SONY Str 7055 main caps voltage drop to 26V and burned resistor

While last measurments when I said I soldered everything back I ment components. All boards feedings are disconected by pins been desoldered. Only headphones, rca inputs and output transistor remains. So basicly I think it is just tranny, dial lights and powerboard. Even with outputs pulled out there is no change. In a few hours (it is a middle of the night here 02:39) I will replace main caps and test again. I could borrow some extra dmm tommorrow.
Did you read vhat guys from diyaudio says. Basicly that mismatch of components is causing this. While I respect all opinions it is hard to belive such scenario. Mostly because I did 2 sonys the same model and It goes smoothly. Gremlins ftw.

Thx guys anyway I am more than gratefull.
 
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Uuh and I noticed that two zeners (d801,d802) installed are 6,8V 0.5w and it should be 7.5V... and bd140-16 instead of ksc2690. 80V vs 160V I think. But should handle 50V rail...? Other components installed are by the bible...
I know a best tech in our country who can do new amp from "kitchen waste" with all of the eq. needed and I could use his service if I needed.
 
Do you think maybe it is time for a reset/recap? Honestly I'm not sure of what is or is not connected at this point.
 
first do sanity checks like make sure your multimeter is working properly .
then using a variac try setting the voltage selector of the unit on the lower voltages then see what the secondaries read .
also you could try replacing the fuses even if they do test good
 
I am done with main caps. Due to smaller diameter of caps I used my pill bottle top for extra diameter.
Will do some test and then solder back everything.

Edit thx Pete but I sadly dont have variac. DBT is everything I can rely on.
IMG_20250125_132948.jpgIMG_20250125_132925.jpgIMG_20250125_132840.jpgIMG_20250125_132647.jpgIMG_20250125_132342.jpg
 
Voila. I soldered everything back. I mean everything. By the book. Triple checked.
test: Dimbulb goes dim, and than increase back to bright. Seems like as the caps are filling. Ok.
I remember that on some Hitachi, bulb was bright untill I adjusted bias.
Then I adjust bias here on Sony. Full counterclockwise. Dim bulb goes almost dark.
(Left? bias) - violet-white and red-white is adjustable. 50mV
(Right? bias) violet and red is non adjustable. Stuck at 14mV

And most important R826 is smoking.
 
45Vac on red+red transformer secondary feeding the board, tested with DBT.
Without DBT 54.6Vac on red+red.
Progress?
 
icko30 said:
Voila. I soldered everything back. I mean everything. By the book. Triple checked.
test: Dimbulb goes dim, and than increase back to bright. Seems like as the caps are filling. Ok.
I remember that on some Hitachi, bulb was bright untill I adjusted bias.
Then I adjust bias here on Sony. Full counterclockwise. Dim bulb goes almost dark.
(Left? bias) - violet-white and red-white is adjustable. 50mV
(Right? bias) violet and red is non adjustable. Stuck at 14mV

And most important R826 is smoking.
Click to expand...
Is R826 in the channel that you can't adjust bias on?
 
I think so... I have difficulties to indicate phisicaly the left or right board to channel... but it is located near to trimmer that can not adjust.
I have a secure pdf, else I would send you some board print.
If you like you could get a service manual from elektrotanya I think...
 
R826 is on the +50 volt supply rail and feeds power to both L & R front ends through <19> on the schematic.
R827 is on the -50 supply rail and also feeds power to both L & R channels through <23> on the schematic.

I can't post the image showing both channels and the <19> & <23> wires you will have to look.

What that means is it could be a problem in the L or the R front ends. The front end is in the green box.

You need to check the front end components for both channels. I know you had issues with one of the VD-1121 replacements. I would start with the
front end transistors in the channel where that diode was. As an example transistors Q701, 702 & 703 are in the front end of the L channel.

7055arx.jpg
 
The 'no tolerance to any load' situation reminds me strongly of the problem I discovered with a Sansui AU-719, where one of the transformers had an open circuit primary. It turned out that the magnetic excitation provided by the presence of power at each end of the primary (but a break somewhere in the middle), was enough to give a reasonable voltage output to the secondaries. But, try to load those secondaries and the voltage dropped considerably. Weirdly the amp sounded OK, but push it just a bit hard and the output from the faulty channel collapsed.

I've been reading along, you guys helping have been doing a pretty good job, but I can't see why that particular resistor likes to burn. I'll continue following, and help if I can, but I'm very keen for you to uncover something - I am very curious to know the answer. ;)
 
I replaced all 3 transistors with new ones day or two ago. For the second time. Just in case.
No luck.
You think I could measure the voltage (dc to ground?) on pinouts? With or without DBT?
And another thing. I installed 2× ksc2690y on q801 and q802 ; elsewhere bd 139-16. I speak for 2sc1124 replacements. Problem?
 
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@Hyperion thx for input
When bought "new to me" I pushed receiver real hard to test, with no issues. Just hum noise static.
After transistor replacements everything went wrong and further...
 
To me there are two issues:
  1. The transformer secondary voltages are too low.
  2. Resistor R826 is overheating.
My observations for (1) above:
  1. I think we eliminated the possibility that the transformer primary side had a wiring error or that one of the primary windings was open with the diagram below.
  2. However, the secondary voltage seems too low at ~50VAC (25VAC to Gnd). From Red-Red wire it should be about 100VAC. Some measurements were on the DBT so they might be low, but not that low.
  3. I think at one test, @icko30 measured the ACV of the secondary (red-red) and it was too low.
  4. I thought that maybe one of the fuses was open and only one of the primary windings was working effectively as a 120VAC primary. I think with all of the tracing below, that is not the case.
  5. My thought is that the transformer is bad.
1737822025043.png
As for the burning resistor R826:
  1. R826 feeds both the + and - voltage generation, so it is difficult to determine which side (or both) is bad.
  2. Look at the schematic below. One side of R826 is 31VDC (50V) and the other side is -2.5VDC! A drop of 34V or 5.25W dissipation.
  3. As @mrk229 points out in his post there is a problem in one of the components after R826.
  4. D701 was replaced with 2 1n4148 diodes.
  5. Q701 could be bad since EBC are all -1.7V.
  6. The - voltage on Q701 base is basically coming from the -50V supply and +50V supply.
  7. R704 is dropping the -23V to -1.6V
Remove Q701 and see if the voltage at R826 comes up to 31.4V
  • D701 has a couple of diode drops (2*0.6V = 1.2V) so it should be about 30V, if not I would check R704.
1737822646081.png
 
C816 reversed or otherwise faulty?

Also, referring to the picture in post #110
I don't know if my eyes are deceiving me, or what capacitor this is, (ringed in Orange) but it looks like it might be installed reversed - according to the board markings.

1737825268597.jpeg
 
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