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Bce22 KX-200 Rebuild - First major rebuild

All of the B+ voltages on the can caps against the schematic, and the output tube plates and screens. Also the heater voltages. basically any place in the schematic that has a voltage on it. If there is a voltage chart for the KX-200 (pin by pin) use that.
 
Okay, so I have spent about 4 hours on the unit today and I am definitely having some issues. Right now I have no sound out of the right channel except for a semi-quiet hum that is not impacted by volume or input. After inspecting my work and not seeing anything specifically out of place I started taking voltages. Voltages in V5/V6 specifically are not good at all. I have attached the voltage readings for my unit.

Can you help me diagnose what might be wrong with V5/V6?

Voltages.jpg
 
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I don't have schematic in front of me and I unintentionally led you astray before but a couple of comments. Factory schematic voltages are base on 115v in (typical early 1960s), With that in mind and your current line is likely 122-123v, your voltages are not ridiculously high ( rule of thumb tends to figure +/- 10% normal.
I would pay attention to filament and bias voltages and at least if the filaments are high you should consider adding a CL or a bucking transformer.
Nothing I mentioned so far addresses your channel issue. You might want to try jumping wipers on the volume pot to try and narrow down your issue.
 
Hi NJ! Don't sweat your earlier comments. They did not send me in the wrong direction and I am happy to have your expertise in this thread. I can't thank you enough.

I do have a Variac that I tend to run the Fisher off of set to 115-117V. I decided to test everything off the wall. I am hoping to get some help with the channel issue. Thanks again!!


PS. I've tried swapping tubes in V5 and V6 with a couple of extra 12AX7s I have. i do not have too many extra lying around, but at least I have a couple.

Oh, I am going to refrain from posting more details of my rebuild until I can track down the error(s). Once determined I will continue with the detailed descriptions and pictures.
 
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I'm flattered but not worthy of your comments. If I may, I think the guidance you'll get (not necessarily from me) if you continue to report your progress will be invaluable to you. Keep up the good work . You're closer than you think.
I don't know if you mentioned. All the controls could probably do with a good cleaning. Your problem may be there ( to be sure, it may not). Nothing wrong with reaching for the low hanging fruit first. If that fails, you may want to do a complete visual inspection with good light and magnification. Looking for bent, broken shorting, solder blobs and cold solder joints. If all these things fail. There are several here who have spent quality time under the hood of these. Help is here. Good luck
 
All of your can caps voltages are at least 50V higher than nominal. Tube load (how well they load down the circuit, mainly output tubes ) has a good deal to do with what the voltage is in the end. If the output tubes are old and weak, they won't have a lot of current draw and the voltages will be higher than normal. Are the output tubes you are using good, as in not weak? Remember in some cases tubes can sound quite good right up to the moment they die. Others can test weak and still produce fairly good sound, but are being highly stressed. So make sure your output tubes are up to snuff. Tubes MUST be inserted when checking voltages. If they are not installed, the voltage will be high, as shown above.


Pins 1&3 of V5 and V6 feed off the same CAN CAP Section. C3b is your source. Check the voltage there against the schematic. Then physically check the wire from C3b for open sections of insulation, pinched wire under mounting nuts, etc. Also pull one side of the following resistors and check for value. R45 & 46 (68K), and R47 & R48 (680K). Also make sure nothing is touching another component or chassis.

Overall I don't see anything glaringly wrong other than higher than normal B+ voltages from the 1st section on out, which would indicate to me that either the output tubes are not biased correctly, or they are weak and not drawing down the system like they should, or 1 or more bad CAN CAP sections.
 
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larryderouin said:
All of your can caps voltages are at least 50V higher than nominal. Tube load (how well they load down the circuit, mainly output tubes ) has a good deal to do with what the voltage is in the end. If the output tubes are old and weak, they won't have a lot of current draw and the voltages will be higher than normal. Are the output tubes you are using good, as in not weak? Remember in some cases tubes can sound quite good right up to the moment they die. Others can test weak and still produce fairly good sound, but are being highly stressed. So make sure your output tubes are up to snuff. Tubes MUST be inserted when checking voltages. If they are not installed, the voltage will be high, as shown above.


Pins 1&3 of V5 and V6 feed off the same CAN CAP Section. C3b is your source. Check the voltage there against the schematic. Then physically check the wire from C3b for open sections of insulation, pinched wire under mounting nuts, etc. Also pull one side of the following resistors and check for value. R45 & 46 (68K), and R47 & R48 (680K). Also make sure nothing is touching another component or chassis.

