JVC RX-6042 5 secs then shuts off

[scampo77]

i found a open darlington pair on one of the amp boards and these pieces i cant get locally. i have had to order them from ebay so ill be a week before ill post again. i dont know if i have damaged something because now after i pulled out that amp board and tried to power up i am blowing the 8amp fuse on the board labeled LVA10209-C9.it is a fuse located right where the mains enters its even before the liniear transformer. im not overly concerned about this because i am suspecting it has to do with the missing board but i thought it was worth a mention in case i should check something else while i am waiting for parts

and R1355 is NOT open and flowing in one way only

[999999999]

Removing a board is not likely to blow a fuse (by itself), more likely the amp was repetitively powered with a shorted condition which kept putting thermal stress on the fuse until it blew, OR the damaged part has simply gotten worse.

Something recommended by some who build and repair AC mains powered gear is to use a current limiter during testing. It can be something as simple as a 60W incandescent light bulb wired in series on the AC hot lead, people build test jigs on a board with a power cord, light bulb, and an AC outlet to more easily put it in series with the gear being tested by not having to alter the gear's AC power circuit itself.

[scampo77]

i didnt think that removing the board made any sense to my new fuse problem.

your first hypothosis is invalidated due to the fact i melted 3 fuses in 3 mins after the first failed.

the lightbulb idea is great, low tech, adjustable, easily diagnosed, effective. ill dig around for a light bulb socket

as for now is there anything else i can check to stop this from happening?

[scampo77]

after many trails i am back and think i am close to getting this thing figured out. my fuse problem was the output case touching on the heatsink, i did not realize the case was connected to one of the legs. i have found 2 sets of darlington pairs shorted and have replaced them only to have one fail right away afterwards. on the third install it went up in a blaze of glory with flames and everything. although this wasent good it pointed with a big arrow to where i suspect my problem is.

sandwiched in between the 2 outputs there is a collection of....

3 resistors.
R330 -470 ohm
R326 -360 ohm
R328 - 470 ohm

1 thermistor (i think)
Q310 - 2SD637/QR

1 transistor (i think)

TH302 - QAD0012-202

i can get the radio to come on and play sounds it is just when i try to move the volume up and down that it explodes.

R330 is the worst failure it actually melted right into the board.

i have 5 individual amp boards and my question is if something is failed/failing should i order 5 parts and replace on all 5 amps?

also with these failed parts, are they the failure? or a side effect of something else that has failed?

[budm]

One thing you should do to help you from blowing up new transistors, hook up 75~100 watts bulbs in series with you amplifier, that way if something shorted out the lamp will lifgt up real bright and limit the voltage to your amp. I used thsi setup for 35 years and saved a lot of new fuses and new transistors.
to burn up the 470 Ohms resistor which is part of the Bias circuits (including the transistor) it must have shorted path for a lot of cuurent. You should monitor the bias current of the out put transistor by measure the DC voltage on the 0.22 Ohm resistor, typical class AB bias should be around 20~25mA (no input signal), so @25mA flowing through 0.22 Ohm, you should have about 5mV DC across the emitter resistor. Also check the DC offset, it should be in mV range. Pot VR302 is for the bias current adjustment. The service manual should tell you how much it should be.

[scampo77]

i put the lightbulb between the actual 110v plug and the wall outlet?

i tried looking up a bias circuit online and most of it was well over my head, is it possible to explain the function of this circuit without getting too technical?

what should i do next? order and replaced burnt parts? what i am GUESSING has happened is the transistor at loc q310 has shorted causeing everything else to fail around it. can someone confirm or deny this theroy please.

if my idea is true should i replace all the Q310's in all 5 amps?

i cant find anything about what the trim pot should be set to all i can find is that it is a 1k pot this one is measuring at 300 ohms and the amp board next to it is measuring at 330

[ben7]

Originally Posted by scampo77

after many trails i am back and think i am close to getting this thing figured out. my fuse problem was the output case touching on the heatsink, i did not realize the case was connected to one of the legs. i have found 2 sets of darlington pairs shorted and have replaced them only to have one fail right away afterwards. on the third install it went up in a blaze of glory with flames and everything. although this wasent good it pointed with a big arrow to where i suspect my problem is.

sandwiched in between the 2 outputs there is a collection of....

