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Fisher 1800 Receiver Information thread and roll call

Discussion in 'Fisher' started by RS Steve, Jul 14, 2016.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Hi Dave - I'll look for my notes on that restore, but I think their filed away. In any event, I'll look for them. I remember the Stereo Beacon lamp issue in his unit finally coming down to the lamp socket, as the Beacon would not light consistently when it was supposed to. The Beacon should come on -- and stay on at all times -- when the Selector switch is turned FM Stereo Filter. This is a forced stereo setting, so the lamp will be on even if tuned to a mono station. If it will not come on when you believe it should, first make sure that there is good strong reception. Then make sure that there is in fact stereo information being properly decoded by the MPX chassis. After that, check my thread on "Getting the most out of.........." where in I address the dancing eye tube in the 400 receiver. It is applicable to the 500C and 800C as well.

    If the center of channel seems to not be in the center of channel, then a full alignment may be called for. At the very least, the Ratio Detector transformer is very likely out of alignment with the rest of the IF strip.

    Sounds like you've got a good project on your hands -- but oh so worth it when you're finished!

    Dave
     

     

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  2. RS Steve

    RS Steve Tube Junkie Subscriber

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    Here is a picture of the bias control if it refreshes your memory.

    Fisher 1800 bias control.jpg
     
  3. Dave451

    Dave451 AK Subscriber Subscriber

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    Thanks much, guys--excellent stuff. I've been through Dave G's 1800 restore thread and the 800C service bulletin, so I'll be following up on all that. I'm up to my elbows in the B+ and bias/filament supply right now. One of the replacement can caps was loose in the socket and a key chassis ground (attached to one of the rivets on the paper-covered divider cap) was loose and had to be drilled out and re-seated with a screw/nut/star washer. I didn't like all the flying leads, so dealing with that as well. Removed the 2x1000 uf bias cap clip to make room for the IBAM board. I decided to mount the new 1000 uF bias filter caps on the IBAM board as well. I've already put in new 7591 cathode resistors and the 100 ohm screen resistors, using a Pin 1 daisy chain on the 7591's.

    HERE"S ANOTHER QUESTION: the power amp couplers are nice Russian PIO's at 0.047 uF, but also come with the original 330k grid resistors. I'd like to not have to replace the 0.047 uF couplers with 0.1 uF if I change the resistors to 200K or 220K. With IBAM in place, how worried should I be about replacing the 330K's? Thanks!
     
  4. RS Steve

    RS Steve Tube Junkie Subscriber

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    I would replace with .068's and 220K then save the PIO's for something else.
     
  5. Dave451

    Dave451 AK Subscriber Subscriber

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    Steve,
    That's exactly what I did. More to come....
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Dave451; Here's a detailed explanation of the Major mods to 400-500/800C (incl the 1800 now) from Dave G. It's about 5 years old so disregard the .1uf coupling cap size, as .068uf is actually closer to the ideal of .072uf (he determined this later) when used with 220K ohm resistors. So just substitute .068uf for .1uf.

    START EXPLANATION
    400-500C-800C MODS for BIAS and POWER SUPPLY
    1.) replace the 4 coupling caps with .1uf/630V caps (OP Choice of cap)
    2.) replace 300k grid resistors with 220K R 1/2Watt.
    3.) install 10oHm 1/4W resistors to ground @ pin 5 on each output tube.
    4.) install 100oHm 1/4W screen stabilization resistors to jumper @ pin 3&8 on each tube. ( I'll give them a try)
    5.) install 10K variable resistor (pot) in BIAS POWER lead to facillitate some adjustment in the Bias circuit.

    Explanation by Dave Gillespie;
    If I may, here is some detail behind the recommendations for these modifications:

    1. "Reducing the grid resistors to 220K": The published maximum DC resistance for grid #1 of 7591 class tubes is 300K (by RCA, Sylvania, and others). This is the maximum value allowable when the tube is operated under conditions of maximum dissipation -- which many designs nearly did -- so yes, using 330K is using a value that is right on the ragged edge.

