Music Man Rp-2100 212 (GP-1 Board)

[Mr_bibbles]

Hey Y'all-


Have an issue that seems straightforward, yet I am stumped.


Music Man RP-2100 (the phaser one with the GP-1 board: https://el34world.com/charts/Schemat...rp_2100_rp.pdf)

1. When plugged into the front of the amp, there's only a blatty signal, and only if you pluck pretty hard. It dies away almost instantly.
2. Line in inputs work fine, decent sound from power amp.
3. Traced signal up to IC1 Pin 3, it's present on that pin. Could not find ANY signal on IC1 Pin 1.
4. Injected signal at treble pot, sounds fine, phaser included.
5. Injected signal at IC1 Pin 3, no sound passed through to power section.
6. Tested IC, I am getting -15 and +15 rails, so no problem there.
7. Checked r3 for ground connection / OL, tested healthy.

So, to summarize, we can guess that IC1 has a failure around it. Signal dissapears right at the chip, injected signal directly after it seems fine.

So, I Replaced the Lm1458 with a compatable LM4558 I had on hand, no change in any test results. What in the good god darned heck am I looking for? Also, I don't understand what D1 and d2 are doing in this stage....


Thank you for your wisdom.

[g1]

Every once in a blue moon, a 4558 will not work for a 1458.
Not saying that is the case, but might as well be sure. If they are socketed, try putting the 1458 from the reverb section into the IC1 position.

[The Dude]

What are the rest of the voltages on the op amp? Check those clamping diodes between pins 2 & 3 of the op amp.

[J M Fahey]

1. When plugged into the front of the amp, there's only a blatty signal, and only if you pluck pretty hard. It dies away almost instantly.
2. Line in inputs work fine, decent sound from power amp.
3. Traced signal up to IC1 Pin 3, it's present on that pin. Could not find ANY signal on IC1 Pin 1.
4. Injected signal at treble pot, sounds fine, phaser included.
5. Injected signal at IC1 Pin 3, no sound passed through to power section. <<< you can´t inject audio there, it´s a zero impedance output
6. Tested IC, I am getting -15 and +15 rails, so no problem there.
7. Checked r3 for ground connection / OL, tested healthy.

So, to summarize, we can guess that IC1 has a failure around it. Signal dissapears right at the chip, injected signal directly after it seems fine.

Please post DC voltage on ALL IC1 pins.
Both with original Op Amp and the replacement, you may try RC4550 and TL072 there, both should work

[Mr_bibbles]

Thank you everyone-

J M-

Thanks, good to know about outputs.

Turns out I made a mistake, I assumed the rails were healthy at 13v (I was using a limiter) But when I remeasured without the limiter I still only have 13v / -13...

IC1- tl072:

1 : 12.3vdc
2 : -50mv
3 : 0
4 : -13vdc
5 : 0
6 : .9mv
7: -1v
8 : 13v

IC 1 with 1458:

1: 12.4
2:-84mv
3: -9mv
4: -13.4vdc
5: 0
6:.9mv
7: -1vdc
8: 13.8VDC

I'm not very familiar with IC technology, so these don't really mean much to me...

BUT

Noting the 3vdc difference in the rails, I checked the 16v zeners (d10/d11), both measure 13vdc/-13vdc. Does this mean.....

A: Something is pulling down the supply? Possibly a bad IC down the line??
B: The supply itself is damaged? Zeners? Possibly a cap ...c30-c33?


I'll poke around, but any leads to this mystery are appreciated!

Thanks again.

[Enzo]

Should work just fine at 13v, you just lose a little headroom, that's all.

Note the glaring 12v instead of about zero on pin 1. That is likely a bad IC.

[Mr_bibbles]

Thanks Enzo-

Those are the measurements for 2 seperate IC chips, the original and a new TL072. So, assuming that both IC's are not bad, I should check for an accidental bridge between Pin 1 and Pin 8? Or other local low voltage supplies?

[Enzo]

Pull the IC from the socket and measure the voltage at the socket pin. Is the 12v still there or is it gone?

[Mr_bibbles]

Without any IC inserted, there's only .6mv on pin 1.

With the TL072, 4558, and original 1458, there is -12v on pin one. Enzo, you were saying this is indicative of a failed IC.

So, assuming that all three IC's were not dead when inserted into the socket, what could have caused this?

Since there is no voltage without anything inserted, can we rule out a shorted pin/solder point?

[Enzo]

We know the 12v is coming from the IC rather than the rest of the circuit. It may well be the IC is good and is making the 12v because of something at its input.

Lets test some ICs. Just to be sure. IC2 is the reverb, yes? When the amp is powered, if you rock the reverb pan, does spring crash noise come out the speaker? In other words is the reverb circuit working? If so, use that socket as a tester. SUb each IC into that socket one at a time and see if it also functions. Only takes a couple seconds each.

I note that pin 7 also has a volt of offset, and that stage is totally isolated by caps from other sources of DC, so the ICs are still suspects.

[Mr_bibbles]

Thanks Enzo, I'll check this tommorow and let you know!

[Mr_bibbles]

Reverb is still present when I swap each of the IC's in the amplifier into position 2. I used a new 4558 to replace the "missing" one each time I swapped it into the reverb slot.

[Enzo]

Then it sounds like the ICs themselves are OK.

[Mr_bibbles]

Word. So, now that we have eliminated the IC's from the equation, what could be causing the 12v leakage onto pin 1? Enzo, you mentioned that it could be creating this voltage because of something at it's input. What kind of failure could cause this to happen??? Is it possible that those clamping diodes have failed and are leaking the + voltage that's supposed to be on pin 3 through?

[Enzo]

WHAT + voltage on pin 3?