Randall RG100 ES solid state questions

[Randall]

Gosh, I sure could use some guidance on this. The preamp channels are now getting a healthy signal to the FX jacks and then to the base of Q8, but it stops there and doesn't get to the base of Q10. I have changed the diff pair of Q8 and Q9, (with BC546) but no change. Voltages seem to be in line, and I have checked out all the electrolytics. Any further help would be appreciated.

Reposting the schematic
http://music-electronics-forum.com/a...ll-rg100es.jpg

[The Dude]

Have you tried isolating the speaker output jacks from ground? Maybe just hang them away from the chassis temporarily? I'm wondering about what is happening to Q9 bias with that string of R52/C36 being grounded on the speaker side instead of lifted by .18 ohm resistor.

[Randall]

Indeed, this exactly how it is right now. Hanging away from chassis. No change. I obtained some isolated jacks, but have not installed them as yet.

[g1]

Q10 checks good?
Can you post DC voltages for EBC of Q10?

[Randall]

Q10 tests good out of circuit.

E = 42.4v
B - 41.8v
C = 1.4v

No signal present at Q10 base, but there is signal at collector, although I can't see how.

[J M Fahey]

No signal present at Q10 base, but there is signal at collector, although I can't see how.

Stop searching for voltage signal there, period.
This is not a tube amp but ab transistor one.
Transistors can be driven by voltage or by current, the case there, so uo will not find signal voltage.
You are running in circles.

I suggest:
1) crop the power amp section (including loop jacks) with some photo editing software for clarity
2) measure all DC voltages shown and by each of them write what *you* have.
3) post image with fresh results.

[Randall]

Here are two links to a photo with measured voltages as requested. It doesn't show it, but I measured with a 1KHz 225mV rms signal injected in the FX return jack. Does anything look off?

http://i668.photobucket.com/albums/v...0ES%20copy.jpg

[The Dude]

It would be easier to see the voltages in relation to the circuit rather than on a picture of the board. I've cropped and tweaked the output section below. Can you write the voltages on the schematic and repost them?

[g1]

Also, it would be best to separate DC and AC voltages. So one of schem. with DC voltages no signal, and one with AC voltages (no DC) with your 1k 225mV input to FX return.

[Randall]

Rats! I thought by "photo editing software" Juan wanted a photo. That took me hours. Oh well, I get on the schematics right away. Thank you for sticking with me on this, it's killing me.

[Randall]

OK here they are.

DC, no input.




AC RMS with signal into FX return jack.

[J M Fahey]

Ok.
DC looks fine, so we let that aside.
AC values show me that your amp has very low gain, only 5.5X .
Actual raw gain should be: (8 ohms / 0.18 ohms) so we should *start* with raw 45X gain .... we are starting with the wrong foot , and gain should be multiplied by (1500/330)+1 ..... which amounts to 5.5+1=5.5 X .... not surprisingly the actual gain we are measuring.

This makes me think that R63 is open/not grounded/disconnected from speaker out "cold" terminal ... which is NOT ground.

So: replace output jacks with plastic types or if old style metallics insulate them from chassis with fiber washers and covering jack thread with tape or heatshrink as needed.
For testing you may leave them floating, but make *certain* that speaker "-" terminal is connected to the top side of R63.
Also make certain that bottom leg of R63 is well connected to power supply Ground.

I bet that today you measure 1.3V RMS on *both* speaker out terminals, plus and minus, you should read on the + one 45 times what you get on the - one.

Ok, go and check it.

[Randall]

I already replaced the jacks with plastic insulated types, and R63 is well connected to PS ground and speaker jack negative as shown. It is not open. I measure 1.3v rms on speaker +, and 0.029mV rms on speaker - . Interestingly, 0.029 x 45 = 1.3.

[The Dude]

Q12 looks to be reverse biased, unless you forgot a negative sign. In that condition, it wouldn't be doing anything. I would check D7 & D8. Maybe one or both of them is/are shorted.

[J M Fahey]

Please repeat this test and post results asked in green: