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Old 25th Sep 2020, 9:34 pm   #1
Deltatango
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Default Kenwood PS-33/53 adjustment Help

Rebuilding an old Kenwood 13.8v Power supply which has undergone a bit of a "heat frazzle" fan was stiff, plenty of fluff dry joints shorted output transistors to name but a few things.

It seems to be quite well built except for the connection to its output transistors which consists of a little pcb with a track to link the two collectors via a nut and bolt, well that was subjected to so much heat it had discoloured the track loosened those bolts etc.

Great seeing this supply could supply 22Amps! so it was make a new pcb or as I did fit a brass plate to fit onto.

Well after all this rebuild I find an adjustment shown as PRO which affects its driver transistor to the output pair, which I take to be either over volts or max peak current output setting can anyone tell me how to adjust this please

Thank you from David
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Old 27th Sep 2020, 1:32 pm   #2
HamishBoxer
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Default Re: Kenwood PS-33/53 adjustment Help

I have found the PS-33 circuit but no mention of the adj, it appears it alters a transistor's bias. Possibly start in the middle or where the existing one is set and see if it alters volts out?
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Old 27th Sep 2020, 2:15 pm   #3
orbanp1
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Default Re: Kenwood PS-33/53 adjustment Help

Hi David,

Plugging it in the circuit into a simulator, VR2 implements a fold-back current limiting adjustment.
Here are the values (I_short at 0.1 Ohm load, I_short is at very beginning of the curves in the picture):

I_short R
0.3A 500k
0.45A 400k
0.69A 300k
1.15A 200k
2.34A 100k
7A 22k

The rest of the load curves look pretty much the same.
Enclosing the simulation pictures.

Regards, Peter
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Old 27th Sep 2020, 7:02 pm   #4
Deltatango
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Default Re: Kenwood PS-33/53 adjustment Help

Many Thanks thats great to know, was thinking it was some limiting or protection system, had even remembered that trick of a transistor like a crowbar across a DC power rail with its base linked via an AC coupling system that would fire in a limited way to snuff out pulses and/or even reduce ripple to avoid using large decoupling capacitors (think that's right).

So I presume on this power supply it will have to be set for a current just above 22Amps!! say about 0.6R at 318W (23A) well its getting colder here anyway.

Many thanks again like to be sure what adjustments do, this poor unit was well fried from a stiff fan, dry joints and many other things perhaps its owner did not take note of the duty cycle of 50% at 16A or 25% at 22A?

Regards from David
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Old 27th Sep 2020, 8:56 pm   #5
orbanp1
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Default Re: Kenwood PS-33/53 adjustment Help

Hi David,

As mentioned the load curves are the same when the load resistor is greater than 0.112 Ohm. With load resistance higher than that the curves are pretty much the same.
The max current the PS would supply is about 44A, VR2 does not change that.
(At 44A the output is above 13V, as can be seen in the curves in the previous posting.
I think you do not set the load resistance, it is what it is, coming from your radio, unless you use this PS for heating purposes ;-)
Changing VR2 only influences the currents at the very beginning of the curves, when the load resistor is less than 0.112 Ohm (I call that a short).

I know that the curves are not presented like "usual" fold-back current limiting curve, but the simulator displays like that.
(I am sure one could work on to produce similar looking output...)

Here are two groups of curves, just showing the current, while stepping through VR2 from 100k to 500k in 100k increments. (The horizontal axis is the load resistance.)
One shows the full curve group (to 1 Ohm load), the other just the beginning of the curves, like zooming in.

Based on this I would set VR2 to 500k, I do not see any advantage of having higher current when in total short.

Regards, Peter
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Old 30th Sep 2020, 9:51 pm   #6
Deltatango
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Default Re: Kenwood PS-33/53 adjustment Help

Thank you for your continued support, I have been experimenting with loading the unit with a 0.6R load @ 13.8v which should be 23A finding a setting point with VR2 where on switching the power on it would not collapse.
Its voltage output though lower to just above 13v, will now tolerate load removal and replacement without a problem.

At this moment I am correcting or limiting its voltage variation between hot and cold which seems to be related with some resistors of 5% or its preset, so changing to 1% resistors along with a ceramic 10% preset which are affected more than I expect from the fan.

I am aware that there is a thermister which is contact with Q5 body but that seems stable compared to its supporting resistors, note that all four transistors of that section have been changed along with zener D4.

Thanks from David
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Old 1st Oct 2020, 1:39 pm   #7
orbanp1
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Default Re: Kenwood PS-33/53 adjustment Help

Hi David,

Glad the PS is working for you!

Looking at the circuit, the simulation results, and your experience, the voltage control circuit in that PS seems to be rather crude.
You might want to think about replacing the control part of that regulator circuit.

You have all the expensive parts of decent quality, the transformer, rectifier, filter, pass and driver transistors, so it might be worthwhile to fix the rest.
(I would keep the above, and the fan control circuit.)
You also might want to add over voltage protection.

You can look for inspiration on the Astron PS web page:
http://www.repeater-builder.com/astr...ron-index.html

I also liked WA0AUQ's rebuild, though it has just "normal", and not a fold-back current limit.
(You do not need four pass transistors, only for 50A rating.)
I am sure it was somewhere on the above web-page, but I could not find it there, but here is the article:
https://ta5fa.blogspot.com/2019/09/w...stron-psu.html

Regards, Peter
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Old 13th Oct 2020, 12:10 am   #8
Deltatango
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Default Re: Kenwood PS-33/53 adjustment Help

Got it stable as it will go, for the current max side gave it a 23Amp load (22Amax stated) and it will hold that load either by connecting it before turning on or after by dabbing its connector when on. Output voltage will dip to above 13.3v but that seems good.

The main problem is it will "vary or breathe" with heat and load so getting an exact voltage point is not possible it seems, with 1% resistors I changed this is now down to +/- 13mV which has a lot to do with the fan blowing a transistor "cuddling" its thermistor in an enforced thermal contact.
A load of about 3 ohms will start at 13.8v rises with heat to 13.83v when its fan fires after a while for cooling the power transistors, this also affects that pair by directly sinking output to about 13.65, fan then stops and voltage rises to repeat.

It will do this with variations of voltages for different loads but I found a 6ohm load enables 13.8v to be set with a good chance it will not reach during one of its cycles, 13.9v or above, which I thought was a maximum voltage to allow it.

Only other mod was to add a 2mm single bare copper wire to the track pcb at its rear section made "crispy" by heat exhaust from the heat sink below, which also acts as a link between primary capacitors negative and output negative.

Been fun, thanks for the help.......

David
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