I had an occasion to spend some time under the deck of one of these beauties, and during the course of my work, determined a number of service points for TA-600 owners to be aware of with their units. While these all relate to the early (7199 driver tube) version, the last point may follow through to later versions as well. These include:
1. Power Supply Dropping Resistor (R19):
Early versions of this receiver specify a 140 Ohm 10 watt device for this component, while later versions used two 280 Ohm 7 watt resistors in parallel to produce a 14 watt rating of the same resistance value. This resistor however dissipates nearly 10 watts under quiescent conditions, leaving zero safety factor in the early units. The safety factor in later units is still marginal when the under hood temps are taken into account. For dependable long term service and extended operation, R19 should be replaced with a 150 Ohm 20 watt device, either as a single resistor, or as two 300 Ohm 10 watt devices in parallel.
2. Lead Dress to Power Supply Resistors R19 and R26:
Early versions used the right two terminals (furthest from the power transformer) on the T-strip containing the NFB components for the Channel A driver tube (V20) to accommodate both R19 & R26. Besides placing about 420 vdc on a terminal that is right next door to a terminal connected to the Channel A 16 Ohm output tap, this allows power supply noise to creep into the cathode circuit of Channel A, via moisture than can contaminate the bakelite material of the T-strip over time. All wiring should be disconnected from these two terminals, and then the terminals should be cut off from the T-strip. Then, a new separate two terminal T-strip should be mounted via the OPT mounting screws to facilitate the two needed terminals. This breaks the noise connection by way of using two separate strips, and adds a significant safety factor to the OPT and any speakers connected to Channel A as well.
Additionally, there are two wires connected to these resistors; one coming from pin #8 of the GZ34 (V11), and one from C37A. As built by Fisher in the early versions, these two wires ran up along the two can caps to the fold in the chassis, made a right turn coming close to pin #7 of V20 (Channel A driver tube), which represents the input to the Channel A power amplifier section, extending down the chassis fold to the appropriate T-strip terminals for R19 and R26. Since these two wires carry significant voltage and ripple currents, their close proximity to V20 adds significant noise to Channel A at any volume control setting above minimum. These wires should be re-routed along the rear of the chassis before extending up to meet their appropriate T-strip terminal points. This revision will make a significant reduction in Channel A hum levels.
3. 7199 Driver Tubes:
Using popular adapters to allow the use of alternate tubes for the driver positions in this unit will typically cause two problems:
A. Notably Increase Quiescent Hum Levels: With the high Gm output tubes employed in this unit, coupled with the high gain of the pentode section of the driver tube, any hum voltages leaking through the heater cathode insulation in the pentode section of these tubes will be sufficiently amplified and become notable in the output as hum. The 7199 is specifically designed to minimize noise via this path, where as the popular alternatives that can be used in this unit with adapters are not. For the blackest background noise levels, 7199 tubes are the tube of choice for this unit.
B. Scratchy Volume Control: Even in their earliest stereo offerings, Fisher followed their time honored design plan of placing the volume control at the input of the power amplifier section, which in this case, means that the wiper of the volume control is connected directly to the control grid in the pentode section of the driver tubes. This scheme works very well as long as the driver tubes develop minimal grid emission current at the control grid of the pentode section. However, the popular adapter facilitated alternative tubes often develop enough grid emission (since they were basically designed for TV video service) to make the volume control sound scratchy with rotation. When this current is applied across the wiper contact where it comes into contact with the carbon track within the control, noise is generated that is amplified and reproduced in the speaker as static whenever the control is rotated. Again, the 7199 is specifically developed to minimize the development of grid emission current, allowing its use in sensitive designs where the pentode grid is connected directly to the wiper of a level control. Under these same conditions, the popular alternative tubes will cause the volume control to produce noisy operation, making again, the 7199 the tube of choice for this unit.
In units where no level controls are involved (such as in many basic power amplifiers for example), it is often possible to use the popular alternative tubes in place of the 7199 with an appropriate adapter, and achieve excellent results from the substitution. In this case however, with the design format that Fisher uses, this is not one of them.
