Musicman RD 50 schematic question

[themusic]

Replaced Filter caps on my RD50. V1 is only getting 14V . Traced back to R61 one side 288V other side 39V ?
I can hardly make out on my schematic ...thanks

[Jazz P Bass]

Did you try swapping out the tube?
The resistor by itself will not have a voltage drop.
It takes current to drop the voltage.

[themusic]

Swapped tube still nothing.. faint distorted signal... line out for pre no problem ...thanks

[Enzo]

Or C35 is really leaky. The tube has plate resistors of 470k and 4.7MEG, so I don;t think the tube willdrag anything down.

[themusic]

Now V1 is 45V C35 110V ...thanks

[themusic]

Back at it... replaced all tubes still very low output, checked C35 seems ok just not getting juice to tube...thanks

[g1]

C35 could "seem ok" but be breaking down under voltage. Try disconnecting it and see if there is any change to the voltage at R61.
Also, you have checked R61 out of circuit and it is 47K ?

[themusic]

Disconnected C35 R61 is now 242V . One side of R11 is 243 the other 144 and thats what I get at V6.
R9 is 197V other side 118V and that's what I'm getting at V1 ???

[g1]

Sounds like much more reasonable voltages, C35 must be bad. How does it sound with C35 disconnected?

[themusic]

Still very low volume... I'll replace C35 in the A.M. thanks

[soundguruman]

Replaced Filter caps on my RD50. V1 is only getting 14V . Traced back to R61 one side 288V other side 39V ?
I can hardly make out on my schematic ...thanks

It's almost always the failure of Q3 or Q4 (JE 1692 transistors)
http://www.blueguitar.org/new/schem/...m_rd50_mod.pdf
The transistors are mounted to the chassis with insulators that fail, and short the case of the transistor to the chassis.
The screws that hold the transistors are supposed to be insulated with plastic washers, but the washers and screws become miss-aligned and short the case of the transistor to the chassis.
The holes for the screws were typically drilled in the wrong spot. This caused the transistor to not line up exactly with the circuit board. It was "offset."
Because of the offset, loose screws, etc, the transistor tends to get out of alignment and the insulators fail, and the volume goes very low.
ALSO, those silly yellow coupling caps in the preamp tend to short out or open up....more often they short out, and fail to block the voltage...
Check with your voltmeter on both sides of the caps. See if there is a voltage drop across the cap, or if the cap has about the same voltage on BOTH sides.
That's all folks.

[Jazz P Bass]

What does Q3 & Q4 have to do with the B+ rail?

[soundguruman]

As I recall the transistors supply bias to the output tubes.
With the bias OFF, the tubes draw waaaaaaaaaaay too much current and drag the B+ down...
Go ahead and pull the output tubes, and see if the B+ comes back up.

[Jazz P Bass]

That is good to know.
But the OP's problem is not the B+, it is the C+.

[Enzo]

SGM, look at the schematic for the amp he is servicing. He already determined that C35 was what was dragging his B+ down. Furthermore, his sag was not along the whole of the B+ string, just the node for the preamp tube.

Please explain then how the cathode drive transistors would selectively drag down the preamp tube B+ node without also pulling down the plate and screen nodes? Please explain how pulling the tubes would allow the voltage at C35 to rise without all the other voltages rising with it.