Another 139XLR power supply problem.

[Low_Boy]

I posted previously about this. The power supply is finished. DC now works plugging in to the back of the radio with my external power supply. No more smoke or melted wires.
Now the problem plugging in to AC the internal supply does not work. Odd that the wires coming out of the transformer to the bridge rectifier both show 12.8V. The wire going from the bridge rectifier + going to the board show 25V. The radio does not power up using the internal supply. Where did I screw up.

 

[Low_Boy]

It appears that the voltage regulator TR401 is bad. The correct 25V going in. Only 8V coming out. Plugging in a external power supply bypasses TR401. But if I am wrong you can comment .

 

[brandon7861]

What is the voltage drop across D301?

 

[Low_Boy]

6V on cathode .005 on anode.

 

[brandon7861]

6V on cathode .005 on anode.

That's where it should be. I had a feeling that had went bad, guess not. What about the voltage on the base of TR302? It should be around 15.2v (for 13.8v output).

 

[Low_Boy]

Only getting what the voltage reg TrR401 is putting out. Everything but TR401 has been replaced.

 

[brandon7861]

Well, that narrows it down lol.

Just to be sure it's TR401 and not something before it that was replaced, I would disconnect TR401's base wire and see if the rest of the regulator is supplying the correct base voltage to the TR401 (about 14.5v). If TR401 were good, it would be possible to measure the voltage on that wire without disconnecting it, but not knowing how it failed, its better to take the wire off and check it. If you get something around 14v going to the 401's base, i'd assume you are correct and order a replacement. If that base wire gives you 8 or 9v after disconnecting it, the problem is somewhere before TR401.

 

[Low_Boy]

I had replaced TR402 with a TIP 41C I believe it is a upgrade from the2SC1419. I like your Idea.

 

[brandon7861]

I would also take note whether that 8v output goes away after disconnecting the base wire. If it did burn out and become a resistor, the output should stay where it is without the base lead connected. However, if that 8v disappears with the base wire disconnected, it is possible that the TIP41C is not supplying enough current to drive TR401. The datasheet for the original part shows a DC current gain of 35 to 320. The datasheet for the TIP41C shows ranges of Hfe a bit lower, 15 to 75. If this problem is a result of a lower current gain part driving the 401, it can still show the proper voltage on the disconnected base wire. Just something to watch out for.

What you could do (if the voltage drops from 8v to 0v with the base lead disconnected) is supply the base pin with an independent voltage from a bench power supply. If a bench supply at 14.5v connected to the TR401's base brings the output voltage up to 13.8, you can suspect the TIP41C of not supplying enough current.

Edit: yet another possibility is that the TR401 has an open collector. In this case, the output of the TIP41C is passing through the BE diode of the 401 and, given that the rest of the radio is acting a load, that load is merely pulling down the voltage from the TIP41c because it cannot handle the current the radio wants. If disconnecting the emitter lead of the 401 brings the voltage coming out of the 401 emitter pin (this time base wire connected) back up to 13v, you can assume the 401 has an open collector and is merely passing on the current from the TIP41

 

[brandon7861]

I would check what I wrote in my edit first because the suggestion in the second paragraph (using an external PS to drive the base) would not tell you if the 401's collector is open (assuming the bench supply is capable of running the radio). If you eliminate the open-collector as a possibility, then go back and try driving the base pin with an external source to see if the issue is with the tip41C's current gain..

 

[Low_Boy]

I do not have the schematic in front of me right now but I believe Power goes to TR401 than to TR301. I may be wrong.

 

[brandon7861]

TR302 and TR401 form a darlington pair. What that means when it is working properly:

TR401 will do everything it can to have an emitter voltage of what is at it's base (minus the 0.7v junction drop). However, if the current being drawn through TR401's collector is more than the current available at the base multiplied by the current gain of the device, it cannot maintain that voltage and both will begin to drop.

TR302 takes the reference voltage at the collector of TR301 (which has very little current capability) and amplifies it to the same voltage (again, -0.7v because of the junction drop) but with much more current handling capability so that the demand of the 401's base doesn't pull it down when it works hard. TR302 is just a voltage follower amplifying the current available at the reference voltage point (collector of TR301).

Now, when things are not working properly:
If the 401's collector is an open circuit (it can fail in different ways), the rest of the 401 (base to emitter) will look like a plain old diode and pass along the voltage and current from the 302. In other words, the power coming out of the 302 will try to run the radio instead of running the high power transistor. And because there is not enough current coming out of the 302 to run the whole radio, the voltage begins to drop as explained above. The current going into the base multiplied by the current gain is how much current can be drawn through the collector before the voltage begins to drop. So, if the 401's collector is open, the 302 will try to run the radio and will not succeed. This will manifest itself as a voltage drop on the output when the output is under (even a small) load.

If the 401's collector is not an open and the transistor is functioning correctly, it's output voltage will depend on whether TR302 is supplying enough current to drive it. This is why I worried about the current gain of the 302, which, for the TIP41C, is lower than the original part. This might not be the issue, but it can be.

This is why I was interested in whether TR401 is acting as a passive resistor or if it has an open collector, because if not (if it is behaving normally), the focus shifts to the 302's current gain as the culprit.

 

[brandon7861]

Long story short, you can just take the 401 out and check for the C-B and E-B diode drops to get a good idea whether it is good or not. If its not showing .7v for both the BC and BE junctions, replace it. If it checks out, suspect the TIP41C's current gain. Sorry for taking the long way around, I was just hoping to figure this out without having you remove the 401, but now I realize that would have been faster.

 

[Low_Boy]

That is no problem I am learning a lot from this. Tomorrow morning I will unhook the wire and tell you what I find.
Thank You

 

[Low_Boy]

So after unhooking the wire that goes from TR401 to the base of TR302 I get no power to the radio at all. TR401 now has 26 volts going to it and I the output on the wire going to the base of TR302 is floating around 0 to .025 volts the other leg of TR401 is 0 volts.