Adding DC Bias/Balance Test Points to the X-202

[dcgillespie]

Ever since my thread of nearly three years ago now about resuscitating my Pitiful X-202 (seen here):

http://www.audiokarma.org/forums/index.php?threads/pitiful-x-202.551073/

I've had a number of requests for more specific information about how to implement one of the modifications I did to my unit, but only generally touched on in that thread. Specifically, it converts two of the rear panel barrier strip terminals in each channel into output tube test points, so that adjustment of the DC Bias and Balance controls can be done far more accurately, and simply too since the modification then means that the bottom panel no longer needs to be removed to facilitate these adjustments. Along with the usual installation of individual 10 Ohm cathode current sampling resistors, this modification also requires removal of the circuits facilitating the antiquated damping control feature of this unit, which then frees up the necessary rear panel terminals. While the installation of the individual cathode resistors is rather routine now for many who frequent this forum, removal of the damping circuits to allow re-purposing of the terminals is not.

I had intended on opening my own unit back up to take more specific pics to support the text of this thread, but then I was contacted by one of this forum's international followers regarding restoration of an X-202 that was going to be purchased here in the states. That led to a coordinated shipment of it directly to me for restoration before ultimate shipment to its new owner over seas. Since the test point modification is part of a suite of modifications being installed into the unit, it made for the perfect opportunity to show the execution of it in detail. So I'll kick it off first with a couple of pics of the unit that's being restored to set the stage. The pics that follow in succeeding posts will then detail the work involved. Since this is a client's piece, you will see other work in progress in some of the pics as well. I'm happy to discuss it, but the focus of the thread is primarily to address the test point modification. In addition to the basic restoration work then, the list of modifications being installed includes:

SAFETY MODIFICATIONS

1. Removal of damping circuits to facilitate rear panel test points.

2. Installation of individual 10 Ohm cathode current sampling resistors.

3. Installation of Screen Stability resistors.

4. Installation of CL-80 current limiter.

PERFORMANCE MODIFICATIONS

1. Removal of the permanent LF filter circuits included throughout the design.

2. Installation of EFB(tm) to maximize performance and output tube life, and reduce generation of heat.

PICS INCLUDE:

Below: Some guys have all the luck! Have you ever seen a more pristine unit? Note that on the later versions like this one that they finally got the filter switches correctly labeled as Low and High Filter switches -- this because they affect their respective frequencies on ALL inputs. On my earlier version unit, even though these switches function identically in the circuit as they do in later versions, they were labeled as Scratch and Rumble Filter switches -- leading you to think that they only operate on the phono settings, which is not true. Even with the earlier versions, these switches affect ALL inputs.


Below: It's also got one of the cleanest top sides as well. Stickers indicate tube testing results.


Below: The unit as built by Fisher -- bone stock.

Next time, we'll get started.

Dave

 

[dcgillespie]

The process starts by first removing ALL of the original wiring to the cathode terminal (pin #3) of each output tube. Any lead connected to these pins should be completely disconnected not only at the output tube socket end, but also from where it attaches at its other end, so that the lead (or component) can be completely removed from the set. If your unit is factory stock as this one is, then you will be removing:

1. A covered jumper wire between pin #3 of both output tube sockets in both channels,

2. A bare jumper from pin #3 of one output tube socket to the adjacent T-strip ground terminal in both channels, and

3. A 10 Ohm 2W resistor from each pair of output tubes in both channels. When you are finished, nothing will be connected to pin #3 of any output tube socket, and the four jumpers and two resistors will be sitting on your bench.

Next, remove ALL the wiring connected to the "C" and "X" terminals of the speaker connection barrier strip in both channels. Also, disconnect the other lead of the two .47 Ohm 5W resistors from the bare jumper lead (the jumper itself remains in place), so that these resistors can be completely removed from the set.

PICS INCLUDE:

Below: When you're finished with the preceding work, pin #3 of each output tube socket and the four barrier strip terminals discussed will appear as in this pic. Sharp eyes who examined the stock underside pic posted earlier, will note that the nick in the black lead of the left OPT was brought to you courtesy of a vintage Fisher Radio Corp solder slinger.



Below: Not seen clearly in the previous pic, when the previous work is finished, you will also be left with two green and one black (cloth) leads that are now connected to nothing as shown.



Next, un-snake the two green wires back to their point of origin, where they connect at the other end as shown below:


More next time.

Dave

 

[dcgillespie]

From the previous work, you will now be left with the one black wire that remains disconnected back at the speaker terminal area, which is how we'll leave it for the moment.

