Randall RG200 help, please

[Randall]

This amp has 14vdc on the output, A common problem with these I gather. It makes sound, although weak and distorted until thr master is turned up, and then it jumps in volume. The line output works well. both +/- 70vdc rails look good at the Drains of all MOSFETs, but there is -14vdc on the other two legs of all six MOSFETS. I had -14v give or take a volt or two on pins 1, 2, 4, 6, 7 and 8 of dual OP AMP IC1. In trying to find where the -14vdc is coming from, I lifted D7, D8 and C12, but still had -14v all over the IC1. I am concerning myself that I have -14v on pin 8 of IC, which to my thinking should be the + supply voltage for the IC. I also have -14V at the hole I lifted D8 from labeled V+

I removed IC1, destroying it in the process, and installed a socket. I now only get -14v at pin 4 and 8 with no OP AMP plugged in, as I don't have one at the moment. So, why do I have a neg voltage on my pos supply pin? I cannot figure out where the supply voltages are coming from, and I cannot find a power supply schematic. Doesn't look like they come from the PS board. Removing the IC took the -14v away from the outputs. I wonder if -14v on pin 8 caused IC1 to short and dump offset voltage out through pin 1 in to MOSFET gates, but I confess I struggle with how they actually work.

Thanks guys.

[g1]

schematic pg.2, sector E2, resistors R52, & R57. Measure both sides of both resistors and post results.

[Jazz P Bass]

The + & - 15 Vdc supply comes in at CN2 (page 2- Coordinates E-2).

On the power supply you will find a 7815 & a 7915 voltage regulator.

How pin 8 is getting the negative supply is the challenge.

[loudthud]

Strange circuit although it's like many of the tube driven MOSFET power amps ala Ampeg. The power supply for IC1 needs to be fixed. IC1 acts like a servo to bring the output offset to zero and it can't do that if both rails are -14V. The power supplys seem to come from +V and -V on sheet two. The reason the power supply connections are not shown is that the schematic program automatically connects them when the part in the library defines the power supply pins as goiing to +V and -V.

Most +/- 15V supplys have reverse polarity protection (diodes to ground) although some inexperienced engineers sometimes don't include them when using 3 terminal regulators. This makes me think that there is simply an open connection between the board and the power supply on the +V.

[loudthud]

Thanks to Jazz P for posting the rest of the schematic. I'll bet the 7815 is blown. Since there is a load that goes directly from +15V to -15V, one output can pull the other past ground and that blows the IC. Anybody who read a National data book from the mid 70s knows this, but that was before most of todays engineers were born. This amp needs protection diodes from +/- 15V to ground. 1N4007 would be fine.

Read about Bob here: http://en.wikipedia.org/wiki/Widlar

[Randall]

Yes. I see now that some of the power supply is actually situated on the control panel face plate board, most notably the +/- 15v regulators. A couple of wires snake over there from the PS and disappear unlabeled on the underside of that board. This one must deviate from the drawing, because I have no U1 or U2, instead I have a IC 12 and 13, both of which are working at the output legs. +15v however is not making it's way back to the PA board via CN2, so at least I know where to look now. And that's a big help.

The measurements G-one asked for are :
R52 -14.5v both sides

R57 -15.1v both sides

As always I appreciate all the good help. I'll look for this lost voltage and get back. Once I restore the supply, should I be OK installing a new TL072?

[Randall]

This amp needs protection diodes from +/- 15V to ground. 1N4007 would be fine.

Is this not what D7 and D8 are doing at IC 1?

[g1]

No, D7 and D8 at IC1 are not going to ground, they are protecting the input of IC1.
see lower diagram on pg.21 of attached pdf

[Randall]

Oh, right. That is the second time I have followed that path to the wrong end.

[Randall]

OK, I pulled the control board to find another's hackings. Among some lifted traces, bad jumper wires, and wrong type of solder, I found the wires carrying +/- 15v and ground from the control panel to the PA board. The red +14v wire was not connected properly. Repairing this connection solved that part. With everything soldered back in I have my 14V rail on pin 8 of IC 1, and as far as I can tell no stray -14v, and no offset. I will order some op amps and see where it brings me. A BIG hope it cures the volume problem, because I do not relish working after someone's hacks.

Thanks for the guidance once again.