Fisher 400 Rebuild Project

Discussion in 'Fisher' started by tekuhn, Apr 10, 2018.

  1. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    I have hit the switches and pots multiple times with DeoxIT. Unfortunately, some have responded better than others.

    The .02 coupling caps are ceramic disc. I was using the scope and tracing the signal. When I reached V10, I had signal on both sides of the cap for the left channel (pin 1), but only had signal on the plate side of the cap for the right channel (pin 6). I hoped that meant the cap was open, but it measures the same as the other one, so it must be something nearby pulling the signal down. Most likely a problem in my shielded cabling run to either the tone PC or the high filter switch. Will look more tonight.

    I was waiting on a set of matched tubes before deciding if the bias range needs to be modified. Thank you for the information on which resistor to change.

    I feel like I'm creating a lot of work for you responding to all my comments and questions - I'll try to minimize my posts until I have some positive results to report.

    Thanks for all the help!
     

     

    Please register to disable this ad.

  2. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    I thoroughly checked out the wiring all while comparing readings between right and left channel halves of V10 and could not find any differences. I decided to solder in a couple of the .047 Erofol caps that I removed from the output section and I had audio on both channels. I then reinstalled the .02 ceramics one at a time and I still have audio in both channels. :dunno:. Oh well, problem solved.

    I don't have a frequency generator, so I went to onlinetonegenerator.com and tried using a signal generated by my PC. They provide sine, square, triangle, and sawtooth patterns. Unfortunately, the purity of the square waves looks pretty poor, and as you raise the frequency, you start losing the corners until by the time you reach 10KHz, the square wave is a nice sine wave. I guess it's due to the digital sampling rate, or compression, not sure.

    Here are the 440Hz, 1KHz, and 5Khz screen shots. The upper trace is the signal-in, and the lower is the preamp-out.

    IMG_3799.JPG IMG_3801.JPG IMG_3802.JPG
     
    Last edited: May 18, 2018
  3. fred soop

    fred soop Super Member

    Messages:
    1,915
    A square wave is made up of the fundamental frequency sine wave plus odd harmonics. So, 5 kHz would include 15 kHz, 25 kHz, 35 kHz, etc. But with a typical digital sampling rate of 44 kHz, only the 15 kHz harmonic could be properly generated. That is essentially what your 5 kHz trace is showing (one lump between transitions).
     
  4. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    Interesting! I figured it had something to do with it being a digital signal - 44.1 actually according to the website. I might look for a cheap analog generator.

    I have never read-up about the difference between a sine wave and square wave, I just always assumed the waveform shape represented a near-instantaneous transition between the 0 and 180 phases for a square wave versus a smooth continuous transition through all the phases 0-360 for the sine wave. I didn't realize there was a difference in the frequencies or harmonics although they do certainly sound different.
     
    Last edited: May 19, 2018
  5. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    Status update: Replaced V10 coupling caps, found four 7868 that somewhat reasonably match in current draw, allowed to run about 20 minutes and then made the following measurements:

    Line AC: 124.0v
    B+: 433.4vdc
    Screens: 314.1vdc
    Bias: -16.09vdc (adj max neg)
    DC heater: -24.58vdc (6.145 per filament)
    AC heater: 6.80vac (only 7868's and phase splitters installed - hoping MPX tubes will tame this a bit)
    Buffer supply: -15.51vdc
    V14 cathode: .224vdc
    V15 cathode: .233vdc
    V16 cathode: .177vdc
    V17 cathode: .193vdc

    The sound quality is very good. Trying to decide whether to go with IBAM to get all four 7868 set for same 21ma quiescent current since I will likely never own a nicely matched quad and I have a few more spares that are very strong. It seems the weaker 7868 are the ones drawing less idle current. I assume that is normal. Opinions?
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,607
    Location:
    Glen Burnie Md.
    I would seriously consider building a Buck transformer to bring your inlet voltage DOWN to a more reasonable 117-118 (at least get it below 120). 124 is too high. Stresses the P.S. Tranny more than needed or wanted. Keep an eye on temps.

