Sony STR-7065 Power Supply Rectifier Caps H-O-T

[Johnny_Law]

Heat in the STR-7065 PS appears to be a feature rather than a bug (i.e., a design error), see e.g. thread 1 and thread 2.

The first reference thread mentions a hot R802; the second thread mentions the same but also that C801-C804 were replaced in a previous with 0.033uf / 600V caps. (@Phase700b)

The STR-7065 on my bench works great - power supply voltages appear correct within spec. So it's not a repair issue (I don't think), instead it's a design issue. The design issue is, I am seeking to reduce the heat level of C801-C804 and R802. My hypothesis is that installing larger components would help simply because of the increased capacity of the larger heat sinks. Thoughts?

> C801-C804: I have measured with an IR thermometer C801-C804 getting above 300degF (!). The originals are 0.022ohm / 200V mylar caps. Maybe move to large 600V film caps (like orange drops, if I can get them to fit)?

> R802: Doesn't measure quite as hot, but I think increasing its size to a 10 watt 220ohm resistor should help.

Thanks in advance for any thoughts.

 

[Johnny_Law]

So perhaps trying the larger caps and resistor is the first step.

 

[Johnny_Law]

I put in a bigger resistor (200ohm / 10 watt wirewound), higher capacitance & voltage ceramic caps (0.033uF / 250V) mounted on the back side of the PCB, and replaced C805 / C806 with new electrolytics.

It was running a bit cooler - the caps on the back side were barely over 100 deg F; the new R802 is around 220-250 deg F, down from 300+ deg F before.





But all of a sudden, a small pop and no more AM/FM at all. Shoot. Power supply voltage to R802 disappeared!

Turns out a resistor blew. BUT ... it's not in the circuit diagram!

It looks like there's an omission from the circuit diagram cited here (https://www.audiokarma.org/forums/index.php?threads/sony-str-7065-right-channel-print-woes.478212/) ... it is missing a resistor between C803/C804 and R802. In the PCB image, you can see R840 (20 ohms / 1 watt) connects C803/C804 to R802; however, in the circuit diagram, R840 is nowhere to be found.

(Edit: Found another thread that mentions R833: https://audiokarma.org/forums/index.php?threads/sony-str-7065-again.765204/)

Well, that resistor blew in this one. Adding to the confusion, it's labeled R833 on the actual PCB ... R833 is nowhere to be found between C803/C804 and R802 in any circuit diagrams I can find. And the one that came out of this one is 20ohmK / 0.5 watt.

Anyway, so I assume it's supposed to be 20ohm / 1 watt, and I will replace it as such, but I wanted to mention it here for the sake of documentation. (Unless I'm mis-reading it, in which case, please let me know.)

 

[frankosport4]

Hope you resolve your troubles there,

How does your Sony 7065 perform when all is well? There were two favorable reviews of it -- by Audio (Nov 1973) and High Fidelity (Jan 1974)

 

[llwhtt]

Have you checked out the service supplement for this receiver? There are a few added resistors in the power supply.

 

[50 Green]

DO NOT UP THE WATTAGE OF RESISTOR 833. IT IS LIKELY A HALF-WATT FUSIBLE RESISTOR!!!

I have a couple of Str-7055s, which use almost the same PS / Power Amp board. (Many of the component numbers are even identical.) The portion of board in your image looks much closer to the 7055 board. I think Sony may have made changes in the board after publication of the original diagrams (which likely would be in a service supplement.) From the looks of the 7055 boards I have, I think it's likely they used the same board for both models, with different components in some positions.

In both the schematic and component placement diagrams that I have for the 7065, a solid trace is shown between C803/C804 and R802. (A resistor in that position would do nothing.) The 7055 diagram shows that portion of the board laid out exactly as yours is in your image, with a break in that large trace, and does include R833. IT IS A .5 WATT FUSIBLE. HFE has the 7055 manual. It might be a helpful resource, if you're not able to find a service supplement for the 7065.

That area of these boards gets hot. All of the boards I've seen in person (as well as in photos) have heat marking in that area. Higher wattage resistors could be helpful in keeping things cooler, but if that fusible is popping, it's doing its job. Something else is probably causing the issue. I'd want to diagnose that before just upping the size of components.

Edit: Looking closer at the numbers on the resistor you pulled, it appears it is a fusible, and also flameproof. It should be replaced w/ one with identical specs (after finding what caused it to go open.)

