xr680c blows main fuse

[etneccas]

On the 400 B/G board on heatsink

I removed all 8 power transistors from the circuitboard and they tested the same as follows
base to emitter 334kohms and infinity rev pol
case/collector to base(not sure which lead is emitter or base)224kohms and infinty rev pol.
case/collector to emmitter infinity with both meter lead polarities
I'm assuming that this because It's not conducting being out of the circuit
Are these normal readings? I'm inclined to think so, but I'm not sure

2-47ohm resistors meter ok
2-diodes not shorted (I lifted 1 end from ckt)read 170ohms and infinity in rev pol.
6 -10watt .33 resistors read good
2-10watt 1ohm resistors read good
Thermal switch is closed/shorted

On the 400BH driver board
I lifted 1 end of each of the BIG diodes and they metered good no short
I tested all transistors on this board in the circuit and nothing reads shorted
I pulled the 2 4700uf big capacitors and they read close to tolerance with my Fluke 87DMM
the 2 -1/4" speaker jacks are not shorted and look OK
None of the glass diodes appear to be shorted I have not lifted from the circuitboard yet
the IC TL074CN looks alittle discolored but the rest of the board shows no signs of heat and discoloration
The paper inductor/coil meas. reads 2ohms the 22ohm resistor in paralell looks Ok

Any Ideas what I could check next?

would a bad IC TL074CN on the driver board cause the main fuse to blow??????

ENZO you in the house?

[etneccas]

update

update........
I soldered all the transistors and diodes back in place
I left the front preamp and master section disconnected
I plugged the power section and transformer with no speaker hooked to the speaker jack
Fired it up through my light bulb limiter and and the the bulb surged on (.6amps)and then stayed dim at .2amps

How long can I leave it hooked this way without a speaker load?
I'd like to carefully take some volt readings and compare to a 400BH schematic
I suspect the problem may be in the master section and will start checking for shorts

[52 Bill]

You can test it that way as long as you want. The voltages may be lower than expected due to the reduced voltage through the light bulb.

[etneccas]

You can test it that way as long as you want. The voltages may be lower than expected due to the reduced voltage through the light bulb.

Thanks Bill
I'll take some readings tonight

[J M Fahey]

Hi etneccas.
1) your measuring part by part is fine, but not enough, you can spend years at it and still miss some critical problems.
As a side note, you seem to be measuring your transistors and diodes on a resistance scale.
That will catch only the most extreme shorts.
Your multimeter has a "diode" measuring scale ( hint ... hint ) which rather than an "ohms" value, shows what voltage is dropping across the junction when fed a couple mA through it (kindly provided by the multimeter itself). This info is much more useful.
You should have around 0.65V in the forward position (the way the arrow points) and out-of-scale open the other way.
You have no excuse for

(not sure which lead is emitter or base)

Download the datasheets and be sure about it.
2) Let's take some voltage readings. (still with the lamp pimiter)
Rather than some individual parts info, it will show us how the amplifier is working as a system.
The first ones should be:
V+ and V- rails
Voltage from each speaker terminal to ground.
Vce: voltage across power transistors.(collector to emitter)
Vbe: base to emitter voltage on each power transistor.
3) Last but not least, search and post or link to its schematic so we all talk about the same.
4) Good luck.

[etneccas]

TRANSISTOR TESTING

PV 400BH

I better get busy......................

[etneccas]

You can test it that way as long as you want. The voltages may be lower than expected due to the reduced voltage through the light bulb.

took some volt readings starting at the input of the xformer
I'm coming off a standard 120v(124v reading) outlet with a 75w lightbulb limiter
which drops me down to 106vac feeding the input of the xformer

the output on the red winding is 67vac
the output on the orange winding is 36vac

the 67vac feeds the big diode network and goes to R66 and R67 I have 87vdc here on the lineside of these resistors and 30vdc on the loadside way too high
The schematic calls for 52vdc on the lineside and 15vdc on the loadside(I read 30vdc here!).................I haven't took any readings on the other winding(orange) yet
I'll lift the 5w resitors and meter them

[etneccas]

resistors tested ok so did the zener diodes
are the high voltage ratings normal...... only have the power section wired up

[J M Fahey]

the output on the red winding is 67vac

There are *two* red wires, measure from each of them to ground (chassis)

the output on the orange winding is 36vac

There are *two* orange wires, measure from each of them to ground (chassis)

the 67vac feeds the big diode network and goes to R66 and R67 I have 87vdc here on the lineside of these resistors and 30vdc on the loadside way too high

What do you measure on the + side of C35 and the - side of C36? Relative to ground.
What do you measure on the + side of CR20 and the - side of CR21? Relative to ground.
What do you measure on both ends of L1? Relative to ground.
Relative to ground means you touch the black probe to the metallic chassis and touch what you want to read with the red probe.

[etneccas]

I totally missed the center tap on the xformer......doh!
I'll go and take some proper meas

[etneccas]

"There are *two* red wires, measure from each of them to ground (chassis)"

33vac both to grd


"There are *two* orange wires, measure from each of them to ground (chassis)"

18vac both to grd


"What do you measure on the + side of C35"
+43vdc

"and the - side of C36? Relative to ground"
-43vdc.

"What do you measure on the + side of CR20"
+15vdc

"and the - side of CR21? Relative to ground."
-15vdc

What do you measure on both ends of L1? Relative to ground.
0vdc

[etneccas]

Read -43.6vdc on Q12's collector and Q13's base
schematic has .58v as the correct volt reading

The resistor on Q12's collector R58 47ohm FP res meters ok i mas. -43.6 volts across it

Q13 shorted

[etneccas]

My bad .... .58v is the drop across R58 (not the volt reading)
Q13 meters ok
back to work

[J M Fahey]

So far so good, your amp looks fine.
Re-check that you have essentially 0 VDC (less than 1 VDC anyway, always referred to ground unless stated "across something") o on the speaker "hot" pin, also called "output rail" whis is anything connected to Q13 to Q16's collectors or Q3 to Q6's emitter resistors, in this case through a current sensing 0.1 ohm resistor which is often used by Peavey.
If so, you can connect a speaker , inject some audio , the amp should work although it will provide only a couple watts and the series lamp should blink in rythm with the music.
Instant 70's Disco !!
If not, there's still something we need to find.

[etneccas]

Thank you!

Not sure why the fuse blew initially but..........
I'm powered up and passing audio with the light bulb limiter
I'll try it without the limiter tomorrow
but I'd like to thank everyone for their input and patience
I hope I created a archive for less experienced people like myself
to understand the 400bh b/g circuit
Man what a crummy cicuitboard.............