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

[RS Steve]

OK, so if that's true, then the problem (or part of it anyway) must be in the power amplifier section of the right channel (Sams V10, V11, V12). Two things to check:

1. Measure the value of R104 (Sams) -- should be 470K . If it's even close to within 10%, that's fine for now.

2. Check the setting of the Phase Inverter adjustment (with the power turned OFF):

A. Measure the resistance of R110 in your unit (18K), and record it's exact value.

B. Measure the resistance between the V10 side of C61, and chassis ground. Leave the meter leads connected, and adjust the right channel phase inverter control so that your meter reads the same resistance as that measured in step A.

Let us know!

Dave

R-104 measured 428K and the left channel R-52 measured 440K. I set the phase inverter adjustments, they were off slightly. Powered the unit back up and it pretty much sounds the same, although the left channel might sound a little better. Can't wait to get the right channel sounding that good, it still has the cross talk, slightly distorted, and less output and bass on right channel.

 

[RS Steve]

I spent a few hours just tracing the right channel circuit, then studying the schematic, and right there in front of me the cause to the cross talk problem. The previous owner actually had a capacitor soldered to the wrong terminal on a T strip, and because I'm good at taking pieces out and soldering in the new one, I soldered it to the exact same spot.
Now it has very very slight cross talk and no more distortion, the Gordon restored X202 has the same very faint cross talk in the right channel. You have to almost put your ear to it to hear it now. The only issue that is still present is the right channel still being weaker than the left, I have to turn the balance control to 1-3/4 right instead of zero to get equal sound. But man does this amps sound great now, I love progress!

I will go back and test voltages, bias, balance and see if it will help. I did replace the other two can caps today also, so this amp has a total recap now.

 

[dcgillespie]

Progress I should say!

Dave

 

[RS Steve]

After taking some voltage readings this is what I came up with. The left channel is showing the typical higher voltages from 123 V wall voltage and new modern capacitors, the right is showing a low reading on pin 9, not sure if this causes the lower output or not.
right channel
pin - 2 is -10.9 V
pin - 9 is 262 V
pin - 7 is 401 V

left channel
pin - 2 is -11.2 V
pin - 9 is 291 V
pin - 7 is 402 V

 

[larryderouin]

Steve; The difference may be the tube. Swap L&R then take the tube voltages on the same pins again. And check and see if the difference in balance swaps sides. I presume these are on OUTPUT TUBES? Sounds like a weaker output tube as pin 9 voltage is from the Main Filter caps. Check the voltages at the Junction of C46B,R86&R90 for 1 side and junction of C46C and R98. Both should be 290VDC with a supply voltage of 117VAC.

Larry

 

[gadget73]

Easy enough to solder to the terminal next to the terminal. I did that with the output coupling caps on my Sherwood 7000 and didn't even notice for probably 2 years. At very low volume where I normally listen it wasn't obvious that only half of the output stage on one channel was doing anything. Immediately obvious if you turned it up though. Took a bit of time to find too, ended up chasing stuff with the scope and got all the way to one output tube grid and realized I had no grid drive.

 

[RS Steve]

I reset the bias and balance, even changed out the tubes in the right channel, but the right channel stays weaker and thinner sounding.

I notice when I have the balance knob all the way to the right channel, the bass control has very little effect on that channel, treble has more effect but neither have as much effect as when the balance knob is turned to the left. Voltage on pin 9 and pin 2 are constantly less on the right channel compared to the left. Bias adjusted in very nicely at .59 V for each pair of output tubes.

 

[dcgillespie]

You really need to get individual 10 Ohm cathode resistors installed, as for all you know, that 59 mA reading is being achieved by one tube conducting 58 ma, and the other 1 mA. Clearly an extreme example, I know. But made, to make the point.

OK. Time for some troubleshooting efforts: with the balance control centered, use a a clip lead to connect the center terminals of the balance control together (that is, the center terminal within the group of three terminals on each control section). Does the sound even up then?

Dave

 

[RS Steve]

No sign of red plating and both tubes on the right side showing -9.1 V on pin two, but I do understand what your saying. I have all new resistors ordered for those areas that have the big wire wound units, none have tested terribly bad but are off enough for me to want better. I have to study how to remove the circuit and install the separate resistors, I like to have the understanding part digested before hand.

I'll do the test on the balance control later when I get back to it.

Thanks Dave!

 

[RS Steve]

This list below is what I'm studying, looks to be pretty straight forward, then it looks like I have to create good grounding points for the 4 new resistors.

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.

 

[RS Steve]

I replaced all the large resistors, installed Dave's individual cathode resistor mod, and also screen stability resistors. I was too chicken to power it up this evening though, I'll feel much more brave tomorrow.

 

[larryderouin]

NEVER SAY CHICKEN! You'll have the wolves here clucking like a bunch of old Hens.

