Rivera Bias trouble

[The Dude]

C7 is a filter for the node supplying V5 plates. It has nothing to do with bias voltage.

[Jazz P Bass]

Post #13: "Disconnect the orange transformer wires and measure the resistance between the two points where the wires plug into the circuit board."

[Valvehead]

Disconnect the orange transformer wires and measure the resistance between the two points where the wires plug into the circuit board.

open

[Valvehead]

Ok--its getting weird. At the + (banded) side of D5 i have 0.8vac and 0.6 vdc

at the other end of D5 = 0v. i put in a new 1n4007 for D5 to eliminate a bad diode .

grounds are good throughout the bias circuit ...r5 and r6 are getting to Vr1 (continuity) and they test good

[TomCarlos]

In #18, you say "open" .... but it could be very high resistance due to the fact that you have the diode sitting there. If you temporarily put a jumper wire across the diode, what do you get? Again, as Dude asked, measure at the points where the orange wires connect to the board.

Check the pc board traces - very carefully. Make sure you do not have a crack or bad solder connection somewhere.

[Enzo]

Take the transformer wires off abd measure resistance between the wires. Not on the board, measuer resistance of the transformer winding. Your meter read an AC voltage, but it still can be open.

[vintagekiki]

it starts at 30mvdc and creeps down to 0mv over about 20 seconds

the new bias pot has no affect

How much is the VAC on TP42 when the orange wires are hooked on R5 (1k)

1)
Check that are not break on PCB lines between PT (orange wires 41VAC) and R5 (or) D6.

[Marshtech]

messy schematic with that writing over it. momentarily, put a jumper across that fuse/resistor or whatever it is that is labeled FS1 or PS1 polyswitch. I'll bet it's open circuit. Surprised nobody suggested this yet. I bet that restores power to TP42 and bias restored.

OOPS! missed the detail in the first post - it's already jumpered out. Never mind.

[vintagekiki]

#23

#1
The factory did not use the "Poly switch" you see right after the power trans it has a jumper there. This amp has 1 owner and never been opened.

[DrGonz78]

Question for the brain trust. Can the neon bulb test method be used to test bias taps in a situation like this one? I know it’s used for testing a bum OT but the idea just came to me.

[The Dude]

The neon bulb test may not show anything on an open, especially if it's right where the wires exit the transformer. I'd first try Enzo's idea in post #21. It's a quick easy measurement.

[Valvehead]

Disconnect the orange transformer wires and measure the resistance between the two points where the wires plug into the circuit board.

measures OPEN

[Valvehead]

Take the transformer wires off abd measure resistance between the wires. Not on the board, measuer resistance of the transformer winding. Your meter read an AC voltage, but it still can be open.

open

[Valvehead]

How much is the VAC on TP42 when the orange wires are hooked on R5 (1k)

1)
Check that are not break on PCB lines between PT (orange wires 41VAC) and R5 (or) D6.

now i have 30VAC at TP42

and 30vac before and after R5

[The Dude]

If you measure no resistance across the bias winding of the transformer (Enzo's test), you have a transformer problem. I'd remove the end bell(s) and inspect junction where the orange wires tie to the actual transformer winding wires. Check the resistance right at the winding wires and see if you still have an open. It's not impossible to have the winding be open in the middle somewhere, but the bias supply draws very little current, so It's more likely a manufacturing error. It's also possible that there was a short in the bias supply that took out the transformer, but worth checking to see if a repair is possible before replacing the transformer.

Edit: Also, if there are crimp connectors on the transformer wires, check those to make sure you don't have a break or bad crimp on one of those. I'd probably check that first, as it doesn't require taking the transformer apart.