Now -- based on whether you are just doing this modification as a stand alone piece or doing it as part of a larger restoration effort as is the case here -- comes what is potentially the hardest part of the work to perform for this modification. I say potentially because how hard it is will largely be determined by your solder slinging skills, and the desired outcome in terms of finished appearance of the work. These are variables that can be all over the map based on each individual. Therefore, I make the comment with no predetermined judgement as to what your skills are or desired outcome in appearance should be. Rather, I simple offer it as information in the same vane as that of a "Curve Ahead" sign, leaving it up to each individual to negotiate based on what is best for them.

What needs to be done then, is that the green wires need to be disconnected and removed. For a stand alone effort, this can be as simple as snipping them out close to the terminal on the T-strip that they connect to, or completely unsoldering the wire from the terminal. Obviously, the former is much easier to do than the latter is. When done as part of a larger restoration effort however, it becomes more involved because the two small electrolytic cathode bypass caps for the AF amp section of V5 and V10 (C9 & C10 Sams designations) must also be replaced, and one lead of these caps is also connected to the terminals where the green wires connect to.

The work area where the caps reside is extremely tight, and access is quite limited, particularly for C10. Ergo, the curve ahead warning. Electrically, snipping out the body of the caps and tying new ones onto the remaining pigtails of the old ones works just as well as completely unsoldering the old caps from their points of connection and soldering in new ones. Some may actually find it easier to simply remove the volume control to gain better access for C10. Proceed in whatever manner is best for your purposes, but replacement of the original caps is really not optional for a proper restoration. In my experience, these caps rarely short or develop a high power factor, but famously go intermittent over time. When they do and do so in the circuit location these caps reside at, it can cause volume fluctuations in the channel with the intermittent cap.

This is a very busy corner of the 202 to work in, that no doubt caused Fisher their share of headaches as well. I'll offer up a pic to show but one reason why. In any event, as a result, they wisely moved the whole power switch/volume control/loudness switch "mess" up to the upper tier control level on the succeeding B versions to eliminate the problem. Complicated as it is however, I personally prefer the power/volume/loudness control located where it is on the 202.

PICS INCLUDE:

Below: The old (orange plastic covered) C10. It has one lead that connects to the same terminal where a green wire that is to be removed connects to. Space is very tight to work in. The modern electrolytic cap seen secured to the chassis in the bottom right hand corner of this pic is part of the restoration of this unit, becoming the new first filter cap section (section A) of the old C4 can cap (Sams) that makes up the filter network of the DC Heater/Bias supply.


Below: The old C9 also has one lead that connects to where the other green wire that is to be removed connects to. Space is better, but still no picnic.


Below: A new C10 is installed, and the green wire removed. The modern cap secured to the chassis in the upper left portion of this pic is also part of the restoration, becoming the new C4B in the DC Heater/Bias supply.


Below: A new C9 is installed, and the green wire associated with it has also been removed.


Below: When I was first able to safely assess the operation of this unit, the Right channel was MIA, and here's why. The end of R107 connected to pin 6 of V10 was brushing against the rear cover over the power switch portion of the volume control, grounding out the signal. Simply wiggling the tube in its socket or turning the loudness switch on or off (part of the volume control assembly) would either move the resistor or the control enough for the two to make contact, and make the scope presentation go nuts. The location of R107 is as built by Fisher. They later relocated the control assembly up north. Earlier versions of the 202 used a slightly different configuration for R107.


Below: Work for the modification (only) is complete in this area. While much remains to be done to complete the restoration, a new C2 can cap (Sams) can be seen as installed in the bottom left corner of the pic as part of that effort, and both sections of the new C4 (mentioned earlier) as well. Also, the new two terminal T-strip for securing the 15 Ohm 5W resistor (R134) can be seen as well. R134 itself is only temporarily reinstalled at this point to allow greater access in addressing restoration of the phase inverter circuits. Note too, that R107 is now well clear of touching the rear cover of the AC power switch.


Next time, we return to the output stage area.

Dave

 

[AlTinkster92]

Nice job Dave, are the brown coupling caps staying? Always wondered about those?

 

[dcgillespie]

No, they are being replaced as part of the restoration -- but in reality, they really aren't a bad cap. The problem is, they can go intermittent, although it is usually the physically smaller darker colored ones that are most prone to doing that. These could last many more years, but the minute you take that chance.........

Now the older dark brown caps market WIMA on them are famous for developing leakage over time. This unit had two of them, and the darn things were nearly a dead short.........

