E3630A repair driving me nuts. Tricky quiz for you - Page 1
Forum section: electronics / repair

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 07, 2013, 08:49:06 pm
content: Hello there my friends,I own a poor broken E3620A. The repair has taken a week, and I have desoldered a bunch of components in the regulation circuit. It'd be great to hear if you have any ideas about what I may be overlooking before I waste too much time. Perhaps the problem is not with the regulator at all.This is what I know:- If no load is connected across VOUT, the instrument works fine. Ripple of VPP= VINand frequency 100 - 300kHz appear in the circuit as a function of:loadandvoltage drop from VINrelative to VOUT. For small resistive loads, the drop from VINto VOUTmay be large before ripple start. But as the resistive value decreases to, say 100R, the ripple come on at about 1V drop VINto VOUT.- All the components in the schematic marked inyellowwere desoldered and measured/tested. Found no faults so far.- When I desoldered C9, ripple is reduced by about an order of magnitude VPP, but they frequency were tripled.- When VOUT= 10V, service manual says that U4 pin 1 should be 0V. In my case it is 700mV. The service manual says this indicate defective U1 or Q3. This may well be true, but I don't think this could affect my problem?- +-12V and +-5V rails look ok.Question: What more should I check?Thank you for your time.Kind regards,Marius

author: free_electron
author_rank: Super Contributor
timestamp: January 07, 2013, 11:26:59 pm
content: sounds like the chopper isn't tracking... check the thyristors and optocouplers to begin with. check that you have pulses on pin 1 and 7 of the LM339 driving the optos via the transistors.this thing uses a switching pre-regulator. feels like it is not working properly and the linear stage can;t compensate due to lack of headroom.

author: Shuggsy
author_rank: Regular Contributor
timestamp: January 08, 2013, 05:34:39 am
content: Quote from: free_electron on January 07, 2013, 11:26:59 pmI would agree, start with U1/Q3 and U2/Q4 (the optocouplers and thyristors, respectively, that free_electron suggested). The switching pre-regulator has 4 states it switches through based on whatever your set output voltage is. It uses the outputs of U4A/B and the driving transistors Q8 and Q7 to switch in and out different sets of secondary windings via the U1/Q3 and U2/Q4 pairs through to CR2 and CR3 (Output 2's bridge rectifiers). By switching out the transformer windings, different voltages are set at Vin (measured between TP3 and TP4 in the circuit itself, before the pass transistor) to control the voltage that the pass transistor (Q1) has to account for.You mention that a big ripple appears in the circuit when it gets loaded down. Where are you measuring this ripple? Also, does Vin change as you increase the voltage with no load? What voltage does the meter on the instrument indicate? Does this voltage agree with the reading of a multimeter across the output? The meter will present the circuit with a high impedance load, so I wouldn't think it would start to oscillate with just the meter... but I could be wrong.If Vin changes to 4 different states as the voltage is set from 0V to 20V, then I would think the optocouplers ans thyristors are working alright, but the real check would be to test them separately. It seems odd that the output would oscillate if Vin is stably going between all 4 states. Vin shouldn't change much during operation unless you change the set output voltage such that one of the windings needs to be switched in or out.Again, not sure where exactly you're measuring the oscillations, but the removal of C9 causing the oscillations to increase would make sense if the voltage feedback circuit was having issues (this could be caused, again as free_electron stated, but the linear section not being able to compensate if the pre-regulator wasn't functioning properly). C9 looks like it stabilizes the feedback voltage seen by the voltage error amp (U10B). Removing C9 removes that damping/stabilization/filtering (however you want to look at it) and allows the circuit to change more quickly. Likely not your problem overall problem, but this just messes around with how fast the circuit is trying to regulate the output. If the circuit is trying to regulate the output faster than the components allow it to, then it'll just continue to oscillate and never settle.Good luck, hopefully it's just a bad optocoupler or thyristor

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2013, 06:18:01 am
content: Change the optocouplers first, as they are the more likely items to fail by losing transfer ratio. Thyristors are pretty rugged devices, but the optos need to drive them properly or else they do not switch reliably.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 08, 2013, 07:19:50 am
content: Hello guys,I am sorry that I confused anyone about the ripple. The ripple appears in the feedback loop of the regulator driving q1. The current limiting opamp seems stable. The ripple has the highest amplitude at Vout and the node by R32. Past R41, the ripple is smaller. But the voltage regulation opamp has a few volt ripple (like its inputs).It gets worse when I solder in C9. When I remove the cap, it gets better by an order of magnitude.I was varying VINusing my other power supply, because the instrument doesn't seem to switch between the four states by itself.The front panel meter is fine. Shows the same as my voltmeters.Thanks for your help. I eill replace the optocouplers and see what happens.M