SX 780 Won't Come out of Protection

treetech said:
Powered on and measured Q21 E +8.0mv C +52.9v B +52.8v (definitely different reading than my other Q's)
Click to expand...

markthefixer said:
It is probably that the zener was duff as well as the now replaced Q19 and Q21.
Click to expand...
^== Heed this WARNING

DANGER: Q21 B read +52.8vdc with E of +8mv when you powered up last when D4 was bad. Way too much for a BE junction. Test Q21 again (I would also retest Q19) BEFORE powering up again. Don't fry any more parts if possible.
 
Whoops, I missed that. (grumble grumble) my head is too deep into this Luxman PA right now.
yes, bench test (out of circuit) Q19 and Q21.
 
MTF, Q21 is an original.
Merlynski: Results of diode testing:
Q21: everything was OL except: Black to B, Red to C .013v, Red to B, Black to C .011v (BAD!??, voltage too low? and not the right connections to get voltage for an NPN?).
Q19: Voltage readings on every connection except two, so that makes it BAD.....should have only had voltages on 2 of 6 connections. (I did check old regulator pulled out and it read GOOD.)
Q20: No conduction on any legs at all....BAD...( I did check the old regulator I pulled out and it read as GOOD.
Q25: It read GOOD: voltage on Red to B, Black to C .582v, Red to B, Black to E .583v, no voltages on any other connections. (Old regulator also tested good, similar voltages on same connections).

Tested C311 on ohms and the values slowly counted up......it did not read as 0 ohms. Sound good?

With your help I know D4 is shorted and I'm waiting on replacement. Will have to order a Q21 (C945) . Will probably use old regulators at Q19 and Q20 unless that is a bad idea as they tested fine.

I am a little disappointed and baffled that some of the new replacements regulators are bad and old ones are good...….
 
@treetech Here is a link to a method for 6-way testing transistors that may help your understanding: Bipolar Junction Transistor Testing Basics
I would replace any transistors I had removed with new modern transistors, sometimes the meter diode test says they are good but they still fail under full voltage and load. This becomes a circlular cycle of failure. I look at the diode test as an easy way to find the 'bad' ones, if they test bad they are bad, but if they don't test bad they still may have failed or been over-stressed. Small transistors are cheap, when you place an order buy extras and save shipping by avoiding repeat small orders. In the voltage regulators like Q19,Q21, D4 I replace all three with new at one time. Leave Q25 out for now. As MTF said earlier most of the power supplies depend on the Q19,Q21, D4 regulator to be working properly first, as it is the reference for the others.
 
Here is a BOM with caps and transistors for an SX-780 rebuild. It includers Mouser part numbers, may save you some time, but may need re-vetted. Does not have any diodes in it.

ASSY, Capacitance, Voltage, Type, Quantity, Identifier, Mouser number, RPL values, RPL w/Film?, Comment
GWK-118, 0.22, 50, CEA NL, 2, "C227, C228", 647-UKT1HR33MDD, 0.33/50V, y, "RPL Cap Val Does Not Match, Non-Stocked, 2mm lead spacing on rpl cap"
GWK-118, 0.47, 50, CEA, 4, "C401, C402, C403, C404 ", 647-UPM1HR47MDD, 0.47/50V, Y?, "Non-Stocked, 2mm lead spacing on rpl cap"
GWK-118, 1, 50, CEA NL, 2, "C207, C208", 647-UKL1H010KDDANA, 1/50V, y,
GWK-118, 2.2, 50, CEA NL, 8, "C103, C104, C117, C118, C229, C230, C309, C310 ", 647-UKL1H2R2KDDANA, 2.2/50V, y,

GWK-118, 4.7, 25, CEA NL, 2, "C217, C218 ", 647-UPW1E4R7MDD, 4.7/25V, y,
GWK-118, 4.7, 35, CEA, 7, "C201, C202, C324, C325, C326, C327, ", 647-UPW1H4R7MDD, 4.7/50V, (+unmarked position just to the right of C309 and the value was 4.7 uF/35 volt.
GWK-118, 10, 16, CEA, 2, "C318, C405 ", 647-UPW1C100MDD, 10/16V,
GWK-118, 33, 16, CEA, 1, C317, 647-UPW1C330MDD, 33/16V,
GWK-118, 47, 6, CEA, 2, "C213, C214 ", 647-UPW0J470MDD6, 470/6.3V,
GWK-118, 47, 16, CEA, 1, C319 , 647-UPW1C470MDD, 47/16V,
GWK-118, 47, 35, CEA, 2, "C305, C306", 647-UKT1V470MDD, 47/35V,
GWK-118, 47, 50, CEA, 1, C316, 647-UPW1H470MED, 47/50V,
GWK-118, 100, 25, CEA, 4, "C301, C302, C303, C304", 647-UPW1E101MED, 100/25V,
GWK-118, 100, 50, CEA, 2, "C307, C308", 647-UPW1H101MPD, 100/50V,
GWK-118, 220, 6.3, CEA, 1, C406, 647-UPW0J221MED, 220/6.3V,
GWK-118, 220, 16, CEA, 1, C311, 647-UPW1C221MPD, 220/16V,
GWK-118, 220, 63, CEA, 1, C315, 647-UPW1J221MPD6, 220/63V,
GWK-118, 470, 6.3, CEA, 2, "C105, C106", 647-UPW0J471MPD, 470/6.3,
GWK-118, 470, 63, CEA, 1, C314, 647-UPW1J471MHD6, 470/63V,

