Got a Yamaha AX-900U that wont come out of protection. If I measure the idle voltage i get -38vdc on the right channel and 0vdc on the left channel and neither side will adjust but I think thats because it is in protection mode. Wondering if anyone could take a quick look at the schematic and give some direction on where to start looking.
Thanks,
Phil
Thanks,
Phil
If you have turned the presets then I would set both to minimum resistance (you can't tell by looking whether that is clockwise or anti clockwise as it depends how the preset is configured). Doing this will force zero bias. The amp will work normally in that state if no others issues exist.
You should also use a DBT (dim bulb tester) when fault finding.
With the amp OFF check for any shorted output transistors and check the four 0.22 ohm emitter resistors and the four 4.7 ohm base resistors.
What you do next depends on the outcome of those checks.
You should also use a DBT (dim bulb tester) when fault finding.
With the amp OFF check for any shorted output transistors and check the four 0.22 ohm emitter resistors and the four 4.7 ohm base resistors.
What you do next depends on the outcome of those checks.
Background information. My son who is taking electronics class played with the amp. He said he was trying to set the idling adjustment to the 6mv and dropped his DMM lead on one of the main output transistors, seen a spark and shut it down. I was a little upset but we all have made mistakes. After that it stopped emgaging the relay. The idle measured .2 mv on one side and 40vdc on the other that's resistors R373 and r374. I decided to do a check on the unit and found several burnt resistors throughout the unit, I don't remember the number of each resistor location but can provide them if necessary.
Last night I measured Q328 and Q324 and ECB all measured 40vdc.
He said he dropped the lead across the leads of Q319. I replaced q319 with a like replacement as I have a whole AX-900 spare for parts.
Now I am at this point of not coming out of protection and the high voltage at R374
Last night I measured Q328 and Q324 and ECB all measured 40vdc.
He said he dropped the lead across the leads of Q319. I replaced q319 with a like replacement as I have a whole AX-900 spare for parts.
Now I am at this point of not coming out of protection and the high voltage at R374
40 volts across any of those resistors shows they are open circuit. You need to check all four because there are two parallel pairs of transistors and resistors per channel. Make sure you understand the term across which is not the same as measuring to the resistor from ground.
So as you say, we've all done things like this such as dropping something onto a live board...
Two parallel pairs means it should actually work, at least for testing with just one good pair fitted. I think you are looking failure of at least some of the output transistors and certainly the emitter resistors that have 40 volts across them. Its a classic failure mode. Then there is the collateral damage of other parts taken out by the failure.
So any combination of these and also the driver transistors. You can do basic checks with the amp OFF and seeing if any transistors read short circuit from C to E. Also check all those low value resistors on a low ohms range on your meter.
So ten parts to do basic static tests on:
I always use a DBT until I am absolutley sure just like the Nikko we fixed. I will get on that later as the wifey grocery shiopped and I got a slew of groceries to put up, 🙁.
My son feels bad but I have reassured him if he plans on getting into electronics/computer repair that this will certainly not be the last time a oopsie daisy occurs in repair.
On another note, I wil pull the old tranny's out of my parts unit and check to make sure the are operational.
Phil
My son feels bad but I have reassured him if he plans on getting into electronics/computer repair that this will certainly not be the last time a oopsie daisy occurs in repair.
On another note, I wil pull the old tranny's out of my parts unit and check to make sure the are operational.
Phil
I've done pretty much the same thing as your son... 🙄 Not dropping the lead but probing and slipping. Worst thing was that the parts were impossible to replace, so the unit is still in storage.
Doing the basic checks with the amp off. R374 and R366 measure open with meter set on low ohm reading. Measured R373 and R365 and get .6 ohms on each resistor.
Measuring C to E on Q328 and get .4 ohms. On Q327 get OL on the meter
Measuring C to E on Q328 and get .4 ohms. On Q327 get OL on the meter
Replaced Q328 with my spare unit and also R374 and R366. Now i get .2 mv across R374 and R373. DBT bulb gets very dim almost as if it is off. No relay engagement.
