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Allrighty got everything swapped. Made a small Heatsink for Q10. But im still showing a short with the bulb current limiter. Q16 Q17 and Q14 are all showing measurements of 0.8ohms to ground on the Collectors. The schematics all show the collectors going to the +rail. So C20 is highly suspect and so is C3 but we know c3 is ok. I cant see anything else
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Take the two screws out of the two outputs in question, and lean the transistors away from the heat sink wall. Now see if your +57 is shorted to ground still. You might have a failed or missing insulator behind one of the transistors. A cap can short, but either of those two being shorted would be Very rare. Having said that, it is simple to unsolder them and lift them out of the circuit to see.Education is what you're left with after you have forgotten what you have learned.Comment
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Bingo I did exactly that. unscrewed and pryed away from silicon pad. no longer shorts. It looks like i He-man tightened the screws and it created a small lip of metal thats getting through the silicone pad!!Originally posted by Enzo View Post
So for clarification after i get the short finalized I will check both power supply rails for the +/- 56V and once thats confirmed i can attached a load and cross my fingers? ha i mean measure the output voltage?Comment
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Once the amp is stable and no DC on the output, only THEN put on a load.Education is what you're left with after you have forgotten what you have learned.Comment
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Am i looking for DC on the B, C and E's of the output BJT's or just the Emitters?Originally posted by Enzo View Post
thnx again Enzo!!!
I learned alot this round with SS electronics
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On the speaker output, which is just about the same as any of the Emitters of the 4 output devices.Originally posted by ThisLifeILead View PostComment
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allrighty, i measures no DC voltage on the speaker outs. well i got -115mv of dc. So i guess its speaker hookup time and time to watch for heat on the mpsa92 Q10. how long should i leave it powered on before i check for heat? do i need to play a bass through it or is idle fine?Comment
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Oh always let an amp idle for a while. if it overheats at idle, it will surely do so when playing. How long? I'd be checking suspicious parts immediately. 100mv is just fine.Education is what you're left with after you have forgotten what you have learned.Comment
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Well i let it run with a resistive load for 30mins and found no heat issues. so i hooked up the speaker and tried it....NO SOUND! ah fudge i guess its time to take some voltage measurements.Comment
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the signal coming into the output board is good. all voltage rails are present.
So i started at the front end of the output board.Q1 collector (Q2 base) is TP6. (-10vdc) im getting nothing. But on the opposite (anode) side of D13 i have the -15v rail. Also at TP7 (-40.2vdc) im getting the full (-)rail -56vdc... i checked all the resistors and diodes in the area and found nothing bad. Will have to pull and measure Q1 and Q2.Comment
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Found more bad transistors. Q9 and Q13 but i need to find an equivalent again for a non stocked unit. 2SC2362K. the NTE equivalent im told is NTE199 but the specs dont add up to me. its only rated for 70v not 120v.
I found ZTX614 at digikey and its rated for 100v.thats probably more then enough im thinkingComment
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OKAY! got it stable without a load attached. I swear i was chasing my tail for alittle while there with shifting blowing transistors.
BUT i got +40vdc on the speaker output. i replaced this last batch Q10, Q11, Q16, Q17, Q18, Q19 and Q14 to get where i am at now. potentially Q9 or Q12 are the trouble makers? I will have to re check the unchanged transistors. From alittle research i did, if DC is present on the output and its not blowing fuses its probably the driver transistor? but being a SS newb how does one identify the driver?
i really do appreciate the help guys! im so close i can taste it :-)Comment
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Q14 'drives' the + output Q16/17
Q15 'drives' the - output Q18/19Comment
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Gotcha! thanks Jazz! So the drivers are directly coupled to the output Base pins!Originally posted by Jazz P Bass View Post
Is that a safe assumption then that Q14 would be a suspect ? (for example if we saw -Vdc on the speaker out we would suspect Q15?) i will have to double check thats one of the ones i replaced. are there any other reasons to see that much DC on the speaker out? I replaced one of the fractional resistors on the outputs 0.15ohms 2W. should i replace the rest (i ordered enough to replace them all but only replaced the blown one) i was doing some trans voltage measurements and i shorted my lead across Q6 which blew one of the fraction resistors and lord knows what else :-PComment
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