Need help with SVT II Pro

[Roberto Lasco]

Have one (SLM US version, s/n BJIDS10001) on the bench, I was testing it at full power then one component on the graphic eq board took fire...
Looks like was a resistor...but burned beyond recognition. I tried to trace it to the schematic, but it just don't make sense.

It sits just between C2 (1uF/50V cap) and the top right screw of the board. Actually one of its legs is connected to this very screw (ground supposedly). The other leg goes to a JW6 jumper (can't find this on schem), that is connected to the gate (pin3) of a J175 together with R30 (100K).
This is completely wrong according to the schematic I have (drawing 07S728-03 rev3), which shows the gate of the JFet connected with R45 (47K) and R30 in a different place too.

Am I the only one that hate working on SVTs ?
thanks for any help,
roberto

[Enzo]

See if my schematic looks like yours, or post yours.

Instead of the gate of that J175, how about it might be the source or drain. And that is circuit ground. And the mystery resistor would be 10 ohm R31. R31 on the drawing sits all by itself, next to that little voltage chart.

Note both ends of it are connected to a "ground." That would be consistent with one end grounding to chassis through a post, while the other end connects to a grounded terminal on the J175. Note that R30 connects to the same ground as the J175 source. That backs me up some. FInd R45 on your board and see if it indeed connects to one of the other terminals of that J175. Note the gate is the center leg on this data sheet, not pin 3.
http://www.fairchildsemi.com/ds/J1/J175.pdf


I'll bet my lunch money that is your resistor.


Now then, you need to determine why current flowed through that resistor to burn it out. It became the path to ground for current that normally used a different path.

[Roberto Lasco]

Enzo, my schematic is the same as yours, not so for the jfet, which fooled me and explains the rest. Your lunch is guaranteed indeed.

Maybe that 10ohm resistor has some fuse role ? Though I doubt it was SLM plans...
This amp arrived with the faulty stdby led (turning red and shut down) complain, although I was unable to reproduce it, worked fine until the flames...

There's an unmatching on some tubes, one side can't be biased on the "green" zone, but I'm not sure that would be enough to trigger the protection circuit.

any thoughts ?
thanks for your time,
roberto

[Enzo]

No, the 10 ohm is not for fuse action, it assures a ground circuit in the absence of parallel connections elsewhere. This is not rare, look for example at the Peavey Classic 30, there are two 47 ohm resistors connecting three different grounds together, bottom center of the schematic. In the amp, they are shorted across by way of jacks grounding to the chassis, but should the jack lose contact with the chassis, the resistor will make sure the circuit grounds still connect.

Since you received the amp not working, don't assume things are right inside. When the power tubes will not balance, in my personal experience the vast majority of the time it is a bad 12AU7 driver tube, at least once we know the power tubes are OK. Not only that, MAKE SURE the proper tubes are in the proper sockets. On the power amp board, V1 is a 12AX7 and V2,V3 are 12AU7. People replace the 12AU7 with a 12AX7 without looking, and they also get the 12AU7s in the wrong sockets. Due to the two tube types having the same connections, the amp will function, but not correctly. Check that.

V1 12AX7 is in the socket next to the SLAVE OUT jack. V2, V3 12AU7s are in the two sockets behind the bias controls.

Look at the matching circuit, page 1, lower right. Just to the left of all that is IC2b. That is the fault detector, and power tube problems most certainly could trigger shutdown.

[Roberto Lasco]

12AU7s are in the right place, test ok (with matched triodes !).
You mean IC5B, right ? I have a matched sextet at hand, major concern now is to know if any other component went south with this overcurrent; when troubleshooting resumes to the solid state stuff, I'm kinda lost...

thanks again,
roberto

[Roberto Lasco]

follow-up

Two of the 6550s were nearly exausted. No other major harm done. Replaced the burnt resistor, put in a matched trio, amp is up and running...

[Ronsonic]

Am I the only one that hate working on SVTs ?

I don't hate it. I just charge by the hour.