Huh

I, and I presume anyone else trying to rectify the original vulnerable supply, do NOT want to start experimenting and risk blowing up the rig.  Since you obviously have the expertise to suggest changes, can you describe in detail, a "crowbar" circuit that we can use (part by part).....as the article you found does?

Which parts of the mod shown in the article would you change or move Huh  If you would LEAVE ALL OF THOSE as described, what other parts would be needed to MOVE (or add) "Crowbar"-in order to accomplish what you recommend Huh

Asking a lot of you, I know, but this is such an important change in protecting the TS-940 (not sure that it will also work, part for part, in the TS-930?).....hoping that you can provide "The Holy Grail".
« Last Edit: November 13, 2017, 08:22:39 AM by N6QWP » Logged
WA1RNE
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« Reply #3 on: November 13, 2017, 09:37:35 AM »

Quote
I would however rather crowbar the 40V supply just after the new fuse location and before the 28V pass transistors.

 This crowbar mod assumes the pass transistors or some other component have failed, likely taking out the pass transistors anyway.

The crowbar circuit should include a gate to cathode resistor and a 0.005 uF ceramic disc capacitor in parallel with it to load the gate against possible radiated or conducted noise which could cause
false tripping of the Triac.

 If a false trip should occur, the fuse characteristics are too slow to protect the pass transistors from crowbar activation - a near direct short to ground - since the circuit has NO current limiting, which is the first flaw of this power supply.

 A simple current limiter should also be added which will fix that issue.


 ..WA1RNE
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N6QWP
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« Reply #4 on: November 13, 2017, 02:09:28 PM »

Thanx RNE--all input is greatly appreciated.  However, I am requesting direct, step by step, part by part mods (as the previous one provided by A5LX does in great detail).  If there are improvements or other mods that will help preserve these old 930/940 radios, there are many of us who seek such information.....but are limited in our experience and knowledge.

If those who really understand the mods can point to schematics that are available and give us a specific series of what parts to replace, add or delete (and where).....that will be the most helpful.  

While not really educated enough in electronics to qualify for doing many of the repairs and mods that have been put forth, I am willing to jump in and follow instructions--if they are complete and detailed enough.  I have the heart.....just not all the practical knowledge.

Since I already have the radios, I (and I am sure, many others) would like to be pointed in the right direction to keep them going.  I (we?) am seeking "proven" mods that will (hopefully) prevent the prevalent destructive failures of these two great old radios.  Mods to the existing mods are very much desired--if we can use the information.....even if not fully understanding them.

Hopefully, those of you that do have the knowledge to critique those published mods and make improvements (including ZS5WC), will be able to provide detailed documentation or specific instructions using the schematics already out there.  Thanx again to those of you willing to put up with the frustrating task of helping out those of us ("appliance operators") who really need it.

Just spent $25 at Staples to print out the article in color by A5LX, so I have the basic blueprint and useable schematic to work from.
« Last Edit: November 13, 2017, 02:34:12 PM by N6QWP » Logged
WA1RNE
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« Reply #5 on: November 14, 2017, 09:43:30 AM »

Quote
However, I am requesting direct, step by step, part by part mods (as the previous one provided by A5LX does in great detail).


 Step by step is great, as long as the design modifications take the various potential scenarios into account.

 The gate to cathode resistor also allows the zener to draw at least the knee current when it turns on which it needs in order to be stable.

 Unfortunately,  this crowbar mod does not - IMO - provide the protection that's needed to guard against the scenarios I described which could easily put you right back to square 1.

 ...WA1RNE
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N6QWP
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« Reply #6 on: November 14, 2017, 02:00:15 PM »

WA1RNE--Could you possibly draw one up and post it for us all to use (with parts values and actual locations where it would be connected to in the schematic)?  If it only involves several parts, I hope that won't be too much trouble.

It would be a shame to keep that knowledge to one's self and take such a practical application to the land of SK's.....none of us are getting any younger.   So many of the Old Timers that used to keep the old gear afloat have since passed....and there is a real need for those that still can help, to do so.  

If you share something in these forums, it remains in the public domain forever.....and for all to use and benefit from your expertise.  Thanx
« Last Edit: November 14, 2017, 02:20:50 PM by N6QWP » Logged
WA1RNE
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« Reply #7 on: November 15, 2017, 09:11:20 AM »

Quote
WA1RNE--Could you possibly draw one up and post it for us all to use (with parts values and actual locations where it would be connected to in the schematic)?  If it only involves several parts, I hope that won't be too much trouble.

 
 I could and it surely doesn't involve a lot of parts.

Problem is, I don't own a working 930 or a 940 so I have no means of verifying actual circuit parameters, i.e. no load/full load input voltage and full load output current at low/nominal/high line and then testing the circuit mod prior to some poor soul installing it in his/her radio and finding out the hard way there's a slight tweak required to make the modification work correctly.

 Not sure if I saved the copy of the service manual but I'll take another look and see what I can come up with.  Of course someone else will have to test it and do so with the output of the AVR disconnected from the radio  - and will need to create an external resistive load to test the power supply afterwards.