    The reason for the maximum resistance specification is to limit the value of reverse grid current voltage that can build up under conditions of maximum dissipation (read that, maximum heat). But there are other concerns that can aggravate this issue as well. Higher line voltages push the heater voltages higher, which means that today, even more heat is generated. The basic concern is that excessive heat (however it's generated) can cause cathode material to boil off and deposit itself onto the grid. This means the grid has now become a partial emitter, with any current flow it provides developing a voltage across the grid resistor that is opposite in polarity to the negative voltage applied to the bottom of the grid resistor. This negative grid current voltage then works against the negative bias voltage, causing the tube to conduct more current, generating even more heat, boiling off more cathode material, and ultimately causing thermal runaway. The production of gas comes into play with these events as well to further aggravate the issue. Therefore, to help control this condition, a maximum DC resistance is specified for the control grid to limit the amount of ill effects that can be caused by reverse grid current.

    As a result, with the high temps that these units typically run at inside their cases, and importantly, because of today's higher line voltages, and because the tubes are biased "on" pretty hard to begin with, it makes very good sense to pull the value of the grid resistors back a little to help minimize the chances of developing any significant reverse grid current voltage, and the possible thermal runaway that can produce -- and this is even before the quality of modern production tubes is considered!

    2. "Double the value of the coupling caps": This has nothing to do with loading on the driver stage, in fact, lowering the value of the grid resistors actually increases the load on the driver stage. The reason the cap values are increased is to keep nearly the same time constant in the RC coupling circuit between the driver stage and the output stage with the new lower value grid resistors installed.

    3. "Install 10 Ohm cathode resistors": This makes for the easiest way to monitor output tube current draw, and therefore allows for the easiest way to set the bias and balance of the output stage (if such controls exist), or check to see that matched output tubes are in fact matched. These resistors are typically used for balancing currents under quiescent conditions, but can also be used to check for balance under dynamic conditions if the proper equipment is available. Balancing output tube quiescent current maximizes OPT low frequency power handling ability (by minimizing core saturation), while balancing dynamic currents minimizes overall distortion.

    4. "Installing a 10K bias pot": Modern tubes are manufactured with far less precision than the best tubes from yesteryear were, and so, fixed bias amplifiers operating with tubes of modern manufacture on higher line voltages will usually need some adjustment from the original fixed setting a manufacturer provided. Installing some type of adjustable control is therefore highly desirable, and adding a DC balancing feature to that is even better. Installing these controls so that they cannot be inadvertently altered from their setting goes hand in hand with the installation of any such controls.

    5. "Installing 100 ohm Screen Stability Resistors": So many folks have posted on various forums -- from all degrees of experience -- how everything is as it should be, but the output tubes will arc for no reason that they can determine. How expensive does that get when output tubes can be ruined in an instant?

    Long ago, I determined that these arcs were in fact occurring from the screen grid. Many pieces of old hifi gear were designed with very little resistance in the screen circuit, which usually didn't present too many problems as the power supply caps of the day had much higher ESR values, which tended to help keep things stable. Even at that however, some designs (Eico HF-89 for example) were simply notorious for arcing output tubes because of the larger overall power supply capacitance values they employed.

    Today it is all too common, with the installation of new power supply caps that have much lower ESR values, and which are so often increased in size as well (all installed on new drop in power supply boards), to have all manner of output tube arcing events occur, which leaves folks puzzled because it just never used to do that before. New manufacture tubes got the lion's share of the blame -- until even NOS tubes were arcing in these refurbished pieces of equipment as well.

    With some form of transient trigger (music, an open ground, whatever), the lower ESR value caps can set up incredibly strong oscillations in what is effectively a tank circuit that's formed with the screen grid, and can cause an arc at any time. Installation of 100 ohm Screen Stability resistors (for 7591 class tubes) stops this problem virtually every time. I've never had anyone tell me that installation of these resistors did not stop the problem. Personally, I've never had an output tube arc event in nearly twenty years now, where before I had boxes of blown tubes until I could determine what the cause was, and develop an appropriate cure. In equipment such as this Fisher represents, it is very cheap insurance against such events, and produces no audible or measurable difference in performance.

