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Another 139XLR power supply problem.

guitar_199 said:
About D302.......
I am royally confused.......
How could he be seeing 19.2 v on the cathode of D302.....
When both collectors on TR302/TR401 are 0.25v?

Where is that 19 volts coming from???
The way this circuit looks.... the nominal 24.8 that SHOULD be on the two collectors drops down through R305 (220R) ..and on to the zener.
But if those collectors are at 0.25V..... that zener wouldn't have anything on it at all.

Something hiding in there......
Click to expand...
guitar_199 said:
If it CAN be done.... I still see value in the test Exit 13 suggested and here is why.....
Yes he has connected a DC supply to the DC input jack and the radio works....
but .... the 13.8V DC feed from the AC power supply hooks up to the opposite side of a switch. When you plug the AC cord in ... it toggles a switch that selects whether DC ... OR regulated DC from the power supply....is connected to the rest of the radio.... if something is gooched in the side of the switch that handles the AC power supply... it could affect this.
Granted.... a few checks with an ohm meter could rule this out too.... but if something is loading down that 13.8 Regulated supply it would sure show up in this test. ( I would also imagine though that TR302/TR401 would be radiating some heat if it were the case.... but still )
Click to expand...
Yea, why not, it is easy to do, just pull fuse F402 and check the orange wire side for output voltage....

But that does not explain the abnormally high voltage on the zener cathode (as you just noticed, editing my reply as we go)... Something is popping it which is why I want the datasheet for the replacement used so I can look at the current handling ability and maximum voltage ratings.
 
I am thinking I would like to see what this does with TR301 pulled.
This is closed loop feedback and we aren't sure what holds this whole thing down.
TR302/TR401 and all of THEIR biasing parts (R305, D302, R304, C302) make up the pass element and even with the feedback connected....the zener voltage (16.95v) up through R304 and the two bases... should give you something near 13.8 ish. That would break the feedback loop and test ONLY the regulating elements. If that works..... you know that the problem comes in when you start sensing feedback from the loading effects.
Still..... the fact that there is only 0.25 v on the two collectors....but 19.32 showing up on the zener makes me itch......

I'll shut up now!!!!!!
 
First D302 checked good and on a diode tester it gave the same readings. As for data sheet I just bought 10 of these on e bay listed as 16 volt 1 watt diodes. Same results

I then unhooked the main power wire off the power supply board. Now I have 10V instead of 9 on the base and emitter of TR41.
 
There is no chance that C302 (220uF on the base of TR302) is either shorting or backwards is there? That could be holding those bases down..... possibly.....
And I still think that pulling TR301 to break the feedback loop could tell you something.... or at least help isolate what is going on........
And (tip o'the hat to Brandon7861) if I did this... I'd pull fuse F402 so that if the voltage spikes it will not be fed through to the radio! The pass transistors will have a minimal load from the resistors down through D301 & the resistors that are there to bias TR301.
THat way... you could insure that the basic pass element part of this thing is working...... (or fix it)... and then hook the negatvie feedback in (TR301 reinstalled!) and go from there.
 
guitar_199 said:
There is no chance that C302 (220uF on the base of TR302) is either shorting or backwards is there? That could be holding those bases down..... possibly.....
And I still think that pulling TR301 to break the feedback loop could tell you something.... or at least help isolate what is going on........
And (tip o'the hat to Brandon7861) if I did this... I'd pull fuse F402 so that if the voltage spikes it will not be fed through to the radio! The pass transistors will have a minimal load from the resistors down through D301 & the resistors that are there to bias TR301.
THat way... you could insure that the basic pass element part of this thing is working...... (or fix it)... and then hook the negatvie feedback in (TR301 reinstalled!) and go from there.
Click to expand...
 
Low_Boy said:
First D302 checked good and on a diode tester it gave the same readings. As for data sheet I just bought 10 of these on e bay listed as 16 volt 1 watt diodes. Same results
Click to expand...
This could be the problem because it needs every bit of that 1 watt to work, and if ebay sold you knockoffs, that would do it. And that happens a lot!!!!!!

For example, the supply is at 25v, the zener is at 16v, that leaves 9v across the 220Ω resistor (which tells us the current). 9v/220Ω=41mA of current. 41mA*16.1v=659mW through the zener. Jumping on ebay and seeing a 10 pack of 1N4745's ("16v 1w" zeners), the datasheet shows a maximum current of 57mA (or 16v*57mA=.912w) which would be just barely ok, but they quickly de-rate with temperature... All it takes is for a shady ebay seller to sell you a slightly inferior/rebranded product that handles 16mA less and its gonna blow right away (or slowly warm to failure).

And if that C302 replacement was of the amazon variety, good chance it is riding its max ESR right out of the box...
 
