Finally acquired a X-202, but it has an issue

Could it be the outer foil on the film caps picking up interference? Turn them around one at a time on the side you are picking up the interference and see if there is any improvement.
 
Steve -- go back to post 45 of this thread and open the link I provided that takes you to a thread I did on a recent X202. In post #5 of that thread, look at the series of pics I provided. They clearly show the .01uF I'm talking about, as installed at the factory. They connect to pin #7 of each phase inverter tube. Those are the caps I'm talking about.

Dave
 
Steve -- go back to post 45 of this thread and open the link I provided that takes you to a thread I did on a recent X202. In post #5 of that thread, look at the series of pics I provided. They clearly show the .01uF I'm talking about, as installed at the factory. They connect to pin #7 of each phase inverter tube. Those are the caps I'm talking about.

Dave
Believe it or not, it looks like C-31 and C-58 were never there when I received this amp. The only caps I replaced around these two tubes were the white 25mf 6V units. Every capacitor I removed was replaced before I went to the next, so the previous owner must of removed them and forgot to replace them. No wonder my brain has been turned upside down. Now I have to figure where they hook to I guess, so missing capacitors like these be causing the issue?
 
Could it be the outer foil on the film caps picking up interference? Turn them around one at a time on the side you are picking up the interference and see if there is any improvement.
Maybe that's why the tar substance was on two of the caps.
 
It's starting to make sense why this amp has the problem it does, pin 7 on V10 and V5 are supposed to have a .01 capacitor and 820K resistor hooked to it according to pictures I have seen and the schematic. But this unit seems to only have a jumper installed with black insulation. I'm at a loss why this section is like this, or what would cause someone to change it. I would think this amp would have issues on both channels, so I guess I will stop here until I'm sure what to do next. I took pictures to try and show what it actually there. I wonder if they changed these up to use a different tube or something. :eek:

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Interesting. The jumpers look original, like the caps were never there -- but the Fisher manual for the range your serial number falls into shows the caps as still part of the circuit. Question: On pin #8 of the phase inverter tubes, there is a resistor, and a lead from a cap you have replaced. What is the value (or color code) of the resistors on pin 8 of both phase inverter tubes?
 
Huh. A 4.7K resistor. That's what I thought when I saw the yellow band, but couldn't see the rest of the resistor. That's a non standard value based on every unit I've ever come into contact with -- but it also explains why the .01 caps are not included, either: with the elevated bias created by the larger cathode bias resistor (it's normally a 3.9K), the .01uF blocking caps are simply not needed. At least the caps aren't there to pick up any cross talk. Looks like you're going to have to do some troubleshooting. So first test: When you set up the unit to produce the crosstalk, if you use a jumper clip to ground pin 7 of V10 (Sams designation), does the crosstalk go away?

Dave
 
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Huh. A 4.7K resistor. That's what I thought when I saw the yellow band, but couldn't see the rest of the resistor. That's a non standard value based on every unit I've ever come into contact with -- but it also explains why the .01 caps are not included, either: with the elevated bias created by the larger cathode bias resistor (it's normally a 3.9K), the .01uF blocking caps are simply not needed. At least the caps aren't there to pick up any cross talk. Looks like you're going to have to do some troubleshooting. So first test: When you set up the unit to produce the crosstalk, if you use a jumper clip to ground pin 7 of V10 (Sams designation), does the crosstalk go away?

Dave

It's looking like I will need to re vamp this area to make it back to stock. It baffles me why it's changed, just goes to show that there are surprises at every turn.
 
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Steve -- when the volume is full down, how loud is the cross-talk in the right channel? I'm sure it is rather tinny sounding, but how loud is it? Would it be noticeable say to someone walking into an average sized bedroom where two people were having a casual conversation?

Dave
 
Steve -- when the volume is full down, how loud is the cross-talk in the right channel? I'm sure it is rather tinny sounding, but how loud is it? Would it be noticeable say to someone walking into an average sized bedroom where two people were having a casual conversation?

Dave
It is even sounding to the left channel if I turn the volume control up about 3/16, seems to be pretty noticeable. When I turn the unit on and it warms up enough to make sound, you hear the right channel start making sound with the volume control full off, then when you shut the unit off, the sound slowly bleeds away. It seems that the cross-talk diminishes a little after the unit is fully warmed up around 15 minutes or so, but it is still noticeable. It's not like you need to put your ear to the speaker to hear, you can be 10 feet away and notice it.
 
