A/D/S power plate 80 and 100

[Rabbit 16v]

150ish mV AC which makes no sense at all to me. Its also fluctuating and the waveform is very erratic.

 

[Perry Babin]

How does the waveform compare to the input on the other rectifier. They should have the same input from the transformer if the rectifiers are center-tapped types.

Use the secondary ground or the negative speaker terminals (assuming that the amp isn't bridgeable) for the scope reference.

 

[Rabbit 16v]

Here are a couple of pics. There is only one bridge rectifier on the amp. I am probably missing something here... Ive been measuring at the output from the bridge rectifier as shown that has two wires going from it to the audio board. here are a couple pics of either side of the power supply board. The blue and red wires are the ones I am presuming are the rail voltage outputs from the rectifier. I am not certain the function of the b829s at this point.

 

[Perry Babin]

The 2SB829 transistors drive the primary windings of the power transformer.

Bridge rectifiers are rare nowadays. If it has negative rail but no positive rail on the terminals of the rectifier, it likely has failed internally. You can't use a standard bridge rectifier in the amp. You need a high speed rectifier. Standard bridge rectifiers will quickly overheat and fail at the higher frequencies and square wave of a switching power supply.

 

[Rabbit 16v]

Sorry pics were being difficult to add. They should be up now. Ive tested the rectifier outside of the circuit and it tests fine.

 

[Perry Babin]

The connection internally could be intermittent. Did you reinstall it to see if you again had +rail?

 

[Rabbit 16v]

Yup, and swapped in another bridge rectifier that I had. Same results. no more than maybe 3VDC pos on the opposite rail.

 

[Perry Babin]

Is the input waveform swinging equally above and below the secondary ground?

 

[Rabbit 16v]

Its pretty erratic. See attached. Might need to fire up the Lectrotech just to see a better waveform. The fluke is mainly for automotive use, so probably not up to par for this task.

 

[Perry Babin]

I don't know how the auto varies but the 97 is a 50mHz scope which is good enough.

Set the scope to 10us and 5v/div and post the waveforms on the collectors of the 2 PS transistors.

 

[Rabbit 16v]

so there is this odd spike that happens every second or so. Both transistor collectors report this. Pics are from only one transistor. I think scope resolution might be a little low at those settings to see anything, unless that tells you something?

 

[Rabbit 16v]

Ugh, doesnt look like the camera got the spike...

 

[Perry Babin]

The power supply isn't operating. Are you sure that you have negative rail voltage?

The supply may simply be charging the capacitor on the pulses that you see but didn't capture.

Place a meter on the negative rail and see if it remains constant or changes slightly corresponding with the pulses.

Do the same with the positive rail.

 

[Rabbit 16v]

Unless I am testing it wrong. There is a cap on each leg that both join to one pad on the board. I am using this as reference. So, neg lead on the common pad where both caps are soldered and pos lead on one side and its a steady 33v. No fluctuation. Scope shows a little noise on it though. On the other side, its all over the place, but never goes about 8v or so.

 

[Perry Babin]

Are you measuring on the terminals of C806 and C807 (referenced to the PS5 diagram) with the black scope probe on the circuit common to both of those capacitors (inverted triangle ground symbol)?

Do you have any spare electrolytic capacitors rated for 50v?

 

[Rabbit 16v]

Yes, those are the test points I am using and black lead on common connection for the caps.

I just looked and no spares. I would probably steal them out of the other amp, but I think if you are suggesting that I swap them out, I should just probably buy a few of them just to have in my stash of parts. Does that sound like a logical approach?

 

[Perry Babin]

Do you have a 110v incandescent lamp (not LED, not CFL)? Lower wattage is better.

 

[Rabbit 16v]

Most likely, yes.

 

[Perry Babin]

Try connecting it across the cap with 33v. Power up and watch the current draw from the 12v supply. Does it increase with the lamp connected?

What is the voltage across that cap with the lamp connected?

Do this with the PS transistors tightly clamped to the heatsink.

Switch the amp on only long enough to answer the questions.

 

[Rabbit 16v]

Ok, just so I am getting this right. Connect it in parallel across both sides of the cap that has -33vdc, observe action on meter on power supply, and check voltage on the same points I have been checking to obtain the -33vdc?