Noise, Heat, and Sparks: Oh My!

[MrPotatoSalad]

Got an RCA Cleartop and voltages are good and noise is gone. Volume control is improved as well. Simply beautiful. I'll send my weak tube in for replacement tomorrow.

Thanks a lot PB.  ;D

 

[Natural Sound]

Got an RCA Cleartop and voltages are good and noise is gone. Volume control is improved as well. Simply beautiful. I'll send my weak tube in for replacement tomorrow.

Thanks a lot PB.  ;D

Did you replace the transistor AND the tube OR just the tube?

 

[MrPotatoSalad]

Did you replace the transistor AND the tube OR just the tube?

Just the tube for now. The Speedball is out until tomorrow when I get my transistors.

 

[MrPotatoSalad]

Switched out the lower TIP50 and 2N2222A transistors and I am still getting bright glowing LEDs on lower main board as well as the LED at A8 lighting up but normally.

The spark that caused this to happen originally was at T7, so would that affect the lower, upper, or whole main board. Should I try replacing upper transistors?

 

[Paul Birkeland]

What are your current voltages?

 

[MrPotatoSalad]

Didn't measure them since I didn't want to leave it on in fear of damage. Is it safe to measure them?

 

[MrPotatoSalad]

OK, after the test, the 5W capacitors were quite hot.
1:57
2:70
3:0
4:67
5:16
6:0
7:66
8:0
9:16
10:0
11:0
12:0
13:67
14:0
15:138
19:0
20:195
A1:18
A2:0
A3:1.4
A4:0
A5:0
A6:74
A7:0
A8:1.6
A9:0
B1:74
B2:67
B3:75
B4:19
B5:67
B6:17
B7:0
B8:0

 

[Paul Birkeland]

You still have a short somewhere drawing down the power supply.  If you run it long enough, you will blow out the 270 Ohm resistors again.

Are you triple sure that the TIP-50 transistors aren't touching the heatsinks in any way?

-PB

 

[MrPotatoSalad]

You still have a short somewhere drawing down the power supply.  If you run it long enough, you will blow out the 270 Ohm resistors again.

Are you triple sure that the TIP-50 transistors aren't touching the heatsinks in any way?

-PB

I have 49 more 270K 270 ohm resistors so no worries there. Don't want to ruin a capacitor though.

And yes, the plastic fully surrounds the 4 edges of the TIP50s and carbon washers are between the screw and TIP50.

 

[MrPotatoSalad]

I'm getting a 60K ohm reading on the collector to base on the new TIP50, which is on the bottom of the board.

 

[Doc B]

Just to clarify it's the 270 ohm 5W resistor PB is talking about, not a 270K ohm resistor.

 

[MrPotatoSalad]

Just to clarify it's the 270 ohm 5W resistor PB is talking about, not a 270K ohm resistor.

Oops, I did mean the 270 ohm 5W resistor.

 

[MrPotatoSalad]

So my TIP50 is attached to the heatsink by screw head>fibre washer>TIP50 plate>thermal pad>heatsink>flat washer>lock washer>bolt. Could it be possible that the screw is touching the TIP50's metal plate as it passes through the plates's hole, which would give it connection to the heatsink via the flat washer?

Also, even if the TIP50 is shorting, the LEDs should still light up. The LEDs light up when I first powered it up even though there were no thermal pads. The spark is what caused the LEDs to go out, so whatever the spark did is still not fixed.

 

[MrPotatoSalad]

When I attach the black probe to the heatsink and red probe to either the collector, emitter, or TIP50 plate I get a resistance of 400M ohm for both TIP50s. The screw to heatsink and screw to TIP50 plate show no connection.

 

[Paul Birkeland]

At this juncture, the best bet is photos, and lots of them.

I would unscrew the big PC board and tip it to the side, then take a shot of the bottom.  Also, the general wiring around the octal socket would be helpful.

-PB

 

[MrPotatoSalad]

Hopefully this works.

 

[MrPotatoSalad]

And if you thought that was the end...

 

[Paul Birkeland]

I thought of an easy test.

Pull the mounting kits off both TIP50 transistors and bend them away from the heatsinks.

Run the amp for ~30 seconds (not much more), then measure the power supply voltage at B2/B5.

If it stays up around ~160V, we have confirmed that you have a short between TIP50 and heatsink.  In this case, maybe fresh mounting kits would be the way to go.

If it still pulls way down, I'll keep pouring over the photos.

-PB

 

[MrPotatoSalad]

B2/B5: 67V
The problem has to be the SB since the stock crack ran fine on its own, so you could probably ignore half the photos.

 

[Paul Birkeland]

OK, let's do some process of elimination stuff here.

Go ahead and remount the TIP50's to the heatsinks with the kits.

There's a red jumper that goes from B+ to B+ on the big Speedball board.  Yank that, then remeasure B2/B5.

-PB