Fisher 500C restoration and modification project
- Thread starter daylite
- Start date
Nice job. But honestly I can't remember ever making a post on one. With almost 26K posts, I guess my internal hardrive needs defragging, and is probably at it's capacity limit........
daylite
Active Member
I came across that post because I had to get a crash course on alignment and front end tracking so I was reading about fm dial pointer stuff. I had discovered in mid-October I had a problem - probably alignment. See posts starting around #22 and ending (after a lot of welcome advice from Dave) at #44. Anyway your mention of the lazy susan hit me as a good idea. Thanks again for that contribution - one of 25,905 and counting!
daylite
Active Member
It's been 5 weeks (and a few major struggles) since my last update (post #9). It took 10-12 days and Dave's help to get me through the troubles and finally a successful alignment to fix a tuning problem. Since then I have been back to working on the project (2 steps forward - 1 step back).
Here's an update to post #9 continuing the list of what has been done and what is planned:
The biggest event (and lesson learned) was getting the EFB built and installed. I have fabricated PCBs for various projects off and on for the past 12 years. I went ahead and made one for the EFB. When I attached it to B+ I got quite a light show - sparks and flares came and went all over the board. Looked like a video taken at night of clouds from above during a thunderstorm. It even had thunder - the loud crack of caps discharging over air gaps. So I learned the wire widths and clearances that worked fine for +/-30 volts don't work so well with 430 volts!! I now use 40 mil wire and pad clearance. The redesigned EFB is installed and working fine. Thanks for a great design, Dave!
Here's an update to post #9 continuing the list of what has been done and what is planned:
The biggest event (and lesson learned) was getting the EFB built and installed. I have fabricated PCBs for various projects off and on for the past 12 years. I went ahead and made one for the EFB. When I attached it to B+ I got quite a light show - sparks and flares came and went all over the board. Looked like a video taken at night of clouds from above during a thunderstorm. It even had thunder - the loud crack of caps discharging over air gaps. So I learned the wire widths and clearances that worked fine for +/-30 volts don't work so well with 430 volts!! I now use 40 mil wire and pad clearance. The redesigned EFB is installed and working fine. Thanks for a great design, Dave!
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Tim D
Super Member
It's been 5 weeks (and a few major struggles) since my last update (post #9). It took 10-12 days and Dave's help to get me through the troubles and finally a successful alignment to fix a tuning problem. Since then I have been back to working on the project (2 steps forward - 1 step back).
Here's an update to post #9 continuing the list of what has been done and what is planned:
The biggest event (and lesson learned) was getting the EFB built and installed. I have fabricated PCBs for various projects off and on for the past 12 years. I went ahead and made one for the EFB. When I attached it to B+ I got quite a light show - sparks and flares came and went all over the board. Looked like a video taken at night of clouds from above during a thunderstorm. It even had thunder - the loud crack of caps discharging over air gaps. So I learned the wire widths and clearances that worked fine for +/-30 volts don't work so well with 430 volts!! I now use 40 mil wire and pad clearance. The redesigned EFB is installed and working fine. Thanks for a great design, Dave!
Good to know about watching trace clearances when working on tube units. I had forgotten that too and have plans to make several different PCB's for my Fishers.
daylite
Active Member
Here I’ll try using a schematic instead of lists to describe and record progress so far…
First let me give all credit for the map/schematic in this post to the group that created the first such map of IBAM and EFB and other mods. Their work (Fisher 500c EFB mod 14.pdf) is found in the AK Database here. The original drawing was made by Joseph W. Strickland. Of course, Dave Gillespie is our chief engineer and the inventor of the EFB, IBBA, and Phase Inverter mods. The original map credits Drew Bolce and Terry Dewick as originators of the IBAM design.
EDIT 11/27: I missed some important prior work. Luis Eduardo Sanchez did a fantastic job on the first schematic of 400 mods starting in 2013 working with Dave. See posts here. (Dave – You’re welcome to send me corrections needed for the schematic here.)
Many thanks to all! I’m sure there were other contributors that helped during discussions as the image was refined - I'm sincerely sorry if I missed others' contributions.
This project map (below) was made to help me keep track of changes completed and look ahead to what’s needed next. It started with the latest Joe Strickland drawing I could find (link above) which was based on early 500C models (#10001 to #19999). His work was then modified to match the schematic for a late 500C models (#30001 to #49999). For example, the late model 500C uses 6CW5 nuvistors in the tuner, so that section needed some changes. Then mods that are listed in the mod lists posted above, as built, whether completed or upcoming, were highlighted or noted in the drawing.
