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Sony STR 7055 overheating problem

K

Kutiltim

New Member
Hi there,

I'm new at the forum.
I'd like to find some information or advice how to solve overheating problem on my receiver Sony STR 7055, which is known as quite common in Sony STR 70xx line.
Overheating occurs in rear left corner at the Power Supply Board. There are resistors, which cause heating and electric smell.
The Receiver Sony STR 7055 has great sound and I'd like to have this piece in my living room, but the smell is annoying my wife, so my Sony is now in the garage waiting for repair.

Thank you for answer

Greeting from Czech Republic
 
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Hi, l would start by checking the bias settings for both channels. Have you visibly inspected the components on the power supply?
 
Kutiltim said:
Hi there,

I'm new at the forum.
I'd like to find some information or advice how to solve overheating problem on my receiver Sony STR 7055, which is known as quite common in Sony STR 70xx line.
Overheating occurs in rear left corner at the Power Supply Board. There are resistors, which cause heating and electric smell.
The Receiver Sony STR 7055 has great sound and I'd like to have this piece in my living room, but the smell is annoying my wife, so my Sony is now in the garage waiting for repair.

Thank you for answer

Greeting from Czech Republic
Click to expand...

I don't know how similar the 7055 and 7065's are but here are a couple of threads on the hot 7065 PS boards:

http://www.audiokarma.org/forums/index.php?threads/sony-str-7065-right-channel-print-woes.478212/
http://audiokarma.org/forums/index.php?threads/help-with-sony-str-7065.526346/

If your 7055 hasn't been run for many years, maybe some of the smell is dust burning off the resistors? My 7065 was pretty smelly when I first got it but is much better after cleaning the boards and running it awhile.
 
Many thanks for replies,

I will try your advices, but I'm amateur in electronics. I have basic knowledge from school and have basic tools - multimeter, soldering iron and service manual with scheme from internet.
Its challenge for me to do this job.
According to scheme the PS board looks very similar or the same at 7055 and 7065.
My 7055 is quite clean with no dust in the hot area
I will write my next steps.

Don't you know if recapping may sort out the problem?

Thanks
 
To bias setting

I have found the procedure of Bias adjustment in the manual, but I don't have Variable transformer and European voltage is 220 V not 120 V,
Should be the reading 50 mV the same at 220 V?

Thanks
 

Attachments

Hi, you won't require the variable transformer, just use your 220v wall voltage as usual. Yes the 50 mV is the same regardless of the AC operating voltage.

Personally l also wouldn't turn the trim pots fully counter clockwise as the manual states. l have come across a few amps where one pot was backwards to the other (one being a Sony TA-1150) and also to what was stated in the manual.

If you are not planning on replacing the trim pots l would mark their current position carefully, rotate them back and forth many times to clean the tracks, then return back to their original position before measuring bias and adjusting to spec.

Use mini grabbers on the posts to measure the bias as you don't want to risk a slip or something touching and causing major damage.
 
So,
I have done the Bias adjustment.
The readings were very high, 85 and 109 mV.
Then I have it adjusted to 50mV. The resistors were still hot. After that I have dropped the values to 30mV.
Resistors are still hot. Maybe not so hot as before, but hot enough to smell.

I have thought, that it would be easier to solve the problem.
It seems, that the Sony is not for me. I have not skills to repair it and professional repair is more expensive than cost of the receiver.

Thank for help guys
 
Kutiltim said:
The resistors were still hot
Click to expand...

The power supply resistors will always run hot, maybe they have some sort of residue on them causing the smell. In any case you have done the receiver a BIG favour in setting the bias as the numbers you had were very high as you say.

Best to put bias back to factory specs (50mV). I understand if you don't want to pursue this any further, all the best with your audio journey.
 
Oldsansui441 said:
The power supply resistors will always run hot, maybe they have some sort of residue on them causing the smell. In any case you have done the receiver a BIG favour in setting the bias as the numbers you had were very high as you say.

Best to put bias back to factory specs (50mV). I understand if you don't want to pursue this any further, all the best with your audio journey.
Click to expand...

On my recently acquired 7065, I noticed the very hot resistor on the PS board and thought the bias must certainly be too high. However, when I checked it, both channels were stable and within 1mV of the SM spec of 50mV. I decided not to mess with the setting. Maybe it's living on borrowed time but I've been using it regularly for a couple of months now and it no longer smells.

It seems like that resistor is a reliability fail waiting to happen but apparently they were designed to run hot..
 
@Kutiltim

Some components within an amp do run hot. While this could possibly indicate a problem, it could also be normal.

Are you able to get a voltage reading across the hot resistor? If so you can then use Ohms law to first calculate current through, and then power dissipated by, that resistor. Then look up the part in the service manual parts list to see what its power rating is. Is the actual power being dissipated by the resistor less than its power rating, or is it more?


Also, it would be wise to inspect the solder joints of those hot running resistors. Inspect very closely with good light and a magnifying glass if you have one.
 
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It's obvious, that hot resistors are normal at this SONY receivers.
I have set back the Bias values at 50 mV and resoldered almost all joints on the board.
Some vintage smell is usual on these old receivers, but my wife is hypersensitive, so the sell is only way for me.
 
It may be normal, but it is not certain that it is. You would sell the receiver to someone not being sure that it is operating normally? Obviously, you have the ability to measure voltage across a resistor because you did this to adjust bias. I will do the calculations for you here if you will get a voltage reading across the hottest resistor. Will also need the ohms value and power rating of the resistor.
 
Don't want to sell this way.
If i would sell it I will found buyer whom I will tell the weakness of this model for modest price.
To somebody more professional than me.
The receiver isn't bad, but old enough to need some work.

I'm not experienced, every step in this job is new for me :)
Is it necessary to cut the leg of the resistor to measure the voltage?

Thanks
 
From what you describe, there's a good chance nothing's wrong with your STR-7055.

That resistor is functioning as designed. It's mounted that far off the PCB and in that position to take advantage of the same cooling air flow that cools the internal portions of the output transistor heat sinks ...and to keep it from burning the board. It emits heat in normal operation.

I'd suspect, as already suggested, that the odor is caused by accumulated debris burning off. Is it dusty inside? Was this unit previously owned by a smoker?

Neither of the STR-7055s I own emit any odor that I've detected (and I've owned one of them since new, in 1973.) Cleaning and / or running the unit until the odor is gone may take care of the problem for you. Or, perhaps your wife is extremely sensitive and detects odors that others wouldn't notice. (In the latter case, even brand new units might be unpleasant for her... I've certainly smelled my share of "new electronics smell.")
 
Kutiltim said:
but the smell is annoying my wife,
Click to expand...
Women have way more acute sensitivity to odors than men.. equivalent to like cats or dogs. Your sony maybe in the garage for sometime. you have my sympathy.. you're not the only one with 'banished' equipment and not for smell but appearance, too.
 
If the settings are close, I'd leave it on in the garage for an hour or so every day and see if that takes care of the smell eventually.
 
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