Fisher 500B Re-Hab

Discussion in 'Fisher' started by Dave451, Feb 10, 2017.

  1. Dave451

    Dave451 AK Subscriber Subscriber

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    Starting a thread on the last "cold weather" project on hand, restoring a Fisher 500B that was purchased in working condition, but with a couple of challenges (lifted from my "Newly Acquired Fisher 400" thread):

    "Picked up a Fisher 500B today from CL in working condition and not in bad shape (I have brights for the knobs, at least!). Was playing into a pair of large Advent speakers and sounding quite good when I picked it up. Good stereo FM, eye closing totally on close by strong FM signals, excellent sound on CD input. Dial cord has slipped. Of course, noticed an odd tube right away--turns out to be a Fender 6V6 stuck in among what appear to be Westinghouse 7591's! I didn't even know that would work! It was playing, though, and sounding good. I haven't been underneath yet to see if any modifications were made to use the 6V6GTA [none were]. The owner said he 'had a tech go over it' and that it had played reliably and well since. One tone control 12AX7 replaced with Russian Tung-Sol reissue (was actually a 12AU7!). Phono tubes are "Made In Holland" Fisher labels. Other 12AX7's are Telefunken ribbed plates, except for a GE 6201 (12AT7) and TF 12AT7 in the multiplexer."

    Well, we established that the 6V6, of course, was mainly a prop but, fortunately, didn't seem to damage anything. I have a new quad of Tung Sol Russian re-issues on hand to install when the initial work is through.
    I'm not planning any serious mods right now, just a basic careful going-over and restore.

    The bias in the 500B is "fixed fixed" and there is no provision for adjustment whatsoever. I'll at least put in screen protection resistors and (probably can't resist) some way of adjusting bias for the individual tubes (IBAM) and 10 ohm cathode resistors.

    I checked the non-output tubes and they are all good. All of the 6AU6 IF tubes are Fisher-labeled Mullard (by codes and appearance). The three 7591A's look like they've been used pretty hard. One tests bad on my tester and the other two aren't real strong, so will replace them all. I'll power test the good pair in the amp just to see how strong they are, because you can't tell everything by the tester results.

    Today's work included:
    1. Checked all four can caps with Sprague TO-5 cap checker by laboriously unhooking each terminal and measuring capacitance value, AC power factor, and leakage current at maximum working voltage. I also checked ESR with my Blue ESR Tester. Every one tested good, and this is pretty rigorous testing. I know some would just replace them all out of hand, but I'm going with these results for now and test the 500B with these in place after the rest of the work.
    2. Due to the kindness of another AK member, I got a replacement Z5 ratio detector transformer at no cost for the one I 'donated' from the 500B to my Fisher 400 project to replace the bad one in it (same part number). I installed it today in the 500B and the "hole in the chassis" is no more!
    3. Replaced the HV rectifiers with 1N4007 and installed new AC caps, with the safety cap being XY rated. The 0.01 uF 600V film caps originally installed were just a bit leaky by this point.
    4. Installed a CL-80 inrush protector in one of the PT primary legs.
    5. Replaced the 8 uF 50V C74 ratio detector resonance cap while I was working around Z5
    6. Replaced the Siemans selenium bridge rectifier in the bias/filament supply with a 3A 400 PIV silicon unit. Replaced the C85 100 uF 25VDC bias supply cap with a Nichicon 100 uF 50VDC unit. The old unit was pretty leaky (electrically).
    7. Replaced the 1 uF 350VDC C210 cap in the multiplex with a same-rated Nichicon unit. This cap was also a little leaky.

    That's enough for today. Pictures include a couple of "before" shots, a shot of some of the power supply work, the new silicon bias bridge rectifier, and the bottom side of the new Z5 transformer.
    Dave
     

    Attached Files:

  2. bhamham

    bhamham Super Member

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    Nice work Dave! It looks to be one of the gold face 500Bs - very nice. That's curious about the 6V6, I guess to a lot of folks a tube is a tube is a tube. Good luck with the rest of the restoration! Bruce
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    It would be interesting to know if the 6V6 itself survived: With screen grid level voltage applied to its cathode, yet the heater referencing ground, the insulation between those two elements must have taken a beating! As well, with the control grid referencing ground, and there being zero potential difference between the screen grid and cathode elements, the tube was biased into an absolutely insane level of cut-off. If the heater/cathode insulation within the tube held, the Fisher shouldn't have been damaged, and just maybe, the tube survived as well........

    Dave
     
  4. Dave451

    Dave451 AK Subscriber Subscriber

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    Thanks, Bruce. I hope this one is a little more straightforward than the 400 work, what with all the mods and the detective work on the bad Z5, helped in great part by Dave G!

