An Attempt At Fisher 400C Transformation...

[Tysen]

Hey folks!

I have an early version of the 400c (no dual tone knobs). I recently had a tech (who will go nameless) restore it (caps, out of spec resistors, and selenium rectifier)...I got it back and it worked fine for literally 2 weeks. This was back in November. While working, it produced a lot of noise which I read prior to the restoration was normal for this unit.

Well, now I have a pre that produces barely audible distorted noise, even at full volume.

I've completed a couple of projects since then and I feel confident I can fix this. I also feel confident I can perform dcgilliespie's mods, but not without help from this great community!

So, I'm starting this thread to document the mods in case I run into problems.

Here's my starting point:




I'll be starting the restoration on Wednesday, but I hope to gather base voltage readings beforehand to see if I can pinpoint my current problem of almost inaudible distorted output. I know it's not the rest of system because everything else works with my passive pre in place. I also should point out that the distorted sound happened somewhat gradually—at first it was a volume imbalance so I swapped tubes between V3 & V6 thinking that my 12AT7s were fried. The imbalance remained. I'm nervous to put in another new set of my 12AT7s and risk killing them too...

Dave suggested that I inspect the integrity of the tape monitor jacks. Which I still need to do. I also will try cleaning the volume pots to see if that helps.


Cheers!

—Tysen

 

[larryderouin]

Those yellow Wax/paper caps need to GO, Unless they are new construction.Gradual degradation of sound can be tubes (not too likely) and leaky caps. The noise you are hearing is most likely from the yellow caps . Get some good quality Polyprop caps (AES has a good selection of Illinois axial caps) and change out the Filter caps if they haven't been changed out. I'd also replace those electrolytics with Nichicon UKL (signal path) or UPW/UHE in the power supply. Panasonic FC series are also popular among some guys.

Clean ALL POTS, SWITCHES on the Front with DeOxit D5 and follow up with Faderlube F5 or Shield S5. Faderlube or Shield will replace the DIRTY lubricant that the D-5 washed out. See this thread " The Idiots Guide to De-Oxit...Revisited ".

 

[dcgillespie]

Humm. Looks to me like the original can cap for the DC heater supply is still being used. It has been shown to have a very long life, but has also shown that when it goes, it goes, and fails completely.

Dave

 

[Tysen]

Those yellow Wax/paper caps need to GO, Unless they are new construction.Gradual degradation of sound can be tubes (not too likely) and leaky caps. The noise you are hearing is most likely from the yellow caps . Get some good quality Polyprop caps (AES has a good selection of Illinois axial caps) and change out the Filter caps if they haven't been changed out. I'd also replace those electrolytics with Nichicon UKL (signal path) or UPW/UHE in the power supply. Panasonic FC series are also popular among some guys.

Clean ALL POTS, SWITCHES on the Front with DeOxit D5 and follow up with Faderlube F5 or Shield S5. Faderlube or Shield will replace the DIRTY lubricant that the D-5 washed out. See this thread " The Idiots Guide to De-Oxit...Revisited ".

Thanks for the link and tip! In regards to the yellow cap, they're actually poly film caps made by C.D.E. (in the 90s I think ?) I have purchased NOS Russian PIO caps to replace most of the C.D.E's to flatten the frequency response, per Dave's suggestion.




I noticed that too, @dcgillespie. I purchased new discrete caps to replace whatever is going on there. Gonna keep the can on top though for looks!

 

[Tysen]

Okay, cleared up the DC cap situation—the original terminals were used as tie points, and then cut off from the actual can. It's appears safe, but I'm going to install some new 1000uf caps with the ground lifted. @dcgillespie you mentioned in @notdigital 's thread that "When you install my modifications, the voltage that the filter caps in the DC heater supply actually "see" remains unchanged from that of the original design." So, 35v is okay, correct?

My last question for this evening is related to C49. All 3 caps are still connected to chassis ground, right? Using 47uf, is okay too?


Edit: I'm using a 8a 400v bridge, that's okay too, right?

 

[dcgillespie]

A 35V rating is perfect for the caps use at C50. 47uF for the three sections of C49 is fine as well, and yes, these sections are still grounded. Only the caps at C50 no longer connect to ground. An 8A bridge is a bit of overkill for a circuit that draws about 1A maximum. As long as it is not so overly large physically so that it won't radiate PS noise into the adjacent audio circuits, it will be fine

Dave

 

[Tysen]

Thanks Dave! the 8A appears similar in size to the one you used. Couldn't find a 4A that you could screw into the chassis. I'm going to post photos of my progress tonight.

Tysen

 

[Tysen]

Well, I think I'm done with the DC Bias, along with the new bridge rectifier.

I had to get creative adding the third 47uf cap as the JJ only had two sections...I think I've hooked it up correctly. And don't worry, the leg of the 1.2k resistor is safe from the negative lead of the cap (has tubing on it).

If anybody sees any egregious errors in my work so far, let me know!


Now off to the DC supply...really wish I would've just purchased a can cap for C50....

 

[Tysen]

Update. I believe I've completed the DC power supply section. It's not necessarily pretty, but considering I didn't have a can cap, I'm pretty happy. (Note, it's hard to tell in the photos but I do have clear shrink tubing in tight areas as a precaution).


To the positive leads of C50: A) 10ohm 2W between the terminals; B) The three white wires and the + wire from the light panel to the same 1000uf; & C) The positive leg of the shielded wire coming from bridge is connected to the other 1000uf cap.

To the lifted terminal: A) negative side of the 10uf cap from pin #2 of the rectifier socket; B) negative side of the shielded wire from the bridge; C) Heater wire from the phono section; & D) ground wire from the light panel.

