EFB installation trouble

streetwise

Active Member
Today I attempted to install an efb board following as closely as possible to the "modifiy the fisher 400 with efb" forum. I followed the schematics as close as I could also looking at pictures as well as I could to make sure things were in the right spot anyways, I fired up the receiver and took a voltage test, everything was out of control, smoke started coming out of one of the can caps? anything noticable from my pictures where do I start to track this down?

wires from efb are
color "EFB board location" - connection location
green and black tape - ac winding
green - ac winding
black with green pair - ground

red and black tape from 22uf and stf10 - output screen grids
red from 4.7ufcap - 330k grid resistors
black with red pair from 1.8k resistor - CR2 bias supply
 

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If it is a fixed bias amp and you have a capacitor smoking/blowing, then my bet would be something connected backward in the bias supply, most often the capacitor(s). Bear in mind the bias supply is typ. negative so capacitor (+) goes to ground while capacitor (-) connects to the bias supply voltage.
 
Are those new caps, or are you working with originals? If smoke is coming out of originals, its entirely possible that they have simply expired. If they are repops or re-stuffed, maybe the caps were installed backwards inside the can.
 
caps are restuffed but about a year ago. this Is a development since I attemepted to install Daves EFB board, has anyone installed daves EFB board? how does the wiring look. Did i forget to disconnect anything?
 
I have checked over my wiring on the EFB board and I don't see any issues polarities are correct transistor wiring looks good checked the 10k trimmer pot value changes and isn't stuck. My only guess is that I don't have something disconnected. checking voltage at the tubes things were going up to 1volt plus on startup.
 
Street -- I understand that everything went haywire, but can you be more specific as to what exactly happened? Which can cap started to smoke? Did the fuse blow? Do any components look damaged? Was the EFB(tm) circuit the only modification made since the unit last worked? As a basic point, it would be very hard indeed for the EFB circuit to cause a can cap to smoke. More likely, there is a short somewhere in the wiring that is causing a resistor associated with the can caps to let out its own magic smoke. If possible, can you identify which resistor that might be? Any and all details will help!

Dave
 
When I started the unit 15 to 30 seconds later the burning smell and a smoke seeming to come from the cap R38-40 on it C52 im pretty certain. As far as I can tell I don't see any damaged components, no smoke damage charring whatsoever. I don't have any blown fuse It seems like nothing is effected. Attempted to retry the unit and tested voltages around the caps they seemed somewhat normal but I shut it off once I start to smell a burnt smell.
 
Update I think I spotted the actual source not sure how this happened? I opened the FM section and spotted C7 the 24 cap has exploded let loose some gooey crap not sure if this was due to the transistor mounting screw not properly holding the section below. I will be going to radio shack later hopefully finding a replacement ceramic and then retesting the unit for any other issues.
 
Good luck with Ratshack. The ones around here carry about 1/10 of what they carried 6 months to a year ago. And their ceramic selection was poor at best.
 
Absolutely Larry looks like another order for mouser coming up. Is there a way to easily disconnect the fm section for now to test the rest of the unit before I get the replacement cap?
 
Pull the RF/IF tubes? If you can trace the B+ for the tuner to the caps, you may be able to disconnect it IF it doesn't affect the preamp/amp.
 
Wow! That is a strange one. If you used the same mounting screw I did to mount the Mosfet, it doesn't even penetrate the FM Front End box, so it is doubtful that the EFB installation had anything to do with it -- unless there's more to this story.

I would make SURE that that cap actually exploded. That cap provides the feedback path so that the local oscillator can in fact oscillate. As a result, the alignment of the front end is VERY sensitive to any movement of that cap once the alignment has been performed. As a result, Fisher -- during production -- glued this cap to the frame of the front end box, so that it could not move after installation. That way, it guaranteed that the alignment would remain stable. If in fact you DO need to replace that cap, you will need to get one with the exact same temp coefficient as the original one displays, or frequency drift as the unit warms up could be significant. My bet is that the cap is just fine, and the goo you saw is really just the glue holding it in place.

On the other hand, if you do need to kill the power to the FM tuner section, removing the lead from C57A will do it.

Dave
 
Also, just as a reference to help you, R38-R40 have nothing to do with the EFB(tm) circuits, but if you installed the new phase inverter circuit for the power amplifier section, that WOULD affect those resistors, as the power for the phase inverter tube is drawn through at least R40, and R39 as well with the modified phase inverter design.

Dave
 
Just to close the thread I figured out the issue. I had the TO-92 transistors hooked up wrong on two legs. I was looking at the wrong schematic for the layout of those transistors. Dave you were right just hard crusty glue on the ceramic cap no need for replacement. The smoke I saw had to have channeled its way through the receiver from the EFB board. The caps on the first board attempt stink like burnt electronics. The EFB and power supply are well worth the time as far as im concerned. Much more openness in presentation and tighter bass are the main changes in the sound and cooler tubes!
 
Glad you got it going! Lead designations can be tough on SS devices if you don't clearly know if you're looking at a top view or bottom view of the lead configuration.

It's great that you stuck with it to figure it out!!

Dave
 
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