Xtant 3300c low output.

[simplebliss]

ok... so years ago i got this amp in a trade knowing that some of the magic smoke had been let out of it.
life got busy an it was set aside.. a few months ago i pulled it out of the closet an pulled the cover off to find it had a bad cap... no big deal... replaced it, hooked it up and everything was good... nice sound very LOUD! been running it everyday no issues until yesterday... powered on just fine.. got maybe 20s into a song at low volume an then i heard two little thumps as if the amp was powering on or something and lost all but a lil volume... i can turn volume all the way up an can just be able to hear it.
nothing looks blown... i didnt smell anything bad.... no smoke..
i've seen a few threads on here where ppl are repairing this amp an was hoping someone could point me in the right direction.. maybe what/where to look on the amp... im not new to repairing electronics... but mostly it's just if i can see the damaged parts.

Any help on this would be greatly appreciated

 

[Perry Babin]

There are several transistors marked J108 near the power amplifier section. What is the DC Voltage on leg 3 of those transistors?

Have you set the jumpers to provide the maximum gain?

Are the gain controls set to the maximum position?

 

[simplebliss]

ill check the volyage on those here in a bit..
an answer to the other question... yes ive tried the input gain jumpers in every setting an turned the gain knobs for left right an sub out.. they all function like there supposed to.. its just the audio coming out is like something you would here from a nice set of headphones a few inchs from your ears.. ive hooked up multiple things to it... same outcome everytime...

 

[simplebliss]

ok...
so i found 3 transistors marked j108.. i think.. really small print
wasn't to sure on what to set my meter on so i took to readings on each one on 20v setting an one on 200m setting.

listing 20v as first number
q37 0.05/53.0
q60 0.05/52.4
q87 0.05/52..5

an i don't know if it makes a diff or not.. but i don't have a power supply and i am doing all testing hooked up to my car battery.. car is not running

 

[simplebliss]

also i would like to thank you for trying to help me with this...

 

[simplebliss]

forgot to say that meter said negative voltage

 

[Perry Babin]

The voltage should be near -15v.

Do you have positive and negative 15v on the power supply pins of the op-amps?

 

[simplebliss]

ok... not sure what op amps those are.. but i found 9 of those lil guys on left side of the board...
number j2068d i think it said
not much diff in voltage on most of them
u22 both - an + on both sides - .04v
u23 same as above
u1 -.03 on both sides both pins
u6 -.04 on both sides both pins
u8 pin 5 +.08 pin 6 +.16 pins 2 an 3 -.04
u10 -.04 both sides both pins
u2 pin 2 .03 pin 3 .04 pin 5 .03 pin 6 .05
u4 pin 2 .05 pin 3 .04 pin 5 .02 pin 6 .03
u9 -.04 on both sides both pins
really hope those are the right ones...
seems like a long ways from 15v...
or im lookin at the wrong ones.. but i looked up all the lil chips on the board an those are the only op amps...

thanks for your time...
craig

 

[Perry Babin]

Do you have voltage ranging from 25-45v on at least one leg of each of the output transistors? The output transistors are the ones with the large blue resistors in front of them. Do NOT let the probe short to the clamp (or to anything else) when measuring the voltage.

 

[simplebliss]

does it matter which leg?

 

[Perry Babin]

You need to check enough of them to find ±25 and ±45 (approximately). I'm trying to determine if the power supply is working properly. If you don't find anything near those voltages or only find ±25 and not ±45, that will lead us towards the problem, hopefully. There is another power supply that you'll also need to check after this one.

 

[simplebliss]

ok.. well no voltage on any of the ones with the big blue resistor in front of it..
my lil meter goes from 20v to 200v.. first time thru on 200v... nothing so i switched to 20v... still nothing

 

[simplebliss]

oh.. i forgot to point out that this time when i powered the amp up... i heard a really faint high pitched noise seems to be coming from the area with the giant coils.
i live really close to a state highway... rare that there is no cars goin by.. so it couldve been there the hole time an this time it was just quiet enough for me to hear it

 

[Perry Babin]

Post the DC voltage on all pins of U18. Look up the datasheet if you don't know the pin configuration. Copy and paste the following into your reply.

U18
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:

 

[simplebliss]

U18
Pin 1: 2.60
Pin 2: 2.60
Pin 3: .01
Pin 4: .16
Pin 5: 2.05
Pin 6: 3.78
Pin 7: 2.04
Pin 8: 4.62
Pin 9: 2.10
Pin 10: nothing
Pin 11: 2.60 and d27 lights up
Pin 12: nothing
Pin 13: 11.27 and d27 came on for a second then turned off, tried 3 more times.. didnt light up again
Pin 14: 2.60 an d27 lights up
Pin 15: 11.28
Pin 16:5.18

no readings were negative

 

[Perry Babin]

13 and 15 should have produced the same readings. Confirm that they are directly connected (0 ohms between 13 and 15). If they are, recheck the voltage on those pins.

If you have the bottom cover off, check R93 to confirm that it's within tolerance (±5%).

 

[simplebliss]

oh the 200 ohm scale on my meter.. i get 0.5 on pins13 an 15
as a after thought i touched the probes together an got the same reading so...
r93 reads 10.5 on the 200 ohm scale.. lowest meter goes... r93 says 100 on it... i know very little about surface mount stuff...
i swapped out the battery in my meter just to have a fresh one in it...
took readings from 13 an 15 again.. this time let the probe touch for a 8 count
pin 13 11.51v solid
pin 15 11.51v but kept flashing to 11.50v

cant explain the .30 diff from this morning to now.. i did put about 50 miles on my car today after not driving for a few days.. battery probly has more charge now..

 

[Perry Babin]

R93 is OK. The 0.3v difference is insignificant.

Now that pins 13 and 15 are constant 11.5, do you have voltage on the power supply pins of the op-amps? Do you have the ±25-±45 on the output transistors?

 

[simplebliss]

ok..
the op amp voltage went up by .01 an sit a .05 instead of .04
an the output transistors have voltage but nothing like what you are saying max i saw was 3.45v

 

[Perry Babin]

You need to see ±15v (approximately) on the power supply pins of the op-amps.

Post the DC voltage on all of the pins of the SG/UC3525. Copy and paste the following into your post.

Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16: