Problems with JTM45/100W power up

Loganp · Post by **Loganp** » Sun Aug 31, 2008 9:44 pm

Ok I have had this amp done for a while now and it has been sitting around due to me having problems with the power up/testing portion of the amp. And now that I have some more free time I am seeking help on here to see if anyone can help me out. So now to explain my problem.. The amp powers up fine ( power switch only ) and I begin to check the voltages at this point. ( pre amp tubes in... No power tubes and not hooked up to a cabinet.) All the voltages on George's chart are within reason and there is no smoke or any problems that I can see at this point. Now here comes the problem. When I turn on the standby switch on Still with no power tubes in and go through the rest of the chart for the preamp and main voltage testing I can feel a hot spot on the amp and then smoke.. For a while I had no idea where it was coming from but then I noticed the 10K 2 watt resister that is beneath the 32uf cap got so hot it changed color and started to smoke.

Far as i can tell everything on the amp is in working order or is it something I am doing in the testing part that is causing this.

Pictures in link...

http://www.flickr.com/photos/logan_p/se ... 044508574/

Flames1950 · Post by **Flames1950** » Sun Aug 31, 2008 11:52 pm

That 32/32 cap........it's an old one, are you sure it's good? Even if it's "NOS" doesn't mean a cap that old is ready to perform up to snuff.

shakti · Post by **shakti** » Mon Sep 01, 2008 1:34 am

Also, without any tubes in it, the voltages are going to be veeery high in there, perhaps too high for certain components? Preamp cap may not take the voltage, what is the rating? Try plugging in the preamp tubes before you power it up, that should lower the voltage. But I would use some cheap ones just in case...

Loganp · Post by **Loganp** » Mon Sep 01, 2008 2:24 am

The preamp tubes were in while I was testing.

Loganp · Post by **Loganp** » Mon Sep 01, 2008 2:26 am

That 32uf/32uf cap is 1 of 2. The caps are the same and I tried them both with the same result. I will pick up a new 32uf cap and see if that makes a difference

Billy Batz · Post by **Billy Batz** » Mon Sep 01, 2008 12:23 pm

Another thing that couldve happened is that, if you tried to take voltage readings with NO preamp tubes in, then the voltage on that cap wouldve been way higher then its rating which is 350 and it couldve blown the cap which would cause it to short now. Ive had that happen before.

The cap may show no visual signs of being blown and it would suck if thats the case since you found that old NOS ecap but ecaps are bad for NOS because age effects them as well as usage.

Also make sure your underboard jumper wires are going to the correct turrets.

Loganp · Post by **Loganp** » Mon Sep 01, 2008 11:27 pm

Ok, so today I switched the Green 32/32 plessey cap for a 16uf sprague atom cap that I had and I am still feeling the heat around that 10k resistor and smelling a burnt smell but no smoke this time witch allowed me to go over the voltage chart and this is what I got. This was done with preamp tubes in the amp but no Power tubes so i expected the power tube socket voltages to be higher. ( everything is measured in DC at 1000V setting except for the heaters witch are AC )

The only differences between my readings and Georges chart are

V1 P1: 10 ( chart says 245 )
V1 P3: 7 ( chart says 1.9 )
V1 P6: 0 ( chart says 240 )
V1 P7: 10.6 ( chart says 0 )
V1 P8: 6.9 ( chart says 1.9 )

V2 P1: 6 ( chart says 205 )
V2 P3: 0 ( chart says 1.2 )
V2 P6: 11 ( chart says 360 )
V2 P7: 6.3 ( chart says 205 )
V2 P8: 6.8 ( chart says 200 )

Everything else seems to be in and around 10%

gutpile · Post by **gutpile** » Tue Sep 02, 2008 6:05 pm

I can't tell for sure from your pics but it looks you are missing the buss wire bridge between the board cap (on the tube side) and the blue wire turret to the left of it... see the link below....

http://forum.metroamp.com/download.php?id=5784

thousandshirts · Post by **thousandshirts** » Tue Sep 02, 2008 6:59 pm

gutpile wrote:I can't tell for sure from your pics but it looks you are missing the buss wire bridge...

Logan is going with the Dickinson/Hendrix amp setup there, so I think it's ok as he has it. I wonder though, if he did take out the extra 10k, put in a buss wire bridge, and changed from a dual 33 to a single 33. Those green ones sure are nice looking old filter caps, and ruling them out from the possibility of being the culprit would be nice. The pictures are good (love those Nikons, Logan), but I don't see anything lurking on the surface here. Maybe SDM, Flames, or somebody else will see something, though!

Loganp · Post by **Loganp** » Tue Sep 02, 2008 7:35 pm

When I fired up the amp last night I changed to a single sprague atom cap and installed the buss wire bridge. I could feel the heat coming off of the 10 K resistor but I was able to take those readings. I will add more pictures and It sucks I can't use those plessey caps.. I have seen them on several old marshall amps and I am offering them up for sale if anyone wants them.

http://www.flickr.com/photos/logan_p/28 ... 044508574/

thousandshirts · Post by **thousandshirts** » Tue Sep 02, 2008 11:33 pm

And you've changed out preamp tubes with 100% for sure good ones? Is the 100k resistor that is mounted directly to V2 still good?

Loganp · Post by **Loganp** » Wed Sep 03, 2008 12:37 am

Have not tried that. I will pull the preamp tubes from my JTM45 and put a new 100K resistor in.

Loganp · Post by **Loganp** » Wed Sep 03, 2008 8:42 pm

Well i think i found my problem. In the midst of installing a new 100K resistor and changing the preamp tubes I realized while going through the amp I had hooked the green wire coming from Pin 7 of V1 to the junction of the plate resistors instead of the Bright channel input resistors. This would explain the 248V that I was getting and not the Zero I should have been and the fact that the 10K 2w resistor kept smoking because the plate resistors are tied into the 10K resistor and the 32uf cap. The amp seems to be working great and all the voltages checked out on spec

Thank you to everyone who helped me out

thousandshirts · Post by **thousandshirts** » Wed Sep 03, 2008 9:10 pm

Great stuff, glad to hear it.

Really good sounding amps, eh?