Connected to the chassis ground in that area is 1) the shielding for the bridge connecting wires (only connected one side); & 2) the grounding wire from pin #5 of V2.

Am I missing anything? it's hard to tell from @dcgillespie 's original "transformation" thread if anything else is connected to the chassis ground in that area.



Assuming nobody spots any errors; I'll be re-capping & changing the grounding scheme in channel A tomorrow.

 

[dcgillespie]

Ty -- The negative side of the 10 uF 100 volt cap should be grounded. The negative side of the DC heater system should be returned to the POSITIVE side of the 10 uF 100 volt cap. That point represents about 60-65 vdc which then lifts the DC heater supply by that amount above ground to bring the heater/cathode voltage in line with the ratings of the tubes for that parameter. If any point of the negative side of the DC heater supply is grounded, it will short out the bias voltage so that the supply will not be lifted above ground.

Dave

 

[Tysen]

Dave,

I see the error of my ways.

Thank you!

 

[Tysen]

Update:

Drilled out all the rivets for the terminals on Channel A and installed the nylon hardware. I also changed some caps, per Dave's instructions. Just waiting on the the four 1ufs. Should arrive on Monday.

All that's left is to ground the terminals to their counterparts on channel B and install the remaining caps.

Attached are some photos. Let me know if anyone spots anything that's off.

 

[Tysen]

Update:

Think I'm done wiring and installing everything (except for the phono mods).

Gonna do a resistance check, and all that seems fine, I'll fire her up (with my variac)!

 

[Tysen]

Second set of photos.

 

[Tysen]

Well, I turned the pre on and only V1, V4,V5, V6 & V7 light up...V2 & V3 don't (turned off the lights to confirm). V6 shines very brightly, and V5, you can BARELY tell that it's lit.

I also got no sound. No popping, or anything, just silence.

@dcgillespie Any suggestions regarding connections I may have missed?

 

[larryderouin]

Pull V3 and V2. Ohm meter between pins 4 & 5. IF you show an open, replace the tube(s). V6 may be getting too much voltage in comparison with V5. V5 may be weak in the filament, (too much resistance) . Do voltage checks between pins 4&5 on each of the six tubes (V1 thru V6). Each should have about 10.5 to 12.6 Volts across each tube. If one is higher than the other in each pair (look at the schematic)the lower one is weak. In each pair the 1st tube will show pin 4 @ 24v with pin5 @ 12v. The black meter probe on chassis ground. The 2nd tube in each pair will show pin4 @ 12v and pin 5.
Pairs as follows:
V3 - V2
V6 - V4
V5 - V1

 

[Tysen]

Finally had a chance to take some voltages. If someone (or @dcgillespie ) can help me look over these, I'd really appreciate it! All voltages taken with my semi cheap RS multimeter with respect to the chassis. Side note, I found a bad solder joint between c49 can cap and ground. Fixed that before taking the voltages. Taken with 117v out of my variac. (Last note: none of the lights are working because I need to add solder to extend then, however one of them must have been screwed in enough because when I engaged the input selector associated with it, the light bulb popped).


C-9 382
C-18c 382
C-32c 382
C-42c 382

C-49-A 387
C-49-B 385
C-49-C 383

C-50-A 195
C-50-B 162

V-3

Pin1 382
Pin2 377
Pin3 fluctuates around 1mv
Pin4 158 (but continues to go down)
Pin5 103
Pin6 383
Pin7 fluctuates around 1mv
Pin8 fluctuates around 1mv
Pin9 131

V-6
Pin1 382
Pin2 fluctuates around 1mv
Pin3 fluctuates around 1mv
Pin4 157
Pin5 158
Pin6 382
Pin7 377
Pin8 fluctuates around 1mv
Pin9 155mv

V-2
Pin1 382
Pin2 fluctuates around 1mv
Pin3 fluctuates around 1mv
Pin4 102
Pin5 .5mv
Pin6 377
Pin7 x2mv
Pin8 .3mv
Pin9 50mv

V-5
Pin1 382
Pin2 fluctuates around 1mv
Pin3 fluctuates around 1mv
Pin4 153
Pin5 153
Pin6 377
Pin7 fluctuates around 1mv
Pin8 fluctuates around 1mv
Pin9 150

V-7
Pin2 -36
Pin3/4 390
Pin6 -18
Pin9 390

 

[larryderouin]

50A and 50B shouldn't see anymore than 30V as this is the heater circuit for V1 thru V6. Check the output side of the rectifier diodes(may be a selenium bridge) Positive and negative.

In DCV (Highest setting), probe pins 3 & 4 on V7. Then probe (ACV HIGHEST SETTING) Pins 1 and 7 on V7. Then shut it down. Report back with readings.

In the Service manual for the 30000 series there is a resistance chart using the points on the tubes and caps. Follow the setup instructions, UNPLUG it, Drain the caps, and draw out a grid, then take resistance resistance readings with reference to Chassis ground on each tube pin and Multi cap. Write it all down and take a pic and post it. (easier than trying to cope with the Forums draconian editing style.)

Manual here. http://www.fisherconsoles.com/non console manuals/fisher 400c 30001 39999 sm.pdf
AND
http://www.fisherconsoles.com/non console manuals/fisher 400c 40001 49999 sm.pdf

 

[Tysen]

thanks for the tips @larryderouin. Unfortunately I don't have a probe, but I'll check the resistance points.

I'll also check voltage at the positive output of the selenium bridge. Report back soon.

 

[Tysen]

Okay, got out my fluke and used that instead of the Radioshack MM.

Resistance readings

Pretty much all prints that are supposed to be 350k+ all fluctuated about .5meg


Voltage readings:


I don't understand why all the plates are getting such high voltages.

Also, some of these readings may be unclear in that I put the AC voltage. I did that in places where no DC was detected.