Switched wires on 16u and 17u [resolved]

[Bottle Rocket]

Hello all,

I'm pretty stumped with my build of one of my Paramount monoblocks.  I recently found that I had miswired the ttp wires coming from the power PC board to 16u and 17u (connecting the red to 17 instead of 16 and the black to 16 instead of 17) and clearly blew a resistor or two on my soft start PC board.  I had the board replaced and just finished my build and attachment of the new PC board.   

The resistances appear to check out, but when I power up the LED's are not lit up and there is a 0 VDC reading on OA.  No apparent voltage readings on any of the connections for the soft start PC board.  I checked some other voltages (such as on terminals 1, 5, 9, 16, and 19) and they are way off. 

So I'm wondering what else I must have damaged by crossing these two wires on 16 and 17.  I appreciate any suggestions. 

Thanks,

Michael

[Doc B.]

It's possible that one or more of the Zener diodes on T11-15 blew. Check each one with the diode test setting on your meter or the 20K ohm resistance range. They should read over limit one way, then when you swap test leads they should each read a low resistance the other way. If they read very low both ways they are blown and need to be replaced. Don't run the amp until you check this because the shorted diodes can pull too much current and stress the power supply.

[Bottle Rocket]

Thanks Doc.  I checked them all and they seem to check out (one side testing at over 20k ohm, and the other way at or near 0) except for the diode from B6 to terminal 11u which tests at 18.87.  Could this be the issue?

[Paul Birkeland]

You can remove one zener diode and test the amp to see if it is working, though it should not be left that way long term.

[Bottle Rocket]

Should I take out the diode going from B6 to 11u or another one?

[Paul Birkeland]

Should I take out the diode going from B6 to 11u or another one?

It doesn't really matter.  Removing one breaks the whole string.  If this solves your problem, I would replace all 5.

-PB

[Bottle Rocket]

Okay, so I removed one of the diodes and things still seem to be pretty off.  Here are the VDC readings I got with the diode removed:

1 - 0.93
5 - 0
9 - 0
10 - 0
16 - 1.17
17 - 0
18 - 0
19 - 0
A1 - 3.06
A2 - 1.39
A3 - 0.99
A4 - 1.97

[Bottle Rocket]

I did another round of resistance checks (with the diode chain intact) and again everything seemed to be within range.  The diode terminals read as:

11 - 19.29
12 - 0
13 - 0
14 - 0
15 - 0

I also was concerned about damage to the power PC board and resistors.  I checked the 270ohm 5W resistor and it was zero one way and seemed to vary the other way (starting around 32.00 and then decreasing steadily). 

[Paul Birkeland]

Is this the first or second monoblock?  It might be pertinent, if you have another working amp, to take the driver board out of the working amp and install it in the other one.  This will narrow down the problem significantly, so you'll know whether you just need to start over with a new soft start PCB, or potentially sort out an issue in the rest of the circuitry.

-PB

[Bottle Rocket]

This is the second monoblock and the first is operating correctly.  So just to be clear, I have already replaced this second monoblock with a new soft start PC board and am still having these issues.  Also did the checks on each diode (which appear to have proper resistances), and have issues when one diode is removed from the chain. 

And Paul, you're saying remove the power PC board from the first monoblock and plug it into the second (non-working) monoblock, correct?  Just want to make sure before I start disconnecting wires. 

Thanks 

[Paul Birkeland]

This is the second monoblock and the first is operating correctly.

Well, that's a helpful reference.

Are these 300B or 2A3 Paramounts?

[Bottle Rocket]

They're the 300B version.  And upon further checking the readings of the resistors on the power supply PC board, they are definitely not functioning correctly. 

By the way, I've been trying to disconnect the non-working power PC board and having a heck of a time since there are so many soldered connections. 

[Paul Birkeland]

Try running the amp without the PC board installed at all.  On the wires that you have that would have connected to the board, stand them all up vertically so they don't touch each other.

Having done that, how are your voltages at the 4 pin socket?

-PB

[Bottle Rocket]

Okay, got the power PC board out, turned the power on, and measured the voltages for the four pin socket:

A1 - 0
A2 - 0.097 then decreasing steadily
A3 - 0
A4 - 0

[Doc B.]

You would indeed get zeros everywhere if you removed the power supply PC board. PB meant remove the driver PC board over the nine pin socket.