Please help with ROHDE SCHWARZ SMS power supply

[george79]

Hello,
i made an attemp to recap the power supply of my SMS generator because of the age of the capacitors. I have replaced all the electrolytics and the tantalum ones with the correct voltage and capacity. After the replace i thought that it is wise to test the power supply before connect it to the instrument. And found something strange.
All the voltages are correct and adjustabled by the trimmer that they have for every voltage except the first +5v. In this voltage i have the exact output +9.6v like the input (a little higher that the 8.2v) that tell on the MP1 test point. But in the MP2 test point i have the same +9.6v. And i cannot adjust it with the R2 trimmer. i have tried to swap the B1 and B2 fro the other voltages, and i have the same results. I have tested the 2N3055 and its good. I think that the trimmer is faulty or T4.
Please can you tell me some quick test that i can do before i will start to desolder and test each component?
I think to start meter at pin 5 of the B1, from the pin 6 i have and i tested 7.15v reference, so with the trimmer adjust i think that i will have adjustable voltage at 5.

Please any help acceptable
Thank you
George

[tom66]

Have you tried applying a load to the output?  It is quite possible it will only regulate to 5V when loaded with something like a few hundred mA.

You can see if the regulator controller is driving the transistor correctly.  In regulation, I would expect the base of the transistor to sit somewhere around 600mV above the output voltage, and T4's base should be another 600mV above that. 

However, bets are off if there is no load, because leakage current can bias the output up from the base.  It looks like the circuit only has ca. 16k ohm load from the feedback dividers, so more than ~300uA of base current could drive the output high.  That seems quite plausible for an older linear regulator circuit.

[george79]

Thanks for your help Tom66,
yes under load the regulation is OK. I have tried first with a resistor, because its a bad thing to burn a lot of electronics in this generator. So everything fine, and i can continue replacing the tantalum capacitors now.
Thanks again

[xavier60]

That usually happens when the design has a bypass resistor across the pass transistor.
What's the voltage on pin 10 of IC B1?