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Fender Stage Lead II 2-12 blowing main fuse

Started by remaker, March 18, 2024, 02:02:18 AM

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remaker

I rescued a Fender Stage Lead II 2-12 blowing the main fuse. Trying to diagnose.

I have a grainy schematic.

Stage Lead II Schematics

I started with a dim bulb tester.

I have disconnected the preamp and controls, leaving only the power amp connected. Still a bright light (fuse blower). The power amp section has a fused lower-voltage section and an unfused higher voltage section (~63V, +/- ~34V). When the 63 V section is not getting power (wires pulled from molex connector), the fault goes away. When the fuses for the low voltage section are fulled, the fault persists. Voltage measures correctly on transformer leads (63VAC) when disconnected so the transformer looks OK

When in fault mode with the dim bulb tester in place, the AC voltage across the big rectifier is 2VAC if thatr is any clue.
 
Any suggestions for hunting the faulty parts? All of the electrolytics show well in range on my ESR meter, no shorts or open. Nothing obviously burnt. Not a lot of parts there!

I'm focused on the schematic on the bottom half of the second page on the linked images.



Tassieviking

I don't think my copy of the schematic is any better on the power amp stage.
Can you take a picture of the PCB so we can see the board and components ?

Disconnect speakers before any testing
The first thing to check with power off is the bridge rectifier with a multimeter set to diode checking.
If the bridge seems ok then check the power transistors for shorts from collector to emitters. (Q110-Q113)
Check the power resistors as well (0R5 5watt).(R123 - R126)
Power on and check if there is any voltage on the + or - 63v rails to ground (0V).
Check for DC voltage on the speaker output
Do this quickly so you don't have the power on too long,turn power off and check if any of the power transistors or power resistors got warm or hot.

Hopefully one of the more experienced guys will give you more/better advice.
Cheers
Mick

There are no stupid questions.
There are only stupid mistakes.

remaker

Thank you, Tassieviking, for the reply!

I have the head removed and on my bench. Speakers and reverb disconnected.

C->E diode tests in situ:
(Q111 and Q113 and 0.590V; Q110 and Q112 are 1.2V and climbing, never reaching a stable place.)

All of the 0.5 ohm resistors check out but the 0.1 ohm resistors as 0.0, though that could just be my cheap meter.

I pulled out the DBA100C rectifier to test. top AC to plus is 0.5, and its shows Open Circuit between one of the AC and minus. Plus to minus is open, minus to plus is 0.9. So it looks like the rectifier has failed, but I'm not sure if that will have killed more downstream components.
 
I suppose I should start with a new rectifier!

remaker

#3
OK, the rectifier was not the problem. I tested it stupidly (in circuit). Once I pulled it out, it was fine. After a long misguided route of testing all of the transistors (all good). I went "back to basics" looking for a dead short.

Since the resistance was ZERO from ground to the negative rail, all the transistors were likely good since they all had the big 0.5 ohm restors on the emitter.

I ultimately came back to the big capacitors which had "tested good" on the ESR meter. And sure enough, one was a dead short! Once pulled, there was no longer a short.

So, I ordered 4x SLP332M063E3P3 CDE brand capacitors which are slightly less diameter but same height, 3000 hours at 105C. Ordered from Digi Key (part number 338-1498-ND)

They work, and the amp lives again.

I also accidentally damaged some small transistors Q102 and Q103 which were the now-discontinued 2SC-2362. Another forum advised that KSC1845FTA (DigiKey KSC1845FTACT-ND)is an adequate substitute. They seem to all be working fine.

I leave this here for future seekers.

Thanks, Tassieviking, for the advice!

saturated

I ask stupid questions
and make stupid mistakes

criticism, critique, derision, flaming, verbal abuse welcome