Echoplex EP-3 Pilot Lamp Problems

[viperpiper]

Hi everyone one, I picked up a half decent Echoplex EP-3 in a very good condition case as well. The only issue, someone removed the pilot life but left the wires. So I measured the lamp voltage, turns out it's 36V, ordered a Marshall Amp Pilot light assembly and an 1822 36V Bayonet bulb. When I plug the bulb into the socket though, I lose the 36 V and it won't light up. If I remove the bulb the 36V can be measured on the new lamp holder I put in. Any suggestions...appreciated..thanks

[Helmholtz]

Seems the lamp supply breaks down because it can't deliver current.

Can you post the schematic?

[The Dude]

My guess is that there is a current limiting resistor in the lamp circuit that has gone towards open. That's just a guess, though. As Helmholtz says, a schematic would help.

[Leo_Gnardo]

I've looked at the schemo and don't see a pilot. I know I need new glasses... Anyway here's a schemo straight from some obscure website called MEF.

[viperpiper]

I've been looking at the schematic as well, and can't seem to find the Pilot lamp. One side of the lamp is soldered to the playback 1/2" jack the other is a long yellow wire that goes all the way to the PCB by the actual motor. I'll start on that board to see if I can find something wrong there.

There are three different schematics for the EP-3 depending on the serial number. Of course mine is missing the serial number. I can post the three versions.

[viperpiper]

http://www.synfo.nl/servicemanuals/Maestro/ECHOPLEX_OWNER-SERVICE_MANUAL.p

[Helmholtz]

There is only one circuit point where you have 36VDC, the first power supply node. That's where a 36V lamp should go, other end connected to ground.
Alternatively you could wire it directly across the secondary winding.

Are you sure a 36V lamp was used originally?

[viperpiper]

I just thought it was 36V when I replaced the missing lamp socket and put my DMM on it without the lamp. I measured 36V DC. This morning I did find a bad resistor, the 680 ohm, was cracked. So I removed it, put in a new resistor. Installed the 36V T3 Lamp and actually got a glow out of it but the voltage now measures just over 6 V when the lamp is in and 36V no lamp in. So may be something else wrong on that one board besides the 680ohm resistor.

[Helmholtz]

I just thought it was 36V when I replaced the missing lamp socket and put my DMM on it without the lamp. I measured 36V DC.

That doesn't tell as much, as there is a series resistor and the lamp isn't operating at full power.

Installed the 36V T3 Lamp and actually got a glow out of it but the voltage now measures just over 6 V when the lamp is in and 36V no lamp in.

That's what you would expect with a 680R series resistor, as that will drop most of the voltage caused by the lamp current.
Try connecting the lamp to the other end of the 680R. That should give you full brightness. The lamp doesn't need a series resistor if the voltage is not above 36V.

BTW, is the motor running?

[viperpiper]

Yep, the motor is running just fine. It's just the pilot lamp problems. When I bought it , it was missing, perhaps this is the reason why.

If someone has an original pilot lamp what voltage T3 lamp are you using. I can't seem to find that information anywhere on the web, what the original lamp would of been.

[Helmholtz]

Yep, the motor is running just fine. It's just the pilot lamp problems. When I bought it , it was missing, perhaps this is the reason why.

I was asking because the voltage at the end of the 680R were the lamp connects is supposed to be only 14V (with lamp out) and connects to a symbol (capital M in a circle) which looks like a motor.
But there is also a motor symbol at the primary. Does the unit have 2 motors or is it a motor with 4 wires?

[Leo_Gnardo]

I was asking because the voltage at the end of the 680R were the lamp connects is supposed to be only 14V (with lamp out) and connects to a symbol (capital M in a circle) which looks like a motor.

Sneaky of them to put a circle M where the pilot lamp is.

Q: what's the wattage of that 680 ohm resistor?

The 680R with lamp in series form a voltage divider. And if the schematic is a clue we should see 14V on the lamp leg of that divider.

I'm looking at a # 756 lamp, rated 80 mA at 14V, with standard bayonet base T3-1/4. But the 680R would have to have to be 1W to sustain that current without risk of failure. Calculated wattage dissipation 0.71W for that R. Helmholtz, what do you think?

[viperpiper]

I think the M is the lamp. There is only one motor and it only has two wires. It is a 1w resistor, so I can always try out a 756 Lamp and see if that solves the issue. I think it may. The orginal 680ohm resistor looked fine, but after taking it out, it had a fine crack in it.

[Helmholtz]

Sneaky of them to put a circle M where the pilot lamp is.

Q: what's the wattage of that 680 ohm resistor?

The 680R with lamp in series form a voltage divider. And if the schematic is a clue we should see 14V on the lamp leg of that divider.

I'm looking at a # 756 lamp, rated 80 mA at 14V, with standard bayonet base T3-1/4. But the 680R would have to have to be 1W to sustain that current without risk of failure. Calculated wattage dissipation 0.71W for that R. Helmholtz, what do you think?

To lower the lamp voltage from 36V to 14V requires a (lamp) current of 32mA with a series R of 680R. So we are looking for a lamp with a wattage of 0.45W at 14V. That could be a lamp having a higher rated voltage than 14V.

Any actual lamp current above 50mA will essentially pull down the lamp voltage to zero.

[viperpiper]

I was assuming that the original lamp was a T3 since I can't seem to find anyone online that can tell me any different. I''ve looked everywhere for a T3 bayonet base lamp that is .45watts with no luck. Perhaps, I'll have to look at another type of lamp assembly. I was trying to keep the jewel lens look that was originally on the echoplex.