Help Saving 400-CX SA-100 and 100-R Tuner

Discussion in 'Fisher' started by moondogtn, Oct 10, 2017.

  1. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    354
    Location:
    Pleasanton, TX
    Good evening to all the fisher gurus. I am hoping y'all are willing to help a first timer do this as I have never worked on tubes. I have done some solid state repairs with guidance from members here. I am willing to listen. I can solder and know some about electronics. Test equipment I currently have are Goldstar O-scope, DBT, Fluke DMM, Eico Power Supply, and two tube testers. I am pretty stoked to attempt this project providing I can get the much needed guidance necessary not to butcher these units. I believe they are in decent cosmetic shape under the dirt and dust. So far, I didn't do anything to them at all, not even wipe off dust as I want to do this in steps.

    Backstory, I traded a friend for some stuff he wanted and I got this trilogy of Fisher tube equipment. He purchased at an auction and it has not been tested. I am excited to get this and want to do my best to get it back in action. He did wipe down the amp. The preamp has a transformer that isn't original but he included a (functional) triad original transformer in the trade for install.

    I don't have a variac but plan to get one soon. I have a DBT. I am not in a rush as I have work, school, and a family but just want to take my time and do it right.

    I attached some pics. Here is a link to the google album.

    https://photos.app.goo.gl/zKpqyv1AOSTiWUP02

    If more are wanted, please just ask. Thanks

    Edited as I figured I would add the test equipment although I have never used tube tester so I will need to figure that out. Also, added google album.


    Bill
     

    Attached Files:

    Last edited: Oct 11, 2017
  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,846
    Location:
    Ball Ground, GA
    Moon -- You'll find plenty of good folks here to help you with your restoration efforts when you get to them. Properly restored, you will really enjoy the pieces you have.

    Until you get to that though, from my post #26 of this thread:

    http://audiokarma.org/forums/index.php?threads/1961-fisher-president-vi-restoration.766491/page-2

    A pic of the bottom of your 400CX would be greatly appreciated. In particular, when turned upside down, with the rear facing you, focusing on the components that are connected to the third tube from the right.

    Otherwise, when you need some help, just ask!

    Welcome to the AK Fisher forum!

    Dave
     
  3. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

    Messages:
    31,456
    Location:
    SW Missouri
    If the tuner chassis is in good shape, keep an eye out on eBay for a replacement faceplate. Sometimes they're found on otherwise damaged units that are parted out. Helluva nice system you'll have there when you're done!
     
  4. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    354
    Location:
    Pleasanton, TX
    Dave, I appreciate your optimism. I’m hopeful but in uncharted territory. I attached a few pics and also added to the album in google photos. Please let me know if these will suffice or I can get more


    I would like to get started but not sure where to begin.

    Cleanup?
    Power test?
    Purchase variac?

    I noticed one of the 12AT7 tubes is missing on the preamp.

    Sam, I’ll set up a watch on eBay for one and keep an eye out.

    Bill
     

    Attached Files:

  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,846
    Location:
    Ball Ground, GA
    Hi Bill -- Thanks so much for the pics. It confirmed what I was looking for.

    Since you're new to tubes, a step at a time is the rule of the day. Certainly get things cleaned up first. Check the stickies on this site about doing that properly (i.e. safely), so that you don't damage any graphics or metalwork in the process. You'll certainly want to replace the one missing 12AT7. Depending on which one is missing, either one or the other channels won't work, or both won't work with one of those tubes missing.

    Your skills and ambition will determine whether to get a variac, or not. Both it and a DBT will let you slowly restart a vintage unit without causing any undue damage. You've already got a DBT, so I'd work with that for the moment. Otherwise, a variac with a builtin watt meter is indispensable if you plan on doing enough work in the future to warrant the purchase.

    It might be best for you to start with the preamp first, because it appears to be the biggest challenge at this point. It appears that someone had to replace the power transformer with external components of unknown specification. As well, unfortunately, there is no Service Manual or schematic available for this rare unit -- unless you came upon one when you acquired these units -- but that shouldn't stop you from doing a basic restoration on it. There will normally be two components that are invariably already bad on this unit -- the selenium rectifier -- originally mounted up high on the inside of the unit near where the phase switch is located, and the can filter capacitor associated with this rectifier that is mounted right in front of where the power transformer normally resides. It appears that the rectifier for this power supply now must be located externally with the new power transformer, because there does not appear to be any inside the unit anymore. For the can, you can either have one custom manufactured (nobody makes an inventory item to replace the original can), or use two discrete caps to replace the original, while leaving the original in place for appearance purposes. These components form a low voltage DC power supply to power most (but not all) of the heaters in the tubes of the unit. When they go bad, the unit will either not operate, or take a very long time to warm up due to the very low voltage produced by the supply for the heaters. You will ultimately want to replace the B+ filter caps as well, but they usually last longer than the DC Heater supply caps do, which is why you start with them first.

