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Just because you can't run with tubes in it. I'm pretty sure the issue is not in the amplifier section. By connecting a remote amp to the tape loop you should be able to get sound through all other inputs and be able to tell which input chain has a short, because it probably will be silent or...

A couple of thoughts to isolate your problem: 1. What is your Selector switch position for all the above tests? If it is not in AUX, switch to that and try getting your measurements (without power tubes) again. If voltages are normal, your short is most likely in the tuner section. Post any...

If you only worked in the area of the PEC installation, it's possible that some small tube in that area got plate grounded or shorted to another component due to a bent leg. That will cause a short, but not immediate short for Power Transformer. After all tubes come on board and start...

Something is taxing your Power Transformer. EFB "seems" to be working...With output tubes out your plates are only 387v at 115vac input? That is too low. Should be around 430v loaded, so without output tubes, you should see close to 450v or so. Unplug the power cord and discharge the main PS...

My mistake, it should be around -27v at EFB output (H1) or IBAM input. We really need to know what voltage is registered at the junction of D3 and D4 anodes on EFB board but it may not be easy to access. It should register in -50v or higher.

Also, it would be good to know what is the negative voltage at the Input of your IBAM (EFB Grid supply Output), it should be something close to -21v.

Early original Fortes with round terminal cups ware "marked" as 4ohm. The later version of Forte 1 with a square terminal cup were "marked" 8ohm. I don't know if there was any difference between those versions electronically though. I have 2 sets of early version and I always ran them on 4ohm...

All pin voltages, of course

Pin 1 should be your raw Screen supply voltage out of the EFB board and it should have the highest reading, compared to P4 and P8, because those two are tied together for screen cooling and get their supply through 100ohm screen stability resistor. Those pin voltages don't line up to that...

That is true but I think the opposite happens. Tubes draw too much, overload the PT capacity, voltages drop, and fuse pop. I don't know how well the bias supply section on the EFB board tracks the screen voltage in the event that the screen supply equals the B+ (which will be a few volts higher...

This clay-colored R10 resistor that you replaced on the EFB board is not installed correctly. The way it is installed, it could easily be your issue. It provides the voltage drop (and current limit) for the Mosfet. If its legs touched, Mosfet will over-dissipate and fail. Or, the Screen supply...

3 pages of the thread and we still don't know if grids actually have negative voltage at all times. It will be impossible to even suggest speculation about where the issue is until all output tube pin voltages are posted. With all output tubes out, power up on variac to about 110vac and let it...

The edge of the PCB can have an exposed foil, which is most likely a ground plane around the entire board. Make sure it doesn't touch any nearby components. Other than that, you probably moved something, and possibly some component, a resistor most likely, got compromised on the first power-up...

OP has an early serial model. Were there instances when those came equipped with other than 6.6k OTs? I have 2 late model 400s and both have "AX" version transformers. In fact, one of the OT doesn't have any markings on the bells at all. I ran AC through it and it came out nearly identical to...

Corrected my post above. Late model 400 used 10k outputs, not early model.

My understanding is, that Dave's EFB will help any PP amp stage by controlling ratios between components under load. EFB components would require tweaking to suit each particular application. In late model 400, Fisher used 10Kohm Output Transformers that did not match the output tube family...

Can you link to that listing?

At 17 I was an electrician apprentice and was sent to remove the disconnected arial 3-phase cable between buildings. I was on the roof of one of those buildings at the roof ledge, holding the cable in one hand, snapping the steel support cable with another... Somehow I kept my balance and was...

The following suggestion is obvious for most people here but may help beginners to avoid disappointing mistakes. Drilling large holes in sheet metal, with step drill bits can be difficult. Bit binds so using a wood block below the metal helps reduce binding. Trick is to make sure that the next...

Sounds like a winding problem, unless the regulator source voltage doesn't drop too. Can you build this load externally and power it from the regulator source with the original circuit disconnected?

Do you know what year that took place?

Regardless of the original problem, capacitor in question has a positive leg bent at the angle where it could touch the capacitor body. It is insulated but good practice is to bend the negative leg and leave positive away from the capacitor body. In OP’s case it’s the opposite. Maybe not what’s...

I have not tried to adjust anything on this unit. The fact that it worked perfectly fine the last time it was used makes me think that even if arm adjustment will make it work, that is a band-aid and there's a problem. On the other hand, if adjustment makes it work, I'd know that it's likely a...

Color bands appear to be different on cathode resistors between channels. Any info on why?

That's not easy for me to tell. There are probably some similarities but I doubt it would be an identical circuit. Was your stop button inactive too? Was your lift working if the lift button was pushed?