A lot of this is very close to Joe’s early 500C mod version. Examples of differences are that this map documents the IBBA circuit instead of the IBAM, and it illustrates some added mods to the wiring on the primary side of the power transformer. I also tried to show the ‘noose removal’ and final phase inverter adjust circuits recommended by Dave.
EDIT 11/27: Dave (and Luis’s drawing) gives -62 V for the rectifier-doubler voltage supply for the EFB. I changed this drawing to show that because it’s closer to what I find (-63.4 V). Also added Luis’ name to the schematic to credit his original work.
EDIT 11/27: Replaced power relay schematic with correct IDEC schematic.
The mods that have been completed are colored blue. So those areas show the state of the hardware now, after my efforts to follow where others have led. The mods that are upcoming are colored green. As I complete new work, I change green to blue. A few things that I haven’t decided on are highlighted with orange. EDIT: Voltages shown in parentheses are measured @120VAC incoming. Please reply if something looks wrong. I'll check the schematic against the hardware and correct any discrepancy.
The image illustrates the very impressive amount of development this group has brought to the 500C! The enhancements extend over a large part of the chassis. I’ll keep up with the map, and closer to the end I’ll post a final schematic with and without highlights to document the mods done for this 500C project. -d
First let me give all credit for the map/schematic in this post to the group that created the first such map of IBAM and EFB and other mods. Their work (Fisher 500c EFB mod 14.pdf) is found in the AK Database here. The original drawing was made by Joseph W. Strickland. Of course, Dave Gillespie is our chief engineer and the inventor of the EFB, IBBA, and Phase Inverter mods. The original map credits Drew Bolce and Terry Dewick as originators of the IBAM design.
EDIT 11/27: I missed some important prior work. Luis Eduardo Sanchez did a fantastic job on the first schematic of 400 mods starting in 2013 working with Dave. See posts here. (Dave – You’re welcome to send me corrections needed for the schematic here.)
Many thanks to all! I’m sure there were other contributors that helped during discussions as the image was refined - I'm sincerely sorry if I missed others' contributions.
This project map (below) was made to help me keep track of changes completed and look ahead to what’s needed next. It started with the latest Joe Strickland drawing I could find (link above) which was based on early 500C models (#10001 to #19999). His work was then modified to match the schematic for a late 500C models (#30001 to #49999). For example, the late model 500C uses 6CW5 nuvistors in the tuner, so that section needed some changes. Then mods that are listed in the mod lists posted above, as built, whether completed or upcoming, were highlighted or noted in the drawing.
A lot of this is very close to Joe’s early 500C mod version. Examples of differences are that this map documents the IBBA circuit instead of the IBAM, and it illustrates some added mods to the wiring on the primary side of the power transformer. I also tried to show the ‘noose removal’ and final phase inverter adjust circuits recommended by Dave.
EDIT 11/27: Dave (and Luis’s drawing) gives -62 V for the rectifier-doubler voltage supply for the EFB. I changed this drawing to show that because it’s closer to what I find (-63.4 V). Also added Luis’ name to the schematic to credit his original work.
EDIT 11/27: Replaced power relay schematic with correct IDEC schematic.
The mods that have been completed are colored blue. So those areas show the state of the hardware now, after my efforts to follow where others have led. The mods that are upcoming are colored green. As I complete new work, I change green to blue. A few things that I haven’t decided on are highlighted with orange. EDIT: Voltages shown in parentheses are measured @120VAC incoming. Please reply if something looks wrong. I'll check the schematic against the hardware and correct any discrepancy.
The image illustrates the very impressive amount of development this group has brought to the 500C! The enhancements extend over a large part of the chassis. I’ll keep up with the map, and closer to the end I’ll post a final schematic with and without highlights to document the mods done for this 500C project. -d
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daylite
Active Member
Tim - The clearance for pads on some components might be small for 400 V. I edited the mask after printing to file to provide more room.
Attachments
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daylite
Active Member
Here’s a report to provide a visual record of progress on this project. On the left of the image is the view from under the chassis the day I received the 500c (August 28, 2021). On the right is the same view but the photo was taken recently (November 18, 2021). With the image is a key providing a few words about each of the highlighted areas A-K where changes have been completed.
It was a pleasure to see the last of those old replacement electrolytics that were scattered around the chassis. Hope you enjoy browsing through the changed landscape. Have a Happy Thanksgiving!
It was a pleasure to see the last of those old replacement electrolytics that were scattered around the chassis. Hope you enjoy browsing through the changed landscape. Have a Happy Thanksgiving!