    Dave, I did test the 6V6 in my B&K 707 tester and it tested good. No shorts or grid emission and a "good" rating, but for certain power tubes (the 6V6 is one), the tester only tests emission, not mutual conductance. I wouldn't hesitate to try it out if I had any equipment that takes it.

    By the way, Dave, I had a very nice session last evening with the 400 listening to a local college station. It's about the only one on the dial with the (now) rare combination of great music and fidelity and the 400, into my AR-4x speakers, sounds just superb on FM. Great detail and sound stage. I'm lucky to have this station around to be able to experience good, Hi-Fi grade FM (jazz, classical, roots, great new stuff and the occasional low-fi sporting event!). The music sounded so good I grabbed another glass of wine and kept listening. Good stuff!

    Dave
     
  5. Dave451

    Dave451 AK Subscriber Subscriber

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    Last couple of days completed the following rehab/mods:

    1. Replaced the "BT" 0.047 400 and 250 V coupler caps between the PI/Drivers and output tubes with 0.047 uF Illinois Capacitor MPW 630V (C57/58/60/61). Replaced 0.047 uF 400V C43/C44 caps in phono preamp with same IC units. Replaced tone control 0.022 uF 250VDC C14/15 with CDE 716 series Orange Drop 400VDC.
    2. After the 6V6 adventure, checked the filament balance resistors R121/122. Both good.
    3. Removed 7591 output tube pin 5 cathode jumpers and installed matched 10 ohm cathode resistors (1/2 watt carbon film) on each tube.
    4. Removed screen jumpers between the 7591 tubes (the ones that daisy chain pins 4 and 8 together) and installed a 16 AWG copper screen voltage bus between two one-terminal tie strips mounted to chassis tube shield screws (see photo). Left the pin 4/8 jumpers in each tube.
    5. Connected the 1.2K 7W power resistor to the near end of the new bus and connected 100 ohm, 1/2 W carbon film screen protection resistors between the bus and pin 4 for each 7591 output tube.
    6. Disconnected the 348V ("C") B+ source wire from pin 8 on V13 and moved to the far end of the new screen bus (didn't want the current from the remaining B+ string going through the V13 screen protection 100 ohm resistor!). This was a hard-to-see connection on the original set up in the original wiring and I was glad I looked close before finalizing the wiring.
    7. Powered up to check the B+ and bias voltages with the new silicon bias supply bridge and PS diodes without tubes. All were as you would expect without tubes in place and operating (output tube plates about 435VDC and screens at about 400V). Put in V7/8 and V9/10 12AX7s and checked bias supply voltages after the tubes warmed up, as follows: Exit bridge = -25.1 VDC; 12AX7 filament supply = -25.1 VDC; Bias supply voltage ("V") = -16.1 VDC; 7591 grids = -15.6 VDC.

    The -15.6V grid voltage compared to -14.7 VDC on the schematic, so we may be a little too negative with respect to specs, but I want to finish the IBAM and have it in place with the tubes on line before I think about changing R128 (15 ohm 5W) in the bias supply to reduce it. The Tung Sol's can run a little hot, so I may want the additional negative bias voltage for them once I see the adjustment spread I get with the new bias controls.

    I laid out the IBAM board (from Dave G's Fisher 400 design I used on my 400) and will get it finished and installed under the chassis as the next step. I prefer to keep the adjustments underneath for now.

    Photos: 1 and 2 are views of new output tube couplers and mods for cathode and screen resistors; 3 is the IBAM layout to finish and install.

    Dave
     

    Attached Files:

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  6. Dave451

    Dave451 AK Subscriber Subscriber

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    IBAM INSTALLED AND TEST RUN

    Completed the IBAM unit and installed under the chassis between the two tube banks (photos). After thorough passive (no voltage) checkout, I centered it and hooked it up via jumpers to the -25V (measured at -25.1 VDC) supply (with 12AX7's installed) and had -17.2VDC on each pair with the bias level pot centered and the DC balance centered. Span was -15.4 to -18.5 VDC. Installed the unit on a stand off and tested again before putting any other tubes in and got -15.1 to -18.3 span.

    So, took a breath and put all the tubes back in, including a new matched quad of Tung Sol 7591A Russia re-issues. Cranked the bias to most negative setting and powered her up. Summary is: 1) Tubes are already drawing fairly high current with max negative bias (Tung Sols are hot running, in my experience), 2) Stereo sound (tuner) is coming out of both channels and sound is good, 3) Amp is dead quiet on Tape and Aux settings, with significant tube rush on Tape Head and Phono settings at mid to max volume but absolutely no hum or buzz (need to roll tubes or check/change pre-amp plate resistors, probably), 4) tuner will need aligned (frequency is off, a little deaf, and bit of distortion on strong stations). Also good news, the 'transplant' Z5 unit is functioning--we'll see how she tunes up and, 5) All B+ voltages are running hot compared to the schematic, probably due to higher line voltage and (mainly) less voltage drop in the new 1N4007 diodes giving higher output voltage from the doubler, I figure.

