?Testing Transistors?

[Ylli]

OK, sounds like the high side outputs are good.

When you power up with a 60 watt bulb in the DBT, how bright is it? Is it flickering at all or steady? Is the Kill-A-Watt still blinking?

 

[rswojo]

Kill-o-Wat not blinking now. Dim bulb flashes bright then dims down but brighter than it should be.
Steady.
Maybe it isn't brighter than it should be, I usually use a 100W bulb in my DBT. That goes out entirely.

 

[Ylli]

Let's confirm the issue is in the right amplifier. With power on, measure the voltage at the high end of C801. Shut down and allow C801 to discharge fully!!! Lift one lead of R764. This is one of the 0.24 ohm resistor near Q802. Power back up and measure the voltage at the high end of C801.

 

[rswojo]

C801 56.2V. R764 lifted C801 62.7V Dim bulb almost out.

 

[Ylli]

That's getting closer, but certainly not an indication of a short being removed. As long as nothing seems to be getting hot (or smoking ) put the 100W bulb back in the DBT and check the voltage on C801.

 

[rswojo]

That's getting closer, but certainly not an indication of a short being removed. As long as nothing seems to be getting hot (or smoking ) put the 100W bulb back in the DBT and check the voltage on C801.

Not very bright flash, dims down to basically nothing, 69.9V. That is what I am used to seeing.

 

[Ylli]

Still strikes me as a awful lot of voltage loss on that cap for a 100W bulb. What voltage are you seeing on the Kill-A-Watt? Let's also lift one lead of R664 and try again.
[please excuse the delay in response, I'm also working a Kenwood KR 5030 on the other line].

 

[rswojo]

120V on Kill-a-Watt. R664 lifted 79.1V, big improvement.
No problem, I'm sort of enjoying the slower pace. I'm going to step aside for 10 minutes.

 

[Ylli]

So it looks like the left channel amp is the one that is drawing too much current. Wasn't it the right channel that seemed bad before? Spooky. Next step is to resolder R764 and confirm the voltage stays near 80. Then try injecting some hum into the right amplifier, probably the best place would be R737 (tone board).

While we are at it, check the voltage on the emitter of Q802 - should be 1 volt or so below the voltage on C801.

 

[rswojo]

R764 back in place. C801 - 69.5V Q802 emitter - 68.2 Q802 collector - 69.5
Inject hum, right channel working. Looks like trouble both sides?

 

[Ylli]

Don't think so, may be chasing a non-problem. Can you measure the voltage across R764? Should be in the neighborhood of 10 - 15 mV

 

[rswojo]

Voltage drop on R764 43mV.

 

[Ylli]

OOW, Rather high. Find pot RV602. monitor the voltage across R764 and attempt to adjust it to 13.7 mV

43 mV translates to an idling bias of 180 mA, should be 50 mA.

 

[rswojo]

Got it at 13.8mV, close enough. Put R664 back and same for other side?

 

[Ylli]

yes. Then check for functionality with the hum injection on that channel.

 

[rswojo]

Adjusted second one to 13.8V, flipped it over to read C801 voltage and it went to 13.2mV, touchy.
C801 happier now at 73.5V.
Those pots should be replaced.

 

[Ylli]

Yes, those bias adjustments always seem to be touchy. That's one reason many replace them with multiturn pots. Anything 12 - 14 mV is OK. Did the hum test work? I'll assume so, and need to call it a night.

 

[rswojo]

Plugged in an iPod, music on right and left channels but with a hum included that is caused by iPod 3.5mm to RCA connectors cable. Junk cable evidently.
I second that motion, class dismissed for the night. Last day of day job tomorrow, starting early ending early, maybe 2 pm.

Good night.

 

[Ylli]

Curious, what was the voltage across R664 before you adjusted it?

I think you can back up to the MPX board again now. C415 and C416 are still removed from the board. Switch the mode to FM and inject hum at R424 and R425 to confirm we signal flow from those points to the outputs.

Getting back to the original problem..........

 

[gadget73]

watching this is like half of my own troubleshooting and repair efforts. Start on one problem, realize part way through there is a new and different problem in addition to the original.