Recently, an inquiry was made about all the various things done to restore a typical Fisher receiver, and what all the various modifications available were, since there was no one place to go to get all of this information. Having recently finished up a 400 receiver (early version in this case) restoration for a client, that also had requested nearly all of the available modifications to be installed, it made for a perfect unit to provide detail pics of, so that those contemplating work done on their unit can get a better idea of what is involved in each case. Therefore, this will largely be a thread of pics, with a few comments thrown in to support each pic presented. The work presented is component level work, in addition to the customary cleaning of tube sockets, switches and controls, and the testing/replacement of tubes.
The pics will spotlight three basic areas of work done/to be done relative specifically to a 400 receiver, with most items applicable to Fisher's other receivers as well. These include:
1. Standard Restoration Work: This work replaces those items that are well documented to fail over time, or have already failed to prevent proper operation of the unit as designed. Replacement of these items will return a unit to the performance and dependability as designed by Fisher, understanding that some wear items (i.e., switches and controls) will be assessed/addressed as necessary on a unit by unit basis. This includes the replacement of:
A. All electrolytic caps in the B+ power supply, DC Heater/Bias supply, MPX, and Ratio Detector circuits. New top side can caps are installed to minimize the intrusion/need of otherwise new under-chassis components required, which helps to minimize clutter and maintain the look of the original Fisher build.
B. Both HV (B+) rectifier diodes.
C. The selenium rectifier with a silicon unit if so equipped.
D. Both AC Bypass caps.
E. Phono preamp and MPX mylar coupling caps.
F. Any other component showing signs of stress, is out of production tolerance, or through troubleshooting has been determined to prevent less than designed/expected performance.
2. Basic Modification Package: This work is considered to be needed work, and addresses known circuit issues, modifies a unit to provide added dependability/component protections beyond that provided by Fisher, and modifies a unit to operate with original intended performance while taking into account of the realities of today's electronic world. Areas addressed includes the reality of today's: modern components, available tubes, AC Power, and FM broadcasting. This includes the installation of:
A. A CL-80 In-rush current limiter.
B. Precision 10Ω .25 watt 1% cathode current sampling resistors at each output tube socket.
C. 100Ω .25 watt Screen Stability resistors at each output tube socket.
D. Some form of bias adjustment for controlling output tube current draw, either via a single bias control for all four tubes, or a circuit allowing individual adjustment of each tube, using either an IBAM, or IBBA as desired.
E. The headphone fix, to correct the incorrect headphone channel identity as built.
F. A voltage dropping network to provide the MPX B+ 1 rail with an ideal 240 vdc.
G. Stereo indicating threshold adjustment.
H. Phase Inverter/Driver stage modifications.
3. Optional Performance/Feature Enhancing Modifications: These are self explanatory, and include:
A. Removal of Aux/Tape Mon bleed circuit. Allows both of these inputs to operate independently of each other.
B. Electrically relocate Tape Mon circuit to same signal point as Input Selector switch. Allows Tape Monitor input to display the same sensitivity as Aux input.
C. Convert Tape Head input into an additional high level input. In conjunction with 3A and 3B above, provides for three separate and independent high level inputs of equal sensitivity.
D. Remove permanent LF filters.
E. Change out Phono Preamp ceramic inter-stage coupling caps with mylar caps. Improves phono performance not because mylar is audibly better than ceramic, but because the +/- 20% ceramic caps used in this position are invariably in spec 20% below value. Mylar caps are typically much closer to specified value. Installing an on target value then improves LF performance.
F. Tone Control Engage/Bypass switch. Allows instant and accurate flat performance if desired.
G. Conversion of Reverb In/Out jacks to Preamp Out/Power Amp In jacks. Allows unit to act as either a tuner/ preamp for a separate external power amp, or a power amp for a separate external preamp.
H. Installation of EFB™. Produces more power at lower distortion, and allows the use of lower output tube quiescent current. Enhances tube life and promotes cooler overall operation. Strongly recommended for late version units due to the change in operating conditions Fisher created when changing the OPT to a 10.2KΩ unit. Under high level operating conditions, this places great stress on the output tubes, which the installation of EFB can correct.