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daylite
Active Member
Here’s a list identifying most of the parts used in this 500C restoration/modification project. The example below is half of the first page. The complete ‘clickable’ list is in the attached PDF. The PDF has active links for about 50 parts purchased since August. You can see most of those parts in the progress report image posted above.
I would never say a choice made for a particular position is the best choice. If choosing again I might try something else and, in some cases, when the part arrived, I decided to use an alternative found around the bench. Other members will have their own favorite ‘go to’ components. Sometimes the final choice is just about being able to get everything needed from a single source.
For the record, these product lines are currently on my favorites list:
Happy Thanksgiving to you and yours.
I would never say a choice made for a particular position is the best choice. If choosing again I might try something else and, in some cases, when the part arrived, I decided to use an alternative found around the bench. Other members will have their own favorite ‘go to’ components. Sometimes the final choice is just about being able to get everything needed from a single source.
For the record, these product lines are currently on my favorites list:
Happy Thanksgiving to you and yours.
daylite
Active Member
For a Fisher K-100 project (discussed briefly here) I built a a transistor buffered zener regulated power supply and an IBAM to control bias. Here I wanted to try Dave's EFB and IBBA which offers both balance and bias-level control. Wednesday’s report showing the underside with highlighted areas for the mods (post #71) couldn’t show the IBBA. It’s mounted on the top of the chassis in the area often used by others here.
Highlighted area B (post #71) showed where the cathode sense wires and the bias supply wires are bundled after leaving the output tube corral. They are combined with a ground lead (BLK) and the voltage lead (RED) from the EFB. That group is divided into two sets of 5 wires. As many have shown, the wires can come to the topside using holes around C92.
Here’s some photos of the IBBA make/install process.
Today might see the start of work on the RIAA equalization circuit. -d
Highlighted area B (post #71) showed where the cathode sense wires and the bias supply wires are bundled after leaving the output tube corral. They are combined with a ground lead (BLK) and the voltage lead (RED) from the EFB. That group is divided into two sets of 5 wires. As many have shown, the wires can come to the topside using holes around C92.
Here’s some photos of the IBBA make/install process.
Today might see the start of work on the RIAA equalization circuit. -d
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daylite
Active Member
I’ve been final-checking all voltages. Yesterday while looking into voltages to be expected in the EFB build, I found I missed some valuable prior work on schematics. Luis Eduardo Sanchez did a fantastic job on the schematic of original 400 mods starting in 2013 working with Dave. See posts in this important-thread. Many thanks to Luis for his pioneering work. I have added an edit to the post of the schematic above (post #69) to cite Luis’ work. Also updated the schematic to add Luis’ name to credit his work and to update the power relay drawing and the measured EFB voltages.
Dave gives -62 V for the rectifier-doubler voltage supply for the EFB. I changed the schematic (post #69) to show that number because it’s closer to what I find (-63.4 V). I think the output of the supply would be little affected by 400-vs-500C differences in the components around the PNPs. -d
Dave gives -62 V for the rectifier-doubler voltage supply for the EFB. I changed the schematic (post #69) to show that number because it’s closer to what I find (-63.4 V). I think the output of the supply would be little affected by 400-vs-500C differences in the components around the PNPs. -d
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daylite
Active Member
This post reports on the EFB which is the most recently added PCB. The circuit followed Dave Gillespie’s plan (Copyright D Gillespie Designs / DGD). Recent discussion by Dave is here. The schematic for the EFB is shown in the complete record for this 500C project in post #69 above and is also included in the figure below that I’m posting to provide a few notes on the make/install and to document the project.
The EFB board (2.6”x1.6”) fits in the open space next to the oscillator shield. Standoffs for the PCB are mounted in existing holes next to the power transformer mounting bolts. The location is close to the AC source required (see black wires that run from rectifier at the very bottom to the EFB). The other EFB connections are: B+ IN (yellow); SCREEN VOLTAGE OUT (green); IBBA VOLTAGE SUPPLY OUT (red). A ground terminal is provided for the PCB but is not needed because metal standoffs were used. The Panasonic 0.47 uF film cap (Mfr #: ECW-FA2J474J4) is a good size for this application. The heat sink is Aavid # 513102B02500G. Other part information is provided in the list attached to post #72. -d
The EFB board (2.6”x1.6”) fits in the open space next to the oscillator shield. Standoffs for the PCB are mounted in existing holes next to the power transformer mounting bolts. The location is close to the AC source required (see black wires that run from rectifier at the very bottom to the EFB). The other EFB connections are: B+ IN (yellow); SCREEN VOLTAGE OUT (green); IBBA VOLTAGE SUPPLY OUT (red). A ground terminal is provided for the PCB but is not needed because metal standoffs were used. The Panasonic 0.47 uF film cap (Mfr #: ECW-FA2J474J4) is a good size for this application. The heat sink is Aavid # 513102B02500G. Other part information is provided in the list attached to post #72. -d
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daylite
Active Member
Here is a short summary on the make/install of the PCB built for the phase inverter bias circuit. The circuit follows Dave Gillespie’s plan (Copyright D Gillespie Designs / DGD). The schematic used for the PCB is the one shown in the map for this project in post #69 and again in the figure below.
