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 Post subject: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:03 am 
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Joined: Dec Sun 06, 2020 10:33 pm
Posts: 207
Location: Waterbury, CT. 06704
Here's my problem,

I'm working on a Ten-Tec Omni D and it's corresponding model 255 power supply and I'm having problems with the power supply cutting out. What I would like to do is to build some sort of a load that would draw 15-18 amps from the power supply to see if the protection circuit is my problem or if I have another issue. Attached are single page scans of the power supply schematic and the radio schematic. Idea's please, is there another avenue of approach that I should be looking at. Help and idea's would be appreciated.

Attachment:
7_PDFsam_255.pdf [61.34 KiB]
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Attachment:
31_PDFsam_Omni_A_-_D_Owner_Manual1534.pdf [44.03 KiB]
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Attachment:
32_PDFsam_Omni_A_-_D_Owner_Manual1534.pdf [42 KiB]
Downloaded 8 times


P.S. I can upload both full manuals in PDF format if that would be more helpful

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:12 am 
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Under what conditions does the power supply 'cut out', whatever that means? You can make a load from old automobile headlamps or whatever resistors you have of low enough resistance and high enough power rating. You are talking about 200 Watts or more.


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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:21 am 
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Yep i just went to the junkyard last week and picked up 5 of them and a nasty cut from one of the bezels.

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:25 am 
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Location: S. Dartmouth MA 02748-1225 USA
There is a TenTec user group: https://groups.io/g/TenTec

I have one of these from an estate. Currently in storage, I have no idea of its electrical condition...

Rather complex supply, I would look for spikes caused by poor connections, the IC or "bad" condensers. The spikes would trip the SCR. If incorrect substitutions for the condensers (type) they could be causing the noise. Substitution of any condenser with a metalized that does not have a dv/dt rating can also cause noise.

If you can run the supply with the rig on receive or lightly load the PA hang a probe on the 12 volts an set a sensitive trigger to catch the noise. Then trace back to where the pulse is the greatest offending component/connection is there. That is all I can offer, unless someone has a case history of this transceiver...

My sloppy calcs say the load is .67 ohm at 220 watts more or less. Some salvage resistors from an electric fork truck? Or raid my junque box of these useless beasts...Unless the load is somewhat variable, it will simply "cut-out" when the load is applied... chas

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Last edited by Chas on Apr Fri 09, 2021 4:30 am, edited 1 time in total.

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:27 am 
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Location: Waterbury, CT. 06704
bob91343 wrote:
Under what conditions does the power supply 'cut out', whatever that means? You can make a load from old automobile headlamps or whatever resistors you have of low enough resistance and high enough power rating. You are talking about 200 Watts or more.

Sometimes it shuts off (cuts-out) as soon as I turn it on and sometimes it shuts off within a few seconds. I'm leery to hook up the radio to another power supply because of the special protection circuit (magnetic trip). They say that you run a high risk of blowing the finals if you do not use this protection circuit.


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7_PDFsam_Omni_A_-_D_Owner_Manual1534.pdf [61.92 KiB]
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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:28 am 
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Chas wrote:
There is a TenTec user group: https://groups.io/g/TenTec


I found it and applied to join, just waiting to be accepted.

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:36 am 
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Joined: Dec Sun 06, 2020 10:33 pm
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Location: Waterbury, CT. 06704
When I was in the service the guys who repaired the big generators had this huge load bank that they could dial in the current draw. It's been 40 plus years but I remember these huge fans that cooled these big banks of what I'm guessing was resistors but some of these generators were for missile sites that used 400 hertz if I'm not mistaken. I want something that I can use to bring this thing right to the edge and hold it there so that I can see if the supply is stable. It appears as if the output transistors have been replaced and I'm not sure if these are problematic or not. .

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 4:53 am 
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The wattage of the load could be distributed among several resistors of all the same resistance and wattage. For example four 2 ohm resistors of 50 watts. or eight resistors of 4 ohms at 25 watts and so on. You will not need a fractional value by exact calculation but you will need to do a similar calc to define any switches used and the wire size of the connections. If those aluminum body resistors are used the metal back plate will need cooling. It is a matter finding the best bargain and engineer around that. The load will probably have future application such manually controlled battery discharge... chas

I "think" I have the original paper manual for the Omni-D but if you have already copied it to PDF an upload to the forum would be welcome if it will go or I will PM my email address. Don't copy for my sake.

WA1JFD

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 5:00 am 
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When you turn the radio on I assume it goes into normal receive mode, so the current draw should be low. If the supply shuts down when you do that, it's either a power supply problem or you have a short in the radio.


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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Fri 09, 2021 7:31 am 
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Greetings to scott.hettel and the Forum:

The big thing now in test equipment is "smart loads" or "electronic loads". These can be configured to draw any current you wish and to ramp the load up slowly or quickly. Basically, they are just bi-polar power transistors on a big heat sink with some base drive circuitry to set the current draw and compensate for changes in the transistor characteristics with temperature... i.e. to stabilize the load. Large heat sinks can be somewhat expensive but so can large power resistors. I would think that it would be possible to home-brew such a load for your purposes without too much trouble.