Overall I don't see anything glaringly wrong other than higher than normal B+ voltages from the 1st section on out, which would indicate to me that either the output tubes are not biased correctly, or they are weak and not drawing down the system like they should, or 1 or more bad CAN CAP sections.
Click to expand...

Hi Larry,

Thanks so much for your assistance.

My output tubes are brand new Tung-Sols from Jim McShane. All tubes were installed when voltages were checked. I set Bias and Balance before checking voltage and was able to set them easily.

I will check C3B and then pull R45/46 & R47/48 and report back. Thanks!
 
larryderouin said:
Also pull one side of the following resistors and check for value. R45 & 46 (68K), and R47 & R48 (680K). Also make sure nothing is touching another component or chassis.
Click to expand...

R45 AND R46 were 680K. I replaced R45 with a 68K resistor and then took voltages again. The updated voltages are below.

Updated Voltages.jpg

I haven't turned it on to see if I still do not have sound out of the right channel. Going to check this now.

By the way, should I be concerned with the elevated B+ voltages when I have new output tubes?
 
Okay, still have very very low sound out of the Right Channel. When I turn the balance pot there is no fading of the Left Channel sound at all, until it is set to Max Right then the Left channel cuts out immediately and completely. What are the next areas to look at?
 
The KX-200 has that dimension control. You'll need to adjust that in conjunction with the volume and balance control. I had problems with one on a x202 that I could never understand. That could be part of the problem.
 
Something else interesting. I was glancing at the x 200 (for all intents and purposes the same unit (very minor differences). The Power supply voltages in the x200 manual are pretty much dead on for you. With the same PS , i'm not sure why that would be, but i'd at least pause and wonder if maybe there's an error in the manual (it happens) . Again doesn't address your channel issue but if we could put the PS to rest we can concentrate on channel problem.
http://akdatabase.com/AKview/albums...Service Manual Serial No_ beginning 40001.pdf
 
larryderouin said:
The KX-200 has that dimension control. You'll need to adjust that in conjunction with the volume and balance control. I had problems with one on a x202 that I could never understand. That could be part of the problem.
Click to expand...

Hi Larry,

I don't really understand what you are trying to describe. I have the Stereo Dimension knob set to the stereo position. If I start to turn it towards mono the right channel begins playing. I am sure there is something I don't understand about the dimension control, however I don't recall having issues with balance before my restoration. Something is amiss for sure.

Since Fixing the resistor issue this morning, I decided to remeasure the voltages on the V7-V10. I've updated my spreadsheet here.

Voltages 3.jpg
 
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Correct me if i'm wrong . if you're getting sound in both channels when adjusting the dimension control. That would mean than your problem is somewhere between the input and the dimension control on the bad channel. It narrows your search area. (similar to what i was suggesting before with the volume wipers.
 
Okay, now I am not getting any sound out of the right channel (except very very low when set to mono) even when setting it to Mono or Channel A. The only thing that I have done is inspect my work around the tone control/Dimension pot and slightly re-position some of the K40 caps to make sure they don't short anything out.

I'm giving up for the day as I am not making any headway. No worries as tomorrow is another day and a good night sleep can't hurt!!!
 
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Any thoughts on what I should be checking next in the hunt to fix the audio issue in the right channel? Once again this is all inputs.
 
I have a nice desktop computer next to my bench. I can probably use the computer to input a specified signal, though without a scope I don't know how much that will benefit.
 
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Again better people here to guide you.
If it were me, I'd make sure everything was clean, controls, jacks, tube sockets.
Back in the day they made inexpensive signal tracers. I'm not sure what's available today. Same idea as the scope. You have 2 virtually identical channels. One works, one doesn't. Send same signal into each. Use (scope/ tracer to follow signal through each channel. You have working channel as a reference. When the signal stops, you found area of your issue.
There likely is another way (possibly joining channels at various points but unless you know what and where, I can't guide you.)
(In case it's not clear the tracer was a little amp speaker combo that just gave you sound of signal instead of image as a scope would).
 
I got it! Turned out to be a Bad Pin 3 on V6. I replaced the pin and now I have sound on both channels and it even sounds nice through my crappy little bench speakers. I must have damaged the pin while installing C22 (0.047uf 630 volt K40). Those K40s are huge for the spot. It's a really tight squeeze.

On a side note, Is R75 (15 ohm 6.5 watt connecting the negatives of C4A and C4B) supposed to be HOT HOT HOT? This resistor is converting alot of energy to heat.
 
Nice work.. Yes the reason that resistor is so large is to dissipate energy (in the form of heat).
Also you're right . The Russian PIOs are big!
It takes a little creativity and effort to fit them.
 
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