 

[Johnny_Law]

Hope you resolve your troubles there,

How does your Sony 7065 perform when all is well? There were two favorable reviews of it -- by Audio (Nov 1973) and High Fidelity (Jan 1974)

It was doing great, just running incredibly hot in the power supply. It is a very, very nice receiver as are the other Sonys of the era.

Have you checked out the service supplement for this receiver? There are a few added resistors in the power supply.

You rock! I looked at the supplement and it's revised with the addition of the 20 ohm / 1 watt resistor (although it's labeled R840, not R833).

DO NOT UP THE WATTAGE OF RESISTOR 833. IT IS LIKELY A HALF-WATT FUSIBLE RESISTOR!!!

I have a couple of Str-7055s, which use almost the same PS / Power Amp board. (Many of the component numbers are even identical.) The portion of board in your image looks much closer to the 7055 board. I think Sony may have made changes in the board after publication of the original diagrams (which likely would be in a service supplement.) From the looks of the 7055 boards I have, I think it's likely they used the same board for both units, with different components in some positions.

In both the schematic and component placement diagrams that I have for the 7065, a solid trace is shown between C803/C804 and R802. (A resistor in that position would do nothing.) The 7055 diagram shows that portion of the board laid out exactly as yours is, in your image, with a break in that large trace, and does include R833. IT IS A .5 WATT FUSIBLE. HFE has the 7055 manual. It might be a helpful resource, if you're not able to find a service supplement for the 7065.

That area of these boards gets hot. All of the boards I've seen in person (as well as in photos) have heat marking in that area. Higher wattage resistors could be helpful in keeping things cooler, but if that fusible is popping, it's doing its job. Something else is probably causing the issue. I'd want to diagnose that before just upping the size of components.

Thank you! I looked at the 7055 service manual ... lo and behold, there's R833! It is 20 ohm / 0.5 watt.



But it's .. exactly the same as the revised 7065 power supply board ... other than the resistor turning into R840 and increasing to 1 watt!



So ... the question is, should I follow the 7065 service supplement (this being a 7065) and use a 1W resistor, or should I follow the 7055 service manual (which matches the R833 designation) and use a 0.5 watt resistor?

 

[50 Green]

The resistor you pulled looks like it's probably original and it's 0.5 watt... Did it look original to you prior to pulling it? (This is one area I'd want to err on the side of a lower-wattage resistor, if I wasn't sure.)

Do you also have the component mounting diagram for the 7055? I wouldn't rely on it for calling out the correct components (some are definitely different) but I think the traces on the PS / Power Amp board may be exactly the same as the 7065, and that could be helpful with component locations, at least.

...Another possibility to consider is that there may be multiple revisions of the 7065. Perhaps there is also a board that lists R804, with increased wattage.

 

[Johnny_Law]

Yes, the 7055 power supply board is exactly the same. Sony must have used the same board in both at a certain point in time ...

Edit: Looks like that resistor is original. The same one is pictured in this thread. https://audiokarma.org/forums/index.php?threads/sony-str-7065-fuses-inside-the-tight-mess.649091/

Specifically: https://audiokarma.org/forums/index.php?attachments/board1-gif.584495/

Edit2: Agree w/ the point about erring on the side of lower watt resistor. Thank you! Will include something like this in my next Mouser order.

 

[50 Green]

In that case, I'd probably stay w/ the 1/2 watt resistor. They may have upped the wattage in a later revision to prevent it from blowing unnecessarily, but unless I was certain that all other components (as well as board and wiring) were the same, I'd rather have a resistor go open, than risk damage elsewhere. The one you pulled survived for 40+ years, so 0.5 is probably adequate.

A couple of years ago, I redid a 7055 I've had since new (so I knew everything was original.) I kept almost everything to the same specs and it has functioned beautifully since. All resistance values remained the same, as did the wattage of the large resistors in the PS area of the board. I did replace carbon comp resistors w/ metal film, and upped the voltage on a few electrolytic caps, but not that many. The only big changes I made were upping the capacitance of the filter caps from 4700 to 6800, and changing out the rectifier diodes to newer, higher voltage / amperage rated parts, just to be on the safe side w/ the increased filter capacitance.

To help combat heat in the PS area of the board, I mounted the diodes and large resistors even further off the board than the originals, positioning them for maximum airflow, and also bundling the wiring above to allow more airflow. One difference between the 7055 boards that I have and the 7065 board you linked to is that R804 is stood much farther off the board - from the factory. It's set up with one very long lead, so the body of the resistor is far from the board and not parallel to it.

