Clean Fisher 400 need advice

Discussion in 'Fisher' started by grindfix, May 31, 2017.

  1. grindfix

    grindfix Active Member

    Messages:
    171
    Hello gang,
    Receiver is from later production and in really clean and nice condition.
    I have performed modifications with the kit purchased from Tube Tycoon. Later I found some people suggesting against him but too late now. All parts replacement went well. I was testing all parts removed and really did not find any that drifted further then 15-20 % off spec values. Tubes were tested by local vintage stereo shop with almost excellent results except for one 12AX7 tested 70/30 which they told me is not going to cause any issues for now. I cleaned all controls and slowly powered receiver with borrowed Variac and everything worked.

    Several concerns:
    1. Aluminum can capacitor next to power transformer gets warm before transformer even start to warm-up.
    2. Cardboard capacitor underneath the chassis has a faint burnt transformer smell.
    3. Receiver has great bottom end sound but seems to lack highs in flat position of the treble pot.
    3.The middle tube on the MPX board was flashing bright and then go dim on power-up. I had tubes rearranged since and it is not doing it now. Possibly, it was the one that tested weak. It is in the Phono section for now.
    4. Tuner is not very clean in stereo. Switching to mono cleans the sound.

    I love the sound of this receiver and want to improve it's abilities and could use guidance.
    What are my next steps?

    Thanks
    Dmitriy
     
  2. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Dmitry. Welcome to the Fisher Forum;

    A COUPLE of the concerns are normal operation of the unit. However we'll go thru them from the top.

    A 12ax7 is a TWIN TRIODE. This means there are TWO operating sections or two tubes in the envelope. The 70/30 means that one side is fairly strong and the other side weak for emissions. This one should be replaced with a tube that has closer matching sections.

    1.) Any can cap that heats up BEFORE anything else is shorting internally and has one foot in the grave and the other on a BANANA PEEL. When it does go it WILL take out many more parts of the power supply. Replace all 4 of the CAN CAPS. www.hayseedhamfest.com has a kit for the 4 can replacements. Plug and play. (Get the Single 4th cap. More expensive, but it's a drop in replacement. The other option is 2 separate caps that replace the can cap and is more for the intermediate to experienced hobbiest. )

    2.) The Underside can is an insulated from the unit can, which is why it has the cardboard cover. They used a tar based adhesive between the cap and the cardboard which over time soaks into the cardboard making it look like it leaks and you smell the old tar. Nothing to worry about, But it should be changed with the other one. There is a Selenium Rectifier (SIEMENS Brand) on the side wall that must be replaced here too).

    3.) This can be from a lot of causes. Weak tubes, leaky coupling caps, out of spec resistors. You'll need to go thru it with a Ohm meter and a cap checker, and find some culprits. The kit from Tubetycoon is a very basic kit and will need some additional parts to get you up to spec.

    4.) Tube flash on cold start-up and dims down is what we euphemistically call a "MULLARD FLASH" It's almost exclusively a British tube phenomenon. It's harmless as it's the initial heatup of the filament for the heater which extends slightly from the assembly. Nothing to worry about.

    5.) To ME, and others will chime in, This sounds like the STEREO MULTIPLEX board needs to be aligned, with new tubes (preferably old stock (your choice of brand)). One thing you can do however and it's more of a check than anything else. On the MULTIPLEX BOARD there is a BLUE POT! It rotates about 300* total. With the unit OFF, Turn the knob to one end of the other to note the end of the knob that will be the indicator. This end will be at the top right or Left corner. Mark that end with a different color sharpie or model paint. When it dries, Turn on the unit, set for a stereo station and then rotate the knob to the CENTER POINT of the pot. Then leave it there. This is as close as you can get to setting the stereo to spec for separation. If you have distortion, try rotating known good tubes thru the 19kc filter and the 38kc Filter positions. The 3rd tube(V102) is just the amplifier for the MPX. If it still sounds like crap, you'll need to finish the rebuild, and have the tuner aligned along with the MPX at a shop.

