New Crackatwoa build. Voltage issues [resolved]

[plasticon47]

Hello:

I completed my build last night.  Glow tests along the way were fine.  Resistance check at the end passed.  Voltages are not passing, though:

Low Current C4S  (Manual  /  ACTUAL)
  IA 150V  /  149V
  OA 60-90V  /  65.6V
  KregA 3-6V    /  6.04V
  bRegA 150V  /  141.7V
  IB 150V  /  141.7V
  OB 60-90V  /  65.2V
  KregB 3-6V  /  16.9V
  bRegB 150V  /  148.9V

High Current C4S (Manual  /  ACTUAL B TUBE  /  ACTUAL D TUBE) 
  IA 190V  /  181.4V  /  144.5V
  OA 150V  /  148.9V  /  140.2V
  bA 0V  /  0V  /  0V 
  IB 0V  /  0V  /  0V
  OB 90-110V  /  91.7V  /  75.1V
  bB 150V  /  148.9V  /  139.7V

Headphone Jack (Manual  /  ACTUAL)
  Tip 0V  /  0V
  Ring 0V  /  0V

All LED's light up, except for ONE:  High current board over socket D, LED closest to bB.  All tubes light up (including both halves of the 12AU7). 

So, some voltages appear to be fine, others not fine.  I don't have pics with me, but can post them later if needed (at work right now).  Can anyone give me some idea of where to start looking to fix?  I ran through the amp last night, checking all solder points, tugging on wires, etc.  It all looks okay to me. 

Thanks in advance for everyone's help.

 

[Paul Birkeland]

High Current C4S (Manual  /  ACTUAL B TUBE  /  ACTUAL D TUBE) 
  IA 190V  /  181.4V  /  144.5V

This is where I would start.  I suspect that perhaps the wire that should be leaving OA on the D side is attached to IA instead.

 

[plasticon47]

Hey Paul:

Thanks very much for your response.  I took a look at the board (high current-D) and just so I am clear, you thought the wire that should be connected from OA to 16L may be connected from IA to 16L?  Or are you talking about the wire on the board from OA to bB?  Either way, I checked those and all appears correct, according to the manual.  In fact, I walked through all steps in the manual for the whole amp and verified connections were correct.  I reflowed the solder joints on the high current - D board in the areas you mentioned (IA, OA, and even did bB).  Just for kicks, I even swapped the two 6AQ5 tubes.  Checked voltages again, and while a little different, the problem areas still remain:

Low Current C4S  (Manual  /  ACTUAL)
  IA 150V  /  149V
  OA 60-90V  /  67V
  KregA 3-6V    /  4.3V
  bRegA 150V  /  134V
  IB 150V  /  134V
  OB 60-90V  /  66V
  KregB 3-6V  /  20V
  bRegB 150V  /  149V

High Current C4S (Manual  /  ACTUAL B TUBE  /  ACTUAL D TUBE)
  IA 190V  /  173V  /  136V
  OA 150V  /  149V  /  133V
  bA 0V  /  0V  /  0V
  IB 0V  /  0V  /  0V
  OB 90-110V  /  92V  /  73V
  bB 150V  /  140V  /  132V

Headphone Jack (Manual  /  ACTUAL)
  Tip 0V  /  0V
  Ring 0V  /  0V

Is there any other spot/voltage I can check that might point to my issue(s)?  I have to say, I built a Mainline a month ago and it seemed like a cakewalk compared to this Crackatwoa.  Again, many thanks for your response.

 

[Paul Birkeland]

You basically have a short somewhere across the A side C4S on the channel with the bad voltages.  I just did a repair today where there was a blob of solder across two transistor legs, and that kind of thing would also cause this kind of problem.

 

[plasticon47]

Hey Paul:

Just want to make sure I am clear: you are saying you think one of the transistors has a short on the HIGH CURRENT C4S BOARD (i.e. - not the LOW CURRENT C4S) on the side with the low voltages (that would be over the D tube socket)?

Assuming I understood correctly, I examined all the legs of the four transistors on that board.  I didn't see any bridges; however, I desoldered all the legs and then resoldered.  I did a continuity test between each of the three legs on the bottom of the board with my multimeter.  I didn't get any continuity (by the way, I don't know if this is a valid test for solder bridges, but I took a shot!).  I fired the amp up, and checked voltages.  Same voltages as previous. 

In light of your post, I am concerned that one of the transistors is bad?  Is there some way to check this?  Perhaps some other test I can do?

