June 3, 2019 at 11:51 am #13618
While measuring the TP VOLTAGES:
TP1 – 176.7
TP2 – 7.2
TP3 – 1.925
TP4 – 154.8
TP5 – o/l
TP6 – 13 and dropping
TP7 – 1.484
TP8 – 168.7
TP9 – 13 and dropping
TP10 – 1.35
TP11 – 132.1
– I once measured TP5 after switching on and saw some brief voltage and then o/l.
– The amp makes sound.
In the interest of full disclosure, the first time I measured I was in my rehearsal space with an AGUILAR DB112NT 8 Ohm cab. While measuring heard the noise at the speaker and after plugging in a guitar ( and bass ) enjoying the sound despite the weird measurements etc. After some duration the sound cut out and r15 exploded. I shut it off quickly and it appears that no damage has been done to board. Cleaned with rubbing alcohol and brought it home. I had a 1/2watt carbon comp that measures close so used it in its place. I don’t appear to have bridged anything and reflowed all the joints I can see from the top. Im now plugging it in with an NS10 8ohm. ( i just don’t have an extension speaker suitable ) and its now making noise again, but wondering what you think of all this and what else to test. Also some of those voltages take a good 30 seconds to settle.
thoughts?June 4, 2019 at 7:53 am #13636
The main thing is we need to figure out what caused R15 to explode. It could be dangerous to use the amp until we figure out what happened. If you are not 100% confident in your solder joints then please take out the board and re-solder them…it’s much better to be safe than dead…or have more components blow up. Also, try tightening your tube socket pins just to make sure everything is making good contact.
All the current the amp uses goes through R15, so something in your amp must have been drawing a lot more current than it’s supposed to. It makes me think that something is shorted….like a bad solder joint or something.
Are you sure that TP6 and TP9 are correct? The bias voltages on those parts of the circuit seem solid, so those test points should be higher. It concerns me that TP5 isn’t giving you a voltage…that’s pretty important to know what that voltage is. Like I said, try tightening your tube sockets, especially pins 8,4, and 9, and re-measure these voltages. Make sure you have the amp plugged into an 8 ohm load (speaker) at all times, even when you take these measurements.
Let me know what you find.