    The RF/IF tubes in the tuner part may show OK, but could be weak. The tuner tubes are cheap and fairly plentiful. I would replace all the 6AU6, 6BA6, and the 6ab4 and 6gk5. If this is a late model version, it could very well have been shipped from the factory with Sylvania tubes which FISHER was using by 1964 or so as they were ramping down their tube unit production and using tubes that cost less but met factory spec. Actually the Sylvania tubes and GE tubes I like better in the RF/IF Sections. Find some old stock RCA, GE, Sylvania, 12ax7's and run those. Make sure the sections are close in gm and emissions on the 12ax7's. Telefunkens, Mullards, Brimars, etc. are becoming ridiculous in price and you can't guarantee they will be matched or even good on EBAY (STAY AWAY FROM EBAY FOR TUBES). Find a tube dealer that deals in old stock tubes and has been around a long time.
     
    Last edited: Jun 1, 2017
  3. grindfix

    grindfix Active Member

    Messages:
    171
    Thank you for your reply, Larry,
    I was looking for replacement tube and capacitor options prior to my post. Your reply confirmed my thoughts.
    Regarding the rectifier, my unit had later production silicone bridge which I've replaced with Tube Tycoon supplied rectifier.
    I'm new to tube gear and would really appreciate directions to find the loss of high frequencies. Desoldering components for testing is a huge pain and if you could help me narrow it down I could get to work. I have correct (so far) service manual provided by Tycoon based on unit's serial number. I have DVOM, Cap tester, Audio Frequency signal generator and desire to get this thing up to speed.
     
  4. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    DeSoldering parts for testing. I'll agree that while it's a Royal PITA, it's a necessary PITA as in-circuit testing will NOT give you an accurate reading on the meter, as the meter is picking up the whole or a good part of the circuit. Loss of hi freq or (some lack of) could be the unit, or could be a speaker crossover problem.

    My math is non existant for trying to figure out electronics. Letters in equations give me migraines, and I literally lock up.

    Keep your value's on caps the same as stock, with one or 3 exceptions. The grid return resistors should be between 200K and 220K 1/2w. And the output couplers increased to .068uf 600v to maintain the R/C timing constant. You won't hear a difference here, but the tube will be running deeper in the SOA. Install 100ohms 1/4 watt screen stability resistors between each tube and the screen supply lead. And add 10ohm 1/4watt cathode resistors.

    Along with the addition of screen stability resistors on each tube, and the cathode resistors, this will keep your 7868's healthy (both Old Stock and the new EH7868) The 7868 is a 7591 in a different base package. The two are considered by most to be the same tube with different bases and envelopes. Performance is predominately the same.
     
  5. grindfix

    grindfix Active Member

    Messages:
    171
    I have to check but I think some of your recommendations regarding resistors were addressed by Tycoon's kit. I will go over it again and add what is missing.
    Just ordered new set of 4 Can Caps and 2 pairs of tubes to try. I've decided to go with new production 12ax7 instead of looking for trusted source of known good vintage tubes. 2 Sovtek 12ax7LPS and EH12ax7 to try. Their specs are different and I should be able to tell the difference. I have a feeling, since none of the removed components replaced by Tycoon's kit were really bad I might be satisfied with sound after replacing can caps and trying several different tube options.
    One question though. Could you please guide me to test all vital points for voltages once I have caps replaced.
    Thanks,
    Dmitriy
     
  6. mrphilco

    mrphilco AK Subscriber Subscriber

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    Ferdinand, IN
    :lurk:

    ...have a 400 of my own coming up in the queue...
     
  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If the kit you used to perform the work you've already done did not include the 1 uF electrolytic cap in the MPX section, that can certainly be the cause of your poor FM Stereo MPX sound as well. That, and the setting of the blue pot are the two biggest points to check when MPX operation is poor.

    Good luck with your 400!

    Dave
     
  8. grindfix

    grindfix Active Member

    Messages:
    171
    1uf cap was part of the kit and was replaced. I will be checking desoldered components in vicinity of can caps when cap kit arrives. I will update results soon. Thanks for your inputs guys!
     
  9. grindfix

    grindfix Active Member

    Messages:
    171
    I've decided to experiment with tubes before can caps replacement. The best sound I got with Russian EH 12AX7 in preamp and Sovtec 12AX7LPS in drivers. I know those are not high end tubes, but... Highs are much improved and no low frequency compromise either. I guess tested good tubes can still be bad? or just not suitable to my taste? I'm more used to SS sound which is generally brighter. Now that I know the tube sound, mainly soft deep bottom end, there is no way back to SS. Next step - recap and connect my stacked Advents in parallel to run from 4 ohm output. I have to try those new tubes in Phono too! With Denon DL-300 should sound much nicer.
     