For the unit whose pics are presented here, all of the above restoration and modification work was requested and installed with the exception of item 3F. The pics then can help the hobbiest/tech get a better idea of the work entailed for each of the items listed:
BELOW: The unit as received. It has seen a bench before: The Balance control was replaced (with a proper Fisher piece), the Bias decoupling cap was replaced, and a used MPX 1 uF composite cap was tacked in place to try and repair the defective MPX section. When that didn't fix it, the old component was (apparently) tacked back in. No other previous service work was observed.
BELOW: Always repairing performance issues that routine restoration work might not correct first, the MPX unit was removed and recapped, and the Stereo/Mono Threshold adjustment modification (resistor) installed as well. The ultimate problems with the unit were determined to be a (literally) dead shorted coupling cap, a broken tube socket receptor pin at V102, and horrible alignment due to previous efforts to see if twisting and turning things would correct the issues. The new 1 uF composite coupling cap, and new .0047 trigger coupling cap (C214) have been installed at this point.
BELOW: The output coupling caps were then replaced. These small .047 630 volt caps are perfect for the tight space allowed.
BELOW: The MPX unit was reinstalled, and the Voltage Dropping Modification installed to allow proper operating voltages for V100 and V102. The S/M Threshold adjustment resistor can also be seen in this shot directly above the new 33 uF 450 volt VD decoupling cap.
BELOW: The Radio Detector electrolytic cap was replaced also.
BELOW: Since the FM dial was more than 1 mHz off of the received frequency, the tuner section was in sore need of an alignment, as was the MPX section. With early version 400 receivers, a really precise alignment is necessary since these units possess less gain and (therefore) limiting action than later four tube IF versions (which are much more tolerant of less than ideal adjustments). A precise alignment then is necessary for the three tube IF design to maximize the sensitivity and limiting action of which it is capable. Of all of Fisher's Wide Band FM Stereo MPX receivers however, early version 400 units do not employ any AVC for the RF or IF sections of the tuner. This aids in maximizes sensitivity and limiting, but also makes for a very hot signal from the Ratio Detector stage with strong signals. With such a hot signal, it can overload V100 in the MPX section when using Russian manufactured tubes in this position. Therefore, while always a good idea, it is mandatory to use good American pieces in this position in early 400 receivers to prevent the overloading caused when Russian tubes are installed. With the MPX repairs, new tubes, full alignment, and modification work all finished up, the receiver is shown receiving WSRV 97.1 out of Atlanta (transmitting tower some 55+ miles away) with a steady eye tube and superb stereo, using nothing but a simple dipole antenna in my basement (underground) lab.
BELOW: Topside, new can caps were installed. Under the old black cardboard cover is a new 50-50-50-50uF @350volt can with all sections wired in parallel to effect an upgrade over the old 100 uF @250 volt can originally occupying that space. The same can is used in the front most position as well to effect the original 200 uF @250 volt can that used to reside there. The two 50 uF 250 volt sections are replaced with discrete caps mounted under the chassis. Also, in this particular unit, all 5 of the tuner section tubes were replaced with new tubes from Jim McShane.
BELOW: Underside, the usual output stage work was done: precision 10Ω cathode current sampling resistors are installed, as are 100Ω Screen Stability resistors. Pin 8 is used as a new Screen Buss to facilitate the Stability resistors. Note too the disappearance of the 220Ω heater balancing resistors normally seen in this location -- this since they have been relocated over to the left channel phase inverter area, where they will receive (the new) heater bias voltage made available from the phase inverter modification. Finally, the gray lead entering near the top of the pic from the right side contains the four leads that provide the individual bias control from the IBAM installed. The original T-Strips provided for the bias connections are still used for two of the tubes, while the other two are connected inside a fully sleeved cover.
BELOW: The Noose Removal (Phase Inverter Modification) has been installed, with new heater balancing resistors provided that connect the heater bias point in the left channel. The input for these stages is from the blk-wht and wht-blk shielded leads running across and just above center of the pic. These leads come from the new Power Amp In jacks -- opposite from where the signal leads used to come from (volume control). Note the small shield fabricated at the input T-Strip for the left channel -- this to eliminate a slight hum with an open input from the heater wires that run right by the Strip.