Humm. OK. I have an idea as to what I think might be going on, but I'll need to run some tests to confirm my suspicions. I've got a client's X-202 here that I can use to make them on, which is perfect. It's due to have its final performance documentation performed later on this coming weekend anyway, so its at a perfect point to serve as a test bed for checking this out. I'll let you know.

Dave
 
Humm. OK. I have an idea as to what I think might be going on, but I'll need to run some tests to confirm my suspicions. I've got a client's X-202 here that I can use to make them on, which is perfect. It's due to have its final performance documentation performed later on this coming weekend anyway, so its at a perfect point to serve as a test bed for checking this out. I'll let you know.

Dave
Thanks Dave, I really appreciate you taking the time to walk me through this. I have all the correct can caps here now, but if you think the problem lies elsewhere I will wait until it is fixed before installing them. Just a note also, when I received this amp, it had 12AT7's installed in those two sockets, not sure if they changed things to accept those or not. I installed correct ones.
 
More testing today. I hooked the speakers up with the jumpers in place that allows the output to go through the phase switch. This hook up causes the right channel to have almost no sound. It will put out about as much sound when turned to full volume as I get from the bleed through hooked the other way.
 
Either bypass the phase switch or pull it out and clean the shit out of it. Make sure all the connections to the switch are correct and tight.
 
Either bypass the phase switch or pull it out and clean the shit out of it. Make sure all the connections to the switch are correct and tight.

Yeah, I have gone through the switches and connections many times. The only thing I have yet to do is a phase invertor adjustment, anyone have a simple procedure for the x202? I couldn't find it in the sticky.
 
It seems every time I turn this amp on to do more testing, the right channel gets progressively worse. It now has become even weaker sounding, and somewhat distorted. I'm leaning towards an out of spec resistor, or one of the can caps I have yet to replace. The left channel sounds super good, it has improved with the cap replacements I have done. I did replace can cap C-1, but still need to do C-2 and C-4, I'm waiting on the sleeve for C-4 to arrive since it was shipped with no cover.
 
Steve -- Something surely seems amiss with the phase reverse switch/wiring. It shouldn't do anything but basically reverse the speaker connections to the right speaker. If the switch allows the right channel speaker to operate in the normal position, but the sound all but disappears when the switch is set to reverse, then either the switch or its connections must be at fault, as the wiring between the switch and the rear connections is always used regardless of which position the switch is in. Be sure to get that resolved before turning the volume up high with a source playing but the right speaker not sounding due to the phase switch being in the reverse position. That scenario effectively amounts to no load being placed on the amplifier, which under high volume conditions is not healthy for any one of a number of components.

OK. Some discussion on the minimum volume signal bleed issue in the X-202: As a basic statement, it is the result of the physical design of the unit, in conjunction with the use of modern high output digital sources.

The X-202 is not the only product to suffer from the "won't turn down" issue. For example, I have a Scott LC21 preamp, that was meticulously built, that would exhibit the same symptoms that Steve is describing with his X-202. There are others as well. At the heart of the issue is (typically) modern CD players, that rather routinely output 2 volts of signal, which is 4 times the rated input voltage for the Scott, and at least 8 times that for the Fisher. Some units, like Heath's AA-141 preamplifier simply roll over and give up -- not displaying any bleed-over issues -- but producing gross amounts of overload distortion at any setting of the volume control. To make that unit really behave and operate properly with modern sources requires nothing less that a complete redesign of the unit. In the X-202, such extremes are not required, but correcting the possibility of minimum volume bleeding does require some deliberate corrective action.

The high source levels can also produce other problems in vintage equipment as well, causing volume controls to operate down "in the weeds", where channel tracking is usually anything but good. A number of folks use external attenuator pieces that cause the volume control to operate up higher in its range of rotation, simply to address the channel inequity that can occur at lower settings. But the attenuators are also helpful in subduing signal bleed issues as well, because they reduce the signal level floating around inside the chassis, and therefore, the tendency to produce bleeding from one circuit, to another. So, one approach to the signal bleed issue is to use external attenuators for all the benefits they can offer, and call it a day.