Putting the 4 rectifier caps (C801-C804) on the back of the board may be helpful in combating heat, but be sure you have air flow on that side. My 7055s have no perforations on the case bottom on that side of the board, and the ventilation slots in the wooden case don't extend to that side of the board.

 

[Johnny_Law]

Wonderful, will order that 0.5 watt resistor and have the peace of mind!

Putting the 4 rectifier caps (C801-C804) on the back of the board may be helpful in combating heat, but be sure you have air flow on that side. My 7055s have no perforations on the case bottom on that side of the board, and the ventilation slots in the wooden case don't extend to that side of the board.

Good point, and yes the 7065 metal case is vented in that area, although the wooden case may not be. I'll have to check on that.

 

[50 Green]

Good point, and yes the 7065 metal case is vented in that area, although the wooden case may not be. I'll have to check on that.

Is there ventilation directly below that side of the board, on the bottom panel? (There isn't on the 7055, and that may also be one part that's common to both models.) For passive convection cooling to work well, there should be a good source of cool air from below.

 

[Johnny_Law]

Is there ventilation directly below that side of the board, on the bottom panel? (There isn't on the 7055, and that may also be one part that's common to both models.) For passive convection cooling to work well, there should be a good source of cool air from below.

Absolutely, a good real-world corollary is a bridge in the winter - this is why the bridge freezes before the road.

In the Sony, there's basically no ventilation internally. Here you can see that aside from the venting below the output transistors, there's no opportunity for a pressure differential / airflow through the case:



However, there is venting in the metal case that allows airflow on the backside of the PCB:

 

[Johnny_Law]

Additionally, I discovered that there actually are some differences between the 7055 power supply and the revised 7065 power supply. Various other resistance values were increased in the 7065 (e.g., R801 went from 22ohms to 82 ohms, R804 went from 220ohms to 330 ohms, etc.). This 7065 has all of those larger 7065 values.

 

[50 Green]

Looks like the ventilation should be fine on the back of the board if you run it without the wooden case.

There are definitely different components in the 7065 PS. The transformer and filter caps are larger, so I would be surprised if Sony didn't up some of the other related components.
I've seen mention elsewhere of at least 3 versions of the 7065, so that fusible resistor may have been increased in wattage on a version later than yours. For myself, I'd want to be very sure that nothing else was different before using a higher wattage resistor... and no matter what, I would be sure to only use a fusible in that position.

 

[avionic]

20ohmK / 0.5 watt.

20 Ω.. K is a tolerance code

 

[50 Green]

Edit2: Agree w/ the point about erring on the side of lower watt resistor. Thank you! Will include something like this in my next Mouser order.

I just noticed the link in your edit... That is NOT a fusible resistor! (Using it in that position would be equivalent to the old "penny in the fuse box." It would give you function but not protection.)

I used FRM-50JT-52-20R (a flameproof, fusible resistor) for R833, but unfortunately it's now an obsolete part.

That resistor is designed to fail, to protect other parts of the circuit. Since yours failed, there is likely something wrong that caused it to blow! To be safe, find and fix that first, then replace it with a flameproof, fusible resistor.

 

[Johnny_Law]

I just noticed the link in your edit... That is NOT a fusible resistor! (Using it in that position would be equivalent to the old "penny in the fuse box." It would give you function but not protection.)

I used FRM-50JT-52-20R (a flameproof, fusible resistor) for R833, but unfortunately it's now an obsolete part.

That resistor is designed to fail, to protect other parts of the circuit. Since yours failed, there is likely something wrong that caused it to blow! To be safe, find and fix that first, then replace it with a flameproof, fusible resistor.

I have done some reading on the topic and as a result replace fusible with metal film. They are more stable under normal operation and won't drift. They will still blow like a fusible resistor under the same conditions.

See, e.g.:

I use metal film of the same wattage and resistance value.

Well, not quite true - fuse resistors are designed to change their value in the presence of electrical stress e.g. too much current - this is their primary function. Unfortunately, fuse resistors seem to drift in value even without an over-current condition - which makes them unusable as far as I am concerned.

Standard resistors primary function is to retain their value, excessive current causes them such severe stress that they eventually burn and 'open'. Overall there isn't much difference except that standard resistors are much more reliable at their primary function.

 

[50 Green]

Your call. Might want to go flameproof metal film, then.

 

[Johnny_Law]

New resistor is in, and the Sony is doing great. Listening to LMU college radio through my Magnepan 1.7, certainly warm but not hot and definitely not smoking. My IR thermometer says it's about 120 deg F.