Stick to the known faulty channel. One pair of these resistors is in the left and one in the right.
This is all classic failure mode symptoms. R366, R368, R374 and R376 must all be 0.22 ohm. Remember your meter lead resistance adds to the total (check by shorting the meter leads together to see what that is).
You can begin with with just one known good pair of transistors and resistors. In other words one good upper NPN transistor and its corresponding resistor and one lower PNP and its resistor will get you a working amp.
If the amp will not come out of protection then check the DC offset on both channels at the output line which is the junction of the 0.22 ohm resistors.
Q328 is definitely failed (0.4 ohm), the other is inconclusive with just that one check but could well be OK.
DC offset at R376 and R374 measured -120mv relative to chassis ground. R373 and R375 measured 10 vdc
-120mv is not an issue in itself but is higher than might be expected for what is quite a sophisticated amp. In absolute terms though its not an issue.
10 volts shows a fault in that channel and there is no easy way to diagnose that without quite a lot of careful voltage checks.
10 volts shows a fault in that channel and there is no easy way to diagnose that without quite a lot of careful voltage checks.
Found fried resistor at R333. I was looking at the trim pots inspecting the areas because right after this occured I had found a slew of burnt resistors and replaced them. Just by chance I happened to see this one. The damage apparently was extensive
That is a problem... and I just wonder if this is going to be outside your comfort zone tbh. It needs extensive initial checks and then voltage measurement. For example copying the circuit and writing down the voltages measured on all the transistor nodes would be a massive help. That should show up many problems.
I look for B to E 0.6 v or -0.6 v. C to E 1 v or higher. Diodes 0.6 forwards or more than 1 backwards. You can check good transistor on other channel as you go. Capacitors non-zero. With the DBT installed rail voltages etc will be low, but should be consistent between the channels if parts are good.
Once an output transistor fries, rail voltage rampages back through a lot of previous parts that are not rated at 63 v. If you find replacing bad stuff mood calming, a useful practice. But not one that is in any sense economic. I replaced 140 parts in one amp, all the way back to the input op amp. Low voltage regulators in the power supply were bad too. I learned a lot that way. I did not have to buy everything, I salvaged a lot of diodes resistors etc from old dead PC's and stuff. Replacing all those parts is more brain stimulating than crossword puzzles, IMHO.
About the probe slip. I find a Pamona grabber has much smaller exposed metal and easier to be precise with than a inch long meter probe. Do not use a ground clip on a scope probe, they can get out of control. One does not need a local ground on a scope probe below 1 mhz anyway.
Once an output transistor fries, rail voltage rampages back through a lot of previous parts that are not rated at 63 v. If you find replacing bad stuff mood calming, a useful practice. But not one that is in any sense economic. I replaced 140 parts in one amp, all the way back to the input op amp. Low voltage regulators in the power supply were bad too. I learned a lot that way. I did not have to buy everything, I salvaged a lot of diodes resistors etc from old dead PC's and stuff. Replacing all those parts is more brain stimulating than crossword puzzles, IMHO.
About the probe slip. I find a Pamona grabber has much smaller exposed metal and easier to be precise with than a inch long meter probe. Do not use a ground clip on a scope probe, they can get out of control. One does not need a local ground on a scope probe below 1 mhz anyway.
You can do both as that will help you see problems. My preferred method on the forum is to see all the transistors listed clearly:
Like this. It has to be clear and easy to assimilate. Also be sure any minus (-) voltages are clearly marked. Two decimal places is enough. And don't be under any illusions given the history and all that has happened to it 🙂 There may well be multiple issues.
Q301
E= 10
B= 9.3
C= 21
Q303
E= 40
B= 40.55
C= 12.05
Q305
E=
B=
C=
All of these: