SOT: SS Amp Repair

[elberto]

So I'm attempting to fix a friend's Fender Deluxe 90, a bigger SS amp.  The problem:  when the volume hits a certain level (turning up the knob while playing), the amp cuts out completely and the power LED flickers a bit, then there's just a low oscillating hum with no guitar signal getting through.  Turning the amp off then on returns the amp to its original state.

Before I tear into this thing (you really have to dismantle the entire amp to get to the solder side of the pcb, and there's a zillion components), any ideas of what to test first?  I'm tempted to think power supply (caps?) or poweramp ICs first, because of the LED flicker.  If this were so, how would I test for a bad IC?  Thanks guys.

[ErikMiller]

It uses IC's? In something bigger than a practice amp? Are ye sure?

I'd give it a thorough chopsticking (poke at each solder joint with a nonconductive probe) first.

You might get some help at www.ampage.org.

I know a pretty decent pro amp tech near you who's not afraid of solid state. :-)

[cjlectronics]

Those amps are notorious for cold solder connections...especially around the pots and jacks.  Just like ErikMiller stated, poke around with a soldering iron and I'm sure you will fix it.

CJ

[elberto]

Thanks for the tip guys, I'll give it a good chopsticking later tonight.  Given the nature of the problem though, am I wrong to be pessimistic that it's a cold solder joint?

[The Tone God]

It could be a power transistor dying. Maybe the power transistor is overheating. Not a very good physical connection to sink enough heat. Maybe the thermal shutoff is kicking in.

If the above advice does not work time to get tracing. Your gonna need the schematic.

Get the schematic here:

http://www.fender.com/support/amp_schematics/

Andrew

[Peter Snowberg]

I'm with Andrew in thinking the problem might be around the thermal shutdown circuitry. I'm assuming that RTC2 is mounted on the output transistor heat sink. Very interesting driver topology in there.

Can you brand yourself with the output heatsink?  :shock:

Try using an audio proble or a scope on either side of R65 (15K). If you don't get variation there when the amp starts failing, you have a good preamp and the problem is somewhere in the phase splitter/driver/output sections.

The driver and outputs are actually two parallel sections (one for push and one for pull) so most points of failure will at worst kill half the signal and not all of it. That points to the phase splitter, which in this case is a combo of U5B, U6A, Q8, Q9, & Q10 along with the thermial protection and mute circuit that feeds Q8.

Watch the voltage on the base of Q8 when the amp fails, that will address if it's from thermal shutdown or not.

Watch the signal on U5 pin 7 and U6 pin 1. What happens during cutout?

Oh yeah... when you turn the knob up what happens? if you dial in 11 and play softly does it work then die when you hit big chords? If you turn the dial down as it's cutting out, does it work fine at lower volumes? Does this happen seconds after powering it up cold or after playing for 20 minutes?

[elberto]

all right, tapping on a transistor has recreated the problem.  It's an MC7089CT voltage regulator.  is there a good method of more definitively testing this guy?

[elberto]

also i have to mention that this Deluxe 90 does not match the schematic that fender provides--the amp I'm working on may be a newer model (it has DSP FX and a different layout).

[elberto]

the output heatsink is the chassis -- not that hot.  The voltage regulator heatsink -- pretty darn hot.

The problem is more like a complete, dramatic, cut in signal.  And once it's gone, you can't get it back, no matter what you do with the knobs, unplug/replug the guitar, etc.  there's just the low, oscillating hum.  

This probably related to thermal shutdown; the problem seemed to happen much more quickly in my sweatbox of a practice space.

[Sir H C]

Most voltage regulators have built in thermal shutdowns.  So if it gets too hot, boom no sound.  Try cleaning the metal tab on the regulator and get some good thermal grease and reconnect it to the heat sink with that.