The board contains all components for the cathode RC tails for V12 and V13, and resistors R120/R85 for the heater string. In a stock chassis R120 and R85 are located on a terminal strip between C97 and C98. The board is mounted on tall standoffs close to V12 and V13 and between the phase inverter adjustment pots and output coupling caps. The board was made very narrow (0.4”/10 mm) so it does not prevent access to components and connections below it.
The two green wires at the left end of the board are from the new R120 and R85 and connect to the heater string (through connection to pins 4/5 and 9; the original R120/R85 resistors are removed). At each end of the board blue wires connect to the 12AX7 cathodes - pins 3 on V12 and V13. During adjustment the wide traces on the board (labeled N$7 and N$1) provide convenient points to contact the (+) side of one of the 22uF caps by using an alligator clip along the board edge. -d
The board contains all components for the cathode RC tails for V12 and V13, and resistors R120/R85 for the heater string. In a stock chassis R120 and R85 are located on a terminal strip between C97 and C98. The board is mounted on tall standoffs close to V12 and V13 and between the phase inverter adjustment pots and output coupling caps. The board was made very narrow (0.4”/10 mm) so it does not prevent access to components and connections below it.
The two green wires at the left end of the board are from the new R120 and R85 and connect to the heater string (through connection to pins 4/5 and 9; the original R120/R85 resistors are removed). At each end of the board blue wires connect to the 12AX7 cathodes - pins 3 on V12 and V13. During adjustment the wide traces on the board (labeled N$7 and N$1) provide convenient points to contact the (+) side of one of the 22uF caps by using an alligator clip along the board edge. -d
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daylite
Active Member
Adjustment of the modified phase inverter bias correction circuits for V12 and V13 (Dave’s circuits - presented here) requires matching two resistances for each tube. For example, for V13 the combined series resistance of R109+R112 (which is measured with the receiver OFF between the (+) side of 91C and pin 1 of V13) is adjusted to match the resistance of the nominally 47K resistor R116B (which is measured with the receiver OFF between the (+) side of the 22 uF cap and pin 3 of V13). The wipers of the adjustment pots R108 and R109 connect directly to 91C(+). Either wiper is a more accessible place for the connection than the cap itself. For more info, see the sticky Larry provided on PI-circuit-mods for several models.
I never built or used a Wheatstone bridge (outside using a VTVM). I remember reading it’s a good way to compare resistances, so for fun I set out to use a bridge circuit to match the resistances for the phase inverter adjustment. On the left below is a drawing of Dave’s P.I. circuit mod (Copyright D Gillespie Designs / DGD). The center figure illustrates the connections for the Wheatstone bridge adjustment method.
It's convenient to have stackable banana plug leads for the DMM and the little rig shown in the upper photo. It has two banana plugs linked by two matched resistors in series. I used 100K 0.1% resistors; 47Ks would give about 10% better sensitivity. Everything was hooked up as shown in the drawing and photos (receiver OFF), and the adjustment requires simply using the pot to null the DMM voltage. That’s it.
With stock pots, getting the response down to 0.5 mV is easy; hitting 0V exactly is difficult. Potentials of 100-200 uV can be reached with a little patience. Sensitivity was measured by adjusting to null (within 400 uV) and then recording the voltage increase when 4.7M, 25M, and 100M resistors are added parallel to 33K resistor R112. The experiment shows there is high sensitivity: A 230-ohm mismatch out of 47K gives an 8.5 mV meter reading. A difference of 45 ohms (0.1%) is easily detected – it gives a 1.9 mV potential on the DMM. A 10-ohm resistance difference gave 0.5 mV on the DMM. Those numbers are very close to what theory predicts. The precision of the resistance measurements at 47K prevents making a good comparison of the two adjustment methods. They are the same within the precision limits.
