How do I find a 5mA bias connection?

Discussion in 'Fisher' started by thornev, May 16, 2018 at 5:39 AM.

  1. thornev

    thornev Active Member

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    Novice question coming...

    I've built the Unity Gain mod on a small board and I'm ready to install it in my 500C. I know the left and right input leads attach to the center tabs of the volume control. What I don't know is (1) what to do with the left and right output leads as I'm not taking advantage of (i.e. connecting to) the preamp OUT and power amp IN circuits and (2) how to find a 5 mA connection to which to connect the IN4007 diode.

    I've not done much of tracing voltage and current in a live unit except for when I installed the IBAM circuit. Is it a matter of probing around components with a digital multimeter until I find 5mA? Since it's a bias connection I assume that's a matter of following a specific circuit, but where do I start?

    And because some people like pictures, see attachment (please excuse the lack of aesthetic symmetry). Black and white leads are left and right OUT. Red and green leads are left and right IN. Grey leads are ground and 5mA connections.
     

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    Last edited: May 16, 2018 at 5:48 AM

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If you're NOT doing the PRE/OUT - MAIN IN mod, then the OUTPUT's go to the shielded cables that are 1st disconnected from the Volume control that lead off to the phase inverters. In essence the Unity gain buffer is between the Volume control (which is the end point of the PREAMP, and the Driver/phase inverters (which start the AMP Section. Remove the cables from the Center tap of the Volume control, tie in the INPUTS to the BUFFER, then tie in the cables to the output's of the Buffer.

    If the 5ma BIAS voltage is Negative you can take off the BIAS Feed to the IBAM board. I would change the color of the 5ma wire so you don't get it and grnd cornfused.
     
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  3. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Thorne-board looks good. However, you can't poke around for a 5 MA source like you can , say, 6 volts or something. Power supplies put out a set Voltage, and can deliver it at a set current rating. Think in hydraulic terms. A garden hose carrying h2o at a set pressure (voltage) will deliver a lot less water per given amount of time (small current ) than a fire hose at the same pressure (big current).What voltage does that thing need? You likely can find something close in your Frankenstein 500, but may need a dropping resistor & rectumfrier to get it, if you need less than 6 volts. Bear in mind, that's AC, not DC. Stay out of the low-voltage bias supply, though! That's -22volts, making ground look positive. 5 ma is a very small amount of current, maybe tap into 6 volt filament supply to the output toobs or the filament line with the bulbs & v7-13.
    .Not much to pick from on these, just 2 filament secondaries at 6 volts, 24 volt secondary for bias, (NO!) & high-voltage secondary (Fuck No!) Theres a wide variety of voltages in these things, but you risk upsetting things if you tap into a grid or cathode circuit that looks close to the desired voltage. Hope this helps ya a bit, now don't blow anything up! And study your schematic to know where I'm coming from. I'm sure you'll hear from Larry or Dave G now that ive got your ball rollin'! Cat :D
     
  4. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Aaahhh, Larry beat me to it!:thumbsup:
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Power goes into Pin 4 on the IC's. According to the Final 400 Modded schematic the power is taken from the negative side of the original diode bridge (CR1) into the 1n4007 and then into pin 4 on each of the ic's. Here's the FINAL version of the 400 schematic that Luis drew up and Dave approved. As I thought, you can take off power from the IBAM BOARD POWER as it's the original bias supply. That would be the easiest way as you have Sheldons board which makes it hard to tap into.
     

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  6. gadget73

    gadget73 junk junkie Subscriber

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    Does that marking just indicate that it consumes 5ma of power from the bias supply? That would seem to be the sensible thing for it to mean, since there isn't really a spot in a typical amplifier that will provide a specific current level. Also, being a simple 2 channel op-amp buffer 5ma seems like a reasonable power consumption for that task.
     

     

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  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I would think that's the loading of the OpAmp rather than a limit on the supply. 5ma off the bias supply is minimal and won't affect the adjustments of the tube bias.
     
  8. thornev

    thornev Active Member

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    Thanks for the replies, Guys. I had inguinal hernia surgery yesterday so I can't lift my radios/tube tester to work on them for 6 weeks. Ugh. At least I pulled through my 1st ever major medical treatment so I have that for which to be thankful. I guess I shouldn't have lifted all those heavy speaker cabs when I was dead tired after all those gigs all those years. Anyway, thanks again and I'll keep reading all this great techie stuff, most of which is still way over my head but interesting nonetheless. Thorne
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If you had laproscopic repair, you got off easy! I remember when an Inguinal Hernia would be opened with at least a 6" cut near the leg fold. And you'd be out of commission for 8-12 weeks. (This was back in the early to mid 70's):(:(. Now they make 3 or 4 punctures with tubes, blow your gut up with CO2 gas and do the repair internally. Then suck out the gas, 1 or 2 sutures per puncture, a few Minion or Strawberry Shortcake Bandaids, a lollipop if you were good:jump: , and they send you home. No young nurses stacked to the 9's, no bedpans, noone to take your vital signs every 4 hours(and wake you from a dead sleep), no gruel in the morning, or Jello that sticks to the wall so well it can be used as an industrial adhesive. Jeez, what's medicine come to!:rolleyes:

    Take it really easy for the next few weeks and ease back into stuff. Get someone else to do the heavy lifting. I heard a rumor that MONGO was hiring out!
     
