Pioneer amp project

[mikebarg]

Well, I haven't been in here for a while but I've gotten back into fixing amps. I've got something different than my usual guitar amps I like to play with. This is a Pioneer SX-253R Stereo Receiver/amp. I'm still looking for a Schematic. A preliminary inspection shows no speaker output on either channel. The front display powers on. I hooked into the headphone output. A very small signal is there. If you turn the main volume up to 100% and run the headphone output into another amp and turn it up to 100% you can hear a very faint signal. I need a schematic. In the mean time I am going to remove the unit from it's case to make everything more accessable.

[nigelwright7557]

You can start by checking power supply voltages and fuses.

Some ohmage checks of output transistor C-E or S-D.

[mikebarg]

I powered up the unit and did a few voltage checks. First, the 2 fuses on the board are good.
I checked the voltage across the 2 main power suplly caps(8200uF 63v)
They both have 53.7 voltage across them.
I found a M4017 voltage regulator. The Vout is 12V.

I checked voltages at the power transistors ( C4689 and A1804)

Channel 1.

C4689 B-C 34.9v C-E 34.9v
A1804 B-C 72v C-e 72v

Channel 2.

C4689 B-C 53.4 C-E 53.9
A1804 B-C 53.4 C-E 53.9

I would think the voltages from channel 1 and channel 2 should match.
I'm I right in thinking the voltages on channel 1 look odd.

[nigelwright7557]

I think i would be checking the channel 1 output transistors next using ohmage checks on BC, BE and CE.
This is best done out of circuit.

[mikebarg]

I removed the CH.1 output transistors and checked . Both are good.

[The Basilisk]

I can't offer advice with your amp, but would also appreciate a schematic or service manual if anyone has one My example works fine, except there is no bias current at idle - hence subtle distortion as it's thus running in pure class B mode! Erk. Now that wouldn't normally be a complex issue except that there's no manual adjustment provided for bias current, so a schematic would really help!
Cheers

[sakis]

best indication of working amp ( regarding the outpout stage ) will be to check all emmiters of output transitors to verify that are near zero volts ( measure emmiter to ground ) anything above 50-80 millivolt will triger the protection and no output

check this first ...then is the rest

[h_a]

Channel 1 is missing the base voltage drop (CB=CE), so the outputs don't turn on at all. I would have a look at VBE multiplier/bias.

Seems to me Channel 1 has a large DC-offset (large imbalance between CE of complimentary outputs) that probably triggers offset protection of both channels.

Have fun, Hannes