800-C Restoration

Discussion in 'Fisher' started by audmod01, Nov 12, 2018.

  1. audmod01

    audmod01 Super Member

    Messages:
    1,405
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    I received some perf-board material from Mouser yesterday, so now I can begin work on the IBAM circuitry. I want to get this unit operational and determine if anything else needs attention.

    Joe
     

     

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  2. thornev

    thornev Well-Known Member

    Messages:
    868
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    Joe - Keep us updated on your IBAM work. I'm particularly interested as I built one. Thorne
     
  3. Omegaman

    Omegaman Ultra Super Member

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  4. audmod01

    audmod01 Super Member

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    Nice to have additional confirmation that some were assembled with the access hole plugged. I still need to find a potential steel bracket I can modify to hold the AM antenna.

    Joe
     
  5. HairySatchel

    HairySatchel Member

    Messages:
    74
    Great thread everybody. Joe, I'm looking to you for inspiration. I just got my 500c that I bought last spring up and running tonight after not being powered on in probably a decade. Hard to believe all those Telefunkin tubes still work after all these years. I have the Metalbone kit and the Hayseed caps and plan to start the restoration soon after I check everything out. My $180.00 500C is alive. :banana:
     
  6. audmod01

    audmod01 Super Member

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    HairySatchel;

    I have a 500-C too, which I need to pull out and check operation and make sure everything is working well. Welcome to the group.

    Joe
     

     

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  7. audmod01

    audmod01 Super Member

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    I decided to change the output tube screen grid bus that I had installed earlier. I found a nice 16AWG wire with solid core that I could use for the bus wire, stripped it (it was already tinned) and located some stranded wire of the same gauge that had orange insulation. I removed pieces of the orange insulation sufficient to slide over areas between where the 100 ohm resistors bridge off to each 7591 screen grid. The 16AWG wire is much stouter and keeps the parts positioned more securely. Adding the orange insulation makes working nearby somewhat less hazardous.
    Screen Bus web 02.jpg

    I have rounded up all the necessary parts for the IBAM circuit and now have the perfboard material on hand. I will post pictures of that when I have it ready to install and when installed. At one time I was considering placing the bias adjustment controls on the top side of the chassis so that access would be easier, but after some thought, I am going to install them under the chassis to keep those who do not know from messing with signal grid bias of the tubes. Some people cannot resist an adjustment and can damage an output tube by misadjustment. Just call it a dumb a** prevention measure.:D

    Joe
     
  8. audmod01

    audmod01 Super Member

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    Today I cut out a piece of perf-board and spent time studying how to mount the IBAM parts to clear the metal spiral wrapped audio cables that start near the output tubes at the rear of the chassis. I finalized the position of parts and ran wires from the signal grid isolation resistors of the output tubes over to where the perf-board is to be mounted. I fabricated some small metal brackets to support the board and fasten the board to the shield around the audio jacks at the rear of the chassis. No. 2-56 hardware is being used and care had to be used to prevent shorting out any components on the board. A lot of hand filing and fitting was required. This took most of the day and became frustrating due to the small size of the parts, so I decided to stop until tomorrow.

    I will take some pictures of the board before installing it.

    Joe
     
  9. audmod01

    audmod01 Super Member

    Messages:
    1,405
    Location:
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    As promised, here are some pictures of the IBAM board:
    IBAM Board web 01.jpg
    Two brackets 1) at left here is made from part of a ground lug from a terminal strip scrounged from a donor chassis; 2) at right is made from a strip of brass that was part of an AC socket from a UPS. Both were reduced in size shaped by filing and drilling, then a hole was drilled into the phenolic board material using a diamond cone shaped Dremel bit. I used 2-56 X 1/4" screws, nuts and star washers to attach them to the board.
    IBAM Board web 02.jpg End view of bottom
    IBAM Board web 03.jpg Other end view - This end attaches to the screw holding the plastic clamp over the audio cables that run along the middle metal ridge under the chassis. The end with the rectangular brass bracket attaches around the folded end of the shield near the audio cables at the back of the chassis.
    IBAM Board web 04.jpg
    This shows the top side with the 12K ohm metal film resistors that ground one end of the 10K ohm multi-turn controls.
    IBAM Board web 05.jpg
    Here is the other side of the top. The white gadget underneath is just a piece of plastic to prop up the board for the pictures.