Dave

 

[dcgillespie]

OK. Back at the output stage area again to wrap this modification up.

The first step is to connect the remaining black wire that was disconnected from the output terminal strip to the ground (mounting) terminal of the 6 terminal T-strip that is located immediately behind the two output tubes closest to the rectifier tube. But just like a kit, don't solder the connection just yet. In fact, make sure the terminal is free enough to accept four more leads (besides the black wire) before soldering.

Next, install four 10 Ohm 1/4 watt resistors, with one lead of each resistor connected to one of the four terminals on the output terminal strip that were previously freed up (i.e., the new test point terminals), and the other lead of all four resistors connected to the same mounting terminal on the 6 terminal T-strip that the black wire is now connected to. These resistors either need to be close tolerance precision units, or hand matched pieces chosen from a batch of looser tolerance pieces -- which is what was done in this case. Either way will get the job done equally well. After all four resistors are installed, then the mounting terminal on the 6 terminal T-strip can be soldered.

Finally, connect a wire from each test point terminal to pin #3 of the output tube socket that the test point is to represent. The test points should be connected so that they represent in the same order as the tube sockets do, left to right. And that completes the modification.

PICS INCLUDE:

Below: The black lead has now been attached to ground, via the mounting terminal of the 6 terminal T-Strip located behind the right channel output tubes. Also note that the original screen grid wiring connected to pin #9 of each output tube socket has been removed, and reinstalled connecting to pin #8 of the output tube sockets. This then allowed for Screen Stability resistors to be installed between pins 8&9 on each socket as a guard against output tube arcing. These are required due to the use of modern low ESR electrolytic caps now installed in the power supply, and in preparation for installing EFB(tm) in this unit as well. The power resistors that are bent up out of the way over the right OPT power the screen grids in the stock unit, and will be temporarily reconnected again so that a baseline of stock performance can be established, and confirmation can be made that the stock unit is operating properly in every way before the remainder of the performance modifications are installed. Finally, also note the nick in the brown lead from the left OPT, similar (but smaller) to that of the nearby black lead noted earlier. It is an indication of how hot the 4000 Ohm power resistors operate in the stock unit. You can see the discoloration of the one resistor where it made contact with the brown lead, causing its insulation to start melting.


Below: Final step. The four matched 10 Ohm current sampling resistors have been installed, with a lead from each of the new test points connecting to pin #3 of the appropriate output tube socket. By mounting the resistors remotely from the tube socket terminals, it prevents the heat of the tube pins from being dissipated into the resistors, and thereby helping them to maintain a stable value over a wide range of operating temperatures. Note also the new can cap comprising C1 on the Sams schematic in the upper right corner of this pic. The additional 22 uF 500V cap secured to the chassis below it augments one section of the new C1, so that all sections of the new C1 represent at least the minimum capacitance value as that for each section of the original Fisher C1 piece.


So there you have it, and all without making any permanent modifications to the unit. I mark each test point with a red marker to indicate its new function. Other than now no longer needing the external ground strap between the "G" and right channel "C" terminals (which is now a test point), the speaker connections remain the same, with the small impedance strap still required between the "S" and appropriate right channel impedance terminal.

With its steel chassis and extra beefy transformers, the X-202 is one of if not the heaviest 6BQ5 family of integrated amps out there. Therefore, having the ability to check and adjust the DC Bias and Balance controls at will without having to remove the bottom cover is a welcomed relief, and huge time saver, too. For those with an X-202, I think you will find this modification well worth the time to execute.

Dave

 

[dcgillespie]

One last but important detail point needs to be mentioned! Once the individual cathode resistors are installed, the target test point voltage when adjusting the DC Bias and Balance controls is now 0.30 vdc, or half the value specified by Fisher. This is based on stock (i.e., non-EFB) operation of the output stage.

Dave

 

[ssouci]

Dave:

I've seen the individual 10 ohm cathode resistors being specified as 1/4 W and 1/2 W... does it make much difference and which do you recommend?... also, do these need to be rated for 350 V?

thanks

 

[ssouci]

PERFORMANCE MODIFICATIONS

1. Removal of the permanent LF filter circuits included throughout the design.

I'm very interested in this modification... do you have any more info on this mod, and what is involved? (assuming it's not some sort of proprietary info of course!)

thanks

 

[dcgillespie]

Whoops -- Sorry I missed this question. The modification is not proprietary, and is something I routinely inform my clients about in general with Fisher gear. Some models it should NOT be removed from (i.e. 500C and 800C receivers), but is generally a good thing to do in the integrated amplifiers, unless your equipment for playing vinyl still requires an extensive LF roll off. For the X-202, there are four such filters -- two active, and two passive. For reference, I'll use the SAMS schematic at its designations to detail the components involved:

1. At V2: C20, and C47should be removed, and a wire installed in their place. You can just short them out for trial purposes if you wish, or just short them as a permanent modification. In an effort to minimize clutter whenever possible, I always remove components no longer being used.