In my own listening room, my Denon CD player has a variable output, whose setting has been set at 50% of its 2 volt rated output for so long that I forget about the fact that its output is reduced. In the case of my own equipment, the reduced setting is not used to address any potential signal bleed, channel imbalance, or gross distortion issues but rather, to simply obtain a smoother volume control "action", versus one that would be rather touchy by comparison. With the client's X-202 that I have here, along with my own X-202, this reduced output doesn't present any observed operational problems, as in, problems that draw your attention as is the case with Steve's unit of this thread. However, with an intentional scrutiny of the volume control's minimum volume action in both my own and my client's unit, I too observed that technically, the right channel does not reach the same minimum volume level that the left channel does. Then, remembering the reduced output that I use, I ran the Denon's output up to maximum (2 volt) output. This definitely made the minimum volume level more pronounced, and more unbalanced between the channels as well -- not to the extreme that Steve is indicating -- but enough that with three units now all displaying the same tendency to one degree or another, it meant that likely, something in the original design or build of these units was contributing to the effect, and therefore not only for my own clients, but the good of the Fisher community as a whole, should be investigated.

In a nutshell then, what's happening is that because of the much higher than ever expected input levels, coupled with the high sensitivity of the X-202 design -- and where the volume control appears in the circuit, it means that there is plenty of opportunity for circuit interaction, where the signal at one point, can jump to other areas it was never intended to be at -- at least, not from that circuit point anyway. This is all aided by the high input impedance levels that vacuum tube amplifier stages typically represent, in close quarters with the two Cathode Follower stages that the X-202 employs. These stages act like blow torch transmitters of the highest signal level that appears within the chassis for other stages and points in the circuit to pick up. At normal listening levels, this bleeding is inconsequential as the actual level of signal that bleeds to other areas is really quite small and very frequency dependent -- unless the input signal level is much high than specification and the volume control is turned to minimum: then you hear the effects of this scenario rather easily, which is only made worse by a small quiet room and high sensitivity speakers. Packing a lot of performance into a relatively small chassis is always a recipe for potential concerns (which is what the X-202 represents), and is exactly the reason that the highest performance systems always include separate pieces to handle small and large signal amplification. The build of the X-202 was certainly good enough for the source levels of the day, but in the audio environment of this day, some examples of it can become challenged.

The answer to the problem is simple: just add some shielding to prevent the signal interaction. That's true, but the trick is to find out where and how much. As is usually the case, the real answer lies in a combination of shielding the components that can be shielded, while relocating those that can't be otherwise shielded to a less sensitive location -- and doing it all in a neat, orderly fashion, so as not to make the modification draw attention to itself. After a couple of rather intense days of going after this issue, I've developed a series of four modifications that when implemented in the X-202, will make any signal bleed between circuits a nonissue. More specifically, they allow a three volt rms signal (overload point) to be applied to any of the high level selector switch or Tape Monitor inputs, and require your ear to be within 1 ft of 101 db efficient speakers to hear any source material from either speaker. Standing in front of these (Cornwall) speakers in a small dead quiet room, the minimum volume signal with this input level applied is inaudible at even a close field listening position. The reduction is significant in units that suffer excessively from this issue.

I will start a new thread to outline the actual procedures of the modifications, but suffice to say, there a few rules that need to be observed when restoring an X-202:

1. Around the tone control amplifier stage of the X-202 (Sams V4), neat, tight work is a must. Long leads are an antenna just asking for bleed type problems to occur.

2. Ditto for using large, oversized boutique or otherwise replacement components in this area. They too act as giant antennas to pick up unwanted signals.

3. The bottom plate is also part of the shielding relative to this issue. Not only does its installation minimize hum and noise in the X-202, but signal bleed as well.

Finally, the X-202B is the continuation of the X-202 line, and it's interesting to note just how compartmentalized the build of that amplifier is -- almost surely to address potential issues of bleeding that would otherwise exist from one unit to the next in a hand wired, mass produced piece of equipment. In that way then, the X-202B is an improved build over that of the X-202. I should have a thread on the modifications that provide the proper shielding posted soon.

Dave
 
For sources that don't have a variable level output, could you simply fit say a -3db attenuator between the source and the amp to make it behave itself better? Should be able to knock one of those together easy enough or I imagine they can be bought pre-made with RCA connectors. I know they exist in BNC form for test gear use.
 
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