Trace Elliot Super Tramp TSC Blowing Fuse's

[gweedo]

Hello, I hope someone can help me, a friend asked me to have a look at his recently aquired, (2nd hand), Trace Elliot Super Tramp Stereo Chorus, as it had started to blow fuse's when he tried to play with the volume turned up. He said the previous owner had it from new and only ever played it in the bedroom at low volume. My Friend said when he played it at home over a number of weeks (at low volume), it sounded fine and didn't have any problems. But then he played at a gig with the volume up and after about 15 minutes the fuse blew, he didnt have another fuse so he put the amp to one side & replaced the fuse the next day, amp played OK (at low volume) he turned the volume up & blew the fuse straight away, replaced the fuse and blew on power up. This amp has 2 output modules, (The Penguin Lead Guitar Version), I will refer to them as left & right. I isolated the problem to the left module and found a shorted BD647 (TR1) and BD648 (TR2) I replaced them both. I carried out cold checks and couldn't see anything obvious. I connected the left module (no speaker load) and checked rail voltages 32v & -32V and had 15V at Z1. I then checked current draw across R2 & R5 (.22 emitter resistors), both 22 mA, I then added speaker load & checked again, R2 4.5 mA & R5 54.5 mA . Here ends the extent of my electronics knowledge, ( just enough to get me into trouble, ha ha!) I thought maybe a bias problem ( but dont understand why only TR3 would draw excessivly & not TR1), I replaced TR4, then TR2 & then TR5, with little or no difference each time, Any thoughts would be appreciated.
NOTE: this is a 1995 model and the attached diagram is for a 1996 model, the only difference is TR2 is a 2N3904 (not TIP31C) and the circuit from the junction of TR1 & R2 to the junction of R3 & R4 is not there, (maybe they had problems with stability & changed it on later models?)

TSC Penguin Output Stage.pdf

[Enzo]

Leave the speaker off for now. It powers up OK with no speaker? You get good power rails and no excess current WITH NO SPEAKER? Now measure for DC voltage across the speaker connections. In other words is the amp putting out DC?

[gweedo]

Hi Enzo, it starts at 170 mV, drops away quickly to 2 mV then slowly creeps back up to around the 100 mV region

[gweedo]

Sorry had my multimeter on autorange, rechecked it tonight with 2 different multimeters, reading sits at DC 90 mV. I did notice that with the red test lead on positive speaker wire & black test lead on negative wire I get a minus reading, reverse the test leads and I get a positive reading, (on both meters, & test leads are in correct sockets), just thought it was worth mentioning. The positive speaker wire does go to the junction of the emitter resistors.