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2002 JCM 2000 DSL. V1, 2, 3, 4, 5 do not light up. Only V6, 7 and 8 light up normally. No output (goes without saying). Makes no difference if something is plugged into the input. Things that I have done……..swapped tubes around and put different tubes in. No change. Swapped the V5 socket with V6…..no change. Checked all around the main board for trace breaks or solder bridges. No obvious signs of foul play. Checked all fuses. Checked the AC between pin 2 and 7 on the output sockets…..V6, 7 and 8 read 6.25VAC…..V5 read .05VAC. Being not a tech by trade was wondering what would be a good starting point…..any suggestions?
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Last edited by The Dude; 02-20-2015, 01:08 AM."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22 -
Just re-read your post. Only V1 and V2 are DC powered. It looks like you lose filament voltage even in part of the AC chain and the rectifier is not getting AC Here's what I would do. Remove the tubes so that you are not getting false readings from the filament windings. Check from socket to socket on the parallel filament path. There must be a break somewhere. I'm going to guess between V5 and V6.Last edited by The Dude; 02-20-2015, 01:08 AM."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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You have a break in the 6.3Vac heater line.
V3, 4, 5, 6, 7 & 8 should all be connected together (heaters in parallel).Comment
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Bear in mind it's been a while for me so correct me if I'm doing this wrong. I measured the AC voltage across pin 2 and 7 of V5 thru 8 and across pins 4 and 5 of V3 and 4. Here is what I read
V8 = 7.3 VAC
V7 = 7.3 VAC
V6 = 7.3 VAC
V5 = 7.3 VAC
V4 = .005 VAC
V3 = .005 VAC
I can barely muddle thru schematics so please break it down in laymen terms for me. I can catch on eventually.
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I have continuity from pin 2 of V5 to pin 4 and 5 of V4........Comment
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On V3 & V4 pins 4/5 are connected together to one side of the heater winding and pin 9 is connected to the other. You'd read across pin 4/5 and pin 9 of all the preamp tubes, pin 2 & 7 for power tubes.
Justin
PS I like the olden ways of drawing heater wiring, with the little pictures. Vox did well, Fender, Gibson... that schem with every wire drawn in for every tube gets so cramped it's just nuts! I can't follow it on a screen."Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -Comment
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Thanks for that heads up Justin!
V4 and V3 across pin 9 and 4/5 = 1.55 VACComment
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Though you say you checked all the fuses, did you do that visually or with an ohmmeter?
Reason I ask is an open F1 would cause the exact symptoms related in the original post (V1-V5 no heater voltage), and sometimes fuses fail without appearing 'blown'.Comment
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When you are testing heater fuses, you MUST remove it from its clip before ohm meter testing. Otherwise the VERY low resistance parallel path through the tubes themselves and the transformer will obscure an open fuse.Education is what you're left with after you have forgotten what you have learned.Comment
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They were tested out of circuit. I removed them all for testingOriginally posted by Enzo View PostComment
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Please check continuity from pin 7 of V5 to pin 9 of V4.Comment
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