    Yes do the IBAM or even better the IBBA board. It may sound a bit like heresy to some to suggest a matched quad of EH 7868's but with dwindling supplies of "old stock 7868's" being matched for cathode draw a crap shoot, that's your best shot at a matched quad, slightly larger pins and all. I've done at least 10-12 400's in the last 8 years (not including my 3) and I've never had a bad or a quad that's more than 2ma high to low from Jim McShane, and 2 of my 3 are still set to 34ma and pulling equally according to measurements I make every 6 months. I've not had to adjust them yet. The 3rd is on the bench for an overhaul.

    Yeah weaker tubes will draw less idle current for a set Bias Voltage. This is normal as they wear.

    2kHz square wave testing per Dave is usually the easiest and best as you get the primary freq, and all the fundamental Harmonics from IIRC 20Hz all the way up to 20kHz.
     

     

    Please register to disable this ad.

  7. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    Thank you for the information, Larry! I think I have a pretty good supply of vintage 7868 (a total of 9 serviceable) to last my lifetime even though they are not matched. I plan to go ahead with the IBAM implementation since it's such a simple mod and I can easily fit in the chassis.

    If I were willing to risk stretching the socket contacts (which I am not at the moment), I have 10 NOS Russian 6P41S that I'd like to give a try. They have the same oversized pins of the EH7868. They are rated for less wattage and lower screen and plate voltages, but many say they are easily able to handle typical 7868 operating points. I'd think that my 400 operating under EFB(tm) control would be a reasonable application and would stay below 14 watts at all but extreme volumes, which I would not subject it to anyway.
     
  8. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    Hi everyone. I have completed adding the IBAM circuits so I can individually adjust the bias on each 7868. I felt it was the simplest way to deal with my mismatched set of output tubes. I must say, this amp is sounding very nice. I plan on letting it accumulate a few hours and then I'll recheck the bias settings and call it good. After that I will go through the FM alignment process and see how the radio section is working.

    Tom Kuhn

    IMG_3837.JPG
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,607
    Location:
    Glen Burnie Md.
    Tom: The alignment instructions for the tuner itself have 2 mistakes. Instruction 1 in the ADJUST Column states Z1,Z2,Z3,top and bottom, Z4 Bottom. Supposed to be Z4 TOP!
    INSTRUCTION 2 in the ADJUST Column reads Z4 TOP adjust. This should read as Z4 BOTTOM. I found this out when i did a customers late model 400 about 5 years ago.
    To recap;
    On Instructions 1 & 2 in the Adjust Column SWAP Z4 TOP AND BOTTOM adjustments.
     
  10. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    I remember reading that there was a mistake in the instructions. Thank you for the details!
     
  11. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    I was hearing some distortion in the bass frequencies in one channel at higher volumes - the channel with the two worst 7868's. I decided to install my best pair in that side to see if that would make a difference. I installed them and powered up and set cathode current on one, and then moved the meter over to the other tube and saw a reading of over 15 volts on the cathode. I immediately turned power off and after thinking it through, measured the resistance of the 10Ω cathode resistor and it was nearly open measuring 5.8M. This is a quality Dale 1watt 0.1% metal film resistor. It does not have a discolored appearance or look any different from the other three. No doubt the new pair of tubes required more negative bias than the pair I had just removed, so it was probably run about a minute biased kinda hot.

    Any thoughts on what might have caused this? The unit seems to be operating great. I was detecting some distortion in the bass of that channel at higher volume. I thought it might be related to the pair of tubes since they are my worst looking in terms of getter appearance - no mirroring left - just brown. I was going to change over to my best pair since I now have the ability to bias them properly.

    I try to let everything cool down at least a few minutes after power-off before turning power back on to avoid a current rush due to hot tubes so I don't think that's what happened here. Any suggestions would be appreciated. I have no more of those resistors, so will be down again for a few days waiting on a Mouser order...
     

