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Sony STR-7025 DC Offset

David_NC

David_NC

Super Member
I picked up this nice little receiver yesterday. It seems to work really well, though I noticed a bit of hum on the left channel. A multimeter tells me there's 24mv of DC on the right channel outputs and 250mv on the left channel. I've adjusted this on an old Marantz receiver, but I'm not finding much in the way of documentation of this unit. Does anybody have any experience with adjusting this unit or a service manual that might point me in the right direction to adjusting this unit?
Even with that issue, this is a really nice sounding receiver with all of its 18 watts. I have it hooked up to a pair of AR Fun Partners (a really bad name for a pair of speakers!) Although the bass is minimal with this setup, as expected, I'm really impressed with it. I was sitting about four feet away from the speakers, which were about 3-3.5 fee apart. With my eyes closed, it sounded like the speakers were a good 10' apart. I swear I heard 'depth' to some of what I was listening to that I don't recall hearing before. The sound from this receiver reminds me quite a bit of a Marantz 1060... smooth, mellow, and enjoyable.
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I have the 18 watt predessor. Other than 2 speaker fuses, all that needed doing was adjust bias back to factory spec. Still had DC offset and the channels were unequal but not as big a difference as what you have.

Seems to me that mine is capacitor coupled. Maybe yours is as well? I could be wrong but I think I read or was told that DC offset considered on the high side is to be expected with those types of amplifiers.
 
62vauxhall said:
I have the 18 watt predessor. Other than 2 speaker fuses, all that needed doing was adjust bias back to factory spec. Still had DC offset and the channels were unequal but not as big a difference as what you have.

Seems to me that mine is capacitor coupled. Maybe yours is as well? I could be wrong but I think I read or was told that DC offset considered on the high side is to be expected with those types of amplifiers.
Click to expand...

This one uses STK-054 power amp modules; did yours have these too? I don't see anywhere to adjust anything on this, other than in the tuner section. The power amp is between that vertical piece of steel and the transformer and is suspended / upside down. I haven't removed the bottom cover and checked for any adjustments yet, but I'll do that shortly.
 
Negative on the STK modules - standard output transistors.

Trying to remember but I was fiddling with an Akai integrated that did have STK modules and a fairly substantial Kenwood too. I seem to remember there being no provision for bias adjustment. Maybe DC offset in those cases might be components deviating in value?
 
David_NC said:
Where would I do this? Is the output of each module the speaker jacks (actually screws, this case.?). That's where I tested initially.
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I believe he means the voltages at the various pins of the output modules (STK-054), rather than just the output voltages.
Hifiengine has a partial schematic for the STR-7025, which shows the the STK-054s in circuit.
There's also someone currently selling a PDF of the full service manual online for 8.99 / offer. I have no connection with that, but would consider it myself if a full manual isn't available elsewhere. (A search will bring it up.)
 
50 Green said:
I believe he means the voltages at the various pins of the output modules (STK-054), rather than just the output voltages.
Hifiengine has a partial schematic for the STR-7025, which shows the the STK-054s in circuit.
There's also someone currently selling a PDF of the full service manual online for 8.99 / offer. I have no connection with that, but would consider it myself if a full manual isn't available elsewhere. (A search will bring it up.)
Click to expand...
I'm not finding manuals - free or paid - for this model. I find quite a few for the next model up, the 7035, though. It would be good to know the voltage values for the pins. I can compare those of the 'good' right channel to those of the 'questionable' left channel. Would these be in reference to chassis ground or some other ground? I wonder if the STK-054's voltage measurements for this model would be the same as those in other models which use it. If so, I'll check to see what other models used the STK-054's. Thanks.
 
I'm really not all that familiar with the amp modules like the STK-054. Do they fail with increased levels of DC on the speaker outputs and increased hum and hiss? One thing I noticed when I was streaming from my iPhone through its headphone jack with the charge cable plugged in is that I got more hum that sounded like a 60Hz hum. Reversing the polarity of the receiver's power cord in the outlet made no difference. The only thing that eliminated it was to unplug the cable or use a battery pack to charge it.

50 Green said:
This comes up on the first page of a google search: https://www.webstore.com/item/Sony-STR7025-Receiver-Service-Manual/80989598
As I mentioned, I have no connection, however were it my receiver, the 8.99 (or whatever offer the seller will accept) would be well worth it for a PDF of a factory service manual.... if it's complete.
Click to expand...
Thanks for that. I'll check into it shortly.
 
David_NC said:
I noticed when I was streaming from my iPhone through its headphone jack with the charge cable plugged in is that I got more hum that sounded like a 60Hz hum. The only thing that eliminated it was to unplug the cable or use a battery pack to charge it.
Click to expand...

Sounds like RF interference from the phone's SMPS. Not likely any fault of your receiver. If you want to use your phone as a source, run it off the battery pack... or try plugging it into an outlet on another circuit from the receiver and use a longer cable to connect to the audio in (to keep the SMPS at a distance.)
 
Appears you have 2 separate issues,
- 250mV dc on left channel
- 60Hz(?) hum

The dc offset may be a case of "bs-in, bs-out" (leaky cap?)
so comparing voltages (L&R channels) at the STK pins may show the "bs-in"

DO NOT probe the STK pins, one slip=toast, probe at adjoining part and
check for any joining links, possibly use minnigrabbers or similiar if
sufficient space. No need to measure pins 1&3 as they are the same (power)
for both channels. Measure dc voltage, black probe connected to ground/chassis

Would be nice to confirm hum is 60Hz or rectified 120Hz.
Is hum present when using an old-school (well grounded) CD player into AUX?
 
50 Green said:
Sounds like RF interference from the phone's SMPS. Not likely any fault of your receiver. If you want to use your phone as a source, run it off the battery pack... or try plugging it into an outlet on another circuit from the receiver and use a longer cable to connect to the audio in (to keep the SMPS at a distance.)
Click to expand...
The odd thing is that I use this phone as a source in about six systems, and I've never had hum. I use it with this particular charger with another receiver, but it's much newer. The hum is present when I put the iPhone in airplane mode, which should turn off the radios. I'll try with another source.

mbz said:
Appears you have 2 separate issues,
- 250mV dc on left channel
- 60Hz(?) hum

The dc offset may be a case of "bs-in, bs-out" (leaky cap?)
so comparing voltages (L&R channels) at the STK pins may show the "bs-in"

DO NOT probe the STK pins, one slip=toast, probe at adjoining part and
check for any joining links, possibly use minnigrabbers or similiar if
sufficient space. No need to measure pins 1&3 as they are the same (power)
for both channels. Measure dc voltage, black probe connected to ground/chassis

Would be nice to confirm hum is 60Hz or rectified 120Hz.
Is hum present when using an old-school (well grounded) CD player into AUX?
Click to expand...
Thanks for the suggestions. I'll check the voltages with mini grabbers when I get back in town next week.
I can certainly check with a CD player, though it's not grounded. Would bonding the receiver and CD player chassis together suffice for this? I'll attempt to verify the frequency in the left channel, too.

Thanks guys!
 
David_NC said:
I can certainly check with a CD player, though it's not grounded. Would bonding the receiver and CD player chassis together suffice for this?
Click to expand...
Try using the CDP normally, it's just I've had hum issues with ixxx devices (only)
 
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