    The preamp is the heart of the system. If it doesn't function, all ability to control the system is lost. But to get it going, you're going to first have to make sense of all the external transformers that were added at some point in an attempt to make the unit operational. Draw out on paper what all that represents. Detail pics of the transformer adventure will let us help you decipher what's going on there. From what I can see, the twisted purple and white leads should be supplying about 30 vdc (positive relative to the chassis) to the unit. Inside the unit, the original purple is the positive lead. The red and orange lead are supplying high voltage to the rectifier tube, while the remaining leads supply 6.3 vac (relative to ground) with a tap at about 5.5 vac to power the indicator lamps.

    If any terminology throws you here just ask. Restoring this equipment will be a long, involved process for you, depending on your skills and learning curve. But this is as good a place to jump in and start at as any. Let us know!

    Dave
     
  6. moondogtn

    moondogtn AK Subscriber Subscriber

    Messages:
    354
    Location:
    Pleasanton, TX
    I’m not sure what they have going on here so I can’t draw out this circuit without more information.

    I attached a picture which may be better if I flip unit right side up and remove the cover but I would like to get large transformer sorted out first. The brown/white twisted pair are attached to a small terminal strip above the master volume switch if it unit were sitting right side up. This is soldered to a purple/white twisted pair and is the soldered to a standard plug?

    I also notice the smaller transformer (2nd to last pic) is not hooked up to anything.

    Large transformer (3rd to last pic) is probably going to get replaced with the original triad transformer but I took pictures of wiring for reference. It’s difficult to differentiate colors so I am planning on using numbers to label each and remove transformer tomorrow if time permits.

    I can power up and test the transformer once I get it disconnected. I will also test the original triad transformer(last pic) to confirm good as I think it’s best to go back with that rather than the behemoth they had installed?

    I hope this helps. I have not had any luck sourcing the schematic for this unit although I asked a friend who is looking through his files.



    Bill
     

    Attached Files:

  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,846
    Location:
    Ball Ground, GA
    Bill -- Now I see how the repair was being executed:

    1. The large power transformer must be supplying the HV to the rectifier tube (310-0-310) for B+, and almost certainly the 6.3 vac to power the rectifier tube heater, and the heater of the 12AX7 on the right end of the unit when it is upright and facing you.

    2. The terminal strip you refer to is an added piece that has four discrete diodes connected to it to replace the original selenium bridge rectifier pack that has been removed.

    3. The 25 volt Stancor transformer must be powering the four diodes to become the DC heater power supply in the unit.

    3. The only absolutely unknown element then is of the yellow-green-blue leads connecting to the terminal strip on the side panel of the unit -- what does the blue lead connect to coming from the transformer package? The Green and Yellow leads should be supplying 6.3 vac to power the heaters mentioned in #1 above, while the Green and Blue leads should be supplying about 5.5 vac to power the front panel indicator lamps -- but I don't see any source for that in the external transformers, unless that is what the disconnected transformer was for. What does the blue lead connect to in the transformer "array"?

    You will definitely want to remove the terminal strip with the four diodes attached and install a silicon bridge rectifier to do that job properly. While the existing approach no doubt works, having so much un-shielded AC floating around the volume control area is a recipe for disaster with regards to hum.

    The "original" Triad transformer may be original to the unit when you or someone before you got this unit, but it is not the original Fisher transformer. Lacking that, it would be best to use the Triad transformer -- but depending on whether that transformer contains a hum strap, it may be better to leave it as an external piece rather than mount it to the chassis. Mounting a transformer without such a strap will increase hum many fold. Check to see if the transformer has one -- I can't tell if it does from the overhead shot provided.

    For reference, I've included a shot of the new silicon bridge rectifier I've install in the unit I have here that replaces the old selenium rectifier pack, and is mounted where the old selenium device was located. You can likely disregard the power resistor associated with that installation. It is there to emulate the voltage drop of the original selenium device so that the peak current drawn from the (still original) power transformer remains as it was when the selenium device was installed and still good. Without the resistor, the lower voltage drop of the new silicon device would cause increased heating in the power transformer which is not good for its long term dependability. Since you are in a sense "rolling your own" transformer configuration, the resistor may not be required in your case, but I highly recommended it when the original transformer is still in place. I'm seeing a number of these units with replaced power transformers -- it may be because such precautions were not taken. The cap associated with the installation is effecting one of the two caps in the DC Heater supply can cap that is bad in this particular unit. The second cap is located near the can cap itself. The can has been left in place for the sake of appearance, but serves no electrical function anymore. The Brown and While wires are the AC leads from the original power transformer (originally connecting to the selenium rectifier pack), and the Black and Purple leads (also originally connecting to the selenium rectifier pack) are the original leads carrying the DC down to the can cap. Anyway, you can use this pic as a reference for a more appropriate way to install a bridge rectifier in place of the four discrete rectifiers currently installed in your unit, which will ensure that no hum is contributed by the heater supply into the audio circuits.

    I hope this helps!

    Dave
    SAM_2249.JPG
     

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