Need Norh SE-9 help

Lux Man

Lux Man

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I just acquired a non-working Nohr SE-9. I have very little history on it other than it would seem a previous owner must have added a fuse because several reviews say it has no fuse. At this point I'm unsure if it was original or added but the note that came with the amp said the fuse blows after 5 minutes or so. The note also says it was a 2 amp fuse. Here again my research shows some discrepancies. The reviews say it consumes 80 watts but the sticker on it says 30 watts. I've read in one forum a 6 amp fuse was used. So what I really want to start out with is what size fuse should be used, because maybe the unit is fine but it was under fused by a previous owner. I could also just replace all the tubes but I'd rather not if it's not required.

So if someone has one and can help me with a starting point I'd really appreciate it. BTW, I tried emailing Norh but the email bounced back immediately, not sure why.

Thank you-
 
80 watts seems about right - that's about 0.7A at 120V. 1A slow-blow or 1.5-2A fast would be about right. There may an EL34 tube with grid leakage - that will get worse, drawing increasing current as it heats up
 
Would there be any "hazard" to the amp if I pulled all the tubes on one side to see if the fuse still blows and if not then begin swapping in the other tubes one at a time to see if I can identify the culprit? I realize that if it were the rectifier tube this approach won't work. It would be just nice to isolate the issue before I re-tube it in case the tubes aren't the issue.
 
Time to begin by replacing the rectifier tube. I installed the 2a fuse and a few seconds later arcing inside the rectifier tube took out the fuse. If that's all it is I'll be a happy camper.
 
Well it wasn't just a bad tube. I bought a new one and the fuse still blows. I also found a bad cap but that wasn't the whole issue either. Still blows the fuse in about 15 seconds and all I have in it is the rectifier tube. I am not real optimistic about fixing this unless someone else has one and can provide some information.
 
Sounds like the first capacitor after the rectifier tube is bad.

Remove the rectifier tube and if it still blows the fuse, the power transformer is suspect.
 
You are right one was bad. I then replaced it but the fuse still blows. I then disconnected the cap entirely and it still blows. Ideas?
 
When the rectifier tube is removed the fuse does not blow. I really wish someone had one of these so they could take a picture of the board so I could know the previous owner(s) didn't make an wiring mistake. Output voltages from the transformers would also be a big help.
 
Well, the rectifier tube can only be wired up one way depending on which plate the high voltage is taken off of. Which rectifier tube are you using?

Is the fuse blackened when it blows or does it look like the wire inside just broke in half?

Why don't you give us some pics and maybe someone here can show some possible discrepancies.
 
I am seeing 780vac across pins 2 and 8 on the 5ar4 rectifier. Isn't that excessively high?
 
Without a load, yes! Measure pin 8 and CT (center tap) or ground. Do the same with pin 2. You should see half of 780.
 
Cademan- yes you are correct, I see 1/2 per side. This is a bit of a mystery as to why the fuse keeps blowing. The rectifier must be seeing something tied to ground. I have the caps and choke pulled and both el34's and the 2-12AX7's. Do tube rectifiers need to see a load? What I mean is is it a mistake to try running this this way just to troubleshoot? Solid state parts don't care if there's no load connected but do tubes?
 
Here are some pictures of both sides of the board. The amplifier consists of 1-5AR4 rectifier, 2 EL34's, 2-12AX7's.

I am surprised the issue isn't more obvious to me. I'm not saying I'm the sharpest tack in the box but this circuit is pretty simple. I will admit that I have not worked on tube gear but I'm open to learning. I'm wondering how do i isolate the rectifier/power supply from the rest of the circuit so I can divide and conquer so to speak?
 

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By removing all the tubes except the rectifier, you won't damage anything for testing purposes. should see DC 370 +/- a few volts at pin 2 of the rectifier. The first capacitor should also be the same voltage.

Usually when the rectifier tube arcs, it is due to a bad or shorted filter capacitor but sometimes it can be a bad rectifier tube itself. I hope you don't have one of those chinese rectifier tubes in there. I have heard that they are not very reliable.

Did the blown fuse look blackened? If so it is because something is drawing too much current. Can be a cathode to heater short but most of the time in my experience it is a bad capacitor or rectifier.

While typing this, I just thought of something that happened to my Sansui 1000A a few years ago that was blowing fuses. It happened to be a shorted output transformer so I would check both output transformer primaries with the ohms setting on your meter, unplugged with no tubes. Both should read the same or very close the same.

You're right! To me, that is one simple circuit and design.
 
The sequence for the fuse blowing is this: I plug it in- 10-15 seconds later the tube arcs and the fuse flashes.

I did replace the tube with a Sovtek which was already in it- same result.

The input caps are both out of the circuit- I pulled one leg as well as the choke.

I'm going to start looking very close for a pinched wire shorted to ground, don't know what else it could be.
 
Does the fuse blow, with all the tubes pulled? (rectifier also)... If not, your primary wiring, and PT seem OK,,, at least they aren't shorted... Then, the problem is rectifier forward...

Edit...I see you tried it with out the rectifier,,,

Regards,
John
 
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It's kinda hard to armchair diagnose over the internet but make sure your capacitors are in the right way and not backwards.
 
I think I'm going to begin creating a schematic starting with the power supply and work forward. Maybe in the process I'll find my issue. I may post a copy of the schematic but it may look more like a pictorial.

Please confirm that there's no issue running tubes without all their connections being made, essentially a open collector or anode.

It's so funny because I really believe if this were a solid state amp I believe it would be fixed by now., or at least that's what I'm going with for now.
 
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