Need info on Yamaha J35 guitar amp power transistors

[fretzburner]

Hi, i got here a japanese made Yamaha J35 amp and i notice the sound is very weak.When i checked
inside the power transistors were replaced with different kind based on the heatsink marks.It has C3298/A1306 which is very low rating for a 40Wrms amp.Maybe somebody here have this amp kindly
check what power transistors installed.
Thanks,
fretz

[mth5044]

Have you tried emailing yamaha?

[R.G.]

Those are a little small for a 40W amp. The outputs having been replaced is a bad sign too.

I can't find the original schematic, but you can replace them with any of the following pairs:

TIP41C/TIP42C
BD244C/BD243C
KSD526/KSB596
KSC2334/KSA1010
Those all have bare heat sink tabs and will need insulating from the heat sink.
These : MJF15030/MJF15031 are the fully insulated packages and don't need special insulation.

All of them will need heat sink compound between them and the heat sink, or they will burn up too.

There may well be other things wrong inside if the first outputs burned out.

[PeterPan]

Though I'm not familiar with this amplifier, my knowledge of transistor circuits tells me that substitute  transistors in the power stage should make make as huge a difference in overall volume as you are seeing. A noticeable difference in tone, a tendency to distort prematurely, or a high temperature even when quiet are the kinds of symptoms I'd expect from badly matched substitute parts. But the overall weakness in signal you speak of suggests a bigger problem... one that the previous owner may have tried to address the same way YOU are considering.

In a case like that, without schematics, I'd consider a more radical but possibly simpler approach, which is to replace the whole power amp section of the circuit. If you do like the amplifier otherwise, this might be a possible choice. You need only consider the existing power supply voltage and current capability, and of course the existing speaker impedance, and from that you will be able to approximate how powerful an amplifier you could substitute. With any luck, you should be able to find a ready made power amp module. Look, for example, for some LM3886 based circuits.

Again, this approach may sound a bit radical. But without documentation on the existing amp, it is a possible way to spare yourself a lot guessing that may prove to be a waste of time and money. I have, in fact, gone this route more than once and in the end was glad I did.

[fretzburner]

I just got a message from the guy i bought this amp from, he said the technician doing the repair mentions
D526/B596 and it coincide with Mr.R.G.'s suggestion.I gotta try this pair.Hope i could source this easily.
@Peterpan Yes the sound is weak(no muscle) and .distort prematurely.I have an LM3886 ready made power amp and will use this as my last resort alternative.

Thanks a lot for all your knowledgeable suggestions guys, will post updates regarding this problem
soon.
Have a nice day!!
fretz

update:Just opened a J25 amp and output trannys are D526/B596 pair.J25 is rated 30W

[fretzburner]

Update:
installed D526/B596 transistors and same problem thin sounding and weak. as i check further i found
that the speaker cone is not moving.probably the magnet shifted , i can't even move the cone with my hand.I guess this is the main cause of my problem, the speaker but that output transistor needs to be replaced too and that's taken cared already..
Any tips on how to re-align the magnet? I don't have any experience yet with speaker repair.

thanks,
fretz

[PeterPan]

I've not had a lot of luck re-aligning speaker cones. But, magnets DO NOT shift position. So I would check and make sure there is no DC (or at least very little) going into that speaker. A short or severe circuit imbalance causing DC to go into the speaker would cause the voice coil's enamel insulation to melt, which is often the cause of a speaker seizing up. If that's happening, make sure you solver that first, testing the amplifier into a high wattage resistive load and checking for DC on the output.

After that, you'll have to decide whether the speaker is valuable enough to have it re-coned, and replace it if not. Not good news I know.   

[Rob Strand]

It is highly likely there are other issues (as RG already mentioned).

When you blow the output transistors (which has occurred in this amp) it is quite common to blow the driver transistors, ie. the transistors that connect the output transistors, and also the voltage gain transistor before the drivers.

I would be disconnecting the speaker then checking the DC voltage across the speaker terminals is less than say 100mV DC.   If there is a significant voltage say more than 1V then something is usually fried.   There usually no point connecting a speaker until the DC output is good.

After that check all the transistor voltage and see if they look OK.  If you don't know how to do this - get hold of a schematic of a similar amp with voltages marked to help.

Plenty of transistors will work in that circuit.

From what I can work out from on-line info the amp is 40W rms into 8 ohms.
The power rails are probably in the order of +/- 35V DC in the idle state.

[fretzburner]

Speaker terminal measured with digital multimeter direct without load reads 30v DC.No input signal and zero volume setting.

[Quackzed]

as i check further i found
that the speaker cone is not moving.probably the magnet shifted , i can't even move the cone with my hand.

i just did this with a speaker for use as a bass drum mic... the glue holding the magnet to the speaker baffle had failed i basically looked at the glue line on each side and marked it to line up, then gently (magnetic pull in mind) placed it to match its previous alignment then slid it 'slowly/gently 'up down left right till i got the voice coil to slide into its round slot, then more slight up down left right shifting till i got the coil to move freely without scraping, listening/feeling for where it was rubbing/ magnet too left up etc... just till i got it to where the voice coil wouldnt rub when pushing the cone in/out... when i got it i traced thin sharpie around the magnet on the basket close as i could, and 4 lines to line up rotation of magnet/baffle alignment, then separated/ glue applied and did it all again with the help of the guide lines... took a bit but i got it working again... left it to dry... ymmv some voice coils are less robust and wont take this kind of abuse, but you cant ruin a ruined speaker so no way to go but up. good luck. it took a bit of time...

[PeterPan]

Speaker terminal measured with digital multimeter direct without load reads 30v DC.No input signal and zero volume setting.

So now you know what fried the speaker.

[Rob Strand]

I suggest checking the transistors in circuit, starting from the output move back towards the input.
- using dmm diode checker vbe ~ 0.6V, and vce ~ 0.6V
(remember to reverse the meter leads or NPN vs PNP)
- using ohms check no shorts from collector to emitter

You can do this to some degree with the power off and in-circuit.
If you get weird readings remove the transistor and check again out of circuit.
As soon as you find a bad part pull it out to prevent false reading on the other transistors.

This will help identify blow transistors.

Also write down the transistor p/n you removed from each location.