soundstream lw4.500 in protect..
I have been working my way through Perrys CD and am about to start diagnosing a lil wonder 500.4. This will be my first amp that is at stage3 repair level. It flashes sound then goes into protect. All output transistors (Tip35+36) diode check on all legs combinations fine.
In the power supply, I have a clean square wave to the rectifiers, and rail voltage is + and clean at 1 rectifier, and - and clean at the other. (but it is only for 1/2 second or so)
In the next day or so, I will be checking for dc at the speaker outputs, follow up with a actual rail voltage, and also putting the multi meter and the scope on the KIA494 PWM chip that i think is the problem.
I will follow Perrys CD and report my findings.
1. Power connected to amp, remote disconnected, Power FETs have power on centre legs and 0v on right and left legs.
2. Connecting remote, sees voltage increase to around 5.4v over around 4 seconds on left leg of power FETs. Then amp goes into protection.
3. Channel 1/2 bridged has 0v offset, channel 3/4 bridged shows around 0.5v offset reducing to 0.2v offset just before shutdown.
4. To see if I can further isolate offset, ch4 shows 0.5v and ch3 shows 0.01v
At this point I am tempted to cut the leads on ch4 output transistors and see if amp will fire up..
Checking outputs with battery disconnected.
5. using diode check on legs 2-3 sees the numbers rise and fall a bit, but on channels 1,2 and 3 sees figures around 0.7 and 0.8. channel 4 has 0.4 and 0.8.
I didnt think this number would be bad enough to throw the amp into protect, but perhaps this lower figure indicates the output is leaking slightly.
I suppose I can power the amp up and measure voltage on the output leg of the... no, they are joined and wont show anything...
um, well 1 of the outputs is + and the other is - so we see if the DC offset is + or - to give a clue as to if it is the tip35 or tip36 that is leaking...
We have +offset on channel 4.
I now measure for the rail voltage on centre leg of outputs. +29v and -29v.
Interestingly the the -29v was on the TIP that was 0.8 on the diode check, and the +29v was on the odd reading TIP with 0.4
I am going to pull the channel 4 TIPs from the amp and see if it will fire up
Still no good, and still have offset on channel 3/4. i going to pull the TIPs from ch3 as well.
Amp powers up and runs and stays out of protect. I have no TIPs here with me so I am stuck for now.
I will run it for 10 min or so and see if it stays ok
I have ordered 5 of each TIP required, not sure how many I have at home, but it wont hurt. Iknow i only have to order the TIPs for channels 3 and 4, but i will replace all TIPs on all channels. Doesnt take long and maybe they will be better matched...
By the time I get home, the TIPs will have arrived, I will swap them out and test the amp to capacity then.
My power supply at home is 35A, I think it might be getting a bit small, but i will test it just below clipping anyways.
Please post a photo of the board (overall) and a second photo of the area around the power supply driver IC. If you can take high quality photos (real camera), email them to me.
I'd suggest that you desolder R169, clamp all components to the heatsink tightly, insert a 15 amp fuse in the power line (or use a current limiter) and power up the amp.
Be aware that this will defeat much of the protection circuit (from what I can tell from the photos) so you have to be careful/observant.
Does the amp power up?
Do all of the channels produce clean audio?
So, I have 0v bias on channels 1 and 2, the TIPs are pulled on channel 3 and 4.
The amp fires up and idles fine, and stays out of protect.
I didnt fire it up as it was in the pics, I was going to desolder the TIPs cut legs, and started to pull it down for access. I have decided to pull it down at the time I replace the outputs to save losing parts.
I think I can leave well enough alone till I replace the TIPs on 3 and 4. Then I will retest it at idle and then take it home for full load tests.
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