In response to a recent thread by RS Steve regarding one of these models that would not quiet appropriately in the right channel when turned to minimum volume, I took the symptoms of his unit, and using his test conditions, checked to see if my own unit, as well as that of a client's would display the same behavior. While not to the extent that Steve's unit did, they too both display the same characteristic: When turned to minimum volume, the right channel is distinctly playing at a level that is louder than the left. And what's more, the symptom was not consistent from unit to unit. Hummmm.
The facts surrounding the problem are addressed in post #99 of Steve's thread, which can be found here:
http://audiokarma.org/forums/index.php?threads/finally-acquired-a-x-202-but-it-has-an-issue.754793/
Also discussed is one possible solution, which involves using external attenuators to reduce modern digital source signal levels to that which is more appropriate for which the unit was designed. This has the added benefit of ensuring that the input of the amplifier will never be overloaded as well. Others however will see that as a band-aid, wanting to address the root cause of the problem, so as to keep the signal path as pure as possible. As with most things in life, the answer is usually somewhere in the middle, as I suggest it is with this issue as well: A minimum of 3 db external attenuation is recommended to ensure that overload will never rear its ugly head, but addressing the basic cause of minimum volume signal bleed is a good thing as well -- and particularly for those units that are inclined to display this problem more prominently than others.
The basic cause is due to the fact that there is not enough appropriate shielding in place within the build, to ensure that when large signal levels are applied to the unit, every unit will properly "quiet" when turned to a minimum volume position. What follows is four modifications to provide that shielding, which will result in extremely quiet minimum volume levels, and virtually eliminate all signal bleed under that condition. The modifications include:
1. Adding a ground plane shield between the Bass control, and the Hi/Scratch Filter switch.
2. Adding a ground plane shield between the Cathode Follower section and tone control section.
3. Adding shielded cable as appropriate.
4. Relocating two components in each channel.
The work to be done will be presented as a series of pics showing the results of the modifications installed into an early version unit. Since the modifications do not change the circuit in any way, the only components you'll need to make the modifications are 2 feet of miniature single conductor shielded cable, and a small (4in X 10in) sheet of #254 Easy Solder Tin. The latter can be had from Amazon and other outlets. The tin is quite easy to form an solder to, so it makes for ideal shield material. No dimensions are given for the shields; they are simply formed to fit in place which will be self evident from the pics provided. The pics will start with the stock look of a just restored unit for the target areas, and progress from there into the four modifications installed, basically to provide before and after pics for reference. Pics include:
BELOW: For reference, a pic of the finished restored unit before all this nonsense started. While irrelevant to this discussion, this unit as shown has already had as part of its restoration, the following modifications installed:
1. The old damping control circuits removed to free up a total of four terminals along the rear terminal strip.
2. Individual 10 Ohm cathode resistors installed with the freed up rear terminals now re-purposed as test points for setting the bias controls.
3. Output tube Screen Stability resistors installed.
4. All permanent LF filters removed.
5. An AC line current limiter installed.
6. The old selenium bridge rectifier converted to silicon.
7. EFB(tm) installed to improve performance and tube life and reduce operating temps.
BELOW: C58 (Sams) in stock position that will be relocated.
BELOW: C31 (Sams) in stock position that will be relocated. It is the horizontally shown yellow cap. Also shown are R102 and R50 (both Sams), which are 47K resistors that connected between the balance control and the T-Strip. These two resistors will be relocated as well.
BELOW: Shows the relocation of C58/R102 to the back of the balance control, and C31/R50 just above it. The caps are secured in place with some silicone adhesive. Also shown is the shield cable connecting the these components back to the tube pin they were disconnected from. Electrically, nothing has changed. The components are simply relocated to a more "secluded" area relative to available radiated signal for pick up.
BELOW: The other end of the shielded cable coming from the relocated components that connect to V10 (Sams). The cable is only grounded at the balance control end on the ground terminal of that control.
BELOW: The other end of the shielded cable coming from the relocated components that connect to V5 (Sams). The cable is only ground at the tube socket end at the center ground ring of V5.
BELOW: Lead #5 from K2 (Sams) as been sniped back close to the body of K2, and shielded cable used to make the connection from it back to pin 7 of V4 (Sams) where this lead originally connected to. The shielded cable is only grounded at the tube socket end, on the T-strip terminal where the cathode resistor from pin 8 (R49) connects to for its ground connection.
BELOW: A new shield is installed between the bass controls, and the Hi/Scratch Filter switch. It is secured by soldering it to an unused ground lug terminal on the T-strip below it.
BELOW: A new shield is installed between the Cathode Follower circuits, and the tone control circuits. It is secured at each end by soldering it to T-strip ground terminals used for existing connections. The shield should extend from the floor of the chassis, up close to the bottom cover (within ~ 1/4 in) when installed.
