500 Mono power supply woes

Discussion in 'Fisher' started by rufleruf, Jan 28, 2019.

  1. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    I bought a 500 mono off craigslist a few weeks ago. Seller said it played, but distorted. I tubed it up yesterday afternoon and I got nothing. I had all the needed caps on hand so I swapped in all but new can cap electrolytics and tried again. Nothing. I looked at it sitting there powered up and not playing and realized the three 12ax7s did not have heaters going. Checked the manual and the three have their own little heater tap off the power transformer. They are in series using pins 4 and 5. Checked the yellow and red wire that is supposed to feed these - got 0V measured against the chassis.

    So, I have a bad power transformer winding correct? In the event that I can't find a replacement transformer, can I get a little transformer to supply the heaters for those three 12AX7's and hide it below deck? If so, can anyone tell me how to choose a transformer?

    How does a winding go bad?
     
    Last edited: Jan 28, 2019

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    This is actually a tap off the high voltage winding, so if you have no voltage there, something bigger is wrong. Can't sub this with a filament trafo.

    Time to ohm out the power transformer and see whats what here. I have a bad feeling about what you may find. It also will not work if the output tubes are completely dead or otherwise have something going on that causes them to draw no current. This is what is called "back bias", which is basically cathode bias but with the resistor between the HV transformer center tap and ground. Unless the tubes draw current, no voltage difference exists between the -36v line and ground so the 12ax7 tubes will not light.
     
  3. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    I measured the other taps and got ballpark voltage at the 5V4 and 6X4. When I got it it had a 5AR4 and 7581's, but those should be fine in the circuit I think...

    I have a K100 console pull I can take a transformer from, appears to be a TA500 with a tuning eye, but I was hoping to do something with it instead of parting it out.
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    I'd ohm check it, that will tell you if there is an open somewhere. Shouldn't need to unhook anything either, just measure from the bias feed wire to each rectifier plate. If it measures, the transformer is OK. If it was open, you should not get any B+. A short to ground would make it appear that you have no -37v, but you will have B+. You'd also have no bias supply though so the tubes will be in meltdown.

    If the transformer ohms OK and it doesn't have a backwards cap or a short to ground on that -37v line, test the output tubes to make sure they aren't dead. Check the 12ax7's too, an open heater or a heater-cathode short could cause this kind of issue.
     
  5. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    gadget73 - thanks for the thoughtful response. All the tubes in it are tested good tubes now - the heaters are lighting as expected and no melt down is going on - just no heater glow in the 3 12ax7s.

    I spent a little while with it before I went to bed. After being on for a while the voltage crept to -9v where -37v is expected. I took readings on the can capacitors (a 40uf/500v x 40uf 500v & a 40uf 450v x 20uf 350v x two 40uf 250v for those who care) and got close to 0 volts on two sections. Seems like I just have some bad/shorted electrolytic sections.

    I stopped at that point to go to bed since I'm getting over a head cold. Before I turned out the light I ordered discrete caps to replace the multisections. Ordered enough to do K100 and the late model TA500 with stereo hook ups that showed up in the mail yesterday.
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    also worth a mention, the filter caps on the -37v line are "backwards", so + to chassis ground. If one of those is in wrong way round you'll get low voltages.

    If the screen supply to the output tubes is low or out, you'll get low voltage on that circuit too.

    Basically all of this depends on current flow through the tubes. The only real difference between this and traditional cathode bias is this one develops bias voltage from all of the tubes in the amp rather than just the output tubes.
     

     

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  7. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    I'll make sure I got the orientation correct on the two electrolytics I replaced. These early units are providing me with a good learning opportunity.

    Does anyone have info on the Stereo Kit they offered for these? One of the units I have has the three RCA connections on the chassis to hook up to the kit. I heard they are connections to a preamp that allows connection to a second amplifier.

    Also, the Sams folder has the output tubes as KT66/EL34 - I assume this is a typo and they meant EL37?
     
    Last edited: Jan 29, 2019
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    That's what I thought. It would be very strange to connect 3 heaters at 12.6 volts each in series, only to then run them off a winding of the power transformer. Almost assuredly, they are actually powered by current flow through the output tubes. If they have no power, then the output tubes are not passing any current, and that is the issue that needs to be dealt with. The fact that the 12AX7 heaters are not glowing is just a symptom, not the problem.

    Good luck with it!

    Dave
     
  9. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    I think you are saying that if there is an issue with C2 or either side of the C3's the current will be not getting to the three 12AX7's, the symptom, not the problem, and that if the -36v source isn't at -36v the output tubes are not doing anything. This is probably down to bad electrolytics?

    Also, question: looking at the 5V4GA in the diagram, 2 and 8 are the 5V heater, how is that tying in to the 430V section? I haven't seen a good explanation of this part of a power supply.

    500 mono power section.JPG


    output section 500.JPG
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Ruf -- Two things:

    1. Sams has used the wrong tube symbol for the 5V4GA. In that tube, there is a cathode sleeve around the heater, and it is tied to pin #8. They don't show this with the tube symbol drawn. What they show is the tube symbol for a fast heating 5U4GB type rectifier tube, wherein the heater is actually called a "filament", because in that case, the electrons are actually being emitted directly from the heater itself, rather than from a cathode element heated by a "heater".

    2. The design of this unit has all current draw of the entire receiver being drawn through the 12AX7 tubes to provide enough current to properly heat them -- that is to say, not just the current flowing through the output tubes. In the design of this unit, it is highly unlikely that the C3 can is the problem, since it's negative side is connected directly the the CT of the HV winding of the power transformer -- NOT ground. If they are shorted or have a high power factor, then the 5V4GA will either red plate, or go dead. If they open, the set should still likely operate somewhat, but with a significant amount of hum.

