EFP3 problem

[tsingle999]

I swapped caps a few months ago and started having intermittent channel outages. I finally noticed the output side of the cap on one channel had become loose. Its in fine now but the leds on that board wont light up nor the led on the center board on the same side as the bad channel and towards the front of the fp3. I swapped tubes to no avail. This pre had been running flawlessly for years. Start with voltages?

 

[Paul Birkeland]

Yup, voltages will likely point to the issue. 

 

[tsingle999]

26 29 & 30 the voltage climbs. Its supposed to be zero
The first time i turned on the unit ony two lights on the middle board lit up. Second time i turned it on everything lit up. All voltages were on except for the above. Bad solder joint somewhere?

 

[Paul Birkeland]

26 29 & 30 the voltage climbs. Its supposed to be zero

How high does it climb?  These are the inputs, and should be really close to 0V.

 

[tsingle999]

Well it starts around 50 mv n climbs a millivolt every two seconds. It went up to 220 mv. Theres definitely something wrong with power up though. Sometimes it works sometimes it doesnt. Switch?

 

[Paul Birkeland]

Eh, there really has to be something else that's wrong.

You'll want to measure the "I" and "O" voltages on the outer PC boards, as well as the "I", "O", and "K" pads no the center PC board.

 

[Paul Joppa]

Also check your power line voltage.  Do you have the smaller MJE350 or the larger MJE5731A on the center board?

 

[tsingle999]

Power line 120.8
I think its the 350 transistors. All 6 look the same. The packing list was checked off 4 350 and 2 340.
Center board
Ob 148.8
Ib 212.3
Kb 4.18
Ka 3.18
Oa 150
Ia 213.6

Right board
Ib 0
Ob 80.9
Ia 148.9
Oa 80

Left board
Ia 150
Oa76.5
Ib 0
Ob 78

 

[Paul Joppa]

The voltages look good - I assume all the LEDs light properly when you did the measurements.

This says to me that there is nothing wrong when it's working, it's just that the failure is intermittent. It seems to be on the center (regulator) board; when the regulator fails no voltage is abailable for the "bad" channel.

Since both LEDs on one side of the regulator board go out, it may be the supply (I) or ground (G) connection - sometimes solder joints do go bad after a few years. You might try poking with a chopstick while watching the LEDs. If only one of them went out I'd have a different guess, but would probably suggest the same test  :^)

 

[tsingle999]

Thanks guys! I chopsticked a lot but couldnt find anything. But it has been working for two days. Hope it holds.

 

[tsingle999]

So it failed again and upon closer inspection and chopsticking i think the problem is the center board. I got it to work again by chopsticking but then touched up a bunch of solder joints on the board. So far so good! Thanks again for the troubkeshoiting help guys!