weird old morley problem

[pinkjimiphoton]

so i got an old power wah boost, right? if i take the bottom off, it works great if there's ambient light. EVERYthing works as it should.

except the 387 lamp. i figured it was blown, ordered some others. they came in yesterday.

for all intents, there's really nothing in the circuit before the lamp... it has a 220r resistor off the junction of two caps;
i get 38v as i expect at the anode of the lamp.

but as soon as i plug the lamp in and slide the connection onto the anode of the lamp, it ain't lighting up as expected. i mean, ya gotta tap, two caps, a resistor and a lamp, what the hell could go wrong? any ideas?

or should i just bite the bullet and replace the dang thing with an led and be done with it?



voltage on one of the caps seems excessive... reading about 70v or so where the cap is shown as 63 v, but only a 50v cap in there... all are original.

i'm just stumped, how can a lamp not work if it has the voltage it needs? i tried bypassing the socket, same deal.

am i missing something?

[Mike Burgundy]

Just cause I didn't see you state it - you did measure the lamps, right? What if the tungsten filament didn't survive the journey?

[GibsonGM]

Just cause I didn't see you state it - you did measure the lamps, right? What if the tungsten filament didn't survive the journey?

Or, there is another issue, indicated by the 'over-voltage' thing at that cap, which blew the lamp?  *shrug*

[Gus]

Jimi

I suspect one or both caps after the diodes

You are getting most of the voltage across one cap 47uf and low to none across the other 500uf when under load(the lamp plugging in) if I am understanding you post

I think something is wrong with the 500uf cap and possibly the 47uf cap.  That power supply is a voltage doubler. 
Sometimes you will see this type supply with two of the same value caps and with voltage divider resistors across the caps to balance the voltage, this one is set up a little different  (Search for voltage doubler power supply)
68 VDC is close to the full voltage across the two caps with a voltage doubler circuit like that when not under load(the lamp will be most of the load) and that is why I guess the voltages of the two caps are 25VDC and 35VDC. I believe it was designed for the transformer secondary winding resistance and load current.

I would check replace the two caps you can use a 470uf for the 500uf.  I would also check the diodes with a DMM set to diode test

[pinkjimiphoton]

thanks brothers,
i will check all these out. i DID check the lamps, they're reading ok so i'm thinking the same thing as gus,
that them filters have gotta go.
where the expected B+ should be around 68v as noted, it's reading almost 80. these ARE the original caps, so i will
hit the local jobber this morning and grab a set of new electros and repop the board with the correct values (thanks for the tip on the 470 v 500u cap, gus, i know that info well at this point, but always good advice for the newer folks to see)

the biggest bitch is my dehumidifier broke down, so my "dungeon" is a death sentence after about 10 minutes... it "feels" cool, but then the sweating starts...

will report back later... thanks guys!

[Gus]

I looked up the 387 bulb, 28V, 40ma
.04Amp x.04Amp x 220ohms = .352watts
what is the wattage of the 220 ohm resistor and did you check the 220 ohm resistor?

[wavley]

I looked up the 387 bulb, 28V, 40ma
.04Amp x.04Amp x 220ohms = .352watts
what is the wattage of the 220 ohm resistor and did you check the 220 ohm resistor?

I'm gonna agree with Gus on this one.  My Morley PFL runs the lamp the same way, I opened it up one day to tweak this or that and found that resistor completely burned up, funny thing is that my lamp still worked and the resistor still measured fine, but obviously I caught it right in time before it burned open.  That was honestly the worst looking resistor I've ever seen that still somehow functioned.

[pinkjimiphoton]

i just replaced all the electros and the diodes in the power supply, but gotta gig i gotta leave for so i will have to wait till tomorrow. the 220r resistor has def been tampered with, and LOOKS burned.... i'm getting 30 some volts there, but resistor is the only thing left. also, the voltage divider 1k/10k has been changed to 10k on both sides. i gotta look at it more tomorrow with a fresh set of hungover eyes.
thanks guys..
stay tuned

[pinkjimiphoton]

sorry, finally a nite off that i'm not sound asleep already.
been busy with guitars and gigs and life and shit. it's ridiculous sometimes. blessed be this hellride.

anywho, i finally got in here and looked around. the morley peeps were helpful, and suggested some places to look and supplied dcr for the bulbs etc.

i broke my dang bifocals a couple weeks ago and haven't been able to afford fixing them yet... bills come first, you know the drill so i've been putting this off.
i looked at the circuit, replaced all the power supply electros and the diodes etc. the 220r restistor was "modded" by someone to...68k? 680k? no idea, it was crispy to the touch, and .680m which whatever it was is way off... it LOOKED like it was marked orange red yellow.... kinda... very fried, and wayyyyyyyy off from where it should have been. my guess (along with the on/off switch being wired backwards, so "off" is "on") is some heroic individual decided maybe it was "too loud" and tried to brown out the pedal by decreasing the amount of light?

one of them headscratchers. the world may never know.

but the good news is it fired right up, and sounds bloody excellent for a 40 year old-ish pedal. i'm gonna replace the rest of the electros in the circuit tomorrow with some fresher eyes just because. doesn't appear to need this, but i like to avert disaster and don't believe in ancient cap mojo.

i appreciate the help and advice guys, as always, YOU ROCK.
pjp

peace out