BELOW: Under the Phase Inverter Adjustment controls lie the 47 uF 100 volt caps that are part of the Phase Inverter Modification.
Next time, the Standard Restoration and Basic Modification Package gets finished up, and the Optional Modifications get covered.
Dave
The pics will spotlight three basic areas of work done/to be done relative specifically to a 400 receiver, with most items applicable to Fisher's other receivers as well. These include:
1. Standard Restoration Work: This work replaces those items that are well documented to fail over time, or have already failed to prevent proper operation of the unit as designed. Replacement of these items will return a unit to the performance and dependability as designed by Fisher, understanding that some wear items (i.e., switches and controls) will be assessed/addressed as necessary on a unit by unit basis. This includes the replacement of:
A. All electrolytic caps in the B+ power supply, DC Heater/Bias supply, MPX, and Ratio Detector circuits. New top side can caps are installed to minimize the intrusion/need of otherwise new under-chassis components required, which helps to minimize clutter and maintain the look of the original Fisher build.
B. Both HV (B+) rectifier diodes.
C. The selenium rectifier with a silicon unit if so equipped.
D. Both AC Bypass caps.
E. Phono preamp and MPX mylar coupling caps.
F. Any other component showing signs of stress, is out of production tolerance, or through troubleshooting has been determined to prevent less than designed/expected performance.
2. Basic Modification Package: This work is considered to be needed work, and addresses known circuit issues, modifies a unit to provide added dependability/component protections beyond that provided by Fisher, and modifies a unit to operate with original intended performance while taking into account of the realities of today's electronic world. Areas addressed includes the reality of today's: modern components, available tubes, AC Power, and FM broadcasting. This includes the installation of:
A. A CL-80 In-rush current limiter.
B. Precision 10Ω .25 watt 1% cathode current sampling resistors at each output tube socket.
C. 100Ω .25 watt Screen Stability resistors at each output tube socket.
D. Some form of bias adjustment for controlling output tube current draw, either via a single bias control for all four tubes, or a circuit allowing individual adjustment of each tube, using either an IBAM, or IBBA as desired.
E. The headphone fix, to correct the incorrect headphone channel identity as built.
F. A voltage dropping network to provide the MPX B+ 1 rail with an ideal 240 vdc.
G. Stereo indicating threshold adjustment.
H. Phase Inverter/Driver stage modifications.
3. Optional Performance/Feature Enhancing Modifications: These are self explanatory, and include:
A. Removal of Aux/Tape Mon bleed circuit. Allows both of these inputs to operate independently of each other.
B. Electrically relocate Tape Mon circuit to same signal point as Input Selector switch. Allows Tape Monitor input to display the same sensitivity as Aux input.
C. Convert Tape Head input into an additional high level input. In conjunction with 3A and 3B above, provides for three separate and independent high level inputs of equal sensitivity.
D. Remove permanent LF filters.
E. Change out Phono Preamp ceramic inter-stage coupling caps with mylar caps. Improves phono performance not because mylar is audibly better than ceramic, but because the +/- 20% ceramic caps used in this position are invariably in spec 20% below value. Mylar caps are typically much closer to specified value. Installing an on target value then improves LF performance.
F. Tone Control Engage/Bypass switch. Allows instant and accurate flat performance if desired.
G. Conversion of Reverb In/Out jacks to Preamp Out/Power Amp In jacks. Allows unit to act as either a tuner/ preamp for a separate external power amp, or a power amp for a separate external preamp.
H. Installation of EFB™. Produces more power at lower distortion, and allows the use of lower output tube quiescent current. Enhances tube life and promotes cooler overall operation. Strongly recommended for late version units due to the change in operating conditions Fisher created when changing the OPT to a 10.2KΩ unit. Under high level operating conditions, this places great stress on the output tubes, which the installation of EFB can correct.
For the unit whose pics are presented here, all of the above restoration and modification work was requested and installed with the exception of item 3F. The pics then can help the hobbiest/tech get a better idea of the work entailed for each of the items listed:
BELOW: The unit as received. It has seen a bench before: The Balance control was replaced (with a proper Fisher piece), the Bias decoupling cap was replaced, and a used MPX 1 uF composite cap was tacked in place to try and repair the defective MPX section. When that didn't fix it, the old component was (apparently) tacked back in. No other previous service work was observed.