     

    Please register to disable this ad.

  12. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    902
    Location:
    Melbourne, FL
    The cathode currents aren't enough to cause a 1W resistor to fail. Probably a manufacturing defect. Unusual though.
     
  13. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    It's actually a tiny component - looks like the size of a 1/4 watt in the old days. The leads are pretty short on this one, so I might have let it get a little too hot while soldering perhaps. I did find another brand 10Ω that I'll go ahead and install until the next time I place a Mouser order.
     
  14. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    I replaced the 10Ω cathode resistor on V17 and all seems fine. No more distortion in the right channel but I can't be sure if that is from replacing the tubes or because the cathode resistor was failing.

    I remember reading a post from another member commenting that his right and left channels were swapped, and I had the same issue. The balance control panned left and right correctly, but the right input was driving the left speaker and vice versa. Long story short, it seems the schematic is wrong for V11. I did not change any wiring from the "loudness" switch to V11 and the Balance pot, and the right channel is using pins 1,2,3 and the left channel is using 6,7,8 of V11. That is the opposite of what both the factory and modified schematics show. I have the outputs from the center contact of my High Filter Tone Defeat switch swapped because I was using the grid pins shown on the schematic as a guide for my wiring. Could someone else please confirm that is what they see also if you happen to have the cover off on a 400? I kinda like it that way, actually, being the opposite of V10. That way if you suspect a bad tube in either socket, you can swap and see if the problem moves to the other channel.
     
    Last edited: May 30, 2018
  15. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    Can someone point me to a download of the KM-60 Service Manual? The amp and pre-amp sections are finished and sounding great so I'm ready to check the FM tuner. With no antenna attached I have no static at all. I did find that one of the 6AU6's was replaced with a 6BA6 which looks like it might work, but I need to sort through my tube stash and find another 6AU6 to make sure it's right.
     
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,607
    Location:
    Glen Burnie Md.

     

    Please register to disable this ad.

  17. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    Thank you. Reading through the non-instrument alignment procedure, it keeps referencing a meter and switch position which I assume is referring to the built-in tuning meter of the KM-60? What should I use as an indicator for the 400 that only has the Stereo Beam indicator?

    Also, I haven't found a 300Ω dipole antenna yet, but with a single long wire attached to the center terminal, I am not hearing any stations at all, so I might have something more fundamental wrong to troubleshoot first. I have a low to moderate hum when switched to either FM Stereo position and the hum stops when switched to FM Mono where I can hear a very low level of static. I will find the proper antenna before going further.
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,607
    Location:
    Glen Burnie Md.
    Parts-Express has for about $3.00 each. I just buy them in bulk (10+) so I can put a new antenna on my consoles when I'm doing an overhaul. Same for any tuners or receivers.

    On the 400 use the Stereo beacon. Set it to FM MONO for the Alignment of the RF and IF Stages. This bypasses the MPX. Then follow the KM-60 instructions substituting the 400's Beacon for the meter. On a strong signal in Mono, the beacon should close by about 1/2 to 2/3. You should get good results with the KM-60 instructions.
     
  19. tekuhn

    tekuhn Active Member

    Messages:
    103
    Location:
    East Texas
    I attached a proper antenna and I do hear a low level of static, but I can not tune any stations. I have a couple tubes on order, a 6HA5 and a 6AB4. Once I have those I will start trying to troubleshoot the tuner. I have no experience in that side at all, so that will be a nice learning experience - hopefully. I have everything buttoned-up on the audio side and started reassembly. I must say the side-lit glass face is really beautiful and the LED festoon lamps really make it pop.

    IMG_3868.JPG
    IMG_3870.JPG
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,416
    Location:
    Ball Ground, GA
    Have you verified proper voltages on all of the tuner tube terminals? And that all the tubes are lit? I've seen dirty tube sockets kill more than one tuner........

    Dave
     

Share This Page