Kenwood KR-9050, need help with diagnosis (Help me Save this Amp)

[grafxdesin]

Alight, out the TTA004B into circuit and the DBT still lights up, readings below. Maybe the issue in the X07-1690 board?

Q3 E -35.7v
C .6v
B -34.2v

Q5 E 20.1V
C 37.6v
B 19.5v

Q7 E 20.1v
C 39.7v
B 19.5v

Q9 E 37.8
C 1.1v (Still correct)
B 38.6

Q11 E 37.7
C 28.5v (Still the issue)
B 38
Q13 E -37.8
C -1.3
B -38.5

 

[grafxdesin]

OK, quick update. When speaking with some people on another site, someone ask me what wattage bulb I was using. When I told him 100 watt, he suggested I use a 200 watt to allow more current and that the amp might be starving for voltage. He was right, when I swapped out the 100 watt for the 200 watt, it took a few seconds but the speaker relays clicked and I had sound. Now I have a working left channel that I can use to rebuild the right which has a bunch of burnt components.
Now, because I want to continue learning and get better at this hobby, why did this work? Why would a larger wattage/current bulb allow this to work? I would think a smaller bulb like a 15watt would only allow this much and "limit" the current a smaller amount, but still allow it to function.

 

[JyriK]

The proof is in the pudding.
Larger bulb allows enough current for the device to work.

 

[mbz]

when I swapped out the 100 watt for the 200 watt, it took a few seconds but the speaker relays clicked and I had sound.

That surprises me because,

Q9 E 37.8
C 1.1v (Still correct)
B 38.6

Q11 E 37.7
C 28.5v (Still the issue)
B 38

Both Q9 and Q11 seeing same inputs but different Vc's, need to do more reading...
Glad you got it sorted. Appears the 100W DBT was still "choking" the amp of power to the point that one stage (or transistor?) hadn't normalized (turned on). The 15W DBT would reduce power to the amp even further. Maybe think of it as a 15W DBT will allow the amp to see 15W of power (= not enough to start up). 100W DBT gets you 100W, 200W DBT allows 200W power to amp(abouts)

 

[grafxdesin]

Alright, maybe getting down to the root of the problem. On board 1680 B2, it looks like I may have a bad bridge rectifier, I say this because of the following reading. On the left side, which did not need any work on this amp, at the bridge rectifier I am getting 75 VDC at the positive and -75VDC on the negative. On the right side I am only getting 38VDC on the possitive and -112 VDC on the negative side, does this sound right? Bad Bridge Rectifier?

 

[mbz]

I am getting 75 VDC at the positive and -75VDC on the negative. On the right side I am only getting 38VDC on the possitive and -112 VDC

Just playing with numbers...

75 + 75 = 150 then divided by 2 = 75
38 + 112 = 150 then divide by 2 = 75

Makes me think the rectifier is outputting the correct voltage range just that the midpoint/GND has shifted. Another observation, the shift is 112 - 75 = 37V (same as 38V!)

Thinking further, you are very probably right, hard to see any GND fault giving these numbers, Possibly for the +38V only one of the diodes is conducting??? Scope trace would be interesting. Should show up in diode test.

Replace the rectifier.