  10. grindfix

    grindfix Active Member

    Messages:
    171
    Hello guys,

    I just finished replacing all 4 can caps with 4 can option from www.hayseedhamfest.com.
    Fired it up with variac and have no new issues. I haven't tried to hook-up antenna to see if FM is any better after recap. Both aluminum cans have drifted way below specs. 30-45% on average between sections. One of them was leaking but thankfully did not make a huge mess. I read some of posts on Fisher 400 and was wondering if someone would be willing to help me with the next steps. I did not see bias checking procedure in the manual and if I understand it correctly, it is set and not adjustable. First I'd like to check that and some other vital voltage points that you could recommend. Then I'm considering building board Dave described in older posts. I'm new to tubes (have been influenced by and restoring vintage SS gear for last couple of years) but can follow instructions and have necessary equipment available. I'm auto repair tech with 25 years of automotive electrical and electronic experience. This is my hobby and I'm obviously not on the same page with many of experienced guys here and may need more detailed guidance to make things work. I'm thinking to keep this one for a while, so I'd like to get it restored/upgraded to make it safe for everyday use.

    Thank you for your help,
    Dmitriy
     
  11. grindfix

    grindfix Active Member

    Messages:
    171
    My wall is 121.8v
    Bias at wall voltage is -21.5v
    at 117v variac Bias is -20.7v
    Manual spec is -17v
    Is this variance big enough to worry about?

    Thanks
     
  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    Dmitriy -- No. In fact, if you are using modern manufacture output tubes, the greater bias voltage is typically needed, and the safe way to error with bias voltage. The real problem is, you just don't know anything until you install a close tolerance 10Ω resistor between the cathode terminal of each output tube (pin #3), and ground. With these installed, then you will be able to determine how much current each tube is drawing, to know if your bias voltage is correct or not. The manual spec is only accurate for the tubes that Fisher told it's tube suppliers to supply. Today, using tubes with a tolerance range rather than those of a specific characteristic, you just don't know anything until you can actually measure the current through each tube, which requires the installation of the cathode resistors. Therefore, you want to get those resistors install asap so you can then know how to best proceed regarding any modifications you are considering to make the bias voltage adjustable.

    When you install the resistors, since terminal #3 of each socket was originally grounded, other connections are often connected to terminal #3 -- not because they need to connect to terminal #3, but because they need to be grounded. Therefore when installing the cathode resistors, disconnect ALL connections made to terminal #3 of each output tube socket, so that these terminals are completely bare. Then, install the 10Ω resistors as discussed. THEY SHOULD BE THE ONLY CONNECTIONS MADE TO TERMINAL #3 AT EACH OUTPUT TUBE SOCKET. If another connection was originally connected to terminal #3 of any output tube socket as a convenient point for grounding that connection, then it must still be connected TO GROUND -- not terminal #3 anymore. Another ground point will need to be found for such a connection.

    It is also highly advisable to install the Screen Stability resistors now that you've installed your new power supply can caps -- this to protect your output tubes. There are plenty of pics showing this work using the search function.

    I hope this helps!

    Dave
     
  13. grindfix

    grindfix Active Member

    Messages:
    171
    Dave,
    Thanks for your post!
    My outputs are original Fishers. I have new reproduction 12ax7 tubes in pre-amp and driver sections.
    I do have 10 ohm cathode resistors installed per Tube Tycoon's kit. While re-checking my work yesterday I found an error on V14 tube 120 ohm resistor is soldered to pin 3 as it was grounded prior to my modification. I will move it to ground point tonight. Thanks for your note that alerted me to recheck.

    Back to bias. Do I measure current draw across 10 ohm cathode resistors? What kind of amperage range am I expecting there?

    Grid stability resistors (100 ohm 1/2 w) need to be installed between grid supply (365v) and each output tube terminals 1/7, correct?

    Any other recommendations?

    Thanks,
    Dmitriy
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Correct on the stability resistors. Across each 10Ω resistor, you're looking for about 0.35 vdc with no signal passing through the unit. Less than about 0.30 vdc, and crossover distortion starts to rise rather quickly, while over 0.40 vdc, tube safety becomes a concern. Ohm's Law (in this case): I= E/R, where I is current in Amps, E is voltage in volts, and R is resistance in Ohms. Therefore 0.35 volts /10Ω = .035 Amp = 35 mA, which is the target.

