Setting DC Offset, Sony STR-7055

[50 Green]

Same thing has happened to me many times. Get to posting and folk come by, answer a few questions then nobody comes back...Don't know why, I guess they forget to watch the thread. Go ahead and adjust the bias on the channel that is way out of range but do so very slowly/carefully report your results for both channel bias/idle current and DC offset here and we will see what we can do from there. See you soon.

Will do. Thanks!

Rummaged around the electrical section in the big orange store, and found what I need to put together a DBT. Will do that before adjusting the bias..

 

[vsat88]

Of course you know not to try and use that when setting the bias right ?

 

[50 Green]

Of course you know not to try and use that when setting the bias right ?

Yes, thanks!

My plan was to turn the bias down with the receiver off, counterclockwise per the service manual, then bring it up on the DBT, with the DVM connected, to verify that's the right direction.

...I don't know how touchy those pots are or aren't, so I want to be sure which direction is down, before adjusting.

After I'm sure, I'll take it off the DBT, let it warm up 5 mins, and adjust the bias.

Thanks again!

 

[vsat88]

You are smarter than the average Bear..I think.. Mark where your pot is now that is what I was told. Also, if you are going to do it that way you might as well de oxit the pot before you adjust it out. I am not sure if that DBT would save your OPT if you do it that way now that I think it over. I do not know if yo would see it reflected by the bulb.... Hollar back at me.

 

[50 Green]

You are smarter than the average Bear..I think.. Mark where your pot is now that is what I was told. Also, if you are going to do it that way you might as well de oxit the pot before you adjust it out. I am not sure if that DBT would save your OPT if you do it that way now that I think it over. I do not know if yo would see it reflected by the bulb.... Hollar back at me.

Not sure either... I'm hoping the service manual is right. Maybe I'll just turn the pot a bit counterclockwise and check if the gain's gone down. Since it's low already, a little up (if the SM is wrong) should be safe.

I know firsthand that it's possible for the SM not to agree w/ the unit. The color codes on the leads to check the gain are reversed on my receiver. They are the correct leads (I traced them on the schematic) and the positions of the terminals for the check are the same as the SM.... just that the colors are opposite of what the SM calls for.

 

[50 Green]

OK, Big mystery now.

Before attempting to set bias, I checked the readings again. While checking, I realized I had the speakers connected when I took the readings before, so I checked again with the speakers disconnected. With the speakers disconnected, the channel that was reading ~53 now reads ~58, but the other channel (which read ~19 with the speakers connected) now reads 0.00!

I'm guessing that the zero bias reading might be a problem beyond mere adjustment, so I'll hold off on trying to adjust it until I hear from someone with more knowledge.

The bias trim pot is RT-701, and the bias reading is taken between the emitters of Q-901 and Q-901.



Can anyone help with what I should check to try and find the problem?

(Or should I start a new thread, since this seems now to be more about bias than DC offset?)

 

[Binkman]

Keep this thread going.....

If you are measuring correctly at points indicated in SM... the adjustments are usually pretty sensitive to your DMM. a slight turn can read several mv's. Might be advisable to replace with bournes trimmers. I guess I'm hitting at if the 0' pot does not change in readings and the ceramic resistors measure okay?

Just let us know. further test points may be necessary, too.

 

[50 Green]

Keep this thread going.....

If you are measuring correctly at points indicated in SM... the adjustments are usually pretty sensitive to your DMM. a slight turn can read several mv's. Might be advisable to replace with bournes trimmers. I guess I'm hitting at if the 0' pot does not change in readings and the ceramic resistors measure okay?
Just let us know. further test points may be necessary, too.

Thanks, I'll continue on with this thread.

Yes, I am measuring correctly at the points indicated in the SM.

I haven't attempted adjusting the pot on the channel that reads 0.00. I'm thinking the 0.00 reading might indicate something more serious is wrong, so I'm reluctant to make any other changes before trying to isolate the problem.

Which ceramic resistors resistors should I check, and can they be measured in place? (I can post other sections of the schematic, if they're not shown in the section I posted.)

Are there other test points you'd recommend checking?

To check the voltages indicated at various locations on the schematic, do I test between the indicated point and the chassis?

Thanks again!

 

[Binkman]

To check the voltages indicated at various locations on the schematic, do I test between the indicated point and the chassis?

Yes and look for a good ground. some of sony's have ground screws etc. but find neg probe to a pos test point If you read negative v's that's the neg. of that circuit. ie. if you reverse the probes your dmm will read pos. v's. or mvolts

Another thing is your DMM may need a new battery or can't measure accurately. Very important.