Until told otherwise I’ll doubt a 50-ohm (0.1%) error in adjustment would cause distortion I could hear. Could it even be measured? This is not presented to be a better way to do the adjustment* – for me it was just something new to try, and simply fun. I hoped you liked reading about it and that you’ll tell me if I am mistaken about anything here. -d
*I'm not convinced about that.
*PRO: The method reduces variables because it eliminates having to change connections in the course of the adjustment process.
*CON: The rig and battery are added gear. There’s a learning curve.
I never built or used a Wheatstone bridge (outside using a VTVM). I remember reading it’s a good way to compare resistances, so for fun I set out to use a bridge circuit to match the resistances for the phase inverter adjustment. On the left below is a drawing of Dave’s P.I. circuit mod (Copyright D Gillespie Designs / DGD). The center figure illustrates the connections for the Wheatstone bridge adjustment method.
It's convenient to have stackable banana plug leads for the DMM and the little rig shown in the upper photo. It has two banana plugs linked by two matched resistors in series. I used 100K 0.1% resistors; 47Ks would give about 10% better sensitivity. Everything was hooked up as shown in the drawing and photos (receiver OFF), and the adjustment requires simply using the pot to null the DMM voltage. That’s it.
With stock pots, getting the response down to 0.5 mV is easy; hitting 0V exactly is difficult. Potentials of 100-200 uV can be reached with a little patience. Sensitivity was measured by adjusting to null (within 400 uV) and then recording the voltage increase when 4.7M, 25M, and 100M resistors are added parallel to 33K resistor R112. The experiment shows there is high sensitivity: A 230-ohm mismatch out of 47K gives an 8.5 mV meter reading. A difference of 45 ohms (0.1%) is easily detected – it gives a 1.9 mV potential on the DMM. A 10-ohm resistance difference gave 0.5 mV on the DMM. Those numbers are very close to what theory predicts. The precision of the resistance measurements at 47K prevents making a good comparison of the two adjustment methods. They are the same within the precision limits.
Until told otherwise I’ll doubt a 50-ohm (0.1%) error in adjustment would cause distortion I could hear. Could it even be measured? This is not presented to be a better way to do the adjustment* – for me it was just something new to try, and simply fun. I hoped you liked reading about it and that you’ll tell me if I am mistaken about anything here. -d
*I'm not convinced about that.
*PRO: The method reduces variables because it eliminates having to change connections in the course of the adjustment process.
*CON: The rig and battery are added gear. There’s a learning curve.
daylite
Active Member
Dave Gillespie explained here why a buffer is needed at the preamp output when the 400/500C/800C reverb jacks are rewired to become from-pre-amp/to-power-amp ports – a mod he presented for the 400.
The buffer schematic used in this project (see below) is the same SFAIK as originally recommended by Dave and built by many others. The buffer PCB board (photo) is mounted securely using just two standoffs. The standoffs are attached by replacing the hex-head sheet metal screws that hold the tuner flywheel bracket with 4-40 machine screws and screwing the standoffs onto them. This approach saves making new holes in the chassis.
The buffer voltage source (nominally -15V) is constructed on a small terminal strip and tucked behind the central wire bundle on the steel panel. It’s screwed to the panel for support and the panel provides the needed ground connection. It is supplied with -31 V from the rectifier (the one that’s no longer needed for bias because the EFB board now supplies bias voltage and has its own on-board rectifier). The buffer voltage supply components reduce the -31 V supply to -15 V to provide V- for the op amp.
The buffer board was tested before installation using audio frequency input from a Wien oscillator and checking output for amplitude and quality with a scope. Installation and hookup used single strand shielded wire to connect the volume pot to the buffer board, the buffer board to the repurposed reverb jacks, and the reverb jacks to the phase inverter tubes. It’s convenient that the existing wires from the volume pot to the phase inverter tubes are long enough to be used to connect the buffer to the reverb jacks. New wire was needed only for connecting the volume pot to buffer and the jacks to phase inverter tubes.
The buffer schematic used in this project (see below) is the same SFAIK as originally recommended by Dave and built by many others. The buffer PCB board (photo) is mounted securely using just two standoffs. The standoffs are attached by replacing the hex-head sheet metal screws that hold the tuner flywheel bracket with 4-40 machine screws and screwing the standoffs onto them. This approach saves making new holes in the chassis.
