Legend A30 humming

[52 Bill]

I also tested CR61 CR17 CR21 CR22 CR23 CR24 the same way and all read 0.5 and 1.3 on the reverse

Okay, good to know.

Now on to the transistors. Each transistor has three leads. They are kind of like a diode with a third connection. The three terminals are the Base, the Collector and the Emitter. You will test the transistors just like you tested the diodes, by connecting the red lead to a terminal (the Base) and the black lead to the other two terminals (the Collector and the Emitter) and then reversing the meter leads to see what readings you get.

Start with the two large transistors that are mounted to the chassis Q13 and Q14 the output transistors. Facing the transistor with the leads closest to you, the left lead is the Base the center lead is the Collector and the right lead is the Emitter. These leads are bent up from the transistor case and are (or should be) poking up through the pc board. Touch the red meter lead to the left lead (the Base) and then touch the black lead to the the center lead (the Collector) and read the meter. Next move the black meter lead to the right lead (the Emitter) and read the meter. Reverse the meter leads and perform the same tests. Finally place one meter lead to the Collector and one to the Emitter and read the meter. And to finish up, reverse the meter leads and read the meter.

What we expect to see is a high/low reading from B to C and B to E, just like when you tested the diodes. We also expect to see a high only reading from C to E. There are resistors in the circuit that will cause the meter readings to seem wrong, but what is important here is that there are no direct shorted connections or other anomalies.

Next you will test Q12 and Q15 the drivers in the same way. I believe that the drivers are a TO-220 cased transistor with a plastic square shaped body mounted to a metal mounting tab. With the body of the transistor facing towards you leads down to the board, the leads are B-C-E. Do all of the same tests to these transistors.

Let us know what you find.

[bfglp]

Q14 red to base black to collector .480v black to emitter .096v reverse .574v and .096v -- red to C & black to E .646v reverse .499v

Q13 red to B black to C .078v black to E .075v reverse red to C .078v red to E .075v -- red to C & black to E .004v reverse .004v

[52 Bill]

...red to C & black to E .004v reverse .004v

This reading is suspect. The next step would be to test this transistor out of circuit. Do you have desoldering equipment?

[bfglp]

Q12 red to Base black to C .635v blk to E .085v reverse nothing to C and .085v to E -- red to C and blk to E nothing reverse .631v

Q15 blk to base red to C .625v red to E .572v reverse nothing to C and .479v to E -- red to C and blk to E .625v reverse 1.825v

[bfglp]

This reading is suspect. The next step would be to test this transistor out of circuit. Do you have desoldering equipment?

yes, i can pull it tomorrow

[52 Bill]

Q12 red to Base black to C .635v blk to E .085v reverse nothing to C and .085v to E

This is off too, but retest after you remove the output transistor.

Do you have a lightbulb limiter? Have you tested the resistors in the power amp section?

[bfglp]

Q13 removed and tested as red to B blk to C.282v blk to E .278v reverse red to C .300v red to E .297v -- red to C blk to E .005v reverse .005v

Q12 retested red to B blk to C.636v blk to E 1.811v reverse .OL & .OL . red to C blk to E .OL reverse .625v

[bfglp]

This is off too, but retest after you remove the output transistor.

Do you have a lightbulb limiter? Have you tested the resistors in the power amp section?

retested Q12 results above ^
i dont have a lightbulb limiter. i have not tested resistors in the power section yet.

[52 Bill]

Q13 is shorted and needs to be replaced. Q12 is still suspect, so remove it from the board and retest.

Go through the power amp section and test all of the resistors with your ohm meter. The ballast resistors that connect to the output emitters are always suspect when you have a shorted transistor. While you're at it, go through the rest of the power amp and test all of the remaining transistors. I think that there are 5 more little ones in there. It will be good practice for you and will make for one single order of parts if there are any other bad ones in there.

To replace the shorted output transistor, you will need some heat sink compound and will need reuse or replace the mica washer and screw insulator washers.

[bfglp]

i pulled Q12 and retested it failed there is a second Q12 near Q11 they were suspect so i pulled them too and they failed, the remaning transistors were OK

what are the resistor #'s that you think may be suspect? and what is your method of testing i read on line that one leg should be lifted??

i researched the lightbulb limiter, and already have the parts to build one. great idea!

[bfglp]

resistors R66 i get 00.5 and R51 i get 1.020 both are marked 0.33ohm The rest tested close to there values on the schematics

[52 Bill]

resistors R66 i get 00.5 and R51 i get 1.020 both are marked 0.33ohm The rest tested close to there values on the schematics

Yes, the ballast/emitter resistors can be damaged when a transistor shorts out. Touch the two leads of your meter together and read the resistance of the leads themselves. Subtract this resistance from your readings of the low value resistors. The .5 ohm value is probably correct when you subtract the resistance of the leads.

The one that reads 1.02 will need to be replaced.

If you have checked all of the resistors and transistors in the power amp section, you can probably safely assume that you can replace the dead stuff and move on to the next step. You may want to consider replacing both outputs and ballast resistors at the same time.

[bfglp]

If you have checked all of the resistors and transistors in the power amp section, you can probably safely assume that you can replace the dead stuff and move on to the next step. You may want to consider replacing both outputs and ballast resistors at the same time.

i finally got all my parts lined up and some time to install them. fired it up and plugged in, it sounded great for a few min.... i was adjusting the tube gain drive and the hum came back, guitar volume droped then some smoke popped up i quickly shut it down. and found that R54 burned up.

[52 Bill]

i finally got all my parts lined up and some time to install them. fired it up and plugged in, it sounded great for a few min.... i was adjusting the tube gain drive and the hum came back, guitar volume droped then some smoke popped up i quickly shut it down. and found that R54 burned up.

Sorry to hear that. Tell us exactly what did you replace? Did you power up straight or with a light bulb limiter?

You will have to start testing parts again.

[bfglp]

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.