Yamaha Amp troubleshoot

[Prattacaster]

I have a Yamaha G-100 410 that was blowing fuses. It is an all solid state 100 watter. When diagnosing I was pointed towards the output board. So I pulled the output transistors out and the fuse held. So I put the output transistors back in, but didnt bolt them down with the mounting screws, and the fuse again held, Then I plugged my guitar in and it played! So I bolted the output transistors down and then the fuse blew. So I pulled the output transistors back out repalced the fuse and then I got a light show! A Few resistors went up in a blaze of glory. Here is the schematic, the toasted resistors are circled. http://s1122.photobucket.com/albums/...series_I2.jpgI I should also mention I noticed that the screws that mount the output transistors had eventually worn thru a couple coppers traces, Two of them being the B+ supply, so right there is probably the original cause of all this, ground being shorted to the B+ at the output transistor. The amp was working fine for 30+ years until the fuse blew the first time. Remember that the resistors caught fire without the Output transistors in. Questions are:
1) What other components are possibly ruined?
2) How do you test the PNP transistors and Output transistors?
3) How does one go about finding direct replacements?
4)This amp is only worth 200bucks in working condition, Should I just cut my losses,since I wasnt thrilled about the amp anyway. (too heavy to lug around, only does clean sound)

[Jazz P Bass]

A word of caution.
Do not, ever, run power transistors without a heatsink of some sort attached to the case.
They will blow from the heat.
I think that you may have had a transistor popping through the mica insulator that is in back of the transistor.
The mica keeps the transistor case (the collector) away from the heat sink which may be at a different potential.
note: some amps do not use them IE: grounded collector or the heat sink itself is isolated from the chassis.
I think you blew the transistors when you ran the amp without the transistors being attached to the heatsink.
They probably failed when they cooled down.
You can find all of the transistors at BDent.com
B&D Enterprises - Electronic Components Distributor

[olddawg]

If you do not have a variac you are going to have to check every resistor in the output stage plus the drivers, outputs, and any diodes. If it is 30 years old I wouldn't trust any electrolytics either. When you put it back together use new micas and heat sink compound. If the transistor screws have isolation washers make sure you use them. Since you do not have a variac, use a light bulb limiter when you turn it on after the repair. There is also probably a bias adjustment. It's a good idea to replace the old bias pot with a new one. Measure the resistance of the old one out of circuit and set the new one to that reading to start. If the old one is bad set it in the middle.

[Prattacaster]

From what I gathered online, my DMM may not be able to test the transitors in the circuit due to their voltage drop being over 1v. I can test most diodes but not transistors and LED's. Is there another way to test? Do I have to test them out of the circuit. There were three different trim pots. The "idle current" trim caught fire, the rest are within spec, All but two resistors were within 1% spec.

Most of the transistors on this board are obsolete now, and to replace them might be expensive. At what point would advise that the amp meets the trash can. The cab is in great condidtion so I would keep that but if this isnt an easy fix that I can do myself I think its time for the amp to go. My own thoughts....others?

[nashvillebill]

For an experienced repair person, this would likely be a fairly easy fix (it'd still take several hours of labor though). For somebody who is learning as they go, this could appear very daunting.

The "brute force" approach would be just to replace most everything on the power amp board. If someone is limited in test equipment and not experienced in amp repair, this may actually be the easiest approach--I doubt the LTP transistors are damaged but replace everything downstream of the LTP--everything around the outputs including all electrolytics in the amp. I don't remember what the output transistors were when I glanced at the schematic but I suspect common low-cost outputs could be substituted. Probably for $50 to $75 of components most everything could be replaced (buying online from Mouser).

Another approach would be to substitute a chipamp for the output amp board, you'd be limited to about 60 watts though.

Paying a repair shop to fix it would be more costly than the amp is worth indeed.

[teemuk]

Don't replace with chip amp. The G-100 power amp has designed-in soft clipping, chip amps won't sound the same.

[Prattacaster]

Thanks for the replies. I have some experience in tube amp repair. NONE in SS, I dont know the lingo either. I knew enough to track where the problem is, and now i'm looking at replacing all of the transistors and diodes because my lack of testing equip. Can anyone tell me what diodes are being used in this circuit. The one that is labeled T5 51 has "4D" stamped on the actual physical component and was also mounted to the heatsink. Is this a 4A lower voltage(75-120v) diode? The other diode is just a very small switching type. IS it practical to find exact matches for all these components, or can I use cross-referenced items and still keep the samll value this amp has? The Output transistors had been replaced once with NTE388 which after looking are in fact the right replacements at half the cost of the original 2sc1586.

[Jazz P Bass]

I would not do a wholesale replacement of the parts.
#1: you risk damaging the solder pads.
#2: you do not know the replacement values. So if you buy an iffy item & the circuit does not respond well to it, where are you then.
Replace the items that failed.
As I posted,BDent has the transistors.
Forget NTE.

[Prattacaster]

Big Thanks for the link to that supplier, and all who responded.