    In troubleshooting the unit, first verify if the 420 volt source is present. If this voltage source is absent, the set will be dead. If there, then verify that nearly the same amount of voltage is present at pins 3 and 4 of both output tube sockets. The proper tubes are the EL37.

    Let us know!

    Dave
     
  11. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    Alright - after a week and a half spent occasionally thinking about the problem and looking at the schematic I fired it up last night and took some readings.

    Work so far: C1 replaced by a 47uf 250V radial (plan to swap in an axial at some point); C2 replaced by 100uf 50V; all other electrolytics except cans replaced; ALL wax caps replaced; added safety cap.

    5V4 pin 8: 380V
    Node after R82 (labeled R52 on the Sams schematic): 22V
    Most everything else: whacked

    This morning I pulled out R82 (a 50ohm 7 watt brick) and got basically infinite resistance.

    As a stop gap measure I paralleled 2 ~100ohm 10W bricks to make a 56ohm brick, installed and fired it back up. I have a bunch of new 10ohm 20w bricks I could have put in series, but it looked ridiculous.

    Where things now stand (note, all voltages below taken at 117V):

    5V4 pin 8: 430V
    Node after R82: 424V
    Node after R83: 414V

    It plays and sounds good! But...

    With the above voltages now good, checking the Sams schematic I see that C4A is fed by the 412V source through R84 a 10K resistor, and C4B through R49 a 330K resistor after that.
    C4A (triangle): 137V (should be 340V)
    C4B (no cut out): 164V (should be 140V)
    Th elower voltage leg:
    6X4 pin 7: 182V
    C4C (tunnel opening): 303V (should be 135V)
    C4D (square): 106V (should be 155V)

    I think the Sams schematic is wrong. C4C should be C4A. Need some time to trace the wires and figure out the rest.

    Lesson for me: pay more attention to the big power dropping resistors. Can anyone recommend a good kit to buy so I have lots of them on hand?
     
    Last edited: Feb 10, 2019

     

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  12. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    I reviewed the schematic compared to the receiver. The Tunnel Cutout should be C4A, The Square C4B, No cutout C4C and the Triangle C4D.

    I replaced R84 a 10K that measured 16K and R49 a 330K that measured 370K

    AND at 119VAC warmed for 20 minutes
    5V4 Pin 8 should be 430V: 430V
    After R82 should be 425V: 426V
    After R83 should be 420V: 417V

    C4A (by schematic) should be 340V: now 372V
    C4B (by schematic) should be 140V: now 154V

    6X4 pin 7: should be 180V: now 177V
    C4D After R80: should be 155V: now 153V
    C4C After R81: should be 135V: now 122V

    -36V source is now -43V

    Should I call it a day, or is -43V too high?
     
    Last edited: Feb 10, 2019
  13. audmod01

    audmod01 Super Member

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    Matt;

    Notice that there is another resistor in the circuit going to the 12AX7 filaments, R65, a 400 ohm 5W resistor. Both it and the 12AX7 filaments are there to provide the proper load to the negative voltage developed for the negative bias sent to the output tube grids. Check to see if R65 is open. That would cause the negative voltage to rise too high and would be applying too much voltage to the 12AX7 filaments. When R65 is the right value that should bring the negative voltage to -36VDC and allow the output tubes to come to correct negative bias. In this case it will allow them to draw more current and thus develop more power output. You could install 10 ohm 1/4 watt resistors and measure how much current is actually flowing through each output tube.

    Joe
     
    Last edited: Feb 10, 2019
  14. audmod01

    audmod01 Super Member

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    Here is the data on the 5V4 Rectifier:
    RC16-5V4.jpg
     
  15. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    Thanks Joe - I'll have a look at R65 later tonight.
     
  16. tubeactive

    tubeactive AK Subscriber Subscriber

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    I am glad you got it going again...great mono receiver with a versatile phono preamp and nice tuner, Also, the amp is extremely wide bandwidth.

    But, why are you using 119VAC in ? These come from an era when 115 was normal, 117 VAC max. from the wall. While the label might say: 115 to 120VAC, they were only anticipating the 120VAC. Can you adjust for max. 117 ACV ? Either a bucking tranny or variac should help get your voltages even closer to specs. Enjoy that receiver !
     

     

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  17. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    Thanks! I'm glad I did too. Three down, one to go on the 500 front. Last one is a late one with jacks to expand to stereo.

    119VAC was just where it was when I ran the tests - I only put it there so Dave would use it to calculate something and solve something I spent 4 hours scratching my head over.
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    I'm actually sort of glad you posted this, because it gives me ideas on how I might possibly get EFB to run in my TA-600 and retain the phono tube heat supply. I keep forgetting about back biasing, mostly because it doesn't show up too often.
     
  19. rufleruf

    rufleruf Poor Impulse Control Subscriber

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    Glad I could help - do a thread on your TA-600 when you get to it. I have that project TA-600 I need dial glass for I asked you about many moons ago that I plan to upgrade with all the mods someday when the backlog clears a bit.
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    I never did find that either. I'm absolutely certain its with the spare dial glass for my Pilot 590. I'm also more than certain I put it "somewhere safe", which is apparently so secure that I can't even find it, let alone break it.

    The 600 has needed filter caps for a year now. I should just order the blasted things so when I catch a warm day I can just go do it. My shop has heat, but its not terribly well insulated so if its too cold, and especially if its too windy, I just can't keep it warm enough to want to sit out there.
     

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