BELOW: Always repairing performance issues that routine restoration work might not correct first, the MPX unit was removed and recapped, and the Stereo/Mono Threshold adjustment modification (resistor) installed as well. The ultimate problems with the unit were determined to be a (literally) dead shorted coupling cap, a broken tube socket receptor pin at V102, and horrible alignment due to previous efforts to see if twisting and turning things would correct the issues. The new 1 uF composite coupling cap, and new .0047 trigger coupling cap (C214) have been installed at this point.
BELOW: The output coupling caps were then replaced. These small .047 630 volt caps are perfect for the tight space allowed.
BELOW: The MPX unit was reinstalled, and the Voltage Dropping Modification installed to allow proper operating voltages for V100 and V102. The S/M Threshold adjustment resistor can also be seen in this shot directly above the new 33 uF 450 volt VD decoupling cap.
BELOW: The Radio Detector electrolytic cap was replaced also.
BELOW: Since the FM dial was more than 1 mHz off of the received frequency, the tuner section was in sore need of an alignment, as was the MPX section. With early version 400 receivers, a really precise alignment is necessary since these units possess less gain and (therefore) limiting action than later four tube IF versions (which are much more tolerant of less than ideal adjustments). A precise alignment then is necessary for the three tube IF design to maximize the sensitivity and limiting action of which it is capable. Of all of Fisher's Wide Band FM Stereo MPX receivers however, early version 400 units do not employ any AVC for the RF or IF sections of the tuner. This aids in maximizes sensitivity and limiting, but also makes for a very hot signal from the Ratio Detector stage with strong signals. With such a hot signal, it can overload V100 in the MPX section when using Russian manufactured tubes in this position. Therefore, while always a good idea, it is mandatory to use good American pieces in this position in early 400 receivers to prevent the overloading caused when Russian tubes are installed. With the MPX repairs, new tubes, full alignment, and modification work all finished up, the receiver is shown receiving WSRV 97.1 out of Atlanta (transmitting tower some 55+ miles away) with a steady eye tube and superb stereo, using nothing but a simple dipole antenna in my basement (underground) lab.
BELOW: Topside, new can caps were installed. Under the old black cardboard cover is a new 50-50-50-50uF @350volt can with all sections wired in parallel to effect an upgrade over the old 100 uF @250 volt can originally occupying that space. The same can is used in the front most position as well to effect the original 200 uF @250 volt can that used to reside there. The two 50 uF 250 volt sections are replaced with discrete caps mounted under the chassis. Also, in this particular unit, all 5 of the tuner section tubes were replaced with new tubes from Jim McShane.
BELOW: Underside, the usual output stage work was done: precision 10Ω cathode current sampling resistors are installed, as are 100Ω Screen Stability resistors. Pin 8 is used as a new Screen Buss to facilitate the Stability resistors. Note too the disappearance of the 220Ω heater balancing resistors normally seen in this location -- this since they have been relocated over to the left channel phase inverter area, where they will receive (the new) heater bias voltage made available from the phase inverter modification. Finally, the gray lead entering near the top of the pic from the right side contains the four leads that provide the individual bias control from the IBAM installed. The original T-Strips provided for the bias connections are still used for two of the tubes, while the other two are connected inside a fully sleeved cover.
BELOW: The Noose Removal (Phase Inverter Modification) has been installed, with new heater balancing resistors provided that connect the heater bias point in the left channel. The input for these stages is from the blk-wht and wht-blk shielded leads running across and just above center of the pic. These leads come from the new Power Amp In jacks -- opposite from where the signal leads used to come from (volume control). Note the small shield fabricated at the input T-Strip for the left channel -- this to eliminate a slight hum with an open input from the heater wires that run right by the Strip.
BELOW: Under the Phase Inverter Adjustment controls lie the 47 uF 100 volt caps that are part of the Phase Inverter Modification.
Next time, the Standard Restoration and Basic Modification Package gets finished up, and the Optional Modifications get covered.
Dave
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