    Hopefully, this will help with an understanding of why these modifications can make such worthwhile improvements in so many of these wonderful instruments. With some components being virtually unobtainable now, minimizing any damaging events that might occur helps all of us to enjoy this equipment even more.

    Good luck with your receiver!

    Dave

    END EXPLANATION
     
    Last edited: Nov 20, 2017

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Dave -- I found my notes on the adjustable bias circuit I developed for Steve's 1800. His unit originally came with a bias voltage divider made up of an 8.2K series resistor between the bias supply and the bias feed to the output tubes, and a 27K resistor from the bias feed to ground. A .1 uF cap paralleled the 27K resistor.

    Due to space constraints, I used a piece of perf board to mount the original 27K resistor and new .1 uF cap (replacing the old .1 uF since this was a full recap), as well as a 10K linear trim pot, and 47K and 4.7K .25 watt resistors. The pot and two new resistors collectively replace the original 8.2K resistor as follows:

    1. The 47K is connected across the full resistance of the 10K pot (two outside terminals).

    2. The 4.7K is connected between the bias supply side of the 10K pot (one outside terminal) and the wiper.

    3. The other outside terminal of the 10K pot is the output to the 27K resistor, .1 uF cap, and feed to the output tubes.

    The board is designed such that when the control is set full down, the bias voltage to the output tubes is identical to that provided by the original voltage divider components. As the control is turned up, the bias voltage is increased above that supplied by the stock design to accommodate today's higher line voltages and hotter tubes.

    I hope this helps!

    Dave
     
  8. Dave451

    Dave451 AK Subscriber Subscriber

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    Well, been working on the 1800 for the past several days and progress is being made. Work included:

    1. Reworked all the B+ power supply. I checked the remaining stock can cap (C91A-D, 200 uf, 3X50 uF, all 250VDC) and although leakage at working voltage was low, every capacitance value was about 1/2 or less the nameplate value! I cross-checked my TO-5 against other new/known caps and, sure enough, C91 was low across the board so I replaced it with a new CE 4X50 uF, 350VDC using three of the 50 uF sections and a 220 uF 300 VDC F&T axial.

    2. Completely reworked the already installed CE 40x3 uf, 525 VDC cap. The can was loose (only one ground lug soldered and it had broken free of the chassis!) and the stock can tabs had simply been cut off with the wiring still attached and tacked to the new cap lugs. Replaced R115 (82K) and R117 (22K) with 1/2 W metal films.

    3. Pulled the replacement Sprague Atom 40 uF 500VDC screen cap (replacing C87D section). It was adequate for voltage and capacitance, but only rated for 65 degrees C and this worried me a bit, so I replaced it with a 47 uF 500 VDC IC rated for 85 degrees C and moved it toward the output tubes, securing with adhesive to the divider around the input wiring.

    4. Added a CL-80 between the switch wiring and one set of PT primary legs (two sets of primaries wired in parallel for 120 VAC service). Checked the fuse, as Dave G had recommended and, it was a 5A slow blow! Replaced with 3 amp SB (the chassis says 1.5 A because it was originally wired for 240 VAC).

    5. I removed the clip for the original 2X1000 uF 35VDC bias cap and added a couple of terminal strips and rewired the bias/12AX7 filament supply with modifications to add Dave G's IBAM design, which I then did install (removing the -17 volt divider of R119 and R122 and the C92 0.1 uf cap). I changed the 15 ohm 5W resistor to 18 ohm 5W to get about -24 to -25 VDC for the bias/filament supply. I installed the IBAM just above the new 47 uF screen cap on a 1 inch standoff over the new C91 50 uFx3 can cap. I replaced the 10K IBAM 'tail' resistors with 15K, as I had done for the 500B I rehabbed before. See pictures.

