An Attempt At Fisher 400C Transformation...

[dcgillespie]

The plate voltage are high because there are problems in the heater circuit. With the heaters not operating correctly, then the tubes won't conduct any current, and the plate voltages will all be very high.

C50 A/B are the DC Heater Supply filter caps. With the modifications correctly installed, the entire negative side of the DC Heater system is no longer directly grounded, but connected to a new (~) 65 vdc voltage (bias) source. Therefore, the voltage across these caps will normally read (about) 93 vdc and 89 vdc. Of course, since the B+ voltage is high due to no current draw, the bias voltage will appear high as well -- but likely not 100V higher than it should as your first readings show.

You need to find out why the heaters are not lighting. That's problem #1. Once it's solved, then move on to making sure that the negative side of the complete DC Heater Supply is elevated above ground by 60-65 vdc, as provided by the modification to protect the heater/cathode insulation in the 12AT7 tubes.

I hope this helps!

Dave

 

[Tysen]

Thanks Dave. That helps!

I think my mistake is that I've kept the heater return lead from Pin #5 on V2 connected to the chassis ground. This should be lifted to the negative lifted lead on C50, correct?

 

[Tysen]

Thanks Dave. That helps!

I think my mistake is that I've kept the heater return lead from Pin #5 on V2 connected to the chassis ground. This should be lifted to the negative lifted lead on C50, correct?

I believe I answered my own question.

 

[Tysen]

Okay, well, that solves my heater issue—attached the lead from Pin#5 of V2 to the negative lifted lead.


All voltages on V2, V3, & V7 appear to be appropriate. V6 & V5 are showing some weird stuff.

Updated voltages (FYI I reversed the voltages on C9,C18 with C32,C43):

 

[dcgillespie]

The voltages on V5 and V6 are likely high because of no ground connection. Lifting the ground connections for channel B are just the first part of the operation. The second part is then connecting the corresponding ground point of channel B to those of Channel A, so that the electrical ground for Channel B is still maintained, but now via the Channel A grounds. It sounds like the second part of that modification was not performed, which would leave all the plate voltages in that channel high. Make sure you installed the "runners" to connect the Channel B grounds to the Channel A grounds. This allows the two channels to operate without any chassis ground loops.

Dave

 

[Tysen]

Sorry, I'm a little confused now...I sincerely appreciate your patience, Dave.

Just to be clear of what I've done, I lifted all terminal strips on Channel A, reattached using nylon hardware, and then installed runners from Channel A grounds over to Channel B's corresponding grounds. Is that what your referring to?

 

[Tysen]

The only grounding difference I can see between Channels A (which appears to be operating correctly) and B is that there is a 470k resistor connecting one side of my now shorted C49 cap to ground.



I've been staring at this thing for the last hour trying to see what I screwed up...I wish I understood more of this stuff.

 

[Tysen]

I just plugged the pre into my test amp and it's confirmed that Channel A does work—it produces music! Channel B produced nothing. Dead silent, as expected.

I have the two heater leads connected to the lifted leg of the DC Power supply. Because channel A is working, I'm led to believe that my ground runner leads over to Channel B are installed correctly. Perhaps my heater lead from the phono section is properly connected to the lifted leg?

 

[dcgillespie]

If you will, give me some close up shots that cover the entire underside and I'll see if we can sort it out. And just to be clear, when you reattached the T-strips with nylon hardware, you also used an insulating washer, so that the mounting tag for the T-strip does not actually touch the chassis as the mount point, correct?

As for the heaters, there are three sets of two tube heaters wired in series. Therefore, there will be a lead from the negative side of the DC Heater Supply to each of the three two-tube series sets. Now, there may be a lead coming from the negative side that branches off to two of the two-tube series sets, or there may be three individual leads. But one way or another, the negative side of the DC Heater Supply needs to go to each of the two-tube series sets. Same thing for the positive side as well.

Dave

 

[Tysen]

@dcgillespie , thank you.

