gas discharge tube= surge protect?

[drewl]

Hey all, working on a new little SS Fender Jazzmaster amp which has a "gas disharge tube" in it which I'm guessing is being used as a surge protector type device.
The amp uses a switching power supply to avoid using a power trans.

Anybody know what these things would measure with an ohmmeter?
does it read a short, open, or diode breakdown or like a cap?

trying to figure out if it's bad or not, I'm thinking it should provide a path to ground to turn on this power supply.
Thanks.

[Enzo]

I'm interested. How old is this amp? I don't recall seeing a switcher on a Fender?

Gas discharge tubes mean to me more or less a neon bulb. And so should read open. It also conjures up the image of a spark gap device, which is exactly what it sounds like. Enough voltage and it arcs over. Would also measure open.

I'll try to find a schemo.

[Enzo]

Where is this part on it? What is the part callout on the board? I have the schematic now.

[drewl]

I figured you would!
It's labeled "G" something and is used as a surge device.
I spent some time studying them today!
It's on one of the ac lugs of the first rectifier bridge, there's no power tranny in this thing, so it has an isolation coil, then rectifier then on to the high voltage switching power supply.

let me know if you can send me the schem.
Thanks.

[Enzo]

The schematic does not include that SMPS. It is a 250w 40v power supply. It is listed as a part in the mechanical blowup of another amp in the series, but no cigar on a schemo. I just wrote to Fender to see if they'd send me that specifically. Send me an email and ask for the rest of the circuit if you would like. I'll let you know what help I get from Fender.

[mykey]

Hey all, working on a new little SS Fender Jazzmaster amp which has a "gas disharge tube" in it which I'm guessing is being used as a surge protector type device.
The amp uses a switching power supply to avoid using a power trans.

Anybody know what these things would measure with an ohmmeter?
does it read a short, open, or diode breakdown or like a cap?

trying to figure out if it's bad or not, I'm thinking it should provide a path to ground to turn on this power supply.
Thanks.

yes that's what its for, surge. Out of the circuit it reads open as far as I remember and is not the defect. but what is wrong with the amp, loud 60 cycle hum? blowing fuse or what?

[Enzo]

It's in the other thread, hardware fell into the power supply and blew it up.

[drewl]

Not too much physical damage, Q5 (unknown ST MA T827) appears to be bad, I think it's part of the "turn on" control circuit for the switching power supply.

[Steve Conner]

Sure Q5 isn't a SCR or triac? Some switchers have one to control the turn-on inrush.

If the gas discharge tube is a surge protector, the PS should work just fine without it, for a while at least.

[drewl]

No I'm not sure about "Q5" I don't know what it is!
I can't find any info, not even on ST's own site.
And after researching gas discharge tubes i see how it's being used and that the circuit will work without it.


I have alot of the other parts in the switching power supply here at work, since it's quite similiar to supplies in some products I service.