JCM900 model 4500... what would cause this?

[methodofcontrol]

i'm not real familiar w/ these amps. i've got one in front of me that most closely follows this schematic:

http://music-electronics-forum.com/a...0-100w-pwr.gif

R35 is a 100 ohm/5w between a large filter cap and a picofuse. the side at the fuse has left a burn mark
in the PCB. as i understand it... the amp worked one day and didn't the next. when opened, R35 was found
with one side unsoldered (fuse side) and the char marks underneath it. the resistor was resoldered and the amp still doesn't work.

i haven't done anything to the amp yet other than test the power tubes for shorts. i also checked the traces all around the
burn zone for proper continuity and confirmed the 100 ohm resistor's value.

tubes test strong w/ no shorts. traces test properly w/ no carbon bridges. 100 ohm resistor measures 100 ohms.

thoughts?

[nickb]

i'm not real familiar w/ these amps. i've got one in front of me that most closely follows this schematic:

http://music-electronics-forum.com/a...0-100w-pwr.gif

R35 is a 100 ohm/5w between a large filter cap and a picofuse. the side at the fuse has left a burn mark
in the PCB. as i understand it... the amp worked one day and didn't the next. when opened, R35 was found
with one side unsoldered (fuse side) and the char marks underneath it. the resistor was resoldered and the amp still doesn't work.

i haven't done anything to the amp yet other than test the power tubes for shorts. i also checked the traces all around the
burn zone for proper continuity and confirmed the 100 ohm resistor's value.

tubes test strong w/ no shorts. traces test properly w/ no carbon bridges. 100 ohm resistor measures 100 ohms.

thoughts?

Have you checked to see if the picofuse is blown?
What is the DC voltage at the OPT center tap?
What is the DC voltage at junction of R35 and R30?

Schematic:
Marshall_jcm900_25xx_45xx_50w.pdf

[g1]

Yes, check DC on both sides of R35 and R30.
And check the high/low switch as they can have problems/solder issues.

[methodofcontrol]

i should've mentioned this earlier, OT has been replaced w/ mercury magnetics and R30 looks replaced from what i assume to be previous repair w/ 27k.

i've got power tubes pulled:

@ R35 i get 454v and 454v
@ R30 i get 454v and 389v

high/low power switch solder joints look ok and test well w/ continuity. i can resolder them to be certain.

[nickb]

Replace the power tubes then

Any noise from speaker at all? Try switching high/low to see if you get anything.
While you are at it, is the 'blown fuse' indicator lit?
Test the preamp by plugging the fx send to another amp.
Test the power amp by feeding the fx return from another preamp/guitar.

If the power amp is dead then measure the DC at all the power tube pins - btw you should hear some noise in the speaker when you touch the grid pins with your meter probe.

[g1]

@ R35 i get 454v and 454v
@ R30 i get 454v and 389v

Please recheck, measuring across the resistors, not to ground (one probe at each end of resistor). There must be some voltage drop across R35 as it is feeding R30 and you measured a voltage drop across R30.
If the drop across R30 is correct, there is only 2mA flowing for all the preamp tubes combined, definitely not right.
Are all heaters lit?

[nickb]

Please recheck, measuring across the resistors, not to ground (one probe at each end of resistor). There must be some voltage drop across R35 as it is feeding R30 and you measured a voltage drop across R30.
If the drop across R30 is correct, there is only 2mA flowing for all the preamp tubes combined, definitely not right.
Are all heaters lit?

Good catch g-'man'

[methodofcontrol]

with the power tubes still pulled, the voltage drop across R35 is .6v, and across R30 is 147v. (not sure why the R30 drop is so different from before, could be my error.)

with power tubes out, V3 heaters glow immediately as expected, V1 and V2 heaters take about 30-60 seconds to start glowing.
with power tubes in, power tube heaters glow and V3 is the same but V1 and V2 do not glow.

g-one, you're theory about there only being 2mA flowing for the preamp heaters is sounding correct...

so, if the preamp's demand for current is cooking R35 (hence burnt smell and charred area)...
is the problem w/ the preamp's demand? (seems unlikely...)
or the supply? (seems more likely... to me, anyway.)
why is the supply being choked off? i mean, if R35 is bad now (still tests good, but i plan on replacing it before i'm through) what brought that on?

[nickb]

with the power tubes still pulled, the voltage drop across R35 is .6v, and across R30 is 147v. (not sure why the R30 drop is so different from before, could be my error.)

with power tubes out, V3 heaters glow immediately as expected, V1 and V2 heaters take about 30-60 seconds to start glowing.
with power tubes in, power tube heaters glow and V3 is the same but V1 and V2 do not glow.

g-one, you're theory about there only being 2mA flowing for the preamp heaters is sounding correct...

so, if the preamp's demand for current is cooking R35 (hence burnt smell and charred area)...
is the problem w/ the preamp's demand? (seems unlikely...)
or the supply? (seems more likely... to me, anyway.)
why is the supply being choked off? i mean, if R35 is bad now (still tests good, but i plan on replacing it before i'm through) what brought that on?

You've got it mixed up: Less heaters => less current = less voltage drop = no charred resistors. I don't see any reason to replace R35 unless it is in bad shape. There is only a small drop across R35 because is a low value and you've no power tubes in. It's certainly not your main issue. You need to fix the reason for the missing heaters - most likely a bad solder joint on one of the tube bases between V2 & V3 and go from there.

[g1]

R35 will normally run hot. Brown board there is not uncommon.
The R30 & R35 voltages you have now listed sound about right considering power tubes are out. With power tubes in, voltage across R35 should increase a bit.
Find out why your heaters are giving you trouble and you may get sound.

[methodofcontrol]

after inspecting the solder joints on V1, V2 and V3 heaters, i found a couple of suspect ones and wouldn't you know it... i re-soldered them and the amp came alive!

would the intermittent heater connection on the pre-tubes have caused R35 to overheat to the point of coming off the board?

[nickb]

after inspecting the solder joints on V1, V2 and V3 heaters, i found a couple of suspect ones and wouldn't you know it... i re-soldered them and the amp came alive!

would the intermittent heater connection on the pre-tubes have caused R35 to overheat to the point of coming off the board?

No. It's just undersized. It happens more often than it should, sadly.

[methodofcontrol]

No. It's just undersized. It happens more often than it should, sadly.

so... maybe i should replace the R35 w/ a 10w, or even an actual 5w. the current resistor looks to be a 3w even though the schematic calls for a 5w. good looking out, nickb! i appreciate the dialogue!

[nickb]

so... maybe i should replace the R35 w/ a 10w, or even an actual 5w. the current resistor looks to be a 3w even though the schematic calls for a 5w. good looking out, nickb! i appreciate the dialogue!

I would use the 5W. 10W might be a squeeze. It you really want peace of mind you could strap two 200 ohms watters in parallel, one on top of the other. Make sure the top one is tightly secured to the lower one by winding it's leads to the lower one.

[methodofcontrol]

i picked up a 100 ohm /10w today @ radio shack. i figure i can make it work even if it's off board somewhere. i had the parallel resistor thought, but only because i have a bunch of 350 ohm 5w'ers here i wanted to use. i nixed it figuring 175 ohms was too far off.