The buffer voltage source (nominally -15V) is constructed on a small terminal strip and tucked behind the central wire bundle on the steel panel. It’s screwed to the panel for support and the panel provides the needed ground connection. It is supplied with -31 V from the rectifier (the one that’s no longer needed for bias because the EFB board now supplies bias voltage and has its own on-board rectifier). The buffer voltage supply components reduce the -31 V supply to -15 V to provide V- for the op amp.
The buffer board was tested before installation using audio frequency input from a Wien oscillator and checking output for amplitude and quality with a scope. Installation and hookup used single strand shielded wire to connect the volume pot to the buffer board, the buffer board to the repurposed reverb jacks, and the reverb jacks to the phase inverter tubes. It’s convenient that the existing wires from the volume pot to the phase inverter tubes are long enough to be used to connect the buffer to the reverb jacks. New wire was needed only for connecting the volume pot to buffer and the jacks to phase inverter tubes.
daylite
Active Member
This project is getting close to the end. In the next few posts I plan to cover the TAPE HEAD input to AUX input conversion, the TAPE MON to AUX input conversion, and the tone control bypass mod. I'll also provide a final update to the project schematic to show all the 'Audiokarma' changes made to this 500C. I hope to get those posts done this weekend. -d
daylite
Active Member
TAPE MON jacks to AUX input modification
I converted the TAPE MON jacks to use as an additional AUX input a couple of weeks ago after deciding I won’t need the tape monitor function in the future. Many here have done the same. Is it OK to alter Fisher’s ‘art’? For this unit my answer is already clear – like many of you I’ve gone far down the customization road. This project was never about restoring the 500C to be a museum-quality unaltered example of Fisher’s original product. This is more of a renovation than restoration. We want these beautiful receivers to be the best they can be and to have the functions needed for how we’ll use them.
This is a simple modification and the function created is very useful. With a flip of the switch the amp can alternate between a selected source and a second AUX source attached to the TAPE MON jacks.
STEP-BY-STEP (left channel)
Remove the 680K resistor (R52) that connects the left channel RECRDR OUT jack (J10) to GND. Remove the components R33 (330K) and C30 (120 pF) that are part of the original lead from the selector switch terminal 1F-8* to the TAPE MON jack (J9). Connect the TAPE MON jack directly and solely to the ‘ON’ side of the TAPE MONITOR switch. Disconnect selector switch terminal 1F-4 from the 470K/24pF component pair (R38/C31). Connect selector switch terminal 1F-4 to the ‘OFF’ side of the TAPE MONITOR switch. Connect the 470K/24pF pair (R38/C31) that is no longer connected to terminal 1F-4 to the center of the TAPE MONITOR switch. (The TAPE MON to AUX input conversion of the right channel follows the same process.) *See this post for an explanation of how selector switch terminals are named and located.
With these changes the TAPE MON jacks become an alternative AUX input that is active when the TAPE MONITOR switch is in the ‘ON’ position. Thanks to all who pioneered this mod and wrote about it here. -d
I converted the TAPE MON jacks to use as an additional AUX input a couple of weeks ago after deciding I won’t need the tape monitor function in the future. Many here have done the same. Is it OK to alter Fisher’s ‘art’? For this unit my answer is already clear – like many of you I’ve gone far down the customization road. This project was never about restoring the 500C to be a museum-quality unaltered example of Fisher’s original product. This is more of a renovation than restoration. We want these beautiful receivers to be the best they can be and to have the functions needed for how we’ll use them.
This is a simple modification and the function created is very useful. With a flip of the switch the amp can alternate between a selected source and a second AUX source attached to the TAPE MON jacks.
STEP-BY-STEP (left channel)
Remove the 680K resistor (R52) that connects the left channel RECRDR OUT jack (J10) to GND. Remove the components R33 (330K) and C30 (120 pF) that are part of the original lead from the selector switch terminal 1F-8* to the TAPE MON jack (J9). Connect the TAPE MON jack directly and solely to the ‘ON’ side of the TAPE MONITOR switch. Disconnect selector switch terminal 1F-4 from the 470K/24pF component pair (R38/C31). Connect selector switch terminal 1F-4 to the ‘OFF’ side of the TAPE MONITOR switch. Connect the 470K/24pF pair (R38/C31) that is no longer connected to terminal 1F-4 to the center of the TAPE MONITOR switch. (The TAPE MON to AUX input conversion of the right channel follows the same process.) *See this post for an explanation of how selector switch terminals are named and located.
With these changes the TAPE MON jacks become an alternative AUX input that is active when the TAPE MONITOR switch is in the ‘ON’ position. Thanks to all who pioneered this mod and wrote about it here. -d
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