Yep, nylon washer are underneath all the T-strip mounting tags.

FYI, the two heater wires I have going to the negative lead of the DC supply, is the lead off of pin#5 on V2 and the heater lead coming from the phono section.

Attached are some of the shots I took tonight.


Best,
Tysen

 

[Tysen]

Is it possible there is some weird heater ground that I'm missing in the phono module?

I'm unfortunately in a position where I don't have the proper knowledge to proceed without some help from this great forum

 

[Tysen]

One more thought—I've been testing the pre without V1 & V4 (phono tubes) in place. Because the negative heater lead from V2 & V5 is connected to V1, and then connected to the lifted ground on the DC supply, do the tubes need to be in place? That's obviously a beginner's question and hopefully it makes sense. I'm away from my home so I'm not in a position to test this theory, so I figured I'd just ask my question.

 

[larryderouin]

V1 and V4 are 2 of the 6 tubes used in the parallel/series heater circuit, so they have to be installed for testing and operation. Otherwise the voltages will be way off.

 

[dcgillespie]

All tubes absolutely need to be in place -- otherwise all the tubes won't light -- and not just the ones not installed. In this case, with the phono tubes removed, the right channel will be dead to all signals, because the right channel line amp tubes won't be lit either, and will therefore show very high plate voltages (Pin #1 & #6).

Dave

 

[Tysen]

Thanks Larry & Dave!

Preamp appears to functioning well now that I've got the 12ax7s in place!! I even listened to some music yesterday with my test amp and minimus 7s!

Here are my voltage readings from today:


New problem with the light bulbs:
I've lost three of them now. They light up and then blow out. Should I be looking for a ground on the light panel? Or did I purchase several duds?
The pilot light appears to be isolated from the chassis by way of a rubber grommet. Am I seeing this correctly?

 

[larryderouin]

Check the voltage at the center terminal to the lamps. Should be 6.3v or so. I can't figure out how they get the power to the lamps, as the only lamp shown is I (eye) 1 in the 6.3v tap off the transformer. But the question is what the H is I1. It's not a pilot lamp. And there are no other lamps shown on the schematic that i can find. But usually they are 6.3v AC with one end going to the side lamps on a receiver then grounding out on the front facepanel. Probably best thing to do is to trace them back thru the selector switch to the transformer on one side then trace out the other end.

I'm concerned about the bulbs glowing bright then dying. This is an indication of overvoltage.

 

[Tysen]

Thanks for the input, Larry.

I performed Dave's panel light mod, so I installed new bulbs coming off the 25V DC source. I measured the voltage across the positive lead of 50b and lifted negative lead and I show 25V, which is feeding the bulb panel. So the voltage going to the lights is correct. I'm wondering if I didn't extend the tips well enough the bulbs are arcing?

 

[larryderouin]

Ok. But what are the voltages at the actual lamps. 25v is way too much for a 6.3v lamp.It has to be dropped somehow, as S2 (function switCH) SWITHES BETWWN TH

 

[RUmad]

Larry I’m pretty sure he converted the 6.3 volt AC lamps to 26 volt DC operation as per Dave’s ‘Transformation’ thread.
Tysen, I didn’t have very good luck extending those bulbs. I found that the threads on the bulb socket (in the preamp) all had the same flat spot which seemed to be stopping the bulbs from threading in far enough. What I did was GENTLY put needle nose pliers inside the socket and widen them all, just a bit. They all screwed in perfectly and have been functioning perfectly for about a week. I also cut about an inch of aquarium airline hose to screw the bulbs in.

EDIT; I don’t mean that I widened the entire socket! Just pushed that flat spot out far enough for the bulbs to screw in past it!

 

[Tysen]

RU,

Perfect tip. Thank you. I just added some more solder to the tips of the bulbs. Power bulb is working great. Aux 1 is working great, but the others aren't. I'm going to widen the sockets per your suggestion.

Keep you posted,

Tysen