Purchased 800C to restore: currently working perfectly

[Skibrain]

Here's a nice, clear pic:

View attachment 909881

Hi Not Digital,
OK, you posted the picture as I was messing around trying to get mine in to the reply. ( Thanks, most excellent ) Looks like you have changed out quite a few more resistors than I have. I measured mine and they were actually quite close. But they are Old!
Thanks for the clear picture. I think I'm good, but I'll wait for the reply.
I want to turn it on again and start testing voltages.

Ken

 

[notdigital]

One relatively minor detail: In both the text and your drawing, you refer to an 80 ohm resistor on pin5/ground. It should be 10 ohm and that is indeed what you have installed. My personal preference at that location is metal film. They tend to go quickly with gusto in the event of a fault condition. Carbon comps, even carbon film resistors tend to like to cook a little longer before letting you know they're upset.

As far as the pic I've posted: that is not my work but I pic I've kept for reference because it very clearly shows all the necessary connections. So, regarding the changed resistors, the person who restored that particular piece may have had a reason. Or no reason at all other than wanting to. Unless resistors are horribly out of spec, I tend to leave them alone. The only resistors that I would change are those mentioned here:

http://audiokarma.org/forums/index.php?threads/service-bulletin-800c.595931/

Looks good to me

 

[larryderouin]

Agreed on the cathode resistors. 10ohm, not 80ohm. Otherwise you're doing good. DON'T RUSH!!! It results in miswired components, etc. and injuries you (burns or you becoming part of an intentional AED! NOT GOOD!!!). So slow down, and get it right! Everyone here has gotten the "new guy, I want to listen to it yesterday syndrome". It goes away after a few rehabs, but fight it. It's not easy butthe more you fight it, the better the rehab.

Larry

 

[Skibrain]

OK, my distortion is back. Voice sounds good, instruments sound distorted ( Piano... Guitar ) Where do I start on voltage measurments? Before the BBAM? Or?
Thanks,
Ken

 

[larryderouin]

Start at the output side of the coupling caps with a scope and check each side of component as you work back on the schematic. When you get a clear signal the part you just tested (both ends) is faulty. I you dont have a scope use another reciever, a rca cable with a .1uf 600v cap soldered to the end and use it as a probe. Plug it in to Aux, and turn volume down on the testing receiver then bring it up so you hear it clearly. DO BOTH SIDES so you have comparisons. Use headphones or speakers to listen to the signal. As you are working back from the output the bad one will be the one you last tested both sides on.

What tubes are you using???? and what are they biased to??? Are the tubes are balanced by the BBAM???

As for voltages. check the power supply voltages and compare to schematic, Then all of the tube socket pins on the output's. If they are all ok, start taking tube voltages on the 12ax7's working back to the inputs.

Larry

 

[Skibrain]

Start at the output side of the coupling caps with a scope and check each side of component as you work back on the schematic. When you get a clear signal the part you just tested (both ends) is faulty. I you dont have a scope use another reciever, a rca cable with a .1uf 600v cap soldered to the end and use it as a probe. Plug it in to Aux, and turn volume down on the testing receiver then bring it up so you hear it clearly. DO BOTH SIDES so you have comparisons. Use headphones or speakers to listen to the signal. As you are working back from the output the bad one will be the one you last tested both sides on.

What tubes are you using???? and what are they biased to??? Are the tubes are balanced by the BBAM???

As for voltages. check the power supply voltages and compare to schematic, Then all of the tube socket pins on the output's. If they are all ok, start taking tube voltages on the 12ax7's working back to the inputs.

Larry

Hi Larry,

I haven't done any Biasing tests. The tubes are GE- NF 188-5 The only non- Fisher tubes in my 800C. I believe it is a biasing issue, but where do i start in testing?
I am mad as I just got rid of all my electronics gear about 3 years ago thinking I was done- I donated it all to my college I graduated from. Of course these dang tube things grabbed me, now I'm into a new hobby.( Or Addiction / Life Style )

 

[Skibrain]

Hi Larry,

I haven't done any Biasing tests. The tubes are GE- NF 188-5 The only non- Fisher tubes in my 800C. I believe it is a biasing issue, but where do i start in testing?
I am mad as I just got rid of all my electronics gear about 3 years ago thinking I was done- I donated it all to my college I graduated from. Of course these dang tube things grabbed me, now I'm into a new hobby.( Or Addiction / Life Style )

Thanks for the above testing procedure. I think I have all the components for the modified audio oscilloscope!