    6. I got rid of the poorly-installed paralleled 2X100 uF 450VDC axials that had replaced C98 (200 uF 250VDC cardboard insulated can). I removed the original cardboard-covered cap from the phenolic wafer, removed the cardboard cover and scraped out the remaining black mastic from the top, and inserted a 220 uF 385VDC JJ wrapped with Teflon sheeting and a little PVC tape for a good friction fit. Expoxied this onto the original phenolic wafer, which I had left in place. The chassis ground lug held by one of the rivets from this cap was loose, so I drilled it out and replaced with a screwed down terminal strip with a star washer.

    7. Removed the ferrite AM antenna for some restore work. It is electrically sound, but the end caps are deteriorated and broken, so I'll fix these and re-attach with two 1 inch plastic cable clamps, as recommended above.

    8. Replaced with HV rectifiers with 1000 PIV 1.5 A.

    9. As recommended above, I replaced the output tube couplers with the "generic yellow" IC 0.068 uF units, 630VDC. Replaced the 330K grid resistors with 220K on three-point terminal strips to avoid flying leads (see pic).

    10. I did Dave G's "800C Service Bulletin" to replace 1/8 watt R86 (270K) and R83 (820K) feeding the diode switching matrix for the stereo beacon with 1/2 W metal films. The 270K measured OK. The 820K, however, measured OK but there was NO downstream voltage--it was gone. Changing it out gave proper voltage downstream. The Stereo Beacon is still not working, but I also cleaned the relay contacts and it is switching. I think the beacon problem is due to need for alignment (see more late).

    I put in the old 7591's and tried to balance and bias them, but the left side set (V10 and V11) would not balance and achieve anything close to 33-35 mA cathode current on either. Fortunately, I had a pretty good Fisher Label 7591 on hand, replaced the apparent weak sister (V10) and everything balanced and biased well. Set all the tubes to 35 mA cathode current and all is well with the amplifier. Sound is quite good with the new components!

    So everything is playing well, but distortion is still present on FM stereo and non-centered signal capture, so an alignment is called for. I think (hope) this will also deal with the non-functioning Stereo Beacon function. Electrically, I think the unit is in good shape now. Need to spruce up the AM antenna and reinstall.

    Voltages at 121VAC line as follows (schematic values in parens): Line voltage after CL-80 119VAC; B+ 434VDC (430); Screen 393 VDC (375); B3 365 VDC (350); A5 323 VDC (300); 215V supply 222VDC (215); C91D + 215 (214); Post R129 192 VDC (187); Post R121 169 VDC (160). The screens run a little hot, just like the 500B I did before. Bias supply -23.7 VDC. The 1200 ohm 7W screen resistor measures OK, but I will replace it before I'm done (a little discolored, as noted before).

    At the output tubes, plates were 431 VDC (430); screens 393 VDC (375); cathodes at 350 mV (35 mA); Grids running right around -18.x VDC.

    Everything is behaving well at this point--on to the alignment! Pictures next post.

    Dave
     
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  9. Dave451

    Dave451 AK Subscriber Subscriber

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    Pictures for the 1800 Work so far:

    1. "Re-Stuffed" C98 voltage divider cap before re-install
    2 and 3. New terminal strips for 220K grid resistors to avoid 'flying leads'. Strips are anchored by screws through vent holes behind the tube shield.
    4. Power amplifier section with all work done except for the '800C Service Bulletin', including new IBAM
    5. Re-worked bias/12AX7 filament supply
    6. Replaced R86 and R83 from the 'Service Bulletin'

    Dave
     

    Attached Files:

  10. RS Steve

    RS Steve Tube Junkie Subscriber

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    Nice work Dave, you'll have it working better than new in no time! :thumbsup:
     
  11. Dave451

    Dave451 AK Subscriber Subscriber

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    Got the FM alignment done and it lined up like a champ! The ratio detector was quite a bit off, but the RF and IF strip were not off much at all. With the alignment, the Stereo Beacon function began to work again and lit on a few strong stereo signals, even in the basement with just a clip lead for an antenna.

    I reworked the ferrite AM antenna to replace the rotted end caps and re-mount it on the swing arm in back with a couple of one-inch cable clamps. When I checked things on AM, there was very little signal coming through. Thought I had messed up the antenna, but it checked good for continuity, so I hooked up the Sencore SG-165 and got good modulated 455 kHz IF in through the mixer, but only a very faint RF signal through the antenna terminal with the Sencore at full output.