 Of course that would mean making any changes - at their own risk -



 
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N6QWP
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« Reply #8 on: November 15, 2017, 03:30:26 PM »

OK....(Dentistry Huh).....how about just drawing what you think would work and where.  We can try it out (without the final unit being hooked up).....and check it out with a load.  If we get within the "ballpark", it seems like it should hold?  Can't do any harm (from the sounds of it)......other than kicking out too soon--or too late.

The idea is to add ANYTHING that will save the final unit (and hopefully) the AVR board from destruction.....if the vulnerable pass transistor(s) blow.

William--How about your idea of Crowbarring after the fuse?  Circuit?  Values?  Exactly where?  In addition to the articles mod, or instead of it?

At this point, ANY USEABLE ideas would be useful.  Would like to button up the rig and try them out.  I am assuming that the A5LX article's mod would help....to a point.  Any extra circuitry would seem to be a plus?
« Last Edit: November 15, 2017, 03:51:29 PM by N6QWP » Logged
N6QWP
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« Reply #9 on: November 17, 2017, 09:18:17 AM »

This thread started with a request for references to tried and proven PRACTICAL modifications to the 28 volt power supply for the TS-930/TS-940 that would protect the final unit and the AVR board from overvoltage should one or both of the susceptible pass transistors fail.

So far, ONLY ONE modification (by A5LX) has been put forth.  Since it was published (with pictures, schematics and detailed instructions long ago) it seems like a good mod to pursue.  I have heard of others, but can not locate them.

There have been several theoretical and conceptual critiques of that mod....BUT NO PRACTICAL DETAILED INSTRUCTIONS ON HOW OR WHERE TO IMPLIMENT THEM.
 
I am seeking mods that are applicable and useable by the "electronically challenged" who would like something that can be installed with actual parts and circuitry....and simple enough to follow without having to freewheel intellectually in order to make the actual modifications.

Was hoping that those that started to provide help, would return with the information that was requested, but as yet, nothing that one could pick up a soldering iron and proceed to implement--other than the modification by A5LX.

Is there anyone elese out there that can provide any further information-THAT WAS ORIGINALLY REQUESTED?--before I go with the aforementioned modifications Huh  Thanx to those that made input....but I need steps that can be followed to the letter.  From the number of hams who have looked at this thread, I would assume that a good number of them might also be seeking something simple enough to follow.

« Last Edit: November 17, 2017, 09:28:08 AM by N6QWP » Logged
KM1H
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« Reply #10 on: November 18, 2017, 02:44:09 PM »

These were near the top from a simple Google search of "kenwood ts-940" power supply. There are several pages worth looking thru.

https://www.radiomods.co.nz/kenwood/kenwoodts940.html

http://www.g8wrb.org/data/Kenwood/TS-940S/Kenwood_TS-940S_Fixes_Reviews.pdf

Carl
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N6QWP
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« Reply #11 on: November 18, 2017, 03:37:08 PM »

Thanx Carl--I am also less than computer savy......Couldn't find anything in the reference links about crowbar circuitry....but heat issues were good to know about on the AVR and heat sink (as mentioned on the "940 Page".   Couldn't find anything relevant on the other sight as linked.  But, the A5LX mod was in a similarly titled site....so that might be what you are referring to???

Fan replacement seems to be a good preventative measure to keep the pass transistors from blowing in the first place.  Will take that into consideration.  

Appreciate any direction in obtaining info on how to prevent catastrophic failure of either the final unit or the AVR board.
« Last Edit: November 18, 2017, 03:41:13 PM by N6QWP » Logged
ZS5WC
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« Reply #12 on: November 18, 2017, 11:17:04 PM »

 :)Hi Brad,
I have TWO TS-940's I have to apply the mods to, I am working out of country at the moment, but will en-devour to do the first mod before Xmas.
After that I could supply more detail once tried and tested.

Crowbar circuits are finicky, they could falsely trip, or trip too late so the setup is critical.
In my mind a high current shunt combined with trip could be better.
Will Think about that in more detail.

73 de William
ZS4L / ZS5WC



 
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N6QWP
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« Reply #13 on: November 19, 2017, 06:48:46 AM »

Thanx William--That sounds very promising.....and helpful.  I will wait until hearing back from you before proceeding.  Someone who actually has the 930 or 940, and who is savvy enough to design, implement, test and evaluate any mods (and pass along the results and instructions) is exactly what I am looking for.  

I am sure that, with over 1000 readers of this thread so far, many others will also be anxiously awaiting your results.  Thanx from all of us.
« Last Edit: November 19, 2017, 07:10:26 AM by N6QWP » Logged
KM1H
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« Reply #14 on: November 19, 2017, 04:26:35 PM »

With a serious upgrade to the deficiencies of the 940 PS I see no need of a crowbar and havent had any more problems since the late 80's and I have never had driver of final failures......and that includes a decade + of full blown DX and WPX contesting with a high percentage of winning scores.

Since it isnt mentioned I sorta guess others feel the same way...time to move on Grin

Carl
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