  10. thornev

    thornev Active Member

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    Yep, laparoscopic. That propoful (sp?) knocked me out before I could recognize I was being knocked out ! The incisions hurt more than the groin. And they use crazy glue now to sew you up ! They wouldn't let me go home until I could pee. I had to drink 2 bottles of water and 1 glass of ginger ale just so I could pee and get my ass home to rest. My shoulders and neck hurt which I can't explain. Maybe from the gas? And supposedly I have another one on the other side, but it's very small and hopefully will stay that way. No more straining for me !!
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Propofol is a really effective sedative-hypnotic. It'll put you out quick. Shoulders and neck soreness might be from positioning on the table. Sometimes during Abdominal surgery they'll put you in a mild trendelenburg position (head down, butt up) so they can visualize the area better. Crazy Glue? That's just Nutz. I've seen Grazy glue used on some facial reconstruction. I guess the puncture sites, being small would benefit from glue vs. sutures (cosmetically). What's next? J.B. Weld for broken bones?
     

     

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  12. Catmanboo

    Catmanboo AK Subscriber Subscriber

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    Welcome to the club, thorne! I had an lower abdominal blowout mesh-patched a few years ago myself. Recovery kinda sucked, walking (waddling)around the house for a couple weeks, swinging a grapefruit-sized swollen purple nut sack. Every move hurt, no position comfortable, nobody to bring sammiches & brewskis! Part of getting old & tore up, I guess. As long as we don't have to endure the kind of shiite that Larry has, be thankful! :eek:
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    I still like how heavy duty bone repair operations use things you would expect a carpenter to use on your deck. I saw an x-ray of a friend after he had shattered his pelvis. I swear they went to the hardware store and got a half dozen #10 wood screws to put him back together.

    The sutures uses to tie a ribcage back together are pretty gnarly looking too. We sterilize them at work. Looks like a heavy duty fishing hook with piano wire attached to it.
     
  14. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Yeah! 2 Open Heart Surgeries, an Abdominal Hernia where the chest tubes were for the open heart, knee surgery, Appendectomy (the old kind with a 6" scar) Neck vertabrae fusion on C-5 to C-7, Back Fusion on Lower Back L-5 to S1 with internal scaffolding, 30+ angioplasties (balloon cath's) with 18 stents. I can't get within 50 ft of a metal detector without overloading the SOB! And I've probably got Gadget's wires in my chest! Don't let him fool you folks, them there are leaders and hooks for a GREAT WHITE!!!
     
  15. RUmad

    RUmad AK Subscriber Subscriber

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    Dang Larry. You make me feel like NOT complaining anymore!
     
  16. thornev

    thornev Active Member

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    Geez, Larry. You're a walking encyclopedia of medical procedures. Day 2 is much better. I'm wearing a hernia belt to keep the water accumulation at bay. Luckily they didn't have to remove any testicles nor start the 5" opening as the backup plan. I consider my self brave and lucky and happy to be alive today. Vassar Brothers Medical Center did a wonderful job.
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Go ahead and complain. This ain't no contest to see who's got the most scars and holes in them. Right now my hips hurt so bad from all the rain, I'm walking with forearm crutches. Weather back, hips, and knee. I can forecast better than those overpaid weatherguessers at NOAA and the TV Stations.

    The cardiac is genetic. Don't buy the Cheap Gene's at the Dollar Store. The Hips, knees, back and neck are from 30+ years of EMS, and Law Enforcement, plus manhandling lumber, engines, other heavy car parts like trannies, rears, Large appliances without benefit of handtrucks, straps etc. Throwing the wife into the pool, and other things. It wears a guy down eventually. I'm now taller than my Dad. I'm 5'-10" and he was 6'2" now 5'8". Arthritis of the spine. But he's fighting it. I just hope that's NOT genetic. I probably won't be able to see the top of my bench in a few years if it is.
     
    Last edited: May 18, 2018 at 7:41 PM
  18. RUmad

    RUmad AK Subscriber Subscriber

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    Yeah. You and me both. This weather isn’t fit
    For humans. At least not busted up old ones. Now fish, yep. This weather’d be good for fish.
     
  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    specially pan fried!!!
     
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