orion 2150 power supply op amps

[Nutcase21]

orion 2150 power supply photocouplers

just got done rebuilding a PS in a 2150sx and i have a slight problem. when i power the amp (no remote power) it draws 0 amps as expected, but when i power the remote it draws approx 1.4 amps, climbs to approx 2.1 then falls all the way to about 0.4 amps and sits there. the power led stays on until the current draw falls below about .6 amps. all the power supply fets were replaced with irf3205 's as well as all of the emitter resistors replaced too. here are the voltage readings on the tl494:
pin 1 0.0004v
pin 2 0.0772v
pin 3 0.0765v
pin 4 0.0064v
pin 5 1.5735v
pin 6 3.6112v
pin 7 0.0002v
pin 8 10.717v
pin 9 4.4503v
pin 10 4.482v
pin 11 10.741v
pin 12 10.928v
pin 13 5.124v
pin 14 5.1247v
pin 15 0.0775v
pin 16 0.0003v

the mct2 optocouplers read the following:
mct2 #1
pin 1 1.1757v
pin 2 0.0001v
pin 3 0.0030v
pin 4 11.589v
pin 5 11.768v
pin 6 12.240v

mct2 #2
pin 1 1.173v
pin 2 0.002v
pin 3 0.0023v
pin4 -0.866v
pin 5 -0.866v
pin 6 -0.350v

the tl494 voltages look good but the 2nd optocoupler looks suspicious. all voltages were taken with battery ground as the negative test lead. any help would be appreciated.

 

[Perry Babin]

To clarify...
None of those are op-amps.

Does the power LED go out with 4v+ on pins 9 and 10 of the 494?

 

[Nutcase21]

the led starts to die out right around there, yes.

 

[Perry Babin]

Measure the DC voltage in the output section with the black probe on the 3rd leg of one of the 6488s and the red probe on the 3rd leg of one of the 6491s. Watch the voltage from the time you apply remote until the LED goes out. What does the voltage do?

 

[Nutcase21]

once power led is lit the voltage spikes to ~70v and then starts to decrease as the led dims out.

 

[Perry Babin]

Double-check to confirm the voltage on pins 9 and 10 of the 494 after the LED goes out.

 

[rodner]

Is the SX similar to the XTR? I ask because I have an XTR and would like to keep this thread for reference.

 

[Perry Babin]

This is a 2150sx.

 

[Nutcase21]

Voltage on pins 9 and 10 starts at ~2.3v and climbs until resting at 4.44v and 4.46v.

 

[Perry Babin]

With the voltage on pins 9 and 10 at 4v+ after the LED goes out, what's the DC voltage on the collector of the NPN driver transistors for the power supply FETs?

 

[Nutcase21]

both npn drivers read 3.701v on their collectors.

 

[Perry Babin]

What is the resistance between the PNP power supply drivers collectors and ground?

 

[Nutcase21]

resistance is 10.17ohms for each pnp collector to ground.

 

[Perry Babin]

Does the waveform on the outer terminals of the primary windings on the power transformer go from a square wave to straight DC when the LED goes out?

 

[Nutcase21]

I don't have a scope to tell, just my fluke 189 meter.

 

[Nutcase21]

my meter will measure frequency and when i check the frequency of both pins 9 and 10 of the tl494 i get ~20.5khz (this is using the battery negative as ground). when i measure the frequency on the secondary winding at start up i get 61khz and ~25v ac and then it decreases to about 20.5khz and 3.34v ac when the led goes out. there is no DC voltage on the secondary windings after the led goes out.

 

[Perry Babin]

There is a 2N6488 standing near the opto-couplers. Remove it. Power it up while monitoring the DC voltage on the outputs. If it goes over the rated voltage for the rail caps, reduce the 12v supply voltage until the rail voltage is below the rail cap voltage. If nothing overheats, check the amp to see if the amp produces clean audio.

 

[Nutcase21]

pulled the 2n6488 and powered it up. rail voltage spikes up to 44 volts (50volt rail caps) and then decreases while the led dies out and settles at ~3.4volts. power supply voltage is ~12.15v

 

[Perry Babin]

Does the voltage on the collectors of the NPN driver transistors remain near 12v?

 

[Nutcase21]

yes