 

[RS Steve]

NEVER SAY CHICKEN! You'll have the wolves here clucking like a bunch of old Hens.

I enjoy getting chuckles at my expense, I laugh at myself quite often . I just wanted to start fresh today by double checking all the work, and make sure my solder slinging was good. Sure is a big pile of old parts sitting here now, none which really show they are way out of spec though.

 

[gadget73]

Walking away for a while and coming back with a clear head is a good way to keep the smoke in. I've broken things for want of doing that more than once.

 

[RS Steve]

I achieved more positive progress, the amp powered up with no issue, biased and balanced in with ease, and sounds more powerful than before. I still have the right channel lacking output compared to the left though. I'm very pleased to be improving sound quality with every step, it is sounding very, very good. I think this imbalance issue is the only remaining problem.

I must say, the new bias circuit is the bomb, Dave you are amazing!

Also, I notice that the tone controls have much less effect on the right channel than the left, even the loudness switch barely makes a difference.

 

[dcgillespie]

Glad the new biasing arrangement works well for you!

If your last statement is absolutely correct, then you must have multiple issues going on: When I had you short the two center terminals of the balance control together, this point appears after the tone controls, volume control, and balance control in the circuit, effectively shorting the two channels together right at the point where the signal is applied to the power amp sections, meaning that both power amp sections were then being supplied with the same identical signal in every aspect. When the test was performed, you indicated that the right channel was still weak, indicating that there must be a problem in the right channel's power output section that is causing the imbalance. However, if you are now also indicating that the tone controls for the right channel seem to produce little change in response when rotated, then something else must be amiss in those circuits as well, since they appear before the point at which the channels were shorted together. In other words, based on what we know at this point, if you correct the channel imbalance in the power output sections (actually, it could be either channel causing the imbalance), that likely will not address the tone control issue you're now noting -- unless the defective power output section is restricting both high and low frequency response.

Regarding the tone controls, check that the front panel low filter switch is functional. It does not produce an in your face difference when turned on or off, but on source material with plenty of LF information present, careful listening will reveal a distinct difference in the setting when speakers of adequate capability are used. If the switch is defectively in the "on" position all the time, it will reduce the effect of the bass control, since the LF filter circuit appears before the tone control circuits.

Dave

 

[RS Steve]

Glad the new biasing arrangement works well for you!

If your last statement is absolutely correct, then you must have multiple issues going on: When I had you short the two center terminals of the balance control together, this point appears after the tone controls, volume control, and balance control in the circuit, effectively shorting the two channels together right at the point where the signal is applied to the power amp sections, meaning that both power amp sections were then being supplied with the same identical signal in every aspect. When the test was performed, you indicated that the right channel was still weak, indicating that there must be a problem in the right channel's power output section that is causing the imbalance. However, if you are now also indicating that the tone controls for the right channel seem to produce little change in response when rotated, then something else must be amiss in those circuits as well, since they appear before the point at which the channels were shorted together. In other words, based on what we know at this point, if you correct the channel imbalance in the power output sections (actually, it could be either channel causing the imbalance), that likely will not address the tone control issue you're now noting -- unless the defective power output section is restricting both high and low frequency response.

Regarding the tone controls, check that the front panel low filter switch is functional. It does not produce an in your face difference when turned on or off, but on source material with plenty of LF information present, careful listening will reveal a distinct difference in the setting when speakers of adequate capability are used. If the switch is defectively in the "on" position all the time, it will reduce the effect of the bass control, since the LF filter circuit appears before the tone control circuits.

Dave

Sorry Dave, I had not tried the jumper on the center balance terminals, so I just tried that. Results show equal output, and the tone controls have the same effect on both channels now. I even tried the low and high filter switches with similar results. The sound didn't seem as good though with the jumper in place, but I imagine it shouldn't when tying them together. So the problem must be before the balance control.

 

[RS Steve]

My next step was to trace things back from the balance control and refer to the schematic in order to make sure all connections were in the proper place. Well, guess what I found, pin 7 of V4 had the end of C58 soldered to it instead of pin 6. So here was another problem the former owner created, that is two connections soldered to the wrong connection point, no wonder he wanted to pass it on.
This amp is now working perfectly!

As frustrating as this has been, it has been a very good learning experience. I get to really understand the circuits along with using the schematic to verify. Special thanks to you Dave, I don't know how you put up with my non electronic brain, but you do. DAVE

 

[gadget73]

The old mixed up pin connection again. Those are honestly a pain in the neck to chase, but if you have a working channel for reference its definitely easier.

Good to hear its singing properly now.

 

[RS Steve]

The old mixed up pin connection again. Those are honestly a pain in the neck to chase, but if you have a working channel for reference its definitely easier.

Good to hear its singing properly now.

I pride myself by not giving up, sure feels great getting it figured out.