2. At V3: C24 should be removed and a wire installed in its place. Also, C26 should be increased to 1.0 uF.

3. At V9: C51 should be removed and a wire installed in its place. Also, C53 should be increased to 1.0 uF.

4. At V5: C31 (a .01 uF incorrectly labeled as .1 uF), should be increased to .027 uF.

5. At V10: C58 (a .01 uF incorrectly labeled as .1 uF) should be increased to .027 uF.

And that's it. Collectively, its a good bit of work, especially to get to some of the components, as the component population within the 202 chassis is pretty dense. However, this modification produces a flat response down to 20 Hz for the high level inputs, while the response below 20Hz still falls fairly quickly on phono inputs, due to the time constants used within the phono preamp design. Also, the front panel Low Filter switch remains fully operational with this modification, so it can still be used if a particular source requires it. For the vast majority of today's users then, this is a good modification to implement, to ensure that the amplifier can pass the full response of today's full range digital sources. However, for those who may still use a changer or who has their equipment installed in a console setting, I would suggest proceeding at a calculated pace with its implementation, to insure that rumble or feedback issues don't crop up.

Finally, at the request of my client, I'll close out the thread in my next post, showing the results of installing the EFB(tm) modification into this unit. Once again, there are no drawbacks, and significant benefits.

Dave

 

[dcgillespie]

Beyond developing EFB circuits for use in an existing design, installing it can be an exercise all unto itself -- particularly if it is to be a neat and orderly installation. The client who owned this beautiful X202 wanted the EFB modification installed, and since I ultimately wanted to develop it anyway for installation into my own unit, the timing of the request was perfect. So, the intention here is to show how it was installed, and what the benefits were as a result of the installation.

As has been shown in so many other installations, the application of EFB produces a number of benefits:

1. OUTPUT TUBE LIFE: The original quiescent cathode current specification for the X-202's 6BQ5 family of output tubes is 30 mA per tube. This level of current flow is required for the output stage to produce minimum distortion when the operating voltages sag with increasing power output. Allowing 2 mA for screen current, that leaves the plate with a current flow of 28 mA under quiescent conditions. With a plate voltage of 390 vdc (Fisher figure, assuming the nominal AC line voltage is applied to the unit), this causes each tube to operate with a plate dissipation of (for all intents and purposes) 11 watts, which represents 91% of the tube's Design Center rating of 12 watts. Since the Design Center rating system represents a more conservative rating system over that of the final Design Maximum rating system used in the waning days of vacuum tube development, it means that although they will be running hot, the tubes will still be within the safe limits established by tube manufacturers for the 6BQ5 family of tubes.

With the application of EFB, there is no need to account for power supply sag, since the EFB circuits automatically adjust output stage operating conditions to maintain optimum performance, regardless of changes in power supply voltages, be they due to changing AC line voltage conditions, or because of changing dynamic conditions (power output produced) in the amplifiers. As a result, output tube quiescent current can be reduced to the same optimum value as that used if the output stages were operated from perfectly regulated power supplies. In the case of the X-202, that means that the new quiescent cathode current target is now 22 mA per tube for optimum (minimum distortion) performance. Now with this reduced current flow by the output stages, power supply voltages rise slightly (about 15 vdc) because in fact the power supply of the 202 is not regulated. But even taking this into account -- and the increased B+ voltage due to today's higher line voltages -- with an effective plate current of just 20 mA, it means that the output tubes now operate with a plate dissipation of just 8.8 watts, representing 73.3% of the conservative Design Center rating.

As a result, not only do the tubes run cooler -- which has been shown to have a direct impact on tube life -- but the cathode current has been reduced by nearly 27% as well. Since it is current flow that normally causes a cathode to wear out, this too has a very positive impact on tube life, with both the reduced current flow and cooler operation forming a one-two punch to aid in extending tube life.


2. POWER TRANSFORMER OPERATING TEMP: When this unit was initially restored, power transformer temps were taken with the stock design still in place, since it has been well noted how hot the power transformer operates in this model. After 2.5 hours of operation in open air (no cabinet) in my 73F listening room, the transformer registered a temp of 139.1F. While perfectly fine for a power transformer, and even on the cool end of transformer operation (hot by transformer standards would be anything over 175F), this would be very hot to the touch -- and with open air operation at that.

