Questions about treble boost mod

Discussion in 'Fisher' started by thornev, Apr 26, 2018.

  1. gadget73

    gadget73 junk junkie Subscriber

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    It goes to "In +". Remember to connect the output back to "In -" or it won't be unity gain.

    Basically what you're doing is duplicating the top part of the schematic, but using the pin numbers on the lower symbol. The power supply part is already taken care of, so its basically 3 resistors and 2 caps you need. Just make sure you connect the 1M resistor like the fully drawn out version of the schematic indicates. Thats what the "To Other Channel" note is for.
     
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  2. thornev

    thornev Well-Known Member

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    Thanks, gadget. This means that I have to duplicate using the right channel op-amp all the parts that are connected to the left channel op-amp, the 3 resistors and 2 caps as you said. The 1M resistor connects to pin 5 on the right channel op-amp, but to what does the OTHER side of that resistor connect? To the same bias lead as the 1M connection in the left channel op-amp?
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    Yes. If you look at where it says "to other channel", that is electrically the same point as the 1M resistor in the upper part of the schematic.
     
  4. thornev

    thornev Well-Known Member

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    OK thanks. So there are 2 wires that connect "From Level Control". Larry said I should use the center tab on that control. It appears I do have 2 center tabs (see attached image). I suspect that one tab is for left channel and the other for right channel so I guess it's important that I get the left and right connections correct.
     

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  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Yep! On the Volume control You have 3 tabs on each pot (which are Left and Right, You'll have to figur that one out tho, My memory for such esoterica is not as good as it was.). Looking at the pot from the back of the unit, the tabs LEFT TO RIGHT are GROUND, Wiper(or output to the Driver/P.I. section of the amp), and input(from the Balance control).

    Disconnect the shielded cables from the Wiper. Leave the grounds as is.
    Connect the wires to the Buffer INPUT to the Wiper Tab (1ea to a pot).
    Connect the shielded cables to the Buffer OUTPUTS maintaining left to right orientation.
    Your Done.
     
    Last edited: Apr 27, 2018
  6. thornev

    thornev Well-Known Member

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    In all the sample installation images of this unity gain mod, I see only 1 e-lytic yet the schematic calls for 2 on the left channel - a 10 uF and a 1000 uF. Why is that? See attached images. Schematic is in post# 1. I want to make sure I build the mod correctly.
     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    probably mounted externally. Wild guess, its the black capacitor to the lower right corner of the pic.
     
  8. thornev

    thornev Well-Known Member

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    Hopefully the final question... connections to the right channel of the op-amp chip... "to other channel" from left channel of chip goes into .1 cap which goes into pin 5 of chip. Also connected to pin 5 of chip is the 1M resistor whose other end is connected to the bias lead. pin 7 of the chip connects to pin 6 of the chip and also connects to the .22 cap, the 100 ohm resistor and the 220K resistor whose other side goes to ground. Do I have it right?

    And for the entire circuit, there are only 3 inputs: the left and right channels of the volume control, and the 5mA bias. Pin 8 of the chip goes to ground. Anyone know from where I can get 5mA bias? Do I have to use a digital multimeter to find where in the entire 500-C I've got 5mA?

    And finally, 7 parts go to ground:
    chip pin 8
    220K x 2
    47K
    .1 50V
    1000 35V
    10 35V
     
    Last edited: Apr 28, 2018
  9. thornev

    thornev Well-Known Member

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    Now I have franken-unity-gain mod to go with my franken-500-c. I haven't figured out how to make pretty designs. Just functional ones. I think it's correct. Some of the connections are on the bottom of the board. The long wires that stick out are the left and right leads to the volume control and the short wire is the lead to the 5mA bias supply. I haven't figured out where to install it. After I trim it, it might go behind the volume control.
     

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  10. thornev

    thornev Well-Known Member

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    I'm about to solder the traces on the back of my Unity Gain mod board. I'd appreciate it if someone could verify that my map of traces is correct based on the schematic. The red lines represent how I will solder the parts together. I apologize if my traces are hard to read. I couldn't think of any other way to map it out for myself.

    One thing I'm unsure of is how to figure out where to tap in to get -7.5V and -15V as written on the schematic.

    There are few places where I'm not sure that I've electronically connected parts correctly. Thanks.
     

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  11. thornev

    thornev Well-Known Member

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    I'm ready to install this board. One final question (I know I keep saying that) before I blow up my 500-C...

    In my last post I presented my map of traces I'll create that connects the components together on the underside of the board. In doing so I see that I've connected a -15V lead to a -7.5V lead. I did that because it seemed efficient to connect some right channel leads to left channel leads simply because the parts are duplicated between the left and right channels. So rather than duplicate the voltages on the left and right channels and since some components are in proximity to each other, I use the voltage present in the left channel for where it's needed in the right channel. But I didn't account for the fact that the left channel on the schematic shows -15V and -7.5V.

    Thinking about it now, it seems like a bad move although I don't know the maximum voltages that all the parts can handle. The capacitors are rated higher than 15V so I think I'm OK with those. An example is that I have the 1M ohm resistor on the right channel (on the schematic for the left channel the portion of the circuit with the 1M ohm resistor displays -7.5V) connected to diode IN4007 which is where the circuit is supposed to get its 5mA which is labeled on the schematic as requiring 15V. I guess the question is "is it alright to have -7.5V and 15V going through the two 1M ohm resistors?"
     

     

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  12. thornev

    thornev Well-Known Member

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    No one? I guess I'll try hooking up my project to a breadboard power supply so that if I've screwed it up, I'll break only the project board and not my entire receiver.
     
