Fisher KX-100 (X-100-B) Restoration

[Tim D]

Oh. BTW, I picked up it's fraternal twin from a fellow AK member via BT. It's a Fisher X100B that needs a complete repair and refurb. This time I'll do the EFB and a few other more advanced changes such as the separate small signal tubes heater power supply, fix the common/ground issue on the OPT secondaries, maybe remove/replace the PEC's in the phono preamp, and a few other mods that I didn't do in my KX-100. Baby steps......

 

[richiem46]

Ok, per Dave's input, the bias circuit was modified per the schematic below, with 22uF/50V caps added to the wiper of each 20K pot. What's not as clear (from the schematic) is that the 220K resistors that I had re-located to the bias board were re-re-located back near the output tubes.

View attachment 1290101

The bias board now looks like this:

View attachment 1290153
And the 220K resistors are now here near the output tubes (I trashed the really small ones that were previously on the bias board since they were too hard to salvage. . .and put new ones here):

View attachment 1290154

And the whole thing looks like this now:

View attachment 1290155

The current through each 10ohm resistor was set at ~42.5mA and the cathode voltage set at 42V.

The other voltages read:
Plate: Forgot to read this one (435 spec)
Screen: 393V (365 spec)
C1: 443V (440 spec)
C2B: 394V (365 spec)
C3A: 383V (350 spec)
C3B: 371V (340 spec)
C3C: 211V (185 spec)

Some of these voltages seem a bit high - not sure if this is an issue or not.

Deoxit'd the pots and will take some scope readings tomorrow hopefully and see if the strange waveforms and channel imbalance are still there or not.

This is an old thread that I have referenced many times to restore several kx-100/x100b Fishers. I am now in the process of restoring a Fisher 400 and am about to build an IBAM board for it and something caught my eye. On the 400 IBAM board the polarity of the 33UF electrolytic capacitor attached between the pot wiper arm and ground has the positive side of the capacitor connected to ground and the negative side attached to the wiper arm. Makes sense I guess since the voltage on the wiper is negative. In this thread the capacitor is attached the opposite way, with the negative side to ground and the positive side to the pot wiper. All of the KX100/x11b's (3) that I have done with this configuration work well. Does the fact that the Fisher 400 is fixed bias affect the orientation of the wiper capacitor? In the case of the kx-100/x101b the polarity of the wiper capacitor matches that of C4 the 100MF cathode bypass capacitor for the four 7868 tubes.

I just want to be sure I understand the difference between these two mods, hate to think I have to go back and fix the three amps I have reforbished!

Thanks
Rich

 

[dcgillespie]

Your understanding is correct. In the fisher 400 receiver, there is an active bias supply generating an output voltage that is negative with respect to ground. Since the output tube cathodes are grounded in that design, a voltage source is needed to operate the grids at less than ground -- which is a negative value. Hence, the need for an active negative bias supply. As such then, any electrolytic caps used in the bias circuit of this design will have their positive terminal grounded. This is a classic fixed bias design.

In the KX100 design cathode bias is employed, with the output tube cathodes operating at several volts above ground, while the grids operate at a lessor potential -- but still above ground. Operationally, the grids are still operating at a negative potential relative to the cathodes but overall, everything about the bias circuit is still operating above ground -- which is always a positive value. Therefore, in the KX-100 bias circuit, any electrolytic caps used in this biasing approach will have their negative terminal grounded. While somewhat convoluted due to the preamp tube heater connection, this is still a classic resistive cathode bias design.

I hope this helps!

Dave

 

[richiem46]

Your understanding is correct. In the fisher 400 receiver, there is an active bias supply generating an output voltage that is negative with respect to ground. Since the output tube cathodes are grounded in that design, a voltage source is needed to operate the grids at less than ground -- which is a negative value. Hence, the need for an active negative bias supply. As such then, any electrolytic caps used in the bias circuit of this design will have their positive terminal grounded. This is a classic fixed bias design.

In the KX100 design cathode bias is employed, with the output tube cathodes operating at several volts above ground, while the grids operate at a lessor potential -- but still above ground. Operationally, the grids are still operating at a negative potential relative to the cathodes but overall, everything about the bias circuit is still operating above ground -- which is always a positive value. Therefore, in the KX-100 bias circuit, any electrolytic caps used in this biasing approach will have their negative terminal grounded. While somewhat convoluted due to the preamp tube heater connection, this is still a classic resistive cathode bias design.

I hope this helps!

Dave

Thanks for clarifying this for me!

Rich

 

[kloon]

The current through each 10ohm resistor was set at ~42.5mA and the cathode voltage set at 42V.

Pardon my math. The original bias circuit R71 of 560 ohm and cathode voltage set to 42V gives bias current of 50mA per tube. With the modification of R71 to 1.8K, the calculated bias current is 37mA per tube (ignoring the current through the bias adjustment pots). But you get 42.5 mA, 15% higher the calculated. After changing the R71, it is still cathode bias. The original design of 50 mA is high, but for cathode bias scheme, the bias current is normally set higher then that for fixed bias.

 

[Prestigomo]

Recapping a X-100-B Question: these 3 components, blue arrows (I believe resistors) are they? what are the values? Also do I really need to replace them?
Thanks Fellas

 

[larryderouin]

Power Resistors. Matter of habit, lift one end out of circuit, test. If off by more than 5%, replace them. BLUE!

Grab them with your fingers and rotate them around the long axis. Spec's are on them. Make damned sure it's OFF, UNPLuGGED, and COLD when you do so. They will wipe your fingerprints off when hot.

CAPS. Replace the BIG Cardboard job(Yellow), and the 4-5 black or Brown Epoxy drops. (GREEN). These usually go bad by becoming intermittent at the lead ends. The ERO-FOILS,(RED or WHITE) usually don't go bad, but test them anyway. If any of them are more than 5% high or low, replace all 4.

 

[Prestigomo]

Power Resistors. Matter of habit, lift one end out of circuit, test. If off by more than 5%, replace them. BLUE!

Grab them with your fingers and rotate them around the long axis. Spec's are on them. Make damned sure it's OFF, UNPLuGGED, and COLD when you do so. They will wipe your fingerprints off when hot.

CAPS. Replace the BIG Cardboard job(Yellow), and the 4-5 black or Brown Epoxy drops. (GREEN). These usually go bad by becoming intermittent at the lead ends. The ERO-FOILS,(RED or WHITE) usually don't go bad, but test them anyway. If any of them are more than 5% high or low, replace all 4.

Awesome …thank you! I really do appreciate your experience and advice

 

[Prestigomo]

Awesome …thank you! I really do appreciate your experience and advice

Merry Christmas!