 

[larryderouin]

Try to balance the pairs, then bias them to about 33-34ma. That should get you in the 70-80 percentile for dissipation. If you can't get them to balance, get as close as you can (5ma is too much. 2-3ma is ok per pair.) Then bias them to 33-34ma. then go thru the whole thing again to see if you can get them balanced before bias

 

[Skibrain]

Hi All,
Here is what I have found tonight. I'll try to add my table with each tested point on the tubes.
OK my table didn't work so I just typed them in.
I also listed the voltage across the 10 Ohm cathode resitor measured on each side of the resistor. Basically on pin 4 and 8.
This is where things got bad. V13 has 2.4 volts across these points and the others are 65 to 150mV. Bad resistor, bad solder joint, bad tube, bad fixer ???
Pin 1 403V 403V 403V 403V
Pin 3 435V 436V 435V 435V
Pin 4 403V 403V 401V 403V
Pin 5 0.17 0.07 0.13 0.08
Pin 6 -24.0 -24.0 -24.0 -24.0
Pin 8 403 403 401 403
150mv 65.3mv 2.4V 72.3mv Voltage across 10 Ohm resistor
V10 V11 V13 V14
Tubes
Bet this is my distortion and hopefully I haven't messed something else up
Or what else should I test?
thanks,
Ken

 

[larryderouin]

Swap V13 with it's paired tube and test it again. If it gives the same readings, it could be a tube OR loose socket pins in the sockets. Tighten the socket pins then test again. It's intermittent so it's either the tube or a loose socket pin, OR a cold solder joint for THAT TUBE, Check your work after the 1st test.

 

[Skibrain]

Swap V13 with it's paired tube and test it again. If it gives the same readings, it could be a tube OR loose socket pins in the sockets. Tighten the socket pins then test again. It's intermittent so it's either the tube or a loose socket pin, OR a cold solder joint for THAT TUBE, Check your work after the 1st test.

Thanks, There is a pin that is loose I know from voltage testing. What is the procedure for tightening pins? I'll do a search also.
Thank you all for the help on my progress.
Ken

 

[larryderouin]

simple. 1.) turn off and unplug unit!
2.) insert a small jewelers screwdriver or a dental pick between the wall of the pin hole and the leaf.
3.) Twist the screwdriver or angle the pick so the leaf moves to the center of the pin hole. It doesn't take much.
4.) Repeat for each leaf in the hole. Usually 2 leaves, but I've seen 3 on larger pin holes. Do all socket holes for shits and giggles.
5.) Reinsert tube. It should feel tighter to insert.
6.) plug in unit and turn on. Redo Voltage checks.
7.) Post results.

 

[notdigital]

150mv 65.3mv 2.4V 72.3mv Voltage across 10 Ohm resistor
Ken

I'm a little confused by what you mean by this measurement

The relevant voltage is at pin 5 of the power tubes. If you are using a bias adjustment tool, then the voltage at pin 6 is variable and, therefore, would affect your important voltages at pin 5. When I found my "garbage" 800-c a light bulb went off after the discussion in this thread:

http://www.audiokarma.org/forums/index.php?threads/part-4-800-c-restore-dialing-her-in.450647/

Seems to me that if you've installed the BBAM and the distortion returned, then you're biasing way too cold. If you've "balanced" the tubes as best you can by swapping them around, then try adjusting the voltage at pin 6 to -20 and see what that does.

 

[Skibrain]

I'm a little confused by what you mean by this measurement

The relevant voltage is at pin 5 of the power tubes. If you are using a bias adjustment tool, then the voltage at pin 6 is variable and, therefore, would affect your important voltages at pin 5. When I found my "garbage" 800-c a light bulb went off after the discussion in this thread:

http://www.audiokarma.org/forums/index.php?threads/part-4-800-c-restore-dialing-her-in.450647/

Seems to me that if you've installed the BBAM and the distortion returned, then you're biasing way too cold. If you've "balanced" the tubes as best you can by swapping them around, then try adjusting the voltage at pin 6 to -20 and see what that does.

OK here is where I am:
No BBAM yet.
AC- 120V
V12- 348V and 132V
V15- 343V and 141V
Measurement are left to right V10, V11, V13, V14
Problem is the voltage measurement across the 10 ohm resistor from Pins 1-4 on V13- at 2.5V
All other 10 ohm measurements are V10-156mv V11- 66mv V13- 2.5V V14- 131mv
Other measurements at 120V AC line in.
Pin 1- 417V all tubes
pin 3- 449V all tubes v13might be closer to 418V it would fluctuate down to 418 sometimes.
pin 4- 417V, 417V, 415V, 417V
pin 5- .183, .085, .093, .180
pin 6- -25V all tubes
pin 8- 418v, 418v, 415v, 418v
Swapped the tubes per Larry V13, V14. No change. problem stayed with socket. tightened pins V13- No change.
Replaced 10 ohm resistor V13- no change. tested 7Watt 1.2K resistor- at 1.21K
Tested voltage on the capacitors_ V12- 348V, 132V. V15- 343V, 141V
Re-soldered tube connections if cold solder. - No Change.
Going to move tubes back now.
Thinking somehow there is a pin connection problem on V13 socket somehow? Thoughts.

Am I biased to low at pin 6 at -25V?
Should I try to balance v12 v15 closer? What voltage- I see 310V on the schematic pin 1 and 130V pin 6? am I even thinking this correctly
redo all soldering on V13 again?
Or bang my head a few times and go back to work?