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Sat 10, 2021 6:36 pm 
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Joined: Dec Sun 06, 2020 10:33 pm
Posts: 207
Location: Waterbury, CT. 06704
Thank you for the help everyone. a lot of good idea's to think about.

Chaz: I can email you the PDF however it available from the link posted

http://www.tentec.com/wp-content/uploads/2016/05/Omni_A_-_D_Owner_Manual1534.pdf

If you cannot download it then shoot me a PM and I will email it. the PDF is about 11 meg.

Again, thanks to all who took the time to help. It is appreciated.

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Sat 10, 2021 6:59 pm 
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Location: Montreal, Quebec
I've wondered about using parts from a scrap computer power supply for an active load. Heatsink and transistors, not much else is needed. I've not looked into it.


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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Sat 10, 2021 8:57 pm 
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Location: Corinth, TX
scott.hettel wrote:
When I was in the service the guys who repaired the big generators had this huge load bank that they could dial in the current draw. It's been 40 plus years but I remember these huge fans that cooled these big banks of what I'm guessing was resistors but some of these generators were for missile sites that used 400 hertz if I'm not mistaken. I want something that I can use to bring this thing right to the edge and hold it there so that I can see if the supply is stable. It appears as if the output transistors have been replaced and I'm not sure if these are problematic or not. .

Yep. Our generator shop had several load banks. The smallest one (20 kw or so) easily fit in the back of a pickup truck. The largest one (200 kw or so) needed a prime mover to tow the trailer it sat in. And yes, missile batteries used 400 Hz generators for main power. And that is why military equipment is specced for operation from 50 Hz (overseas power) to 1000 Hz (aircraft power).

OK, on to the problem:
You say the PS is "cutting out". When it does, I am guessing the 20 amp F1 does NOT blow. The output just drops to zero. Yes? (You need to COMPLETELY define the problem. :) )

The two SCRs, Q2 and Q3 serve as electronic crowbars. Q3 serves as an overvoltage protection circuit. When the output rises above the threshhold of the 15 volt Zener D1, Q3 fires and shorts the PS output to ground, blowing the fuse. Q2 serves as an overcurrent protection, removing the drive to the series pass transistors (I think). U1 was a standard voltage regulator IC 45 years ago. Lot of pros and a few cons.

723 datasheet: https://components101.com/ics/lm723-vol ... gulator-ic
Practical info: https://www.circuitstoday.com/ic-723-voltage-regulators
More practical (much more) info: https://electronicprojectsforfun.wordpr ... regulator/

R12 set the voltage at the output of the PS. R10 sets the overcurrent level (I think). Adjustments should be in the manual.

mblack wrote:
I've wondered about using parts from a scrap computer power supply for an active load. Heatsink and transistors, not much else is needed. I've not looked into it.

Probably not feasible. Computer PSs are designed for efficiency. A modern 1 kw PS (OK, that's a pretty big one) operates at around 95% efficiency. That means it "wastes" about 50 watts as heat. A not so modern 1 kw PS operates at around 90% efficiency.

John


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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Sat 10, 2021 11:05 pm 
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Greetings to John and the Forum:

Based on mblack's description, I am assuming that this is not a switch mode power supply but a conventional series pass supply for a main frame of many years ago. In which case, the efficiency of the original supply is irrelevant.

There are power transistors on a heat sink. The transistors have a maximum voltage rating (Vce) and a maximum power dissipation rating, which is affected by the size and cooling of the heat sink. As long as these values are greater than the output that is to be expected from the power supply under test, they should make an adequate electronic load.

Regards,

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Sat 10, 2021 11:11 pm 
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Resistors are expensive, so using scavenged transistors makes it cheaper. And you might find a way to make it adjustable. Otherwise, there is no advantage in using solid state loads.


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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Sun 11, 2021 1:39 am 
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True, Jim. I just assumed he was talking about a semi-modern switching supply. But, yes, there are a LOT of heavy iron (and aluminum), old fashioned analog power supplies out there.

John


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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Sun 11, 2021 3:16 am 
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Place an Astron RS-20 inside the Ten Tec power supply. About $50 used. Find one with the three conductor plug in computer type cord. These are less than 15 years old.

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Mon 12, 2021 5:54 am 
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Ten-Tek used the power supply to protect the finals.
I guess they could not use the popular "SWR Foldback" circuit due to patent troubles.
Running the station from a big car battery cost me the finals and drivers because I used an inline fuse that did not blow fast enough when the antenna broke.
It sounds like you have lost the finals. Disconnect the power to the final circuit and see if the rest of the radio works.
If so - it is first fix the SWR and then the final assembly.
Good hunting
Pat

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 Post subject: Re: Looking for idea's on building a load
PostPosted: Apr Mon 12, 2021 6:26 am 
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Try it with some other supply or a very small 12 volt battery and keep the drive very low and transmit into a dummy load to see if the finals are working. Even better, buy a twenty amp aircraft type fast acting magnetic circuit breaker and use any power supply or battery you want. TenTec used to sell such a breaker with their name on it for using battery power. I ran an Omni D for years from a deep cycle battery with that type of breaker. They are not expensive and are still manufactured. I thought I posted a similar reply early in this thread but maybe I didn't hit submit or else the monitor didn't like it for some reason. Check old TenTec adds and you will see the breaker as an option.

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