Please help for B+ problem!

[SKLau]

My Magus A-1 tube pre-amp had a loud pop and became noisy and the B+ went up from 300v to 420v. The B+ PS has 2 transistors, the MJE350 and the TIP50. I don’t know how they work but I have the schematic when the MJE350 died a few years ago there was no B+ voltage. I replaced it along with the TIP50 and the zener diodes and noted their vdc readings from the circuit on the schematic for future reference.

I post the schematic with the vdc readings which I noted down from last time and marked where the problem vdc in circle. The notations of the transistors are based on the trace of the PCB.

This time the R38 470R resistor was cracked changing its value to 406R and a bad or broken PCB connection from R60 2K to the TIP50 base and the TIP50 B to E was short. I replaced the 470R and the TIP50 and hard wired the connection from R60 2K to TIP50 B. But I wired after the R60 by mistake. It should be wired between the R38 and R60. When powered up, it cooked the MJE350 and shorted the TIP50 B-E and the VR4 82v zener. I rectified the wiring and replaced the transistors and all zeners however the B+ is still 418v.

The vdc readings on the MJE350 circuit seem correct. The high vdc appears after the MJE350 C from 310v through the R42 10K to 402v and through the R41 1K to 418v and go to the TIP50 C where there should be about 300v.

I am puzzled what have gone wrong. Did I wire the orientations incorrect to the TIP50? Or are the high vdc on R42 and R41 caused by bad components? I am grateful somebody can help me out. Your help in solving this problem would be much appreciated.

Arron

[DougL]

Looking at the Schematic the collector of Q1 looks more or less correct.

I see the issue, you have base and collector reversed on your diagram. Its causing a single diode drop to r41, which would put that node at 438 - .7.

Let me know if you have questions.

Doug

[audiowize]

Replace the MJE-350 with an MJE-5731A. Note that the 5731a fits in backwards compared to the 350. It is a beefier chip that's also less noisy.

[SKLau]

Thanks Doug for the indication.

Do you mean the collector of Q1 should go to base of Q2 and the collector of Q2 should connect between the R38 and R60?

Arron

[DougL]

On Q2, the Schematic drawing does not match your labeling.

You have the Base of Q2 labeled C
You have the collector labeled B

If the transistor in inserted per the labels, it would be expected to operate with the (bad) voltages documented.

HTH

Doug

[SKLau]

Thanks Doug for the explanation.

I reversed the connections of B and C and the voltages are back to normal. Doug, many thanks again for your help in solving this problem.

Thanks Audiowize for the info, I will try the MJE-5731A to substitute the MJE-350 for improvement.

Arron

[cerrem]

A few suggestions for improvement....
This power supply has been around and re-hashed for many years...
For Q2...get 2 of them......and twist them together to make a darlington configuration then re-install.... You will get a much better low frequency resonse from the pre, due to lowering the output impedance of the follower.... Since this is not a closed loop regulated power supply, the output impedance is not as low as it should be....
Also use 2 protection diodes across Q2, 1N4007 will be fine... You need one from E to C and another from C to B.... The anode of both diodes will be at the E..... This will protect the Q2 durring shut-down..... This topology is notorious for blowing up, when no protection diodes are there...
Keep in mind that the Q1 is acting as a CCS for your zeners, which is very good for voltage stability.... The only trouble you may encounter is at time 0 durring start-up, your B+ may be very high before you pull current and you may have to check the transient voltage across Q1 to see how close you are to poping it... Since PNP at this size and beta don't come in really high voltage...I think 300V may be the limit for that part...either way that was another issue with this supply...It relies on time constants for it's survival....durring start-up...

Best Regards
Chris

[SKLau]

Thanks Chris for your suggestion.

These make sense to me and I will implement your suggestion in the next upgrade.

Kindest regards,
Arron