 

[Skibrain]

OK here is where I am:
No BBAM yet.
AC- 120V
V12- 348V and 132V
V15- 343V and 141V
Measurement are left to right V10, V11, V13, V14
Problem is the voltage measurement across the 10 ohm resistor from Pins 1-4 on V13- at 2.5V
All other 10 ohm measurements are V10-156mv V11- 66mv V13- 2.5V V14- 131mv
Other measurements at 120V AC line in.
Pin 1- 417V all tubes
pin 3- 449V all tubes v13might be closer to 418V it would fluctuate down to 418 sometimes.
pin 4- 417V, 417V, 415V, 417V
pin 5- .183, .085, .093, .180
pin 6- -25V all tubes
pin 8- 418v, 418v, 415v, 418v
Swapped the tubes per Larry V13, V14. No change. problem stayed with socket. tightened pins V13- No change.
Replaced 10 ohm resistor V13- no change. tested 7Watt 1.2K resistor- at 1.21K
Tested voltage on the capacitors_ V12- 348V, 132V. V15- 343V, 141V
Re-soldered tube connections if cold solder. - No Change.
Going to move tubes back now.
Thinking somehow there is a pin connection problem on V13 socket somehow? Thoughts.

Am I biased to low at pin 6 at -25V?
Should I try to balance v12 v15 closer? What voltage- I see 310V on the schematic pin 1 and 130V pin 6? am I even thinking this correctly
redo all soldering on V13 again?
Or bang my head a few times and go back to work?

Pin 3 I meant to say it would float down to 448V not the 418V that I had above.

 

[notdigital]

OK here is where I am:
No BBAM yet.


Am I biased to low at pin 6 at -25V?

Or bang my head a few times and go back to work?

I usually kick my own ass when I hit walls like this. I'm pretty flexible that way. Personally, I've never tried the head banging technique. But if it works for you........

After that, I would focus on your pin 6 and pin 5 readings and nothing else. Your tubes are running too cold and my guess is that the distortion you're hearing is due to the fact that not enough current is running through the power tubes. Since you have not installed a way to adjust the bias voltage to the tubes, I'd suggest you install a 10k pot in place of the bias resistor. I forget the number. M-Bone should have identified it. My preference is for a simple 10k pot so you can literally play with it and observe the changes to the pin 5 readings. My guess is that your tubes are worn to different degrees resulting in the differences you see @ -24vdc. At this point it doesn't matter. What is more important is that your tubes creep closer to 30ma @ pin 5. Once you've gotten to the point where you can get the relevant voltages to move, then you can decide to install a more matched quad or install the BBAM or IBAM.

I struggled with the same wall you are at now. That's why I suggested that you read the thread I posted earlier in post 53. You are literally right there! All you have to do is get the pin 5 voltage moving in the right direction. For convention's sake, set your variac to 117vac and, sor now, use that as your operating voltage. That will provide a constant that you can guarantee.

Finish line is right around the corner!!!!!!

 

[Skibrain]

OK I see something that isn't the same as the picture that NotDigital sent earlier in this thread.
There is a 1.2K 1 watt resistor. originally attached to the pin 8. I left it there.
On NotDigital's picture this is attached to V13 pin 1.
There is also the 1.2K 7 Watt resistor that I had attached to V13 on pin 1 per all updates.
I see on the picture that NotDigital sent that this is attached to the V11 on Pin 1.
What is correct?
Maybe this correction and the the 10K pot per NotDigital- I was going to do this per my measurements, so thanks NotDigital. I'll increase voltage to correct level after I fix the other problem above.
NotDigitals picture below: Is this correct?

 

[notdigital]

Review Dave's post #33. However, that is an issue separate from your pin 5 readings.

.... I'll increase voltage to correct level after I fix the other problem above.

If I were standing in your shoes, that's exactly what I would do!

 

[dcgillespie]

I'm confused here. In post #49, you cite one set of voltages from pin 5 of each output tube to ground, and another set of voltages across the 10Ω resistors, which are completely different. This is not possible. The 10Ω resistors are connected from pin 5 to ground at each output tube. Am I missing something here?

Your distortion would appear to be from one output tube in each channel drawing hardly any current at all. From post #49, the tubes producing 65 and 72 mV are only drawing 6.5 and 7.2 mA each -- nowhere near enough to produce any usable audio output. The one tube that is producing 2.4 or 2.5 vdc would mean that the tube is drawing 250 ma -- again, not possible. The tube is either glowing BRIGHT cherry red (melting glass), or the indication is simply wrong. Have you checked the accuracy of the 10Ω resistors installed? The accuracy of your meter?

I don't know if there is even enough bias voltage available to be able to cause the tubes to draw such low current. As a result, the condition of the tubes comes into question as well.

If I'm not understanding what I'm reading, please set me straight!

Dave

 

[Skibrain]

OK to make sure I'm measuring correct points. chassis ground to pin5 very low volume.
Pin5 to ground- .183v .085v .093v .17v

Now if I measure between pin 1 red probe to pin 4 black probe.
156mv 66 mv 2.5volts 131mv

Pin 6 to ground- all are at -25v all tubes

What other test points should I be testing?
I also checked other pictures and I'm moving the 1.2k 7 Watt to v11 pin 1 and leaving 1.2k 1 watt at the original position of v13 pin 8

Point out my mistakes. I kno something is wrong, whether it is my testing or where my connections are.
Below is another picture I found of how it is supposed to be. At least as I am finding.