So I would recommend a lot of shi* but I don't want to screw up the unit. I.e. bias at milliavolts versus some crap meter that can't read mv's. 000?
change the setting to min on mv dc.

Other notes on sony prints are blocked in the sm........ these are called 'basic' voltages. basic is pretty much a guess. can vary a few mv's.. but go back to the rail voltages.

any luck adjusting the pots is Key! 000? You should see the meter mv read out.

 

[50 Green]

Yes and look for a good ground. some of sony's have ground screws etc. but find neg probe to a pos test point If you read negative v's that's the neg. of that circuit. ie. if you reverse the probes your dmm will read pos. v's. or mvolts

Another thing is your DMM may need a new battery or can't measure accurately. Very important.

So I would recommend a lot of shi* but I don't want to screw up the unit. I.e. bias at milliavolts versus some crap meter that can't read mv's. 000?
change the setting to min on mv dc.

Other notes on sony prints are blocked in the sm........ these are called 'basic' voltages. basic is pretty much a guess. can vary a few mv's.. but go back to the rail voltages.

any luck adjusting the pots is Key! 000? You should see the meter mv read out.

Thanks for your reply, Binkman.

My DMM is good, and battery is fresh. It autoranges, and is definitely on mV scale when I take the readings. (I check.) Same DMM immediately reads bias for other channel in mV. I've tested back and forth between the two channels several times to confirm. (I wasn't ready to believe 0.00 at first, either.)

Could you explain what you mean by "Other notes on sony prints are blocked in the sm." I'm not sure I understand. I have the original (USA/Canada version) FSM (actual physical manual) if that helps.

I'll get in there and read voltages, working back. The ground screw on the back of the unit would be a good ground point to use, then?

I'm also reading up on how the bias circuit is set up, hoping to understand better, what/where to check. If you (or anyone) are familiar with this circuit and have any tips, I'll appreciate any an all info.

I'm a bit busy with work for a couple of days, but will get to it as soon as possible, and post back when I have.

Thanks again!

 

[Binkman]

Just to clarify.. indicated dmm hookup test points to set or check bias.

For testing other circuits i.e. rail voltages etc.. noted with the rectangular box are called 'basic' voltages. they may be erroneous because as sony sm's can be a headache. but again the neg probe is grounded to the chassis and pos to point of a circuit to test. You should put some elec tape on the pos probe tip to only reveal the very tip end as to avoid any shorts. Certainly does not hurt to rehearse your testing before powering on as requires a steady hand or placing a shielded test clip with power off. make notes take pics, too. You can be called away and lose your reference. (take your time)

Ground points on the chassis? In many sony's you will see a pcb ground screw. The screw on the back of the unit may not be a good spot. And you should check those pcb ground screws are indeed set well and I choose a spot that is convenient and close to that pcb ground screw.

 

[50 Green]

Thank you. Excellent advice. I like the electrical tape idea on the test probe, as well as rehearsal and notes. I was thinking of testing on the back of the board, since there are fewer chances to accidentally contact something (I think.)

I've definitely taken the bias readings at the test points indicated in the SM. (And double checked them by following the wires to the output transistors.)

I understand that the readings at the test points might not agree exactly w/ the SM. I'll try to compare with the "good" channel, to find a point where readings begin to differ.

I'll have to look for a PCB ground screw. The power / amp board is mounted w/ plastic rivets, so if there's a ground for that board, it must be a wire to the chassis, somewhere.

It'll be a couple of days before I get a chance to get back in there... and maybe more to carefully go through it, but I'll post back as soon as I have (or haven't) found further clues.

Thanks again!

 

[vsat88]

Finding a good ground should be pretty easy. Look for exposed metal and use a clamp ; ) You could use your Ohm/Resistance scale on the meter to check it a different places . The resistors are usually 5 watt square sand resistors (on the amps I have worked on anyway) looks like to me they should read about a 1/2 an Ohm. (0.47) and the Ohm resistance scale on the meter can be used to check those resistors to compare channel to channel while you find your good ground . Please keep in mind I am a hobbyist and know little of Sony receivers. Personally I would mark that Pot and try turning on it a bit. Hell, if its at 0 what could it hurt ??

OH, BTW..Please look at Binkman's post count then compare it to mine. You will see very quickly that I have no where near the knowledge about this that he has. So, If I were you I would take his suggestions before mine. I am mainly here to learn and try to be helpful, If I am not then tell me so. I don't offend easily !