    I was surprised at how long it took to create such a small board. Installation will utilize four wires from the output tubes signal grid isolation resistors. V14 gets a yellow wire, V13 gets an orange wire, V11 gets a brown wire and V10 gets a black wire. They will attach from left to right as viewed in the picture just above. The metal spiral shielded set of cables will pass between the set of controls at left and right in the mostly empty space between the two sets of controls. I have not decided whether to solder the brass bracket to the metal shield in the chassis or drill a hole and use some more no. 2-56 hardware at the left end. The right end bracket is the one that gets fastened to the screw holding the plastic cable clamp closest to the back of the chassis along the metal ridge.

    Joe
     

    Attached Files:

  10. thornev

    thornev Well-Known Member

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    Joe - What about the leads for juice and ground?
     
  11. audmod01

    audmod01 Super Member

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    thornev;

    The white wire that supplies the negative voltage from the bridge rectifier originally goes all the way to the existing OEM 330K ohm signal grid isolation resistors. That wire will be shortened and will be applied to the IBAM board where there is a bus to receive it. The ground end of the circuit will be grounded close by to the chassis. Another black wire will be used for that.

    I do not have any .068uF capacitors nor do I have any 220K ohm 1/2 watt resistors. I do have some 1% 220K ohm 1/4 watt metal film resistors which might be OK, but they are physically so small I may get some 5% carbon film types in a 1/2 watt or 1 watt size. The larger wattage is not necessary, but the physical size is easier to work with. One thing that we should pay attention to is the voltage rating of resistors (yes they do have voltage ratings too) if you choose a resistor with too low a voltage rating particularly in circuits with high AC voltages or high DC voltages the resistor can arc-over internally and destroy itself and potentially other parts in a circuit. I do have some .1uf polypropylene capacitors rated at 630VDC, but installing those would drop the low end knee of the response curve and could lead to some instability. So, I need to place another order for parts. It means another delay before I can even install and adjust the output tubes, but I can deal with that. It is far better to have the right parts and do all the desired changes before trying the output tubes. One of the old coupling capacitors to the output tubes is already leaky and another one could fail if I don't change them. The tubes are too expensive to take the chance.

    Joe
     

     

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  12. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Joe.....I'd like to hear your thoughts on why you decided on IBAM versus IBBA?
    I'm not advocating one over the other, just curious what your reasoning was.

    Tks, Jef
     
  13. thornev

    thornev Well-Known Member

    Messages:
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    Joe - Lately I've seen that mouser.com is providing 2-day Fedex deliveries for only $7.99. Pretty good deal. I don't know if it's because I've been logging in with my credentials and buying a lot of stuff, but it's worth a try. Thorne
     
  14. audmod01

    audmod01 Super Member

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    Jef;

    I have done the IBAM several times already and had some of the parts (variable resistors & 12K ohm fixed resistors) on hand already.

    thornev;

    Mouser delivers parts quickly and I always choose the economy shipping through USPS which is about half the first class fee or the basic FedEx or UPS rates and the items get here just as quick. UPS has a contract with USPS to deliver some items, where UPS delivers to the local post office and the post office does the delivery to the home. I don't know if FedEx has similar arrangements with the USPS or not.

    Today is clouding up and temperatures are dropping - a good day to use inside for work on my projects.

    Joe
     
  15. audmod01

    audmod01 Super Member

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    I just received a Mouser order from a few days ago by USPS and the origin is shown as Matt Bell in Mansfield, TX instead of Mouser. It may be that they are using a contractor now to handle USPS mail - interesting.

    Joe
     
  16. thornev

    thornev Well-Known Member

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    It may be too that for parts they don't have in stock, they vend out to a contractor.
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Matt Bell is Mouser's Shipping contact. Pretty much every pkg I've gotten from Mouser over the last 10 years has come with his name on it. Joe; If you have 0.082uf's they'll work well and be very close to the original RC ratio.
    Larry
     
  18. audmod01

    audmod01 Super Member

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    Larry;

    That's strange. I have been ordering from Mouser even longer and have not seen his name on packages before.
    I do not have any .082uF capacitors either. I did order some .068uF capacitors today. I might order some .082 units next time i order.

    As a follow-up, I found an old box from Mouser in the garage and it has Matt's name on it too. I must have just failed to notice this.

    Joe
     
    Last edited: Jan 11, 2019
  19. audmod01

    audmod01 Super Member

    Messages:
    1,405
    Location:
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    Here's a picture of the IBAM Board installed. I am still waiting on grid capacitors and resistors.

    IBAM Board installed web 01.jpg

    Joe
     
  20. thornev

    thornev Well-Known Member

    Messages:
    868
    Location:
    Mid Hudson Valley, NY
    Congrats on the IBAM 500-C install, Joe. Very economical (space-wise). Thorne

    ps - how many mA do you have dialed in? and which 7591 tubes?
     

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