?Testing Transistors?

[rswojo]

OK I'm finally back. Q802 bad. C - E short. Replaced with "new" Q from parts unit. I managed to confuse myself temporarily but it is in with the base oriented toward R805.

 

[Ylli]

C-E or B-E short? You measured the same voltage on the base and emitter. Anyway....

Do you now have 78 volts on the emitter? What size bulb do you have in your DBT?

 

[rswojo]

My confusion came from assuming the center lead was the collector. So my corrected test results on the transistor would be B=78V, C=36.4V, E=36.4V. (I think) My correct test results on the pulled Q802 were C-E 0.5V, OL B-E short, short, B-C 0.5V,OL. Inexperience, confusion, bad memory all not good.

100W bulb in dim bulb.

 

[Ylli]

Makes more sense.

 

[rswojo]

LOL, sorry. Powering up.

 

[rswojo]

About 63V on all three legs of Q802. Somewhat low.

 

[Ylli]

Got a 60 W? Use the smallest wattage bulb that still allows the unit to start up normally. I usually have an AC voltmeter (actually a Kill-A Watt meter) connected at the output of my DBT and use the smallest bulb that gets me at least 100VAC unit input voltage. If there is some issue that took out that transistor, don't want it to happen again.

 

[Ylli]

About 63V on all three legs of Q802. Somewhat low.

Yes, is low. Can you **carefully** measure the actual AC voltage into the units power plug?

 

[Ylli]

Wondering if there is an issue in the right power amp that is drawing too much current.

 

[rswojo]

Got a 60 W? Use the smallest wattage bulb that still allows the unit to start up normally. I usually have an AC voltmeter (actually a Kill-A Watt meter) connected at the output of my DBT and use the smallest bulb that gets me at least 100VAC unit input voltage. If there is some issue that took out that transistor, don't want it to happen again.

60W bulb in dim bulb. Have to go work on day job for the afternoon. Wife is champing at the bit to get going on that. Back around 4:30. I can hold her off for a few more minutes.

Kilowatt says 122V after 60W bulb in dim bulb with receiver off. Turn receiver on display on Kilowatt says 112V then starts flashing like it is confused.

112V on AC connection inside receiver.

 

[Ylli]

The receiver or the Kill-A-Watt starts blinking? If the receiver is power cycling, then the bulb may be too small - although 112 volts sure should be enough.
Let's leave things alone for now and restart when you get back. Have fun!

 

[rswojo]

The receiver or the Kill-A-Watt starts blinking? If the receiver is power cycling, then the bulb may be too small - although 112 volts sure should be enough.
Let's leave things alone for now and restart when you get back. Have fun!

Kill -A-Watt blinks. 2 days left on day job.

 

[Ylli]

Back in post #78, you measured 78 volts on the collector of Q802 when Q802 was bad. Now with a functional Q802 the voltage is sucked down to 63 volts. That would point to an excessive load on the output side of Q802, or excessive draw on one of the power amps when bias is applied. When you get back, we can make some resistance measurements to check out that idea.

 

[rswojo]

I'mmmmmmm Baaaaaaack!

It looks like this unit was flying on a wing and a prayer?

 

[Ylli]

Did you ever find R801 - 2.7K wirewound? If you did, with power off of course, measure the resistance from each each to ground. Checking to see if there is any static DC path that could be drawing too much current. If you didn't find it, them just measure resistance from the emitter of Q802 to ground, and from the collector of Q704 to ground.

 

[Ylli]

See attached. Find this run on the power amps and measure resistance to ground for both left and right channel. Power off.

 

[rswojo]

Did you ever find R801 - 2.7K wirewound? If you did, with power off of course, measure the resistance from each each to ground. Checking to see if there is any static DC path that could be drawing too much current. If you didn't find it, them just measure resistance from the emitter of Q802 to ground, and from the collector of Q704 to ground.

R801 is supposed to be between the main filter cap and the collector of Q802 from what I can see in the schematic. Not there, brown wire from main filter cap to Q802 only.

emitter of Q802 to ground = 8.29K
collector of Q704 to ground = 5.4K

 

[rswojo]

View attachment 1814402 See attached. Find this run on the power amps and measure resistance to ground for both left and right channel. Power off.

Measuring from R657 and R757 to ground right side increases from 4K to 7K. Left side decreases from about 20K to 7K.

 

[Ylli]

The wires are close on the schematic. It is actually between Q802 emitter and the preamps. But your numbers are good. No short there.

Measuring from R657 and R757 to ground right side increases from 4K to 7K. Left side decreases from about 20K to 7K.

The initial number is dependent on the charges in the various caps. The final number is the important one. That last checked the low side output transistors for C-E shorts. They look good. Let's also check the high side: Measure from the 80 volt buss (top of C801) to the point indicated in my last attachment (#96).

 

[rswojo]

The wires are close on the schematic. It is actually between Q802 emitter and the preamps. But your numbers are good. No short there.
The initial number is dependent on the charges in the various caps. The final number is the important one. That last checked the low side output transistors for C-E shorts. They look good. Let's also check the high side: Measure from the 80 volt buss (top of C801) to the point indicated in my last attachment (#96).

Both sides R657/757 to C801 8.6K