    I suspect the 6BA6 RF amplifier tube. It checked a bit gassy when I tested it and I think the cycling during bench work killed it. Voltages on the front end tubes were all good. Ordering a new 6BA6, 6BE6 for the AM front end from Jim McShane. I'm also going to get a new 6GK5 high-mu triode for the Stereo Beacon to have on hand.

    So, the amplifier and FM radio are great, but I need to get AM going again before I put it back in the case and bring it up to the listening room.

    Couple of pictures of the ferrite antenna repair. It's sturdy and not beautiful, but a lot better looking and structurally sound than what I had before!
    Dave
     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I like it! Beauty is in the eye of the beholder. If it's neat and durable and performs its job well, then that's beautiful. Besides, if you're looking at that side of the 1800 all day, then you're looking at the wrong side!

    Dave
     
  13. Dave451

    Dave451 AK Subscriber Subscriber

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    Well, the AM RF tube did indeed give up the ghost on the bench and a new one resolved the AM reception issue. Put it all back together and here it is. What a beautiful beast! It's in the listening room on the "receiver bookcase" along with my Scott 340B, Dave G mods Fisher 400, and Scott 350B with 299B (the Scotts are idle right now--that's why the tuner is sitting atop the amp).

    Right Beethoven Symphony No. 2 is playing from the local station (vinyl--I can hear the tiny pops and the album is warped very slightly--can hear the 'thump' every now and then). It is a mighty sound through the AR-4x speakers!

    So, this one will stay in rotation for quite a while I think. Since replacing the weak 7591 output tube, the amp is sounding great biased about 35 mA cathode current per tube. I have a couple of ongoing questions, but I'll start another thread.

    Dave

    PS The mounting clamps covered up part of the warning label on the AM antenna; right now it says "Use As A Handle." I'll resist the temptation until I can get a "Do NOT…" on the label! :)
     

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  14. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Home Depot has those clamps in translucent white (you know what I mean) and they let enough of the "DO NOT" thru. OR if you can duplicate the label, just run it over the clamps when installing the antenna on the clamps.
     
  15. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Beautiful, wish I had a rack like that for my gear! Al
     
  16. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Racks are for females built like a Brick sanitary facility, or a BIG BUCK, Al!

    (I know, I got Fire Watch in the Lower level #2 Boiler Room)
     
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  17. Dave451

    Dave451 AK Subscriber Subscriber

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    LOL, Larry--Much of my family has been in the woods looking for the latter! On a more mundane note, I'll probably opt for the re-label on the antenna.
     
  18. Dave451

    Dave451 AK Subscriber Subscriber

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    Steve,
    I re-read Dave G's old post on his 800C and saw that he and Larry were collecting information on 800C output transformers. In my "1800 Stereo Beacon" thread, I posted this info for my 1800, and thought I'd put it here since you called for info:

    Fisher 1800 S/N 52468T (Originally wired for 220-240 VAC--dual voltage PT now wired for 120VAC)
    OPTS: T991-116-2C and T991-116-1C, both also stamped 1005702

    Dave
     
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  19. RS Steve

    RS Steve Tube Junkie Subscriber

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    Another Fisher 1800 popped up on the auction site (no affiliation), looks like someone drilled a hole in the face for a replacement on / off switch at some point. :crazy:

    Serial number will be documented in this thread and seems to fall into the assumed production number of 1,000 units. 51996 T

    51564T - RS Steve
    51611T - lithography
    51674T - online
    51692T - RS Steve
    51694T - for sale online
    51996T- online
    51997T - Ivorydan
    52127T - Tuna
    52154T - Ivorydan
    52293T - RS Steve
    52406T - RS Steve
    52468T - Dave451
    52478T - coryo
    ?????? - Fisherdude


    Fisher 1800 on ebay.jpg
     
    Last edited: Jan 12, 2018
  20. Tom B

    Tom B AK Subscriber Subscriber

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    looks like Steve has 29% of all known 1800's out there.!!!!!
     

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