Changing Pilot Lamp kills Fender Deluxe Reverb

[nevetslab]

Returning to repairs on the Rental Gear inventory, a Fender Deluxe Reverb came over with a gaffer tape explanation that changing the pilot light killed the amp. I moved it next to the bench, plugged it into the variac/power analyzer, and brought up the AC mains, watching for fault current. Came up normal, though no Pilot Light lit. Removed the upper rear panel, saw the 5AR4 rectifier tube was lit, amp drawing around 300mA from the Mains. Switching out of S/B didn't change anything, so I removed the amp chassis to the lab jacks on the test bench to have a closer look. I saw the pilot lamp didn't make it into the holder, but succeeded in shorting it to chassis.



I removed the bulb and straightened the frame of the lamp holder. Then, after checking the bulb to see it was ok, put it back into place, and tried again. I did find plate voltage was present on the output of the rectifier tube, but found no heater glow no the tubes, nor pilot light. I removed the heater wires from the PCB, and checked for AC voltage. I got 7VAC unloaded. Plugged it back in, finding no voltage on the heater buss that passes from the power tubes on down to the rest of the tubes.



My guess was since there's no fuse in line with the heater wiring, the PCB may have substituted a trace for one. I found continuity on only one of the two traces from the heater terminals to the power tube heater terminals, those being the closest from the PCB edge fed by the quick-disconnect terminals. I did find heater voltage at those terminals when I plugged them back in. I haven't yet pulled the PCB's and wiring free from the PCB to verify my suspicions.

Nice to have a built-in 'depth charge' potential on an amp. What's missing is a small piece of adhesive-backed insulation (fish paper) below the pilot light holder! I never stopped to consider how easy this is, though after all these years, this is the first time I've encountered it.

[nevetslab]

OK, let me wipe the egg off of my face. I hadn't pulled up the schematic of the '65 reissue Deluxe Reverb, but, it didn't show any PCB mounted fuse holder. I did see the fuse holder there when it was all still together, but measuring across the fuse, it appeared ok. DID I REMOVE THE FUSE AND CHECK IT OUT OF CIRCUIT? NO! Looked at the rating on the fuse...T5A, same as is on the silk screen image of the PCB. Checked the wiring in the PCB layout, and sure enough, that fuse IS in series with the heater circuits, including the pilot lamp, to save the heater winding in the power xfmr.



Now that I have the PCB's removed/lifted from the chassis, finding the fuse holder IS there to save burnt foil. So, I guess Fender DID have enough failures of PCB traces when folks go to replace the pilot light, with the lamp holder assembly bending as it can, and sometimes do just what this amp did. I guess I should be annoyed by my NOT removing the fuse and checking that the fuse terminals were in series with the heater winding terminals. I've made the mistake before on a different amp, finding the xfmr winding will show up in the measurement looking across the fuse if left in circuit.

So, now to put it all back together and see all is back to normal.

[Leo_Gnardo]

From time to time I've seen a sleeve of what appears to be "cambric" insulation covering the bayonet base of such lamp holders. When that area looks dicey to me, I've fitted a similar short cylinder of - unshrunk - heat shrink tubing to prevent any fumble fingered bulb replacers from gooching up the works. Having witnesses the results of this fix again years later I was surprised the HST hadn't shrunk on its own. Not enough heat from the bulb to do that. No biggie, it's still insulating just fine and doesn't become a hindrance to replacing the lamp in the future.

Another variation on this problem is when the fixture base loosens up, then "windmills" when a new bulb is fitted, causing a short from a 6.3V energized tab on the back of the fixture to chassis. Ran across one of these just a couple days ago in an older Fender. Luckily the amp's AC fuse popped and no further damage. In that case the whole fixture was bent in such a way that it couldn't easily be mended so I swapped in a new one. Lucky I had a couple spares in stock. IIRC on the reissues Fender does put a sleeve of insulation over the solder tabs on the fixture, score a point for them for foresight that's rare to see in recent times.

[nevetslab]

From time to time I've seen a sleeve of what appears to be "cambric" insulation covering the bayonet base of such lamp holders. When that area looks dicey to me, I've fitted a similar short cylinder of - unshrunk - heat shrink tubing to prevent any fumble fingered bulb replacers from gooching up the works. Having witnesses the results of this fix again years later I was surprised the HST hadn't shrunk on its own. Not enough heat from the bulb to do that. No biggie, it's still insulating just fine and doesn't become a hindrance to replacing the lamp in the future.

Another variation on this problem is when the fixture base loosens up, then "windmills" when a new bulb is fitted, causing a short from a 6.3V energized tab on the back of the fixture to chassis. Ran across one of these just a couple days ago in an older Fender. Luckily the amp's AC fuse popped and no further damage. In that case the whole fixture was bent in such a way that it couldn't easily be mended so I swapped in a new one. Lucky I had a couple spares in stock. IIRC on the reissues Fender does put a sleeve of insulation over the solder tabs on the fixture, score a point for them for foresight that's rare to see in recent times.

Has anyone ever come across the correct size open end wrench to tighten up that large Hex Nut in the assembly...and of course hindered by the flange at the top of the panel?

[The Dude]

I've always just used a big needle nose pliers.

[Leo_Gnardo]

Has anyone ever come across the correct size open end wrench to tighten up that large Hex Nut in the assembly...and of course hindered by the flange at the top of the panel?

Not here. I tighten it as much as possible by hand. Then use a technique I learned from John Muir's "How to Keep Your Volkswagen Alive: A Manual of Step by Step Procedures for the Compleat Idiot" a very popular publication in the 1970's. There's a 36mm gland nut in the old VW engine, sometimes you have to remove & replace. John's solution to turn big nuts that you have no wrench for - grab your "utility" screwdriver - a slot screwdriver you can bang on without fear of damaging it - and a hammer. Spin the nut clockwise onto the threads on the bezel piece until finger-tight. Then start tapping a corner of that pesky nut, holding the tip of the screwdriver on a nearby corner of the hex nut, tap with hammer, it will turn. Keep doing that until the nut is secure. For a little extra security you can dab on a dot of fingernail polish or if you dare, thick cyanoacrylate "crazy" glue to keep it in place once you're sure it's tight. Of course, tap that nut in the opposite direction to loosen. You knew that...

Even if I had a proper size wrench I doubt I could fit it in that tight corner. I wonder how Fender installs them?

[Enzo]

Banging on a screwdriver is how electricians tighten those funky nuts on electrical boxes. I used that way on such fuse holders.

[The Dude]

Well, you can fix anything with the right hammer.