    When adjusting the bias voltage to bring the cathode voltage in line, a greater negative voltage reduces the voltage across the 10Ω resistors. Making the bias voltage less negative increases the voltage across the resistors.

    Dave
     
  15. grindfix

    grindfix Active Member

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    171
    Any specific brand/type of resistors for screen stability? Metal film?
    I did not find any photos showing it done. How do you recommend securing supply end of each resistor? It wouldn't be very safe to leave it dangling attached to wire of the supply voltage.
     
  16. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    On the 400 I take and replace the screen supply lead with a piece of 14ga Romex, with the covering intact. I route it above the tube sockets, cut out 4 pieces of covering on the romex to attach the resistors, then attach the resistors (100ohm 1/4w metal film) to the pin 1-7 jumpers, and the other end wrapped around the romex. Keep the resistor lead as short as possible between the screen supply and the resistor, and the resistor and the tube jumper. With the romex it's all fairly stiff and there are no real flying leads to contend with. Let me see if i can find a pic on my computer.
     
  17. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Here's a close up of the 1st one I did. It was uprated to have the cover on the romex later on. Note also that the 1.2K 7w resistor is now connected to the romex(buss bar), in addition to the resistors. I still do it this way on 400's, while 500c and 800c you can add the resistor to the socket. See the rough drawing in the Common FISHER PARTS Sticky in the Subforum page 2 post #40... http://audiokarma.org/forums/index....needed-for-fishers.327561/page-2#post-6480170

    100_2613.JPG
     
  18. grindfix

    grindfix Active Member

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    171
    Makes perfect sense. Conflicting info on resistor wattage. I found someone using 1/2 watt. Dave agreed to that. You suggest 1/4 watt? Do they suppose to open in case of overload?
     
  19. ncwalz

    ncwalz AK Subscriber Subscriber

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    Can't you use pin 8 of the 7868 tubes for the buss for the screen stabilization resistors? Don't think it is connected to anything.
     
  20. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    I've always used 1/4 watt for both screen stab and cathode resistors. The 1/2 watt recommendation may have been an ok to the OP as that's what he may have had in stock. When I 1st started putting in screen and cathode resistors on Dave's recommendations, he quoted to me 1/4w to 1/2w for the screens, however the 1/4w for the cathodes would react faster for any tube fault. So I've always used /14w for both. This is Dave's Treatise on the subject of tube arcing of which screen stab resistors are a direct result. http://www.tronola.com/html/maximize_tube_life.html Download the article as it goes into some details on the whys and wherefor's. In the comments section of this page dated Dec. 05, 2015 in response to a comment on Dec. 04, 2015 the subject of Carbon Comps and metal films is discussed. Daves reply is quoted
    'quote' From a pure theory standpoint, carbon composition resistors get recommended for stopper service because they do not have the inductance associated with them that film type resistors can have. From a practical standpoint however, the effects of any such inductance is a moot point at the frequencies that vacuum tube audio amplifiers operate at. Still, in theory, they are the best type to use in this application, so for control grid stoppers, they would get the nod.

    For Screen Stability service however, the very real and practical qualities of metal film resistors make them the clear choice in this position. With the high voltages that a screen grid operates at, it always invites the possibility of collateral damage should a fault occur. In this application then, a carbon composition resistor needs time to heat up, overheat, and finally burn up to potentially open the circuit. However, a metal film resistor will open rather instantly by comparison, announcing the fault with a distinct "pop", and clear the overload much quicker. This quality gives metal film resistors a definite advantage for Screen Stability applications, as well as in all power supply positions in general.

    Thanks for the interest -- I hope this helps! 'end quote'

    I've since moved to Metal Films for cathode and screens as they tend to pop faster than a carbon comp. Grid stops get carbon film as they tend to be quieter in my experience on the Fisher's.


    As for Pin 8, TDSL shows it as I/C. RCA's RC-23 1964 Tube manual shows in the Key: Basing diagram that I/C is a "DO NOT USE" pin.


    ScreenHunter_76 Jun. 21 10.33.jpg

    ScreenHunter_77 Jun. 21 10.40.jpg
     
    Last edited: Jun 21, 2017

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