Transistors
ASSY, Original, Type, Quantity, Identifier, Mouser #, Mfg Number, Match Pair, Comment
GWK-118, 2sc1775, NPN, 4, "Q5, Q6, Q11, Q12", 512-KSC1845FTA, KSC1845FTA,
GWK-118, 2sc1885, NPN, 2, "Q13, Q14", 512-KSC2383YTA, KSC2383YTA,
GWK-118, 2sa912, PNP, 2, "Q15, Q16", 512-KSA1013YBU, KSA1013YBU,
GWK-118, 2sd325R, NPN, 1, Q19, 512-KSC2073TU, KSC2073TU, 863-MJE15032G
GWK-118, 2sb536L, PNP, 1, Q20, 512-KSA940TU, KSA940TU, 863-MJE15033G
GWK-118, 2SD712, NPN, 1, Q25, 512-KSC2073TU, KSC2073TU, 863-MJE15032G
GWK-118, 2sc1384, NPN, 1, Q26, 512-KSC2690A, KSC2690A,
GWK-118, 2SC2291, dual NPN, 2 (4), "Q9, Q10", 512-KSC1845FTA, KSC1845TFA, Y, bent leads
GWK-118, 2sa979, dual PNP, 2 (4), "Q7, Q8", 512-KSA992FBU, KSA992FBU, Y,

Note: This is a collaborative work with input from many AK members including MTF, Echowars and MattSD
 
Last edited:
Merlysnki, makes good sense to go new on the transistors..... will get them coming this way.....Thanks to you, MTF and others the clouds blocking how this electronic stuff works are starting to lift...will report back when I have Q19, Q20, Q21, and D4 (14v Zener) replaced.....
 
Merlynski, MTF......I see Mouser replacement transistors listed for Q19, Q20 and Q25, and I have those now in hand, along with Zener D4. I must have missed ordering Q21....but looking at the replacement list above.....no Q21.....also no Q21 on service manual pages 39 or 40 of parts list (all other transistors are there). On page 24 of SM there is an "external appearance of resistor" showing it as BCE for the 2SC945A transistor I pulled out. I ordered this on Amazon: " Hilitchi 675-Piece 15 Values 2N2222-S9018 NPN PNP Power General Purpose Transistors Assortment Kit ". It has C945 NPN transistors.71MzmJB35EL._SL1000_.jpg . I was wondering whether this will do it for a Q21 replacement? Also....once in hand.....do I replace the Zener first, then all the other transistors before powering up....or do I replace the D4 Zener then replace one transistor at a time....dim bulbing each addition? Thank you gentlemen...
 
If it actually is a 2sc945 in your assortment give it a try.
Or try this one I found in a quick search: Q21, 2sc945, 512-KSC2383YTA
Since bad semiconductors that are connected together, especially in a power supply, tend to fry each other I would replace Q19, Q21, D4 and Q25 all at the same time.
 
Well after changing out D4, Q19, Q20, Q21 and Q25 and also C309 and C311 turned it on with dim bulb no click but no smoke or fire either. So went to straight wall power and set about checking the DC offset and found I had 25+ volts on pins 6 and 9 (not good). Soon thereafter I heard a small pop and saw some smoke...……. I really appreciate the insight and patient help, but realize I need to give up on this one. I have gotten some good experience with soldering, desoldering using the service manual diagrams etc. Would not have attempted it without this forum and the generous assistance, particularly Merlynski, MTF and Watthour......
 
You got ahead of us, there was more to check before going to full wall power. I would be glad to continue to help you. Repairing an amp is a step-by-step procedure that must be done carefully in the proper order. So far every unit I have helped on has been a success, as long as the owner does not give up. I wish you well . . .
 
You must log in or register to reply here.
Back
Top Bottom