 

[50 Green]

Finding a good ground should be pretty easy. Look for exposed metal and use a clamp ; ) You could use your Ohm/Resistance scale on the meter to check it a different places . The resistors are usually 5 watt square sand resistors (on the amps I have worked on anyway) looks like to me they should read about a 1/2 an Ohm. (0.47) and the Ohm resistance scale on the meter can be used to check those resistors to compare channel to channel while you find your good ground . Please keep in mind I am a hobbyist and know little of Sony receivers. Personally I would mark that Pot and try turning on it a bit. Hell, if its at 0 what could it hurt ??

OH, BTW..Please look at Binkman's post count then compare it to mine. You will see very quickly that I have no where near the knowledge about this that he has. So, If I were you I would take his suggestions before mine. I am mainly here to learn and try to be helpful, If I am not then tell me so. I don't offend easily !

Thanks vsat88.... Appreciate the help!

I haven't had a chance to get in there yet... Want to have a big enough block of time that I'm not rushing... Makes sense to use the ohm meter to find a good ground.
I guess, too, that there's no harm in trying to adjust the bias... Seems odd that it would read 0.0, though. I'll probably trace back a little, at least, in that circuit, to see if I can come across anything that's way out of spec....

I'll post up results as soon as I have something to tell.

Thanks again!

 

[vsat88]

Go for it Brother !

 

[50 Green]

OK, I took some time and took a lot of readings.... Many are way off, so rather than posting up a bunch of numbers, if anyone could give an idea of what the most important readings would be to help diagnose, that would be a huge help!

A couple of readings that might tell something (I hope!): First, the + and - rails, instead of reading 50V / -50V read ~ 38V / -38V.

Also, I can adjust the bias to read 50 mV for each channel, but something must be way off... The test points for each channel should read 50mV between them, with one test point reading +25mV to ground and the other -25mV to ground. When I adjust the bias pots to to get a reading of 50mV between the test points, the test points to ground read -120 / -74.5 for one channel and -143 / -93.5 for the other... (ALL NEGATIVE!)

Thank you in advance for any help you might be able to offer!!

 

[Binkman]

Please note that sony sms can be erroneous. see**** below. Note your dmm with reversed leads +/- will display - or + so these might be erroneous per sm. I also assume that pots and cleaning has been done.

As I interpret the bias adjustment diagram with meter leads to one side and the other are independent. You measure one side then the other. many techs will hook up two meters. there is no ground chassis measurement indicated in the sm. seems like you did adjust 50mv on the pots at full power after warm up. (sm says use a variac.. variable transformer but I assume the DBT test was successful. the bulb dimmed?) back to page one you said you're not blowing fuses so I assume p.s. is okay but no shorts in audio path.

****As far as rail voltage reading 30vdc versus 50vdc may be normal as I've seen that at idle but when load is applied it kicks up to rail v. as far as measurements being off sm says these v's are avg. i.e. you may read 51.1 and 49.9 vdc on another channel per same ic left or right channel.

Take some measurements from pdf pg 40.. note the top of page to output transistors and thermal compensator ic's.. get readings.. as long as fairly even per side i.e. 0.025vdc per outputs noted and the compensator v's.

 

[50 Green]

Thank you Binkman.

Yes, DBT was successful. I measured the bias as specified in the SM, but when it was so far off, I checked the voltages specified on the schematic for each of the individual test points to ground. (Those test points, BTW are the emitters of the output transistor pairs.)

Will get those measurements to you asap, but not sure if I'm looking at the right manual page... Page 40 of the manual's numbering is just the first page of the parts list. and page 40 of the PDF's numbers is the last page of the manual... a blank page.
I'm looking at the PDF that's on hifiengine... Is that the same one you're referring to?

Thanks again!

 

[Goldie99]

I don't see the problem - the emitter voltages you're referring to (+/-25 mV) on the schematic are versus a mid-point value of 0, at the output, but you said in your original post that you had dc-offsets of ca. 100mV in both channels (on the outputs), so I'd expect the +/- 25mV values to be displaced accordingly.
I'd have more concern re. the main rail voltages, but I don't know the amp.

 

[50 Green]

Thank you Goldie99. That gives me some additional perspective. (Please excuse my lack of knowledge. I'm a complete novice, just reading furiously all I can, trying to learn.)

Do I understand correctly that the emitter voltages can be expected to be skewed like they are, due to the DC offset?

Would you also concur that the ~ 95-105mA of DC offset is acceptable, and I should just leave well enough alone.... or is that something you would want to address?

As for the low rail voltage, I'll greatly appreciate any help as to how serious an issue it might be, and possible causes / solutions.

(If it's helpful, I can post the schematic for the power supply / amplifier board.)

Thanks again!!