Damaged Siglent SPD3303X-E PSU ?

[MathWizard]

Hi the main 2 CH's are working fine, but today I went to use the 3rd CH, and I'm only reading 0.5VmilliVolt (not 0.5V) w/ probes on the O/P, for either 2.5/3.3/5V setting. It's on/off light works, and it's overload light is green for OK.

I barely use that CH as it's not current limited (or not settable by the user), so I can't say for sure when it was working last. I tried using that CH in a situation 2 weeks ago were it might have been damaged, or already broke, I figured my circuit wasn't working.

At worst I would have just in effect ran 5V into a 1N4001 for a few moments on my circuit. I certainly never smelled any smoke, or heard anything besides the relay's (I was using all 3 CH's)


No way is this thing under warranty, and ages ago, I hacked it to the 3303X version. So I have no problem opening it up.

[tautech]

Not heard of any Ch3 issues with these.
Things to try:
Exercise the voltage selection switch.
Check the Lock isn't ON.

Service manual:
https://siglentna.com/wp-content/uploads/dlm_uploads/2019/08/SPD3000X-Series-Service-Manual.pdf
P46 is the HW troubleshooting section.

Do post your findings please so to provide help for others that might have similar problems.

[MathWizard]

Thanks, No it should be on. I can't really recall using ch3 after I hacked it but I guessing I tried after that. The rest of it seems to work great.

Looking at it w/ the DMM, when I turn it on I get a peak of 1.1mV that quickly decays, and on the scope, I can see that little raise. But I can't see any spikes or anything happening to show it's trying to build up a real voltage, w/ single shot on any time scale.

So I was just making a LM723 PSU and a L200 PSU, so I better finish them 1st before I open this 1. The L200 is about done, I'm just breadboarding the other 1.

[tautech]

Well one we sold last year did this too ! 

Some emails to factory tech support offered some test tips and info that there certainly is fuse for each channel.
Waiting on values for each of them. 

Please check the below test points:
1.  F301 fuse.  check it whether burnt down
2.  Check the TP4, The normal voltage is 11.2± 1.5V
3.  Check the TP3, voltage of Pin1,2 is about 3V;  and the Pin3,2 is about 15V.
 
Please be careful. Because the board has the high voltage. 

[MathWizard]

Oh thanks Tautech I just got around to taking it apart. The only problem i could find on the power PCB was the 6.3A fuse melted. The sense resistor on the output board measured ok. I was afraid the transformer winding was toast too, but it just reads 80m-ohm anyways.

I replaced the fuse and it all works again.

I believe this happened when I tried powering a stereo, that uses a 6V regulator, as a reference for a bunch of op-amps. And I had already seen that when powering it off a single channel, w/ the correct 7806, but some big capacitors missing...the 7806 would be driven low by the op-amps, which is not something I'd expect, but it did/does.

On the day I think it happened, the 7806 was unhooked, and I went to use the 5V CH in it's place, then I noticed I had no more 5V-CH. That was the 1st time in ages I used it. So that's all I can think of. How that wiould burn out a 6.3A fuze tho, IDK.

For ages I've been using Ch1 for the 18V rail, and Ch2 for the 6V ref rail, instead of a 7806, and that works ok. I need to try that stereo with all the correct caps in place, and then the 6V ref should be powered up before the op-amps get going.

[tautech]

OK good, just a blown ch3 fuse but what baffles me is why a current limited PSU should even blow an output fuse ? 

Shame while you had yours apart you didn't give us all the fuse values and a pic of the PCB. 

[MathWizard]

Well it was the fuse for the ch3 winding, right before the bridge rectifier, there's a series pass mosfet driven from the control board. There's a triac + TL431 providing over-voltage protection.

I never took off the control PCB, I really don't want to break it. So luckily seems to be working again, but I never load tested it yet, but it switches between 2.5/3.3/5V.

It looks great inside tho, and very little dust after ~3years.

I'm surprised some other protection didn't kick in 1st and turn off the mosfet before the fuse went. But I'm not taking it apart to look at the control, unless there's another problem.

[tautech]

Thanks. 
OK then from your schematic it would seem the TRIAC crowbar kicked in for it to blow the ch3 fuse.

[MathWizard]

Yeah thats possible too, I usually would have turned on 16V 1st, and might have had just about that on the 6V rail at the time. The crowbar kicks in around 11.5V, and then there's no logic to turn off the triac.

+6.3 amps certainly never went through my stereo

[tautech]

Ah then to prevent it happening again you would need to correctly sequence the channels to ON with SCPI commands. 

[mawyatt]

If this was the case I'm curious why the use of an old style fuse and not a PTC type fuse which will "reset" when power is recycled and the external over-voltage "fault" removed.

Using a TRIAC to crowbar the output and hopefully protect the PS from external over-voltages is a good design choice by Siglent IMO, but using an old style fuse which must be replaced is not IMO! I'm sure they had "reasons" for this, I certainly would like to know what those "reasons" were. I mean the things I've seen from Siglent isn't the penny-pinching shortcuts you find in the usual lower grade equipment, so doubt it is to save a few cents on a fuse!!

Anyway, looks like you are back up and running again

Best,

[cuspide]

OK good, just a blown ch3 fuse but what baffles me is why a current limited PSU should even blow an output fuse ? 

Shame while you had yours apart you didn't give us all the fuse values and a pic of the PCB. 

I just had to replace the very same fuse (F301), in the hope that that will fix Ch3 (I think I accidentally shorted it and busted the fuse).

If this can help anyone in the future, the fuse is a through hole, slow 6.3A 250V, low breaking capacity - brand LittleFuse (LF T6.3AL250V P)

I think this is the fuse https://www.digikey.com/en/products/detail/littelfuse-inc/021806-3MXEP/777615 (or at least, this is the one I bought    and should receive in a few days - the one that was there I think was its now obsolete predecessor https://www.digikey.com/en/products/detail/littelfuse-inc/021806-3MXE/552899).

BTW, not an expert here, but it's the first time I see through-hole fuses, it kind of seems to defy the point of having a cheap and easy to replace component that saves the circuit if you have to take the generator apart to desolder the fuse. Can someone with more experience comment on why they didn't just use a fuse holder for this, and make our life easier? Thanks!