2 different JCM2000s suffering significant heat damage to filament connections

[SoulFetish]

Over the past week, I worked on a couple of JCM2000s which came in with different issues. In both cases I had to remove the mains quick disconnects from the main board. In the first case, when pulling the first one of the heater wires, it literally was so brittle, the wire insulation crumbled and broke apart when removing it (I just though they used brown color wire.) I burnished the board contacts and made new wire/crimp connects with proper wire as part of the repair, and didn't think much about it again.
When I opened the second amp a day or so later, I could see the heat discoloration on the heater traces on the main board and realized this amp was suffering the same condition. This time, the QD on the mains transformer pulled right out of the seat. After removing the plastic top of the transformer cover, if revealed some discoloration to the insulation on the magnet wire leading up to the terminal connections. I became concerned about breakdown in the insulation, so I applied some MG red insulating varnish to the discolored areas (Insulation Class H to 180˚–3000V/mil) and told the customer that he there was heat damage to the winding insulation; and if that breaks down, he will need a new PT.
Have you guys seen this as an issue affecting these amps, and what do you do in cases like this?

[DrGonz78]

Which exact model of jcm 2000 was it? Can you date the amps?

[SoulFetish]

Which exact model of jcm 2000 was it? Can you date the amps?

hmmm. Off the top of my head, I can't remember. Well actually, I can figure it out. I'll remember which board version if I can log onto the site. lemme check and I'll get back to you...

edit: The service site is an absolute disaster right now. I'll try and find out the particulars today or tomorrow. But, I believe it was the version with the main board ending in "-00" but I'll double check

[Leo_Gnardo]

Have you guys seen this as an issue affecting these amps, and what do you do in cases like this?

Sure have, mostly in big bass amps like SVT-CL & similar, also Fender/Sunn Bassman 300. Where the filament leads attach to the circus boards, often the plastic boots surrounding the push-on wire terminals have gone brown, black, even melted. I solder the filament leads straight to the printed circuit pads & haven't had any further problems. My theory is it's the initial current rush when powering up that's way in excess of what the quick-connect terminals can safely handle. A few hundred cycles of that, and now the connection is, how we say? - "ohmic" - that is corroding, heating up and getting worse with each power-up repeat. QC connects seem to work OK in household appliances & behind dashboards but I'm not too happy with them in amps especially in high current filament circuits. A correctly soldered connection will last way longer than those "convenient" QC spades & shovels. Heck I just had one go bad in a recently made Sonny Boy harp amp - speaker spade connection fell apart after I fixed the amp - even though I never removed or stressed that connection in any way. Now speaker and cable are soldered together & let's hope that amp gives its owner years of satisfactory gigs.

[drewl]

I've seen it on Mesa Boogie amps.

If you didn't know, some of these amps the PC board becomes conductive and the bias current will runaway until it kills the output tubes and blows the fuse.

The early light green boards are the ones.

Monitor your bias points to see if it keeps increasing.

[m1989jmp]

If I remember correctly, it's a well documented issue on TSL and DSL amplifiers; something about DC filaments bridge rectifier getting too hot and losing connection to the PCB traces.

I don't know if it's the cause or consequence though.

[Jazz P Bass]

I had a Crate VC3112 come through.

Customer complaint: "I smelled something burning and then it stopped outputting a signal".

Flipped the main board over and , wallah!.

2 of the Vdc rectifiers where shorted .(1N4007 no less!)
The heater buss from there back was fried.

Never did find the 'why', just the results.

[nevetslab]

Likewise, I've had that similar issue at the PCB where the power xfmr's 1/4" quick-disconnect heater winding terminals plug in. I've had the traces on the PCB lift up on at least one JCM 2000, though usually just find it's the wires that are crimped into the terminals that breaks down. I've NOT seen that back all the way up the flexible leads to where the termination changes to the magnet wires inside the bell housing of the power xfmr. I think that's rare.

What causes it? Poor crimp connections, oxidized flexible lead wires when they're installed. Though it was puzzling to find PCB damage, though when the overheating takes place to damage those connections, that heat has to go someplace, so PCB traces/connection is one of the destinations.

I've had to eliminate the right-angle Fast-ons on many Ampeg SVT-CL's & -VR's from that.

[g1]

Same problem where heaters are run via molex connectors. High current through any type of connector is a challenge. A bit of resistance creates a lot of heat, causing increased resistance, and it just spirals into kind of a thermal runaway situation.

[Leo_Gnardo]

Same problem where heaters are run via molex connectors. High current through any type of connector is a challenge. A bit of resistance creates a lot of heat, causing increased resistance, and it just spirals into kind of a thermal runaway situation.

Molex & similar - I've mentioned on these pages the failures I've found on otherwise good sounding Crate Blue Voodoo amps. Filament connections made thru molex/comb connectors at each end of the mini board that has the output tube sockets on it are a bugaboo. I snip 'em off and make a home run connection to the main board with 18 or 16 ga stranded wire.

[SoulFetish]

Same problem where heaters are run via molex connectors. High current through any type of connector is a challenge. A bit of resistance creates a lot of heat, causing increased resistance, and it just spirals into kind of a thermal runaway situation.

Exactly. Once the oxide builds up due to heat, it goes from bad to worse. I actually like using quality crimp connectors. But if the connector is poor quality, or makes poor contact, the heat will eventually cause a breakdown and failure.

[DrGonz78]

If the circuit boards end in -00, like Soulfetish thought, then they must be older jcm 2000 models. Probably had 1000's of hours of use and just cooked the connections. The other one that springs to mind is the Bugera 6160 or something where they had a batch of failures of molex heater connectors.

Edit: it was Bugera 6260 and possibly the 6262 which had the bad molex connectors.

[Pedro Vecino]

I have never seen that problem. What I have seen is that of the rectifier several times in old units.

If you didn't know, some of these amps the PC board becomes conductive and the bias current will runaway until it kills the output tubes and blows the fuse.
The early light green boards are the ones.

About this problem: there are DSL50/100 and TSL100/122 amps with light green PC boards that do not have that problem. That is: the problem beats in the light green boards but not in all, and I do not mean something circumstantial but because they are different.
The way that I use to detect the susceptible versions without having it in front is identifying the frontal switches.
Those susceptible to problem have the surface of the switch angled (as in the JCM800 and 900) while the non-susceptible have it curved. The typography is also different.
I have changed many boards in all of them and I have never found any that does not comply with that rule. Sometimes in the modern ones (even with a light green board), despite being able to associate the symptoms with the big problem, they have never corresponded to it.
The TSL60/601/602 are free of that problem.