Also, I'm wondering about the other voltages:

On LOW CURRENT C4S:
  bRegA 150V  /  134V  (this is low)
  IB 150V  /  134V  (this is low)
  KregB 3-6V  /  20V  (this is very high)
 
On HIGH CURRENT C4S for the B tube (i.e. - the "non-offending board")
  IA 190V  /  173V  (this is low)
  bB 150V  /  140V  (this is low, though not hugely so)

Would a bad transistor on the D side HIGH CURRENT C4S cause these issues on the other boards as well?  Is there some way to isolate which transistor may be the culprit? 

Thanks again for your help with this.

 

[Paul Birkeland]

To check transistors, you check for continuity between pairs of legs. 

Beyond this, you would want to post build photos or send the kit in for repair and I can figure it out for you.

 

[plasticon47]

Hey Paul.  Thanks for your response.

I really don't want to admit defeat and send this to you to fix.  I can take some pics and post this weekend, but in the meantime, I want to see if I can come up with some other things to try/test.  You mentioned above that you believed there was a short on the A side of the HIGH CURRENT C4S (if I read your brief comments correctly).  I have some older amps/boards I have built lying around and might be able to swap one or both of the transistors on the A side of the board.  I know I have a PN2907A transistor, but I doubt I have a MJE5731A transistor. 

1.  Is it worth it to try to replace the two transistors on the A side of the HIGH CURRENT C4S board? 
2.  Is it possible for me to just purchase the PCB for a HIGH CURRENT C4S board from you guys and try to fix this myself by replacing the whole board?

 

[Paul Birkeland]

Yes you can purchase a new board, but the thing to do first would be to trade the high current C4S boards between sides to see if the problem follows one of the boards.

 

[plasticon47]

Paul:

I swapped the HIGH CURRENT C4S boards this evening.  Voltages:

Low Current C4S  (Manual  /  ACTUAL)
  IA 150V  /  141V
  OA 60-90V  /  65V
  KregA 3-6V    /  16.7V
  bRegA 150V  /  149V
  IB 150V  /  149V
  OB 60-90V  /  65V
  KregB 3-6V  /  6.1V
  bRegB 150V  /  141V

High Current C4S (Manual  /  ACTUAL B TUBE  /  ACTUAL D TUBE)
  IA 190V  /  145V  /  182V
  OA 150V  /  140V  /  149V
  bA 0V  /  0V  /  0V
  IB 0V  /  0V  /  0V
  OB 90-110V  /  74V  /  92V
  bB 150V  /  140V  /  149V

Headphone Jack (Manual  /  ACTUAL)
  Tip 0V  /  0V
  Ring 0V  /  0V

So, it appears to me that the low voltages did follow the board.  When I desoldered the wires to swap the boards, I studied all the solder points with a magnifying glass.  I saw no solder bridges , blobs, or anything suspicious. 

As the voltages followed the boards, can I safely assume the board with the low voltages (or more accurately, one of its components) is bad?  In a previous post, you mentioned you thought there was short on the A side of the HIGH VOLTAGE C4S board.  Is this still the logical conclusion?  Is there something else for me to test?

There aren't really that many components on one side of the HIGH VOLTAGE C4S board.  Is there an easy way to test them?  Also, at this point, is it a good idea to order a new board from you guys? 

Sorry for all the questions.  It is rather frustrating being so close to the end of this project, yet not being able to listen.  It is more frustrating not knowing what the issue might be.  I appreciate your help with this.

 

[Paul Birkeland]

I would just order a replacement board from replacementparts(at)bottlehead(dot)com.  Otherwise if I had your amp on my bench and ran into that, I would use my powered solder sucker to suck all the solder out of the joints for the transistors and LEDs, then I would replace them.

 

[plasticon47]

Hey Paul:
I tool the board out and desoldered the two transistors in the center of the board.  Also desoldered the LEDs.  Found some info online re: checking transistors and LEDs with a multimeter and checked all of them (all checked out fine).  I replaced one of the LEDs (the one that didn't light up) with an HLMP 6000 (only had one extra from a build sometime ago).  Put everything back on the board and fired up:

Low Current C4S  (Manual  /  ACTUAL)
  IA 150V  /  149V
  OA 60-90V  /  66V
  KregA 3-6V    /  4V
  bRegA 150V  /  149V
  IB 150V  /  149V
  OB 60-90V  /  67V
  KregB 3-6V  /  6V
  bRegB 150V  /  149V

High Current C4S (Manual  /  ACTUAL B TUBE  /  ACTUAL D TUBE)
  IA 190V  /  180V  /  180V
  OA 150V  /  149V  /  149V
  bA 0V  /  0V  /  0V
  IB 0V  /  0V  /  0V
  OB 90-110V  /  92V  /  93V
  bB 150V  /  149V  /  149.7V

Headphone Jack (Manual  /  ACTUAL)
  Tip 0V  /  0V
  Ring 0V  /  0V

It looks like all voltages are "in range" (or extremely close) with the exception of IA on both HIGH CURRENT C4S boaards (180V instead of 190V).  Is this voltage okay?  (It looks like it is a hair over 5% off).

