Legend A30 humming

[52 Bill]

i replaced Q11, Q12, Q13, r66, r51 recently, main power caps were replaced earlier

powered up with a light bulb limiter. was on for 5min. or so before it malfunctioned.

R54 is directly related to Q10, so check Q10. Also check everything that you added to the circuit. Hopefully the limiter saved the new parts, but check to be sure.

What did you use as replacements for the transistors?

[bfglp]

transistors were direct replacements. Q12 a GE had an added T to the end of the series.

Q10 tested as Bk to base R to C .629, R to E 1.649
....................R to base Bk to C .1.548 Bk to E .565.........R to C, BK to E .623 reverse .OL

next to Q10 i replaced Q11 & Q12 .............Q11 tested as BK to base R to C .OL, R to E .OL
....................................................R to base Bk to C .665, Bk to E .966 and kept climbing slowly?......R to C, Bk to E .OL reverse .649
Q12 tested as Bk to base R to C .OL, R to E 1.272
....................R to base Bk to C .635, Bk to E .OL...............R to C, Bk to E .OL reverse .661

[Jazz P Bass]

Q10 is toast.

[52 Bill]

transistors were direct replacements. Q12 a GE had an added T to the end of the series.

I'm at a loss here, can you give us some actual part numbers?

Some of your transistor readings are not making any sense to me. Some of it could be circuit interaction and some of it could be wrong transistor leg identification on your part. Please double check the transistor legs and case styles. As before, you may need to remove the transistors from the board to get an accurate test.

In testing parts, you will need to retest all of the transistors, diodes and resistors in the power amp section, not just the ones around Q10.

And before you power up again, disconnect the speaker. Next time start with the limiter just to read voltages and to see that the circuit is stable before adding the output load.

[Jazz P Bass]

Kind of what I was thinking when I saw the post.

Blindly firing up a just repaired amp is simply asking for trouble.

A few voltage measurements, even with the lamp limiter, would tell if the amp was stable or not.

Apparently it was the later case.

[bfglp]

looking back I'm certain i reversed/forgot where the base side was.
there are two Q12 transistors one is 4249 202/pn ( this is next to Q11 that i recently tested ) and the other GE D41E5 i replaced this one with a GE D41E5T
Q11 is a 4249 202/pn
Q10 has a round shape, so what side is the base on?

[Jazz P Bass]

Here is some info that may help.


Transistor Connections

The best thing to do is to look up the specific components datasheet.
On there you will find the pinout diagram

[bfglp]

thanks for the info, i found some more here Datasheets & application notes - Datasheet Archive Search Engine

[bfglp]

Q13 a TIP33A red to Base bk to C .470v bk to E .099 reverse r to C 1.644v r to E .099v ............r to C bk to E 1.610 reverse .480

Q14 a TIP33A r to B bk to C .428 bk to E .097 reverse r to C .587 to E .097 ..............r to C bk to E .656 reverse .503

Q15 a GE D41E5 r to B bk to C .627 bk to E .627 reverse r to C 1.859 to E .OL ..............r to C bk to E .576 reverse .480

Q12 a GE D41E5T r to B bk to C 1.646 bk to E .001 reverse r to C 570 to E .001 ...............r to C bk to E .474 reverse 1.644

Q10 a RCA 40408 r to B bk to C .620 bk to E .584 reverse r to C .OL to E 1.547 ................r to C bk to E .563 reverse 1.645

Q11 a PN4249 r to B bk to C .OL to E .OL reverse r to C .664 to E .648 ................bk to E r to C .949 & climbing reverse .OL

Q12 a PN4249 r to B bk to C .OL to E .OL reverse r to C .657 to E .635 ................bk to E r to C 1.166 & climbing reverse .OL

[52 Bill]

Well, Q12 the D41E5T appears shorted, some of the others are off, but you need to either remove them or at least lift a leg to retest.

Have you tested the diodes and resistors yet?

[bfglp]

i was informed by a vendor that the D42E5t replaced the D14E5 and I'm finding out that both are hard to find. is there a newer version of this transistor?
or a forum to cross reference transistors?

i'm going to test the diodes & resistors today

[52 Bill]

i was informed by a vendor that the D42E5t replaced the D14E5 and I'm finding out that both are hard to find. is there a newer version of this transistor?
or a forum to cross reference transistors?

i'm going to test the diodes & resistors today

Yes, the D41E5 is an obsolete part and because of the case style TO-202, very hard to substitute unless you can fit in a TO-220 style case in there. The legs are in different positions than the original ones, so the transistor will need to be inserted into the board with it's legs bent to match the board.

How do the transistors mount to the board, are they flat on their backs or standing upright? Are there heatsinks attached?

[bfglp]

Yes, the D41E5 is an obsolete part and because of the case style TO-202, very hard to substitute unless you can fit in a TO-220 style case in there. The legs are in different positions than the original ones, so the transistor will need to be inserted into the board with it's legs bent to match the board.

How do the transistors mount to the board, are they flat on their backs or standing upright? Are there heatsinks attached?

they stand upright and the heatsinks are attached to a diode with a cable ti

[bfglp]

i tested all of the diodes on the board lifting a leg when i found an abnormality; multi meter set on diode test Red to anode Bk to cathode all readings were between .515 & .588............leads reversed .OL

[52 Bill]

Upright can work, maybe you can install something like a TIP31C or a TIP32C in place there. The leg order is different but it should fit in there.

You can still mount the diodes in contact with the metal tabs.