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Service Bulletin -- 800C

Discussion in 'Fisher' started by dcgillespie, May 28, 2014.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    Here's one that slipped through the cracks in the engineering department at FRC on at least the 800C receiver I recently acquired (serial# 38069H):

    Symptom: Poor, distorted sound quality when operating in FM mono mode only, either by selector switch selection, or due to FM Automatic mode dropping out of FM Stereo operation. Sound quality in any FM Stereo function normal.

    Check: R86 for open condition. This part is originally specified as a 270K 1/8W resistor, and is the component that was factory installed in my unit. However, under normal operation, this resistor is dissipating at least .305 watt at all times, and more during operation in FM Stereo mode. This represents a continuous dissipation level that is at least 244% of the component's rating, not to mention the elevated under-chassis temps it operates in as well. This causes this resistor to ultimately fail, but not in a visibly obvious way.

    Circuit Action: R86 provides a fixed bias voltage to the FM Automatic matrix switching circuit. When R86 opens, this bias voltage is removed. The switching bias voltage can then only forward bias the matrix diodes for stereo operation, but not mono operation.

    Repair: R86 should be replaced with a resistor of the same value, but rated for at least .5 watt, and preferably a 1 watt rating in all units employing FM Automatic matrix switching.

    It is also observed that R83 (820k 1/8W) operates at 120% of rated dissipation based on normal operation, and the specified component being installed. While the originally installed component has not failed in my unit, conservative design would dictate a .5 watt rated component be used in this position as well.

    For all units using 1/8th watt rated resistors in these locations, replacing both of them with components of the same value, but rated at .5 watt should be part of any quality refurbishment effort.

    Dave
     
    Last edited: May 29, 2014

     

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  2. notdigital

    notdigital AK Subscriber Subscriber

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    Well, out comes the solder!

    Thanks Dave :thmbsp:
     
  3. drewbolce

    drewbolce Audio Analyist

    Messages:
    572
    Location:
    Oak Ridge, TN USA
    I always thought that the small electrolytic C65 was the source of trouble in this area. It should always be replaced anyway, along with C67.

    If you have (not you, Dave) touched C66 (the only polystyrene) with your soldering iron this whole circuit will give strange and wonderful results. You can guess how I know that.

    This circuit is used in the 500c also, so all of this applies there with the necessary component number changes .
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I just checked the 10483A Exec's 800-C. It's got 1/8w. BUT the values are different. R86 is 220k, R83 is 680K and R77 is 820K. R83 and R77 on the 10483A are reversed from my 43355H model, but they are all 1/8w on both units. Looks like a Ratshack run if I don't have any 1/2 watters.

    Other than the values on R83 and R77 being reversed on the 10483A they are identical in the section.

    Would you recommend changing the 10483A to match the 43355H or leave it alone for values???

    10483A EXECUTIVE
    [​IMG]

    43355H Standalone
    [​IMG]


    Larry
     

    Attached Files:

    Last edited: May 29, 2014
  5. drewbolce

    drewbolce Audio Analyist

    Messages:
    572
    Location:
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    R77 and R83 are swapped on the two schematics. I suspect one of them is wrong. I'll let Dave look into this further.

    Yes, there are errors on lots of Fisher schematics.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Larry -- The effect of the earlier resistor values is to apply greater biasing voltages (both fixed and switching) to the matrix diodes relative to ground. However, the voltages shown on the two schematics don't support this, which means the B+ voltage between the two units is in question. In my unit, B2 is specified at 350 vdc, where as in at least your early unit, "B" is unknown from the schematic snippets supplied. Until the B+ supply voltages are known for these two units, I would not arbitrarily change the components. However, even in your earlier unit, R86 as specified is surely being well over-dissipated if the original device is only a 1/8W component.

    Drew -- Excellent point. The small value electrolytic caps are always suspect over time, frankly even more so than those in the PS cans. The effect of C65 developing a high power factor (basically shorting) would be the same as R86 opening up (symptom wise), and would in fact hasten the demise of R86. The effect of C65 going open (most common failure mode for small electrolytics) would be to elevate hum in all modes of FM operation, and reduce separation during FM Stereo operation.

    Dave
     

     

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  7. drewbolce

    drewbolce Audio Analyist

    Messages:
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    Dave, don't get stuck on minor voltage differences. What we have here is a schematic error. I'll look at my 500c and see what the values are for each of the resistors.
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    OK. According to the Executive 950 Schematic the B+ for that area is coming off the "B" marked cap section or C87C with a value of 320V. I get 324V

    On the 30001 to 49999 manual the B+ is also fed off the "B" marker, C87C with a value of 350V. I get 357V.