  13. diddlebunk

    diddlebunk New Member

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    The thread seems to have dropped off. Did you complete the mod, Thorne? How did things turn out?
     
  14. thornev

    thornev Well-Known Member

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    Diddle... Yes I did complete the project, but when I installed the board, all I got was horrible distortion. And when I turned the BALANCE control full right or left, I ended up getting the audio from that channel in both speakers. Perhaps I have too many mods interfering with each other i.e. they don't coexist well. No one offered any feedback except one person who suggested that I fiddle too much and that I should appreciate my accomplishments, stop modding. So I took his suggestion...sort of. Today I replaced the 12AX7 preamp tubes with 1956 Tung-Sol 12AT7WA's. I like their sound, but I have to investigate to ensure I'm not heading toward receiver doom by installing specs from those tubes that the rest of the circuitry can't handle. What's up with your work? Thorne
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    guessing the board isn't assembled right, but its pretty near impossible to tell that from pics. It wouldn't be a case of things interfering with one another unless something wasn't hooked up right.
     
  16. thornev

    thornev Well-Known Member

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    Gadget... I checked the traces on the bottom of the board against the schematic multiple times so if it's that I built it wrong, then it's because I don't understand the schematic. Which is quite possible. Having the IC split in two on the schematic made it difficult for me to understand how the leads in the 2 parts connect to each other and to the outside (outside of the board) world.
     

     

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  17. thornev

    thornev Well-Known Member

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    I'd like to resurrect this thread because I've decided I want to convert the reverb jack J11 into a PREAMP OUT jack. I don't care about the AMP IN jack (J12 and J16). I just purchased a McIntosh MC250 amp and I want to put the 500-C preamp through it. I scoured this forum but could not find instructions on how to do it. I know it's supposedly illustrated in version 13 of Dave G's schematic, but I'm unsure how it's done. In the schematic it looks like if I attach the Unity Gain mod to J11/J15 and J12/J16, I've got what I want. Is that right? For one thing I don't understand that the PREAMP OUT jack is labeled J11 and J15 when there is only one physical jack. Thanks, Thorne
     
  18. Dave451

    Dave451 AK Subscriber Subscriber

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    Hey, Thorne. Was looking at your thread and offer the following:

    1. The description for converting the 'space expander' jacks to 'pre-out' and 'amp-in' jacks is contained in Dave G's "Improving the Fisher 400" thread, as are the instructions, schematics and info on the unity buffer mod installation (you likely know this).

    2. The schematic with the final wiring of the new jacks (which must be very similar between the 400 and the 500C in this part of the circuit from a quick look at the original schematics) is in post #103, which may be the one you are referring to. The description is pretty much verbal in earlier posts in the thread--I don't think there's a step-by-step in this or other posts, but I may be wrong on that. Dave is basically taking the space expander jacks out of the original circuit and re-purposing them in place for the pre-out/amp-in service. This gets woven in with the removal of the audio bleed circuit, tone control bypass, etc and is a little confusing. I didn't do the space expander jack or these other switching mods, so can't give you any direct experience there.

    3. The 500C service manual does indeed show four physical jacks (2 pairs) on top of the chassis with two curved jumpers between them. To install the space expander, you pulled the jumpers and plugged in each channels SE inputs and outputs as appropriate. This is the same scheme for the pre-in/amp-out jacks. Leave the jumpers in (one per side) unless you want to use the pre out or amp in functionality. I don't see how you can do the 'pre-out' without doing the 'amp in' because of the way jumpers are used. Once you break the circuit and install the former space expander jacks in it, you have to have the 'amp in' jacks in place to receive the front end input to the power amp through the jumper when you want to resume normal operation of the 500C.

    4. Later in the thread, Dave does expand on how to wire the single op amp chip (containing dual amplifiers!) based on questions from those following along. The single 1000 uF cap is a remnant of the original bias supply circuit which is repurposed because EFB is installed and the original bias supply is no longer needed in that case, so Dave decided to use it to power the unity buffer circuit. There is only one power supply circuit for both sides of the unity buffer chip, which is why you only see the one large 1000 uF cap and one small 10 uF 35V cap on the little board.

    Hope you get things lined out on your 500C--good luck!
     
    Last edited: Dec 13, 2018
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  19. thornev

    thornev Well-Known Member

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    Of course ! How could I be so dumb not to realize that there are 4 jacks. Oh well. So from the schematic it looks to me that instead of connecting the leads to the center tabs of the volume control, they are connected to the 4 jacks. Is it that simple a difference?
     
  20. Dave451

    Dave451 AK Subscriber Subscriber

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    Per the schematic below, you still connect the INPUTS of the unity gain buffer to the wiper of the Volume control. The buffer OUTPUTS (coming off the 220K resistors) go to the first set of jacks (which become pre-out) and the second set of jacks (which become amp-in) connect to the input grid of the driver tube (note the 10K driver grid/input resistor, which I recall is added). In normal receiver operation, a jumper is installed between each pair of jacks to reconnect the buffer with the driver.

    Note how the single negative power supply serves both op amps on the buffer chip and the pin numbers and wiring for each of the two op amps on the single chip are shown. It shows all the wiring, whereas Dave G's original schematic said "to other channel" for power supply connection and just showed the pins (but not the wiring) for the other op amp circuit.

    In disconnecting the original Space Expander ('reverb') jacks, you'd have to be careful to maintain the connections to the Tape Monitor & Record Out jacks that remain in the circuit.

    upload_2018-12-16_9-55-25.png
     
    Last edited: Dec 16, 2018
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