Marshall Repair Newbie

[jlguler]

Hello all,
I am new to the forum, just wanted to say hi. I recently repaired a Marshall 3315 150 watt head. replaced two pico fuses, darlington transistors, zener diodes, and power resistors. it blew everything in pairs! She works good now, low noise, high gain. I am now working on a MG100DFX. I replaced the 7293 that was cooked. The preamp circuit works, but not the power circuit. I am new to working on electronics, but it's fun. I have already learned a great deal playing around and reading info on this forum. I am building a cheap signal tracer out of a set of powered speakers to help troubleshoot.

John

[Enzo]

When faced with an amp with two TDA7293 chips, when one blows, it is a good idea to replace them both. The "good" one was probably stressed when the other one failed.

TDA7293 are down to $3.42 apiece at Mouser.

If it has good 7293 modules in it and still makes no sound, look on the power supply page of the schematic, bottom center, and there is a transistor TR2 with a cap and a zener in parallel - they make the control signal labelled "STBY."

If any one of those three parts is shorted, the amp will remain in standby mode and thus silent.

And of course make sure your speakers and connections are OK, etc.

[jlguler]

Checked Zener, cap, and transistor out of circuit, all checked good with meter. I put them back in. I noticed when I tested the preamp circuit (by connecting the headphone out to a power amp) it seemed distorted, noisy, and a weak signal, but maybe I had the gain too high. I have 15vdc at all opamps, I have 41vdc at the 7293. Could it be the cap or resistor in series with the inverting input? Thanks for your help, I appreciate it.

John