1992 Fender Blues Deluxe Hit By Lightning

[JacksonRR]

Well it was plugged in when lighting struck the house and now it's down.
The power section works fine.
All my voltages are checking out.
Plugging into either of the inputs gives the same thing. No volume on clean and severely reduced volume on drive.
None of the pots are dead, nor any components and I've checked them all(I guess haha).

You guys have any clue? It's for a friend who's had this puppy down for 4 years. I'm a little lost here.

As always, the scheme: http://www.blueguitar.org/new/schem/...ues_deluxe.gif

Thanks guys. I promise I won't take any credit when I get it back to him working again.

[Prairie Dawg]

Go back to fundamentals. You've missed something somewhere. Your problems are in a small piece of real estate. Pull the circuit board and go over it with a magnifying lamp.

Once you've gone through and validated each system within the amp then you're left with very little. I would be looking for arcing damage.

Lightning strikes can do some really unusual stuff. I worked in the aircraft industry and dealt with them on a not infrequent basis.

[Enzo]

+1 what PrairieDOg said


What does "all your voltages check out" mean? Have you gone down the row of tubes checking each pin? If the powr amp part works, then great, the trouble is the preamp. I assume you plugged a signal into the power amp in jack? With signal plugged in out front at the regular input, is anything coming out the preamp out jack? If preamp out works, the you might just have a bad power amp jack, or a burnt out trace under it. Always check the copper and solder.

The first two tubes are your preamp. Check pins 1 and 6 on each of them. Got something like 150-200VDC there? Well, pins 1,6 of V1, but only pin 1 of V2, the pin 6 side is not used. Just because a tube is OK doesn;t mean it is functioning. Of you get 300-350v there, the tube is not working.

Trace signal. APply a loud steady signal to the input. Sine waves are fine if you have them,but I usually just plug in a line level feed from my shop stereo. All we want to know is where the signal dies, so music is fine as a test signal. Now follow it through the schematic with a scope or AC voltmeter. Is the signal present at those 12AX7 pins? Follow the signal path and check the plates - pins 1 and 6 again - on the two preamp tubes. Present at all three places?

Turn the reverb up some, and rock the amp back and forth to crash the reverb springs. Does that come out the speaker loud?

You indicate you can change channels, so at least some of the ICs have power. But verify all ICs get power, both +15 and -15VDC. Just because the power supplies work, doesn;t mean all the parts that use that power are getting it.

And the one that gets my vote: Q1. Look at the drawing. See Q1 left of the power amp jack? One side is grounded, see if the other side of it shows shorted to ground. If so, then either Q1 is bad, or it is not being turned off at its gate. SO if it shows shorted, look at the gate pin and see if ther is a good voltage there. If no voltage, then Q1 is ON and shorting out the signal. NO, I have no idea which leg is which on Q1. But look at CR2 next to it. Measure resistance to ground from the Cathode of CR2. (The line end) Yes, with the amp running, measure resistance to ground from that point. Well, check for stray voltage there first, if you find 10v or somethiing, then skip resistance, the part is bad. The amp will work without Q1, so if in doubt, remove it and find out. You do want to install a good one, either the old one or a new one, before you set the amp free.

[Prairie Dawg]

That's high praise indeed, Enzo. When I worked for Kal Aero in K'zoo the shop had a Merlin that had taken a lightning strike on one prop. A blob of metal hit the windshield and trashed it but the real fun was finding where the lightning went in and where it came out-every high-ish resistance place was arced including the front and rear main bearings in the number two engine, several places in the airframe and near the static discharger in the tail. And everything ferrous was magnetized. What to do with a permanently attached steel engine mount that was magnetized? I called a place that did industrial magnafluxing and asked them to come out and bring only their demag coil. Wrapped it around the engine mount and fixed it right there. There's no repair manual for stuff like that either-you gotta write your own as you go along.

[JacksonRR]

Thanks guys I did indeed miss something. All of my voltages going to the valves were in order, but the -16 side of the relay feed was down before R86(no voltage present either side) and that resistor had some slight browning underneath. Yes, I used that wonderful search function. Still can't take any credit. I'll be doing a more thorough evaluation after work today, go to Fry's and get whatever and let you guys know how it goes from there. I'm sure this can't be all that's wrong.

[JacksonRR]

Ok boys. Been on a real stretch with work lately. Got back on it tonight. Replaced C38 and CR20 I now have correct voltage on either side of that ceramic resistor... R86 I believe.

Still not working.

So about that Q1 situation... No voltage on any of those pins or either side of CR2 and CR3. I did try measuring the resistance from CR2 to ground with it on and there's no continuity.

All of my ICs are getting their voltages and I tripped the channel switching by being sloppy and sticking the probe on two of the pins.


What do you guys think now?

[Enzo]

OK, now, that means we have verified we have both +15 and -15?

Q1 is a mute. If it is stuck on, it will kill the signal. ALl the rest of the circuit, unless the loop is used, is all tube. SO at this point I;d probaly remove Q1 and unsolder and lift CR2, just to leave no question as to their involvement. If the amp then works, we know where the problem is. If not, then we need to isolate the problem.

Since the powr amp works start going through the preamp. My first step is generally get out the voltmenter, and go down the row of preamp tubes and check pins 1,3,6,8 on each for voltage. Plate pins 1 and 6 ought to have some B+ on them, maybe a couple hundred volts. The cathodes, pins 3,8 ought to have a volt or two on them. Well except the phase splittr, but it apparently works. All this wil tell us if any tubes are not conducting.

There is only one channel through this amp, the drive simple switches some stuff to increase the gain. You solve th problem with one "channel," and I bet the other channel will fall into place.

APply a steady strong signal to the amp input, and trace it stage by stage. Got signqal on a grid? then hopw about at the corresponding plate, then on to the next stage grid, then its plate, etc.

[JacksonRR]

Sorry for the slow update, I've been busy with more work than I care to talk about. It ended up being Q1 in the end. All is well. Thanks boys and thanks Enzo for having the solution all along.