    All the can caps are running cool. Quiescent voltages as follows:

    Line voltage (pre-CL-80): 121.5 VAC; post CL-80 119.5 VAC
    BIAS/12AX7 Filament: Exit bridge and -25V source: -25.1 VDC

    B+ sources (schematic/measured): 395/408, 348/380, 320/360, 290/327, 165/178.

    Output tube voltages (schematic/measured): Plates = 393/404, Screens = 348/380, Grids = -14.7/-17.8

    The cathode voltages (with 10 ohm resistors) are: V13 = 365 mV, V14 = 290 mV, V15 = 355 mV, V16 = 377 mV.

    So, things are a little hot and I'll have to think over the power supply with the new rectifiers. I've got the bias max negative and am still drawing more than I want for quiescent 7591A cathode current (29 to 38 mA). Will probably have to adjust the 10K resistors in the IBAM to get a volt or two more negative bias. Not sure what the story is with V14, as it is drawing significantly less than the other three with same set of voltages. Will burn them in a little and see what happens with that one.

    Any comments or suggestions most welcome!

    Dave
     

    Attached Files:

  7. Dave451

    Dave451 AK Subscriber Subscriber

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    Back in town and re-worked the IBAM unit, replacing the 10K resistors with 22K as I saw Dave G recommended for the 500B (and which I should have installed in the first place!). This gave me a new bias spread of -20.0 to -18.7 with all tubes installed but the output tubes. When I put the Tung Sol 7591A's in, the most negative bias voltage had risen from -17.8VDC to -19.7 VDC, so I was able to easily adjust the bias on each to 30 mA per tube quiescent current now. The tubes turned out to be a little less matched than I might have wanted, but I was able to easily balance all with the IBAM unit.

    7591A grid voltages to obtain 30 mA quiescent cathode current per tube were -19.0 and -18.0 VDC (this one is a bit of a weak sister) for one pair and -19.0 and -19.2 VDC for the other. We'll see how this plays as the tubes burn in a little; right now they are brand new.

    B+ and plate voltages remain high (as above), with line right at 120V after the CL-80, B+ is 412 VDC, plates are 407VDC, screens are 383VDC. These are quite a bit higher than the 500B schematic, but pretty much in line with what you would see on the Fisher 400. Some small tube voltages are high as well, as follows:

    PI/Driver: 329 and 159 VDC, versus 288 and 188 schematic, respectively
    Tone Control: 217 and 168 VDC, versus 170 and 155 schematic, respectively
    Pre-amp and tuner tube plates are pretty close to schematic voltages.

    So, the higher power supply voltages with the new 1N4007 diodes are significantly above the schematic voltages for the 500B, but aren't much out of line with equivalent voltages of the 400 or the 500C, say.

    QUESTION: Should I worry about these higher B+ voltages and, say, put the old silicon PS diodes back in, or is it OK to run the 500B at the kinds of voltages I see above?

    Thanks!
    Dave
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    New B+ rectifiers won't increase the B+ any, since the old ones were silicon as well. The main B+ is fine, but have you investigated why the screen voltage is so high?

    Dave
     
  9. Dave451

    Dave451 AK Subscriber Subscriber

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    Good point, Dave. Based on the schematic, about the only thing it could be is the 1.2K 7W resistor, right? I had it loose with the screen resistor mods, but dang if I remember whether I checked it. That would also explain why the 'downstream' voltages are also high. I'll have a look and report back. Thanks.
    Dave
     
  10. bhamham

    bhamham Super Member

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    Raising the bias might help a little to drop those voltages, but definitely check that big resistor and the other, smaller 1.2k.
     
  11. Dave451

    Dave451 AK Subscriber Subscriber

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    Bruce and Dave, I did check the resistors and the two 1.2K resistors (7W and 1W) are both within 2% of schematic value. The only thing I can figure is the high screen voltage is due to the way the tubes are being operated. Looking at my notes, the screen voltage remained almost the same (382VDC vs. 380VDC), even though the bias voltage was changed from -17.8 to -19.0 VDC. If the PS caps were bad (despite all my testing), the B+ voltage would go down instead of up, right? From the tube data sheet, any reduction in screen current (around 3 mA per tube) pulling on "C" (348 supply, running 380V) wouldn't seem to be enough to raise the voltage to this level, correct? Still scratching my head a little.
    Dave
     
  12. bhamham

    bhamham Super Member

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    I don't know Dave. I had to do what you did to an IBAM to get a set of new issue Tung Sols to balance and bias right. I can't remember if I had elevated grid voltages and sold the unit so can't check. I like those tubes, they're rock steady but with more requiring adjustments for biasing they're losing their appeal to me. I wish the EH's weren't so doggone big! Now, I like to buy good testing old stock when I can afford them.
     
