Final check not happening

[harrislp]

Hello,

I recently built my crack amp and I am having a problem with the last check (the headphone TRS voltage check). I can't seem to get the voltage to stay below 9V or go back to 0V. Whenever I test it on either the tip or ring the voltage jumps around wildly and won't go back to 0V, or at least won't stay there. I have re-flowed all the solder joints and tested all the other voltages (they all check out within the 10-15% range) and resistances too. SO I am a bit stumped and would appreciate some advice and/or help. Thanks in advance 

 

[Doc B]

It could just be poor test lead contact with the test point. Try using clip leads to attach the test lead to the test point. The alligator clips will make a lot better contact than the test lead tip. You can buy a bag of them for a few bucks at Radio Shack. I consider them one of the most important tools we use.

 

[harrislp]

Okay I will give it a go. Thanks. Any other ideas in case it is not the test lead? All the other voltages seem to check out.

 

[Doc B]

There is a 2.49K (if i remember correctly) resistor across each output that serves to bleed of any potential that builds across the output at statrtup. Make sure it is properly connected.

 

[harrislp]

I tried to do some troubleshooting and went back and did the resistance checks with the alligator clips. Now numbers 6 and 10 are showing zero ohms resistance when they should be 2.4K, and I have no idea what I have done wrong. All the other resistances are fine, even b3 and b6. I'm getting frustrated now, as the resistances on 6 and 10 were correct when I first measured them. I did the mod and soldered the two unused headphone jack pins to the ground and carefully soldered the 2.49K resistors to the output terminals, and did another full check of my solder joints. So I am, yet again, stumped. I don't want to move on to voltage checks until this is sorted out.

 

[Paul Birkeland]

I did the mod and soldered the two unused headphone jack pins to the ground and carefully soldered the 2.49K resistors to the output terminals,

Yes, you did the mod that shorts 6 and 10 to ground, so they will measure 0 Ohms.

 

[harrislp]

Okay everything seems to be working, all the tests checked out and I am listening to music on it now. Thanks for your help Doc and PB. Now to build the case.

 

[harrislp]

Okay, new problem 

I am not getting any sound at all, no buzzing, no crackling, nothing. The RCA 6080 tube is glowing brightly and giving off plenty of heat but the 12Au7A is lukewarm to the touch and the filament is only very faintly glowing in one spot. Is this a faulty tube or some other problem? I've tried several sources with the amp and rechecked my solder joints. I don't have any other tubes to test with and it was working fine a few days ago before I made the case. I seem to remember the 12au7 glowing more brightly and being at least hot to touch, but I could be imagining it. What could be the cause of this? Interestingly, when I have the unit plugged in but not switched on, I am getting a buzz when I pull the headphones halfway out of the jack. Could this be a ground issue?

Thanks in advance.

 

[Doc B]

It's possible that a bad or missed solder joint worked loose in moving the chassis around. Recheck all voltages and let us know any that are out of spec.

 

[harrislp]

Okay I checked the resitances and voltages and most of them are out of spec now. I found the red wire at 13U was loose and resoldered it but no change, which was weird as you'd think resoldering a loose wire would at least change something. Maybe I didn't resolder it correctly. Not sure, it's in there tight now though.

The problems I measured are:

RESISTANCE: Terminal 6 reads 0 ohms (should read 2.4K). Terminal 10 reads 0 ohms (should read 2.4K)
                      Terminals 1,2,4 and 5 all read OL (outside limits) where they should have a readable value (they did before when it was working).
                      Terminal 13 also reads OL even after I resoldered the wire from B5.
                      The Right (Red) RCA Jack centre pin reads OL. The left (white) RCA jack centre pin reads 97K ohms, within the expected range.

VOLTAGE: There is no voltage going to the B socket (i.e. B1, B2 etc.) and no voltage on Terminals 1-10 either. I didn't really go into it too much as I got a bit paranoid about it after that and switched it off. I didn't retest voltages after I soldered the wire as the resistances weren't checking out.

 

[Doc B]

Considering the number of OL readings it might be that some wire that is a ground connection has come loose. Take a close look at all of the black wire connections.

 

[harrislp]

Okay I've resoldered quite a few joints and now I'm getting the correct resistances on all terminals except for 6 and 10, which are still reading 0 insteadof 2.4K. Also I'm getting voltages to alterminals that should be getting them but some voltages are well over the spec. And the LED from A8 works intermittently when powered up, when i started testing voltages it did not work but then came on when i tested the octal socket and then went off again. What is my next stage of troubleshooting?

 

[ssssly]

Unless you undid the mod, as CB pointed out above, 6 and 10 should read 0 ohms now. So that isn't your problem.

Check the wiring into and out of the LED at A8. If it is intermittently getting voltage there is something wrong and should be a good starting point.

 

[Paul Birkeland]

I'd post the new voltages.

6 and 10 will still be 0 Ohms.

 

[harrislp]

NEW VOLTAGES (out of spec in RED)

Terminal 1: 150
2: 169       
3: 0
4: 169
5: 84
6: 0
7: 152
8: 0
9: 107
10: 0
11: 0
12: 0
13: 169
14: 0
15: 193
20: 0
21: 219

A1: 84      A2: 0      A3: 1.54    A4: 0      A5: 0    A6: 154    A7: 0    A8: 0 (LED not on)    A9: 0

B1: 154    B2: 169    B3: 154    B4: 84    B5: 169    B6: 108    B7: 0      B8: 0

I resoldered the LED on A8 but no dice. The led does work but not during normal operation.

Thanks,
Harris. 

 

[ssssly]

I would start by rechecking the diode. I would probably just replace it. Then check that nothing is touching something it shouldn't be around T1, 2 and 7. And then that the connection at B3 is good.

I'm sure CB will have a more technical solution. He always seems to get me up and running.

 

[Paul Birkeland]

I'd touch up the solder joints on the LED that doesn't work.

Also touch up the joints on the 22K resistor between terminals 1 and 2.

All the high voltages are related to this, once that LED is glowing, things will drop into line.

-PB

 

[Paul Joppa]

Looks to me like one side of the 12AU7 is not glowing. Could be a bad tube, but more likely a bad connection at A5. Check connections at A4 and A5 (and examine the tube closely while it is on).

 

[harrislp]

Thanks guys for all your helpful replies, they are much appreciated. I tried all the suggestions (apart from a new diode as I don't have one) and nothing worked. My readings are still the same after touching up all the joints on the A and B sockets and the terminals 1,2,7. I even did a few others that looked suspicious. The LED will not come on at all now, so I suspect I will need to ask for a replacement, correct? Then try a new tube? I've resoldered till I'm blue in the face   

Where would I obtain a new LED from?

Any and all suggestions are welcome -- I will get this thing working before my birthday (April 26). I hope.

 

[Paul Birkeland]

You can give us a call and request some replacements, I'm guessing the voltage at T5 is now higher than it should be?