Ohhhhhhhhhhhh I said I'd shut up.......but I can't!!!!!!!!!!!
How I see that regulator working.........
--------------------------------------------
First.... IGNORE TR301. Play like it isn't there.
The supposed 24.8V (that should be on both collectors TR302/TR401) drops through R305 (220R) to D302 which starts conducting...but should plateau at about 16.95 v. The rest (24.8 - 16.95) should be across R305.
--That 16.95 v drives current through R304 (470R) to the base of TR302 which is going to make it conduct. That will cause current to flow to the base of TR401 which will cause it to bias on and make current flow through TR401 to the load. The voltage....will be indirectly controlled by the zener (D302). So... 16.95 .... minus the current * resistance of R304 minus TR302's BE drop (0.7) minus TR401's BE drop (another 0.7) is going to give you the nominal 13.8V. (actually it will be a mite higher than that.... the pot adjustment RT301 will set it to 13.8V!)
--
If you pull TR301 and fuse F402 (for safety!) you should see that 13.8 ish on the output rail from TR401's emitter. (once RT301 is properly adjusted!)
---------
- Now the feedback....
------
TR301's emitter is referenced to D301 (6.1vzener). So the idea is... TR301 is going to be biased off....until the base voltage starts climbing above 6.1v + 0.7 (Vbe drop) and will then start to conduct. What happens when it conducts? It starts pulling the base of TR302 down towards the 6.1 v zener.....meaning.... it "throttles down" the pass transistors....making them drive a little less.
----
So my thinking is that... with NO LOAD at ALL... (fuse pulled) .... you would see TR301 conducting because...since the out voltage is 13.8.... the base of TR301 will be above the zener....and start to turn on....which will pull the base of TR302 down.....throttling the pass elements down.... in an attempt to reduce the voltage. This OF COURSE is adjustable by RT301.
--
at this point.... the NO LOAD conditions will find a quiescent point where it will stabilize..... and you will have 13. something volts "on tap".
-------
NOW..... we put the fuse on (NOT REALLY.... not yet.... just for my example!)...
but we start pulling some serious current out of that supply....especially if we "key up". What is going to happen?????
---
First the current draw will pull that 13.8ish volts down!!!!! Because the pass transistor is currently throttled by TR301. When that voltage drops..... all the voltages on the "voltage divider" made up of R302, RT301 & R301 will also start to drop. That will drop the voltage on the base of TR301. THAT will make TR301 REDUCE it's current flow.
THAT will release the base of TR302 (it will release the clamping effect that pulls it down) and the zener D302 (our 16.95 volt friend) will make the base rise.... which will turn the pass transistors ON a little harder....forcing MORE current to flow to the output.... which will make the voltage RISE back up to that "quiescent point".... or at least try to.
----- when the load is released suddenly.... the output of the supply will try to rise....because of the sudden drop in current draw... so the voltages on the "voltage divider" will all rise..... which will make TR301 conduct..... which will pull down a little on the base of TR302.....which will cause the pass transistors to reduce current output....and the voltage will again....stabilize at the "quiescent point".--
--
--
--
I am sure hoping that this info will be helpful... but I really do think that .... pulling TR301 out (and the fuse!!!!) and fighting the pass elements first.... THEN throw TR301 back in to get the load adjustment working... will get you somewhere.
Okay... NOW I'll shut up!!!!!! ;)
 
Last edited:
Well I usually buy all caps from Mouser. I do not know where this came from. A older friend got out of radio and moved out of town. He gave me a lot of his supply so possibly one of his but it tested stronger than the Rubicon I replaced it with. Is there something I may have here I can test in place of D302?
 
Three of your 5.6v zeners in series would be close, again probably requiring later adjustment of RT301...
edit: and you could compensate for that extra .7v by adding a forward biased diode in series with R304 :) depends on how messy you want things to get lol
 
After dinner I will hook up 3 and just turn the radio on. If the channel display lights up I will know that is the problem. 8 or 9 volts is about enough to just dimly lite the meters. I am hoping that is it. Like I said many times before It worked before I changed the parts out. I can't even try the old diode because I am not sure which one it is. I saved the parts and the 2 diodes look the same. Actually I have pictures. One was a bit darker than the other and one was corroded Possibly I can find it.
 
You can make a homebrew zener tester (because most DMM's don't output high enough voltage to test those). You need three things, a voltmeter, a voltage source higher than the zener voltage, and a 1k resistor. The voltmeter tests the voltage across the diode while the resistor acts to limit the current from the supply so it don't cook the diode. If the diode is good, you will get the normal forward drop of .7v across the diode with one polarity and the zener voltage across the diode with the other polarity.

TR302 might need changing too, because with 19v-9v=10v across the BE junction (max is 5v) it is probably damaged.
 

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