Tektronix 465B 5V power rail help needed - Page 1
Forum section: electronics / repair

author: john_boca
author_rank: Newbie
timestamp: January 20, 2025, 03:37:48 pm
content: Seems my process is try to fix one thing, break another. I was probing a transistor for another issue and shorted two leads. Big pop and my 5V rail blew. I am still new to this and making mistakes. Fortunately, this is not my day job.After replacing the fuse, the power was still off. Can't remember how "off"; that was a few fixes ago. I followed the power supply troubleshooting guide in the SM and disconnected everything it said to disconnect except the calibrator circuit. Figured that was not the source of my problem and didn't want to desolder more parts than necessary as each one is a future possible problem.All other power rails are good, in case that matters. It is only 5V rail that is off.I have replaced Q4312. Twice. (I forgot the mylar insulator between the transistor "tab" and the mounting post, shorting collector to ground and blowing a fuse, so I suspected the transistor also might be bad and replaced again.) I have also temporarily swapped out Q4311 and U4206 (see parts below in case it matters). Since the new parts did not change result, I reverted to the originals.With Q4312 removed, I verified resistance of R4422 and R4325.Also, with Q4312 removed, I also removed Q4311, turned unit on and measured voltage at 5V rail. It was 0.7V. Just wanted to make sure some other circuit was not affecting the power rail.Right now the 5V rail is 8V, not 5V.My DC voltage readings are as follows:Part No Expected Actual CommentQ4312 collector 9.3V 10.2V This is input from filter cap. The higher value is probably due to the 110V vs. 120V line in. All the other power rail inputs from filter caps are high, too.U4206 pin 3 5.0V 5.01V V+ inputU4206 pin 4 -8.0V -8.01V VeeU4206 pin 8 15.0V 15.0V VccU4206 pin 1 6.7V 1.8V Channel 1 out (bad)U4206 pin 2 5.0V 8.05V V- input and also the 5V rail output (bad)With Q4312 collector at 10.2V and base at 1.8V, it should turn on. However, with emitter at 8V (after R4325), it seems that it can't be turned on. This would seem like an open between collector and emitter and voltage drop is just due to the two resistors.Having replaced this transistor twice , though, it doesn't seem like the transistor is the issue. But if it isn't this transistor, what else could cause the voltage to be too high?Finally, in case it matters, my replacement/swap parts are:Part no Original ReplacementQ4312 SPS3414/MPS-U45 CEN-U45Q4311 S038487 2N2222U4206 MC1458N NE5532Any suggestions for what check, likely suspects, etc.?

author: D2236
author_rank: Regular Contributor
timestamp: January 20, 2025, 11:07:49 pm
content: It appears that there should be a minimum current draw (~ 185 mA) on the +5 supply for it to regulate. It is obvious if you remove Q4312 out of the circuit, some minimum current should flow through the 25 ohm resistor to drop the 9.3 V down to 5 V. Resistor R4422 was most probably added to reduce the power dissipation in Q4312. Older models 465s doesn't have this resistor fitted.Generally when you use this method, you will choose the resistor value to supply less than the needed current draw, so it can be brought into regulation with the pass transistor Q4312 turning on in parallel.

author: john_boca
author_rank: Newbie
timestamp: January 21, 2025, 01:10:51 pm
content: Thank you for the reply. That was about 99th on the list of possible reasons for the issue. Hadn't even considered it.Let's see if I'm understanding you correctly... Insufficient current draw = no voltage regulation. Do I have that right? While there is some current draw through R4422, it is less than what is sufficient to bring it into regulation. If so, I think next step is to reconnect the other circuits to get some current draw. Does that seem right?

author: D2236
author_rank: Regular Contributor
timestamp: January 21, 2025, 04:27:58 pm
content: You can also temporary lift one side of R4422, and it should regulate to 5V if the circuit is working.Included is a simulation to show the issue with not enough load current. The one circuit is regulating to 5 V, while the other one with the higher load resistor is out of regulation.

author: john_boca
author_rank: Newbie
timestamp: January 22, 2025, 02:48:05 am
content: Wow! Thank you so much for taking the time to simulate the circuit.I am going to lift one side of R4422 as you suggested soon. (Day job and life taking up my time today.)Still trying to absorb what is going on in the two versions of the circuit, but it seems to make sense. The higher load resistance pulls less current.And what you are saying is the voltage regulator will only regulate properly if there is some minimum amount of load current.Thanks for explanation.

author: D2236
author_rank: Regular Contributor
timestamp: January 22, 2025, 03:30:36 am
content: Think of it this way. Having Q4312 in the circuitcan only reduce the minimum currentthrough R4422 (if some current gets bypassed through Q4312) resulting in less voltage dropped over the resistor. This will cause a higher output voltage to be on the 5 V line. The only othercondition that will increase the voltage dropover R4422 is to have more load current draw.

author: john_boca
author_rank: Newbie
timestamp: January 25, 2025, 06:13:06 pm
content: Yeah, that makes sense. Any current through Q4312 is less current through R4422, so having the Q in circuit can only reduce the minimum current through the R.I did as you suggested and lifted one end of R4422. Result:5V rail = 5.01V! Works for me.Then I reattached all the other lifted jumpers and resistors specified in the 5V rail troubleshooting section of SM. Result: Still 5.01V!I think that this was all one big fire drill. Had I just replaced the fuse and checked the circuit, it would probably have been fine. My gut says that when a fuse blows, it blows for a reason. And that reason likely took some other components with it. And rather than replace fuse, start up, blow fuse again, I thought I was doing the smart thing by checking the rail in isolation.On the up side... I learned something new, thanks to you. Also got some troubleshooting experience and soldering/desoldering practice.Now back to the troubleshooting I was doing before blowing the fuse. Will try to work through that for a while to see what I can figure on my own before reaching out for more help.Thank you so much for your replies and explanations.