4. B+ Feed to R131 and R133:
Downstream from the output tube screen grids, Fisher provided a dual B+ distribution network in the TA-600 -- one for each channel -- to support the driver, line/tone, and phono preamp stages. This was a nice touch in the early days of stereo, when crosstalk was a major concern in the early all-on-one-chassis stereo offerings. In the TA-600, it works great up to the driver tubes. But from that point on downstream, some dyslexic Fisher engineer got their wires crossed -- literally. The wires feeding R131 and R133 need to be reversed at their C37C & C37D connecting terminals. As is, the distribution from the Channel A power amplifier section is powering the Channel B line/tone and phono preamp sections, and visa versa. Reversing these two leads will allow the distribution networks to operate as intended.
Pics include:
1. The unit with the wiring scheme as originally built by Fisher. Note the white and yellow leads coming down the right side of the can caps, connecting to the bottom can cap. These two leads connected as shown reverse the B+ supply between the two channels to the small signal tube stages.
Also note the brown lead from pin #8 of the GZ34 rectifier tube, and the red lead from C37A (7:30 position) extending up to the fold in the chassis, turning right, going across the fold of the chassis near the top driver tube socket (Channel A, V20), and ultimately connecting to R19 and R26, which are out of the picture. The close proximity to this tube (pin #7 in particular) causes notable noise in that channel.
2. Here, the white and yellow leads have been reversed, so that each B+ distribution circuit stays within its own designated channel. The brown and red leads to R19 and R26 have not yet been corrected.
3. The finished power supply revisions. As before, the white and yellow leads have been reversed, but now, the brown and red leads from pin #8 of the GZ34 and C37A respectively, have become two red leads running across the back of the chassis before extending up to connect to the new terminals for R19 and R26.
Note also that R19 and R26 connect only to T-strips designated specifically for them now, and that R19 now consists of two 300 Ohm 10 watt resistors. The right two terminals that used to be part of the T-strip in the center of the pic just under the black tubing that used to facilitate R19 ad R26 have been cut off.
Together, all of these revisions work to increase durability, minimize noise to an inconsequential level (even on sensitive speakers), and maximize stereo performance, while making for quiet volume control operation. I highly recommend that any and all of the points mentioned, as they might apply to any particular TA-600, be implemented to obtain the optimum performance of which the unit is capable.
Dave
1. Power Supply Dropping Resistor (R19):
Early versions of this receiver specify a 140 Ohm 10 watt device for this component, while later versions used two 280 Ohm 7 watt resistors in parallel to produce a 14 watt rating of the same resistance value. This resistor however dissipates nearly 10 watts under quiescent conditions, leaving zero safety factor in the early units. The safety factor in later units is still marginal when the under hood temps are taken into account. For dependable long term service and extended operation, R19 should be replaced with a 150 Ohm 20 watt device, either as a single resistor, or as two 300 Ohm 10 watt devices in parallel.
2. Lead Dress to Power Supply Resistors R19 and R26:
Early versions used the right two terminals (furthest from the power transformer) on the T-strip containing the NFB components for the Channel A driver tube (V20) to accommodate both R19 & R26. Besides placing about 420 vdc on a terminal that is right next door to a terminal connected to the Channel A 16 Ohm output tap, this allows power supply noise to creep into the cathode circuit of Channel A, via moisture than can contaminate the bakelite material of the T-strip over time. All wiring should be disconnected from these two terminals, and then the terminals should be cut off from the T-strip. Then, a new separate two terminal T-strip should be mounted via the OPT mounting screws to facilitate the two needed terminals. This breaks the noise connection by way of using two separate strips, and adds a significant safety factor to the OPT and any speakers connected to Channel A as well.
Additionally, there are two wires connected to these resistors; one coming from pin #8 of the GZ34 (V11), and one from C37A. As built by Fisher in the early versions, these two wires ran up along the two can caps to the fold in the chassis, made a right turn coming close to pin #7 of V20 (Channel A driver tube), which represents the input to the Channel A power amplifier section, extending down the chassis fold to the appropriate T-strip terminals for R19 and R26. Since these two wires carry significant voltage and ripple currents, their close proximity to V20 adds significant noise to Channel A at any volume control setting above minimum. These wires should be re-routed along the rear of the chassis before extending up to meet their appropriate T-strip terminal points. This revision will make a significant reduction in Channel A hum levels.
3. 7199 Driver Tubes:
Using popular adapters to allow the use of alternate tubes for the driver positions in this unit will typically cause two problems:
A. Notably Increase Quiescent Hum Levels: With the high Gm output tubes employed in this unit, coupled with the high gain of the pentode section of the driver tube, any hum voltages leaking through the heater cathode insulation in the pentode section of these tubes will be sufficiently amplified and become notable in the output as hum. The 7199 is specifically designed to minimize noise via this path, where as the popular alternatives that can be used in this unit with adapters are not. For the blackest background noise levels, 7199 tubes are the tube of choice for this unit.
B. Scratchy Volume Control: Even in their earliest stereo offerings, Fisher followed their time honored design plan of placing the volume control at the input of the power amplifier section, which in this case, means that the wiper of the volume control is connected directly to the control grid in the pentode section of the driver tubes. This scheme works very well as long as the driver tubes develop minimal grid emission current at the control grid of the pentode section. However, the popular adapter facilitated alternative tubes often develop enough grid emission (since they were basically designed for TV video service) to make the volume control sound scratchy with rotation. When this current is applied across the wiper contact where it comes into contact with the carbon track within the control, noise is generated that is amplified and reproduced in the speaker as static whenever the control is rotated. Again, the 7199 is specifically developed to minimize the development of grid emission current, allowing its use in sensitive designs where the pentode grid is connected directly to the wiper of a level control. Under these same conditions, the popular alternative tubes will cause the volume control to produce noisy operation, making again, the 7199 the tube of choice for this unit.
In units where no level controls are involved (such as in many basic power amplifiers for example), it is often possible to use the popular alternative tubes in place of the 7199 with an appropriate adapter, and achieve excellent results from the substitution. In this case however, with the design format that Fisher uses, this is not one of them.
4. B+ Feed to R131 and R133:
Downstream from the output tube screen grids, Fisher provided a dual B+ distribution network in the TA-600 -- one for each channel -- to support the driver, line/tone, and phono preamp stages. This was a nice touch in the early days of stereo, when crosstalk was a major concern in the early all-on-one-chassis stereo offerings. In the TA-600, it works great up to the driver tubes. But from that point on downstream, some dyslexic Fisher engineer got their wires crossed -- literally. The wires feeding R131 and R133 need to be reversed at their C37C & C37D connecting terminals. As is, the distribution from the Channel A power amplifier section is powering the Channel B line/tone and phono preamp sections, and visa versa. Reversing these two leads will allow the distribution networks to operate as intended.
Pics include:
1. The unit with the wiring scheme as originally built by Fisher. Note the white and yellow leads coming down the right side of the can caps, connecting to the bottom can cap. These two leads connected as shown reverse the B+ supply between the two channels to the small signal tube stages.
Also note the brown lead from pin #8 of the GZ34 rectifier tube, and the red lead from C37A (7:30 position) extending up to the fold in the chassis, turning right, going across the fold of the chassis near the top driver tube socket (Channel A, V20), and ultimately connecting to R19 and R26, which are out of the picture. The close proximity to this tube (pin #7 in particular) causes notable noise in that channel.
2. Here, the white and yellow leads have been reversed, so that each B+ distribution circuit stays within its own designated channel. The brown and red leads to R19 and R26 have not yet been corrected.
3. The finished power supply revisions. As before, the white and yellow leads have been reversed, but now, the brown and red leads from pin #8 of the GZ34 and C37A respectively, have become two red leads running across the back of the chassis before extending up to connect to the new terminals for R19 and R26.
Note also that R19 and R26 connect only to T-strips designated specifically for them now, and that R19 now consists of two 300 Ohm 10 watt resistors. The right two terminals that used to be part of the T-strip in the center of the pic just under the black tubing that used to facilitate R19 ad R26 have been cut off.
Together, all of these revisions work to increase durability, minimize noise to an inconsequential level (even on sensitive speakers), and maximize stereo performance, while making for quiet volume control operation. I highly recommend that any and all of the points mentioned, as they might apply to any particular TA-600, be implemented to obtain the optimum performance of which the unit is capable.
Dave
