Bottlehead Kits > Crack-a-two-a
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:
--- Quote from: plasticon47 on July 22, 2021, 07:09:55 AM ---High Current C4S (Manual / ACTUAL B TUBE / ACTUAL D TUBE)
IA 190V / 181.4V / 144.5V
--- End quote ---
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.
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