1.  Assuming the voltages above are okay, am I ready to go?

2.  One more question:  I was thinking about swapping the TRRS headphone jack with a balanced XLR 4pin one.  I read a post in this forum where it said you could do this and just bridge the two grounds on the jack.  I just want to confirm this with you.  So, I would have red wire output to pin 3, white wire output to pin 1, and then black wire output to pins 2 and 4?

Many thanks for your continued help with this.
 

 

[Paul Birkeland]

It looks like you're all set.  Yes, that is consistent with how a 4 pin XLR should be wired for headphone use.  You could also just use a short TRS to XLR adapter.

 

[plasticon47]

Hey Paul:

I ran the Crackatwoa with the TRS jack all week, and it worked fine.  (Sounded pretty fantastic as well).  So, many thanks for all of your help.

I'm going to install the XLR jack in place of the TRS as I mentioned above.  When I do that, is it worth installing the diode mod you mentioned in another thread (mod intended to reduce noise if a computer/DAC is near the amp).  The diodes install between the IEC ground and the first ground lug.  From reading that thread, I wasn't clear as to whether this is worth doing on the Crackatwoa.  I would very much appreciate your advice.  Thanks.

 

[Paul Birkeland]

Sure, you could add that to your C2A.

 

[plasticon47]

Hey Paul:
Question:  I added the diodes without issue.  Then I swapped the TRS jack for an XLR 4 pin one....used the same kind as in the Mainline.

Just because I changed the wiring (as well as the diode) I decided to check voltages.  All voltages on the boards were fine.  When I checked the voltage at the WHITE (pin 1) and RED (pin 3) wires at the jack (same wires that were attached to TIP and RING of the TRS jack), I get 78 VDC on each one.  Is this okay?  When I check voltages with the TRS jack, the voltages on both those points (TIP and RING) are 0 VDC.  I want to make sure these voltages for the XLR jack are correct so I don't ruin my headphones.

Thanks in advance for your help.  Once this is done, I promise to leave you alone (for now!).

 

[Thermioniclife]

Yeah, 78vdc is not good, do not plug your phones. You should have 0 vdc as you had before. As for AC volts you should have just a few milivolts. It's as if the coupling caps are not present or they are shorted, allowing Dc volts to pass

 

[plasticon47]

Hey Thermioniclife:

Well, I am really glad I asked my question before plugging in my phones.  Thanks VERY much for your response.  But, this is perplexing.  I literally changed nothing other than the jack from TRS to XLR.  I wired WHITE (formerly TIP) to pin 1, BLACK to pin 2 (with a jumper to pin 4), and RED to pin 3.  I didn't touch the coupling caps and the unit worked fine with the TRS jack last night.  All voltages pass with the XLR jack in there with the exception of checking pins 1 and 3 of the XLR jack (78 VDC on each).

There are resistors from TIP and RING on the TRS jack to ground (on the jack).  Does there need to be resistors on the XLR jack as well?  From looking at the Mainline, there are no resistors on either the TS or the XLR jacks.  I am rather confused.

 

[Thermioniclife]

Stand by while look at the schematic

 

[Thermioniclife]

Ok, there are 2k2 ohm resistors from tip and ring to ground you should install them. Having said that I'm not sure if that is the problem. So if you still have them put them in and recheck voltage. Are you sure you measured 78 volts Dc?

 

[plasticon47]

Thermioniclife:
Thanks for the advice.  And I am positive 78VDC was observed on the WHITE and RED wires.  I was kind of wondering about those resistors.  I went to take them off the TRS jack to put on the XLR one and one broke in half (must be too much heat...rats!).  So, the closest resistor value I have in my "scrap parts box" is 2.49Kohm.  Can I use two of those instead?

And I'm really hoping Paul can respond.  I see his name on the schematic, so his confirmation of your (Thermioniclife's) advice would make me feel MUCH better.  Is the lack of resistors on the XLR jack what is causing my problem?

And once again, Thermioniclife, I really appreciate your response and assistance.