How do I find a 5mA bias connection?

thornev

Super Member
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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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.
 
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 (**** 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
 
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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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.
 
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.
 
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
 
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!
 
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 !!
 
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?
 
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:
 
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.
 
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!!!
 
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.
 
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.
 
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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.
 
I finally installed the Unity Gain mod on my 500-C today and all I get is distortion. I can tune in a radio station and get music, but I get a hum in one channel with the music and both channels are badly distorted even at low volume. Pictures and schematics I used are in the above posts. I attached the IN4007 lead on the Unity Gain board to the same pin 4 on tube V14 which is where my IBAM board gets its bias. Ground lead is attached to ground. The BLK and WHT board leads (which are left and right channel OUTs) are connected to the leads that I disconnected from the 2 center tabs on the VOL control. The RED and GRN board leads (which are left and right channel INs) are connected to the 2 center tabs on the VOL control. I used the headphones setting to monitor sound, not speakers, and I'm not using the preamp OUT nor poweramp IN in the circuit. The bass frequencies are what distorts the worst.

Also I notice that when I turn the BALANCE control all the way right or left, that channel is in both left and right speakers. That's not right.

Anyone know what I can try to debug the problem? The RIAA mod is also installed in case that makes a difference.

UPDATE: I measured voltage coming into the IN4007 diode and it measured -22 VDC. Is that OK? I measured current and got around .25 mA DC. The left and right IN and OUT channel leads from the Unity Gain board are NOT insulated if that matters. I had hum in one channel with the volume all the way down, but I got rid of that by choosing a different ground point.

On a separate note... There's a 1.4K ohm 7W power (I think wire wound) resistor on pin 4 of power tube V9 that gets really hot. It's not shown on the schematic. Is that a sign that the resistor is bad? There's another similar resistor elsewhere that gets really hot. Replace it?

And by the way, there is no pin 4 on both sides of the IC. The single IC has only pins 1 - 8 so I believe that pin 5 on the right channel is where the power comes in and it's the same B- line that goes into pin 4 on the left channel, right? In other words, the 47K resistor next to the 10uF 35V e-cap, the side of the 47K that gets bias is connected to the 1M resistor that in turn feeds pin 5 of the LF353N, right?

UPDATE 2: I uninstalled the Unity Gain mod and still get a little hum with VOL on but all the way off, and those resistors still get very hot so I guess that's normal. I spent 3 hours last night verifying the Unity Gain board is wired correctly and it seems to be, but I must have done something wrong since I'm getting that bad distortion. I hope someone reads this post and can help. HELP !
 
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