  13. Dave451

    Dave451 AK Subscriber Subscriber

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    Interestingly, the voltages I'm seeing for my 500B are not all that different than what you'd see for the 500C & 800C, which uses the same PS component values for the resistors and caps, except the power transformer, of course (correct by the parts list for my 500B) and maybe the PS rectifier diodes.
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The Screen circuit is stock in this one, correct? As in, you did not add EFB to this one, right?

    Dave
     
  15. Dave451

    Dave451 AK Subscriber Subscriber

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    That's correct, Dave.
     
  16. Dave451

    Dave451 AK Subscriber Subscriber

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    So, I rechecked the 1.2K 7W resistor by substitution in case something unlikely was going on at voltage. I also checked C88 by substituting a 220 uF 300 VDC unit. Still read right around 380V on the screens. I biased the tubes to 35 mA per tube and no change to screen voltage. By the way, the 'weak sister' Tung Sol kept me from biasing that pair beyond about 31-32 mA. I pulled it and substituted another new Tung Sol and all was well. The questionable tube was requiring about another -1V of bias for the same current and when I tried to increase it, it was pulling down the pair even at max (least negative) bias. Hopefully I can get a replacement from the vendor. Reset the four to 30 mA apiece and I'm inclined to accept the 380V screen voltage and the other B+ voltages as being acceptable, in line with what you see on the 500C--sound reasonable, or should I keep searching? Any and all thoughts appreciated.

    That said, I'm proceeding to alignment of the tuner. First issue on that score is the dial pointer. It is not running smoothly from end to end. The dial cord that wraps around the spindle on the tuning knob shaft is moving back and forth across the shaft and is overlapping near the ends and stalling the tuning knob. I need to figure out how to get it to stay in the middle of the tuning knob shaft to avoid this. Cleaning it up a little may help, but can't afford to lose a lot of friction at this point, as this is what drives the tuning mechanism. From the electrical to the mechanical!

    Dave
     
  17. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    DaveG. I wonder if this is similar to some late model 400's having a 2..2K 7W resistor(R143on 400/ R133 on500B) btwn the 1st and 2nd B+ voltages vs a 1.2K 7W.
    Dave451; Looking at the 500B, with the wall voltage @ 117 what are the actual voltages along the B+ take-off's all the way from C88 (B) to take off "G" Have all the CAN CAPS been replaced? If you have an analog VTVM take the voltages with it vs. the DMM. The impedances in an analog VTVM are more in line to take voltages than a DMM would be.
     
  18. Dave451

    Dave451 AK Subscriber Subscriber

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    Thanks, Larry. I'll check with the line at 117 VAC using the Variac and report back. I didn't replace any of the can caps, as I laboriously checked each (by disconnecting the wiring from each terminal) for ESR, AC power factor, and (especially) leakage current at max working voltage with my Sprague TO-5 tester. They all run cool and there's not a trace of hum in anything, including the phono circuit at max volume. The unit had been in regular use when I bought it, so the caps all appear to be in good shape. Replaced all the small electrolytics under the chassis.

    No joy on the analogue VTVM--everything I have is digital. Maybe one day....

    Dave
     
  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    One other thing to consider is the conditions under which the schematic voltages were taken -- which is usually indicated as the unit being set for FM Stereo MPX operation, with the tuning pointer all the way to the left, no antenna connected, no station being received, and the volume control at minimum.

    Dave
     
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  20. Dave451

    Dave451 AK Subscriber Subscriber

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    OK, here are the voltages, measured at 117 VAC exit the CL-80 (PT primary) with the volume turned down and FM MPX engaged:

    Source/Schematic VDC Measured (VDC) 500C Schematic

    B+ (395) 402 430
    Screens/348 V 375 375
    320 V 356 350
    290 V 324 295
    G 232 215

    I'll need to check R126, 2.7K 1/2 W, as the jump between the 320 and 290V source looks pretty high, but otherwise the voltages look more comparable to the 500C voltages, which uses the same PS string up to the final resistor (82K for the 500C, vs 100K for the 500B).

    Alignment went well and got the dial cord problem worked out. More on that in next post.

    Dave
     

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