3 resistors.
R330 -470 ohm
R326 -360 ohm
R328 - 470 ohm

1 thermistor (i think)
Q310 - 2SD637/QR

1 transistor (i think)

TH302 - QAD0012-202

i can get the radio to come on and play sounds it is just when i try to move the volume up and down that it explodes.

R330 is the worst failure it actually melted right into the board.

i have 5 individual amp boards and my question is if something is failed/failing should i order 5 parts and replace on all 5 amps?

also with these failed parts, are they the failure? or a side effect of something else that has failed?

Seems like an overvoltage on the resistors, which is causing them to blow.

Check Q308, Q310, and D304 for shorts.

-Ben

[scampo77]

Q308, Q304 both tested fine. but Q310 is the one that had melted down and blew off the board.

i need to know before i order these parts if this is likely a local problem or if these transistors (or anything else) fail as a team. should i replace Q310,Q810,Q909,Q809,Q309? there is one of these transistors on each amp, also 4x thermistors.

the outer legs measure 330 ohms and the rest are all <100k ohms on the remaining 4 amp boards. and all 4 thermistors measure 830 ohms

when i order parts usually the parts are cheap but they kill you on S+H

[ben7]

Originally Posted by scampo77

Q308, Q304 both tested fine. but Q310 is the one that had melted down and blew off the board.

i need to know before i order these parts if this is likely a local problem or if these transistors (or anything else) fail as a team. should i replace Q310,Q810,Q909,Q809,Q309? there is one of these transistors on each amp, also 4x thermistors.

the outer legs measure 330 ohms and the rest are all <100k ohms on the remaining 4 amp boards. and all 4 thermistors measure 830 ohms

when i order parts usually the parts are cheap but they kill you on S+H

Hm, something must have made the resistors burn,
check the output transistors, Q316 and Q318.

If Q310 shorted then the voltage across all the resistors (the ones that blew) would drop, and they wouldn't have blown.
They burn because of overvoltage, the more voltage applied across a resistor, the more current it will draw, and hence the more power it will dissipate as heat. If it dissipates enough energy it will catch fire.

And yes, in amplifiers like this there are cascade failures

-Ben

[budm]

Do you have clearer diagrams, it will be easier to explain how it works?

[budm]

This is what I use to put the lamp in series with the Hot (Line) side of the load (the equipment you are repairing).
http://s807.photobucket.com/albums/yy352/budm/Tools/

[scampo77]

If you look on one of the first pages of this thread you will see all the pdf docs I am working with. There is a parts list, schematic, and owners manual. You can zoom in and print screen and open in mspaint so you can draw all over it.

And my ouptputs are not shorted.

[ben7]

Originally Posted by scampo77

If you look on one of the first pages of this thread you will see all the pdf docs I am working with. There is a parts list, schematic, and owners manual. You can zoom in and print screen and open in mspaint so you can draw all over it.

And my ouptputs are not shorted.

ok, can you check Q314, Q312, and D306, D308?

-Ben

[scampo77]

they seem fine the lowest reading i can get is on Q312/4 i am getting 73 ohm on the outer legs. they both measure the same

[scampo77]

should i order 1x 2sd637 and 1x QAD0012-202 ?

[ben7]

Originally Posted by scampo77

they seem fine the lowest reading i can get is on Q312/4 i am getting 73 ohm on the outer legs. they both measure the same

Hm, did you read the resistance of them in-circuit? If you did then they should be ok; the resistance you measured was from R338 and R342 in parallel.

Try taking Q302 out, if it is open circuit then that is the problem. If it is not the problem, then I don't know what else could be broken.

[scampo77]

i finally found time to mock up a led circuit for this transistor 10k ohm to the base 470 ohm + led on the collector and ground on the emitter and i put a switch in there and i can turn the light on and off.

i think i should order these parts, is there anything else i should replace along with the transistor, 3 resistors and thermistor?

[scampo77]

i have ordered my parts but i am still very curious about the lightbulb inline thing. i dont quite understand how to wire it. i made a picture is one of them correct?

[Agent24]

Inline = in series

You want the bulb in series with the PSU, so you want to use diagram B.

Diagram A shows the bulb in parallel with the PSU.



I think this method is typically done with a standard tungsten-filament incandescent bulb.

In either case, I suggest you have a look at: http://www.repairfaq.org/sam/smpsfaq.htm#smpstslbt

[budm]

I use incandescent bulb, the wattage is depended on how big the amp I work on, normally, 100w bulb is good enough for moat work, that is why I made the box so I can easily change the bulb.