BELOW: The finished under chassis look with the new modifications installed (in addition to those previously mentioned).
And that's it. It does take some time to install, but for those units with excessive signal bleed under minimum volume conditions, these modifications are just the ticket for peace and quiet.
Dave
The facts surrounding the problem are addressed in post #99 of Steve's thread, which can be found here:
http://audiokarma.org/forums/index.php?threads/finally-acquired-a-x-202-but-it-has-an-issue.754793/
Also discussed is one possible solution, which involves using external attenuators to reduce modern digital source signal levels to that which is more appropriate for which the unit was designed. This has the added benefit of ensuring that the input of the amplifier will never be overloaded as well. Others however will see that as a band-aid, wanting to address the root cause of the problem, so as to keep the signal path as pure as possible. As with most things in life, the answer is usually somewhere in the middle, as I suggest it is with this issue as well: A minimum of 3 db external attenuation is recommended to ensure that overload will never rear its ugly head, but addressing the basic cause of minimum volume signal bleed is a good thing as well -- and particularly for those units that are inclined to display this problem more prominently than others.
The basic cause is due to the fact that there is not enough appropriate shielding in place within the build, to ensure that when large signal levels are applied to the unit, every unit will properly "quiet" when turned to a minimum volume position. What follows is four modifications to provide that shielding, which will result in extremely quiet minimum volume levels, and virtually eliminate all signal bleed under that condition. The modifications include:
1. Adding a ground plane shield between the Bass control, and the Hi/Scratch Filter switch.
2. Adding a ground plane shield between the Cathode Follower section and tone control section.
3. Adding shielded cable as appropriate.
4. Relocating two components in each channel.
The work to be done will be presented as a series of pics showing the results of the modifications installed into an early version unit. Since the modifications do not change the circuit in any way, the only components you'll need to make the modifications are 2 feet of miniature single conductor shielded cable, and a small (4in X 10in) sheet of #254 Easy Solder Tin. The latter can be had from Amazon and other outlets. The tin is quite easy to form an solder to, so it makes for ideal shield material. No dimensions are given for the shields; they are simply formed to fit in place which will be self evident from the pics provided. The pics will start with the stock look of a just restored unit for the target areas, and progress from there into the four modifications installed, basically to provide before and after pics for reference. Pics include:
BELOW: For reference, a pic of the finished restored unit before all this nonsense started. While irrelevant to this discussion, this unit as shown has already had as part of its restoration, the following modifications installed:
1. The old damping control circuits removed to free up a total of four terminals along the rear terminal strip.
2. Individual 10 Ohm cathode resistors installed with the freed up rear terminals now re-purposed as test points for setting the bias controls.
3. Output tube Screen Stability resistors installed.
4. All permanent LF filters removed.
5. An AC line current limiter installed.
6. The old selenium bridge rectifier converted to silicon.
7. EFB(tm) installed to improve performance and tube life and reduce operating temps.
BELOW: C58 (Sams) in stock position that will be relocated.
BELOW: C31 (Sams) in stock position that will be relocated. It is the horizontally shown yellow cap. Also shown are R102 and R50 (both Sams), which are 47K resistors that connected between the balance control and the T-Strip. These two resistors will be relocated as well.
BELOW: Shows the relocation of C58/R102 to the back of the balance control, and C31/R50 just above it. The caps are secured in place with some silicone adhesive. Also shown is the shield cable connecting the these components back to the tube pin they were disconnected from. Electrically, nothing has changed. The components are simply relocated to a more "secluded" area relative to available radiated signal for pick up.
BELOW: The other end of the shielded cable coming from the relocated components that connect to V10 (Sams). The cable is only grounded at the balance control end on the ground terminal of that control.
BELOW: The other end of the shielded cable coming from the relocated components that connect to V5 (Sams). The cable is only ground at the tube socket end at the center ground ring of V5.
BELOW: Lead #5 from K2 (Sams) as been sniped back close to the body of K2, and shielded cable used to make the connection from it back to pin 7 of V4 (Sams) where this lead originally connected to. The shielded cable is only grounded at the tube socket end, on the T-strip terminal where the cathode resistor from pin 8 (R49) connects to for its ground connection.
BELOW: A new shield is installed between the bass controls, and the Hi/Scratch Filter switch. It is secured by soldering it to an unused ground lug terminal on the T-strip below it.
BELOW: A new shield is installed between the Cathode Follower circuits, and the tone control circuits. It is secured at each end by soldering it to T-strip ground terminals used for existing connections. The shield should extend from the floor of the chassis, up close to the bottom cover (within ~ 1/4 in) when installed.
BELOW: The finished under chassis look with the new modifications installed (in addition to those previously mentioned).
And that's it. It does take some time to install, but for those units with excessive signal bleed under minimum volume conditions, these modifications are just the ticket for peace and quiet.
Dave