With the installation of EFB, transformer temps fell to an average of 131.5F -- and this was after extended operation in a simulated case enclosure, in the same listen room environment of 73F. The drop in temp was the cumulative effect of reduced current draw from the power transformer, reduced output tube temperatures, and reduced under-chassis heat generation, since the installation of EFB means that all those hot hot power resistors in the stock screen grid bleeder circuit are now gone. Don't get me wrong. This amplifier still gets plenty hot. But now, a significant edge has been taken off the heat produced by the original design, which can only bode very well for extending the life of the restored unit overall.


3. POWER OUTPUT: Fisher rated the X-202 (and its SA-100 cousin) as a 50 watt amplifier -- 25 per channel. Of course, it had no prayer of ever achieving this power level based on current day standards -- even with today's higher AC line voltages: This rating was based on the "Music Power" rating system of the day, based on a single channel driven, and represents the instantaneous power level that the amplifier can theoretically reach under (very) short term (fractional second) conditions. In reality, the real single channel driven RMS power output is 20 watts, with a more honest rating of 16 watts RMS per channel with both channels driven. You can see that those boys in marketing sure did like the Music Power rating system!

In fact, before the installation of EFB, the restored X-202 of this thread mimicked these power ratings exactly, such that when operating from a 121 vac line, it would produce 20 watts RMS from each channel individually from 25 Hz to 20 kHz, and 16 watts RMS across the same band with both channels driven. This is completely in keeping with this performance parameter as measured in my own X-202, and my own SA-100 as well under the same conditions.

With the installation of EFB, the bandwidth of full power output remained unchanged, but now, single channel power output rose to 25.0 watts RMS in either channel, while with both channels driven, each channel would produce a continuous 20.5 watts RMS in each channel, representing an honest 28% increase in power output when operating under the control of EFB.


4. HARMONIC DISTORTION: In stock form, each channel individually would produce just under .5 % THD at the onset of clipping (20 watts RMS) at 1 kHz. This figure doubled to just over 1.0% at the reduced power output available with both channels driven at the onset of clipping (16 watts RMS) at the same frequency.

After the installation of EFB, THD at 25.0 watt RMS at 1 kHz in either channel (driven separately) came in at 0.32%. At 20.5 watts RMS, it rose from 0.30% in either channel when driven individually, to 0.31% with both channels driven. Besides the increase in power output achieved, the virtually identical low distortion performance whether one or both channels are driven within the maximum power rating with both channels driven is a hallmark of what operation with EFB achieves: In spite of the significantly different power supply conditions present when comparing single channel to both channels driven performance, the distortion performance of the amplifier is basically unchanged, because the EFB regulators adjust the operating conditions of the output stages to maintain their optimum operating point in spite of the different power supply conditions present.

So once again, the application of EFB brings multiple advantages to the table, improving both power and distortion, while lowering operating temps and extending tube life. A few pics are included to show how it was installed in the very crowded X-202 chassis:

Below: The EFB Screen Grid Regulator (SGR) circuit is rather simple, with the vast majority of its components located between and around the output transformers, where the old bleeder screen grid power resistors used to reside. Their removal really
helped to cool down the under-hood temps.


Below: The power mosfet, which is the heart of the SGR, is mounted in one of the coolest areas of the amplifier, using one of the two mounting holes originally used to mount the selenium rectifier. The other hole mounts the new silicon bridge that replaces the old selenium unit. Both of these devices remain quite cool (<100F) even under extended operation in simulated case mounting conditions.


Below: The EFB bias supply and Control Grid Regualtor (CGR) are somewhat more complex, and so they are built up on a small perf-board circuit board, which is then secured to the under side of the chassis directly under the power transformer.
The CGR supplies EFB controlled bias voltage to the stock bias and balance controls, which continue to operate as they originally did, except that the new cathode voltage adjustment target is 0.22 vdc for each tube.


Below: The end result of the restoration effort, including the modifications to remove the permanent LF filters, convert four rear panel terminals into individual bias measuring test points, and the installation of EFB.


Below: The power transformer enjoyed the update as well -- even after extended operation.


The X-202 is one of Fisher's finest, built -- I believe -- when Fisher arrived at the height of its heyday with vacuum tubes. The efforts of this thread work to elevate it to be the best it can be in today's audio environment, without losing any of its original presence, function, or a sonic virtues in the listening room. Those who own one, know what I mean!

Dave