    Drew: The 10001-19999 series was exclusive to the 1963(Model 950) Executive Console. The 20001-29999 started the standalones in 1963 model year. The higher numbered of the 20001-29999 may have carried over to the 1964 standalones. The 1964 model 960 Executives used the 30001-39999 series as did the standalones. (Not sure why, probably parts count and commonality of parts, plus the shift over to SS Units in 1964/66.) There are numerous differences between the 10001-19999 and the 20001 and higher. Output transformers, different coupling cap sizes in the tone controls, NFB Circuit, Center Channel setup, etc. I'm tending to think this is one more of the changes between the 950 Executive and the rest of the 800-C series.

    C65 and 67 were changed on both units to 2.2uf and .47. C65 with a Nichicon lytic and C67 with an ECQ Film. I did have to change out C66 (1800pf) with 4 paralleled 470pf wima's on the 43355H. It lost a battle against a soldering iron. replacement works fine.

    Larry
     
    Last edited: May 29, 2014
  9. drewbolce

    drewbolce Audio Analyist

    Messages:
    572
    Location:
    Oak Ridge, TN USA
    Ok, on the 500c the 680k resistor goes to ground. This means that the "43355H Standalone" schematic is the correct one.

    Dave can you verify that your 800c has the 680k grounded?

    Of course Fisher moved this part of the circuit down by the power tubes on the 500c, just to make tracing it more fun.

    Good history Larry, there are many variations of this part of the circuit as you have noted. I think they were having trouble in this area all along. However, in this case I am almost certain it is just a schematic error.

    I think soldering irons are drawn to Polystyrene's, just a hunch.
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    In my 800C, the 680K is in fact grounded.

    Here's the reason I question if there is a simple schematic error: Besides these two resistors being swapped between the schematics, there is also the fact that the earlier unit used a 220K in place of the 270K for R86. This means that BOTH the fixed and switching bias voltages have effectively been raised in the earlier unit. BUT, that increase would only be true if the B+ supply to the circuit remained the same in both units -- but now we know it didn't. The B+ supply to the circuit is lower in the earlier unit, but the bias voltages are (about) the same due to the different component values shown on the earlier schematic. As a result, I'm simply saying that the case can be made that the resistor values were potentially not swapped, but correct as shown.

    Ultimately, we just won't know until someone can take a peek in an earlier unit to verify the circuitry. When the number of schematic errors becomes significant enough, it just makes it very hard to decipher between legitimate engineering changes, and oops......

    Dave
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Dave & Drew; On the 10483A the 820K is grounded as opposed to the 43355H which has the 680K grounded.

    I'm fairly sure Perchdog has a 10xxx series unit also.

    I'm out of all of those values so it looks like I'll have to make a mouser order for them along with some parts for the 160-T (next on my rehab list).

    Larry
     

     

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  12. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I just changed the resistors on the Executive (10483A). The values are as per schematic for the 10001-19999 series. 220K, 820K, 680K. They are laid out electrically per the schematic. I hand traced them from the diode board, and out 2 components on each lead. all values matched schematic.

    I'm thinking FISHER was still trying to get full compatability between their receivers and multiplexers, and consolidating mpx units down to 1-2 for the range of component tuners and receivers. At 1st glance, this Executive 800-C (63) placed next to a standalone 800-C looks identical on top. But soon enough you notice that the opt's are slightly different in size. Open it up it looks at glance just like any other 800-C until you go digging and find differences in the phase inv /output section, mpx, and other minor stuff.

    At this point I'm tempted to say its not a TYPO. Everything on the Matrix circuit is as the schematic says it is.

    I'll try and lug the 45000 up stairs tomorrow and change the resistors out.

    Larry
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I did the 45533H 800-C and am listening to it right now. The resistors were installed as per it's schematic, so there is not a TYPO (AFAIC), with the 820K and 680K reversed from the 10483A version. Both units have the MX-65 mpx unit. I find that both work better with a 12AT7 in V100 position as far as grabbing stereo. Put a 12ax7 in and unless the signal is strong as all get out, the 45533H won't trip to stereo in FM STEREO AUTOMATIC and the 10483A trips only with a signal level higher than 2-1/2. They both need a complete alignment, but there is no one in the baltimore are that can/will do them that I would trust (except Merrylander, but he's retired and not doing alignments anymore).

    This is on a powered Winegard FM / TV Antenna on the Trailer. The amp on the antenna was replaced last year as was the wall jack plate. Even with a DIPOLE outside and on top, I can't get them to trip to Stereo unless the selector is set to FM STEREO FILTER.

    When I get them in the house and on the Big roof antenna we'll see what's what.


    They are good enough for casual listening, so I can live with it.

    Larry
     
    Last edited: Jun 22, 2014
  14. vintelectra

    vintelectra Active Member

    Messages:
    473
    Hello fans. Thanks to this thread and Dave insight, I have solved a big headache in this 500c. R 86 in the 800c is R 75
    In my schematic . Thanks again Martin
     
  15. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Dave is this on the existing 800C you have had or a newer one? Just curious.... Mines a serial # 36K.
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    This first came to light on my own unit, and has been verified on others of around the same serial number.
     

     

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