Randall RG100 ES solid state questions

[J M Fahey]

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.

Voltages in purple are *audio* voltages, not DC.

[The Dude]

Juan,
I was looking at his upload titled "DC, no input". Those are DC voltages and Q12 base is positive instead of negative. The emitter sits at around zero volts, so shouldn't the base be negative in relation to emitter since it's a PNP transistor?

[The Dude]

Hmmm, that still shows 0.5V. But as long as you say it's -0.54V, I won't worry about the picture.

[Randall]

Dude, you must have seen it while I was trying to edit the post. I have corrected the original jpeg to show -0.5v. I will post the new requested ac voltages a bit later. Thanks again for the help. I have a feeling I'm going to be pretty embarrassed by the time this is done!

[Randall]

Here is the new AC map with requested voltage points



C34 after the diff pair is not shorted, it measures OK, and does not change anything if lifted.

[J M Fahey]

Juan,
I was looking at his upload titled "DC, no input". Those are DC voltages and Q12 base is positive instead of negative. The emitter sits at around zero volts, so shouldn't the base be negative in relation to emitter since it's a PNP transistor?

Oh, I found DC voltages reasonable in general so I let it aside (I said so in my post) .
I didnīt focus on Q12 base voltage (now corrected and reposted) since the *whole* amp in general looks good so I jumped straight to Test 2 gain and expected voltage levels at different critical points in the amp.
I guess itīs clear by now that is do not worry much about "bad parts" by themselves (those will point at themselves later) but am a "functionalist" , focus on how the whole system works.
Wrong behaviour will lead me to bad parts.
As in, for example: wrong plate voltage will lead me to open resistor, lack of +V rail , bad or unbiased tube, leaking coupling cap, way faster than testing tube in tube tester or curve tracer, testing cap in a bridge or leakage tester, pulling the plate resistor to measure its resistance, same with cathode resistor or cathode cap.
Yes, eventually a "bad part" will be the physical cause for the problem, but "functional testing" is way faster.

[J M Fahey]

Here is the new AC map with requested voltage points

[ATTACH=CONFIG]43528[/ATTACH]

C34 after the diff pair is not shorted, it measures OK, and does not change anything if lifted.

Oh, C34 *does* change things, a lot ... on a properly functioning Randall amp.
It kills NFB through the "normal" path R45+R46 so the *only* one which remains is through R52.

But now that amp is behaving "as if" the right side (on schematic) of R52 were connected to the hot end of output jacks instead of the cold end (which is not actual ground but the hot end of R63).
Please measure AC voltage at both ends of R52, straight at its legs.



There *must* be some path feeding those 219 mV to point 2, the base of Q9.

To confirm/discard that please repeat the test, injecting same voltage as before.
Check that you still have between 1.22 and 1.25 VAC at speaker out and then measure (8) and (9)

---------------------------------



As a separate experiment, you may feed again those 225mV RMS to amp input, recheck you still have around 1.25VAC ay speaker out ... and then lift said speaker .... output voltage should go through the roof ... in properly working RG100.
I suspect in your it will stay about the same.
But leave this for another day, only if the test I suggest gives unexpected results.

[Randall]

Hooray! I found it! After your instruction to look at R52, I found that you were correct in that the right side of R52 was indeed connected to the hot end of the isolated out put jacks instead of the cold end. I measured 0.222v on the left side of R52, and 0.123v on the right side, which did not equate with the .026v I was seeing on the top of R53 (point #3). So I traced the path from R52 and it ended up on the pos side of the output jack. I had replaced the jacks earlier with isolated jacks, and had I copied the wiring as it had come to me. (someone had tried to repair this amp prior to me, and left me quite a hacked up mess). Switching that wire to the neg side of the output jack saved the day. Now I measure a clean 100 watts into 8 ohms, and everything functions as it should.

Juan, thank you, thank you, thank you! I am humbled at the knowledge and guidance of the senior members of this forum and the unselfishness and willingness to help less talented and experienced members like myself. This was a good problem for me to work through, and I learned more than one thing. The customer is going to be really happy, because he thought perhaps it couldn't be saved, because someone else had already tried to work on it as is evidenced by the many components snipped off to test and then tacked back together with a blob of solder.)

So thanks again to everyone who helped me with this! Now get this thing out of my shop!

[Jazz P Bass]

Awe inspiring troubleshooting!

[nickb]

That was nasty problem to find. Well done spotting the stray wire.