Crest LA901 red status light on CH A

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#23
Outputs and drivers are the same, four outputs and one driver per phase (five bigguns total). The one with brass colored clip is probably the bias xsistor. The TO220 is the predriver. Look on the schematic for anything that if shorted would connect the +rail to the output, D5 for instance.

Craig
 
#24
First of all, thanks to you both for all the guidance you both have provided !

Last night I checked D5 and D6 with the Fluke in diode mode and both appear to be fine. It would be nice if the components were identified on the PCB 🙂

D5 .41/inf
D6 .41/inf

I also went thru all the diodes (in diode mode) and resistors (in ohms mode) and all check ok, Im looking for opens or shorts right ? Pretty much all the values were as stated on the schematic.

Regarding which HS is Channel A, I dont know, how can I verify ?

Is there any chance that the op amps or the two other IC's could be causing this ? Or have I made some earlier error that has lead me in error to this point ? Or how about the last board that looks like the power supply ? There are no signs of physical damage that I can see after careful examination with a light and magnifying glass......

Thanks again !


Rick
 

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#26
Greetings,

Other than the transformer, all PCBs have been removed, dust busted and physical inspected, but even after a side by side Fluke transistor, diode, and cap (didnt lift one side) tests, everything matched between the two heat sinks. No obvious opens or shorts. Does one really need to lift one of the legs to see if the cap is open or shorted ? The fluke pretty much matched except for C3 one of the Power Supply caps read @ 1/2 of the 7300uf units, but logic tells me that if it was that why is only one of the channels showing issues..... ?

The only thing I did notice was when I removed connections for the Status and Clip the wires are just press ins. Not like the rest of the keyed white natural poly inline type connnections. Maybe a loose connection, do you feel lucky, I hoping....

So Im gonna slap it back together for grins and giggles as an old boss once said....if it doesnt work I'll start pullings HS caps unless you mention some other things.....

still dont know which side is A, left or the right HS

Mag
 
#28
Same results :-(

Well its time to pull out the HS's again, Channel A is still red ! Is there anyone that knows which side is A ?

Its time to start pulling one by one the caps that line the rail that leads to the ground rail, as it has to be one of those thats causing the PS voltage making to the output.

Think we've pretty much eliminated the output transistors right ?

Well it looks nice and clean 🙂, now if it'll sound as good as it looks ! After it gets fixed.

Again thanks to you guys for all your help !

Rick
 
#29
One channel is still good, right? Can you run the amp until the good channel HS gets warm? Then you'll know. Otherwise trace the wiring out to the speaker jacks. Also are both HS assys. the same physically? Try swapping the assys to see if it is the assy or not.

Craig
 
#30
Well after reassembly the status for Channel A is still red.

Hear the relay/s clicking, but still the same prob.

Regarding switching the two HS's, didnt think @ that. Looks like this might work, at first thought that the pins that attach to the middle board would not be the same but after looking at it, I'll give that a try, and see if it follows the HS......


Thx again !
 
#31
Well after swapping the two HS around, the problem stayed on Ch A, which states the HS assm does not appear to be the issue.

Now where ?

Either the PS board which has the circuitry for the status or the one of the two other boards. What @ the 65V measured on the output ?
 
#32
Look at the LAOPTO schematic, do you see IC2? Check the outputs 10 & 11 for CH.A and 5 & 6 for CH.B. I'm not sure if there is suppose to be DC there or not but they should be all the same. I thought at first IC1 might cause a DC offset but it is capacitor coupled to IC2.

Craig
 
#33
I think your IC2 is going to be bad, latching up to either supply seems to be it's modus operandi. Check all resistors around that IC, might as well ck everything, there's not that much. MCM Electronics has STK3152s for $14.51, you might try Crest though.

Craig
 
#34
I think your IC2 is going to be bad, latching up to either supply seems to be it's modus operandi. Check all resistors around that IC, might as well ck everything, there's not that much. MCM Electronics has STK3152s for $14.51, you might try Crest though.

Craig
Click to expand...

I checked every single component on all boards so I ordered the Hybrid IC from Peavy/Crest for @20 bucks. I ordered a varistor from them last Tuesday for a Peavy monitor and had it sent standard mail, and it hasn't arrived yet. So I don't expect the Sanyo chip till late next week.

Tried calling Sanyo engineer in San Diego and they tried to get me one from Japan, but no luck..... to old they said....
 
#35
i will just bet you will find bad capacitors are the cause. I have about a dozen crest amps under my belt now. not that specific model but the VS, CA and many of the pro series amps and 75% of them have had bad caps that cause it to stay in protect. in fact every CA series that crosses my bench gets new small value EL caps on the PSU-reg/protection board.


Zc
 
#36
Old thread I know but I have one of these on the bench now. Started with Channel A in protect, checked output before the relay and it's OK, good signal and NO DC offset. Found a shorted electrolytic in the protect circuit not letting relay drive circuit to function. Replaced all four caps in the circuit and we're good to go.

Craig

The culprits were "Surge" brand caps from guess where?
 
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