More Rhodes Satellite amp confusion

[Randall]

I have the amp producing sound, but it is a bit hummy, not a lot but it's there. I find nothing wrong, it's been recapped. So I don't know, are these inherently noisy amps? I have no experience with them.

More specifically, what do pots R 9 and R11 do? I'm wondering if there is some sort of balancing that may be off that may be the source of the hum? Schematic is on page 81 of the manual.

http://www.fenderrhodes.com/org/ch11/fig11-12.jpg

[g1]

Check if there is any DC offset at the output. Try loaded and unloaded.
Then check the DC voltage across R19 and then across R15.
Post your results.
Is there no mention of bias or offset adjust in the manual?

[Randall]

Sorry, bench was clogged up for a bit.

I see 0.2vdc on the output with a load, and 1.9vdc with no load.

Drop across R15 and R19, which are "/\/\" shaped pieces of flown wire are .007 vdc and .012vdc

I find no mention of bias or offset adjust in the manual.

[g1]

I would try (unloaded) tweaking R9 to get R19 voltage down to the same .007 as R15, then see if you now have 0VDC at output.
(that "M" shaped wire is probably nichrome which was sometimes used as resistance wire)

[Randall]

I couldn't get R19 down to .007v, but I did get to match at .013v and that put my unloaded output at .17v and loaded at 0.02v. Still hums, tho.

[Enzo]

Hums.

60Hz or 120Hz - make sure with scope.

How much ripple on main filters?

Short pins 1 and 3 at input to ground input. Do not ground input to chassis. Any change?

Notee that Q1,2 run off separate power supply +25v, the rest runs on main rail. Are +25v clean?

[The Dude]

Is this the same amp that had a shorted/replaced Q1? If so, is Q2 possibly damaged also? What are the voltages on Q2?

[Enzo]

If any were replaced, Q!, Q2 ought to be the same type as each other.

[Randall]

"60Hz or 120Hz - make sure with scope."

120Hz.

"How much ripple on main filters?"

.03 - .05 vac


"Short pins 1 and 3 at input to ground input. Do not ground input to chassis. Any change?"

no difference, but input jack is on a separate panel not connected to chassis.

"Note that Q1,2 run off separate power supply +25v, the rest runs on main rail. Are +25v clean?"

yes.

"Is this the same amp that had a shorted/replaced Q1? If so, is Q2 possibly damaged also? What are the voltages on Q2?"

Yes, this is the same amp that had shorted/replaced Q1. Voltages on Q2 are E = 10.1 v, B = 10.8v, C = 12.1v

"If any were replaced, Q!, Q2 ought to be the same type as each other."

I subbed a NPN MPS6530 for Q1. I assume a misprint on the schematic referring to G1, Q2 as 2M3393. Should I replace Q2 with a MPS6530 as well?

[J M Fahey]

You balanced uppen and lower bias, and as a result you reduced offset to almosr 0: cool.

About humming a little: those are *crude* early technology transformer driven, and rail ripple , through the long bias string appears at Q6 and Q7 power transisors base, which to boot do have voltage gain.
I am surprised not A LOT of hum appears at the output.
I would leave as is.itīs a little too late to re engineer that amp.

Do you want hum free clean sound?
Junk those power boards and install, say, a couple LM3886 amps.
What is rail voltage?

[Randall]

Thank you Juan, you have confirmed my suspicion that there is nothing really wrong with this amp at this point. I did swap out Q2 to match Q1, and it may have reduced some hum, but I am resigned to the fact that these amps are a little inherently noisy. It is now in the hands of the owner.