Hi All, I inherited a Sony STR-7800SD. I am excited to get it up an running, but need some help. It has high offset in the left channel (100mV) but the right channel is less than 10mV. I am new to vintage receiver restoration, and am looking for a copy of the service manual for this model. I have been to HiFi Engine, but their version doesn't have a schematic. Can anyone either point me in the direction to look for the cause of the high offset, or a complete manual?
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slimecity
Super Member
Hey there and welcome. It looks like you may need to get the manual from an AK user - I believe there may be somewhere here who has it.
There should be a couple of adjustable pots on the power amplifier PCB, one will be for bias and the other for DC offset (assuming this is adjustable). Does the unit have a relay and is it clicking the speakers "on?" 100mV isn't huge in the scheme of things and while it could ideally be lower, if the unit is working normally you may be OK until you can work out how to lower it.
There should be a couple of adjustable pots on the power amplifier PCB, one will be for bias and the other for DC offset (assuming this is adjustable). Does the unit have a relay and is it clicking the speakers "on?" 100mV isn't huge in the scheme of things and while it could ideally be lower, if the unit is working normally you may be OK until you can work out how to lower it.
Good afternoon! Does the unit function as is? Does either channel get warmer then the other? Sony used to suffer from heat stressed and cracked connections galore. Poor connections can cause increased resistance and changes in values. If it's playing do both channels sound clear, distortion free and balanced?
Thanks for the responses. I have adjusted the pots for bias according to the abbreviated service manual that I have. It does play, but sounds distorted and under powered. The left channel heat sink seems to heat up faster. There is distortion in the left channel and the speaker relay kicks in just below half volume. I don't believe there are any pots to adjust offset. It was dead when I got it, but was able to get it going with a power resistor and bridge rectifier replacement on the power board.
Ok, thanks for the update. Good news is you have a working channel that will serve as a "guide" for fixing the bad one. I'm pretty sure this is a DC coupled amp, so I always give the cautionary statement, to probe carefully! One wrong move, and they eat themselves! Chain reaction sort of. I also recommend having a variac in order to run amp and lower voltages so problems aren't as taxing on components as at full B+ That said, here we go.
I'm assuming you have checked both amp channels for cracked connections, heat stressed solder joints etc? Sony is not as good at soldering as they are at designing lol. The fact that it sounds "underpowered" almost hints at a driver or output transistor failure. One half of the "push pull" ckt is working. There should be two low value (3 ohms or less) high wattage resistors in the area of the output transistors. check both to make sure they are not open. Use the good channel as your guide. This can all be done while unit is off. Check each output transistor. If you have a "diode" test on your meter that's good to use. You should read approx .5-.7volts between the legs in two positions of the leads. Anything less or readings in both directions, indicate shorted transistor. If you only have one reading.. maybe open. If you get odd readings, I'd remove transistor from ckt and test, or use good channel as the reference. Check each resistor to make sure they are not open. The fact the DC level increases as you apply more signal also makes me think your missing a voltage on either a driver or output. Does this unit have two of the same transistors mounted on the heat sink, or is it one PNP and one NPN per channel? You can look up the numbers listed on each transistor on line.
What resistor did you wind up changing in supply? Is the resistor before the bridge or after? What does the other side of the resistor feed? A schematic would be an awesome help, and may wind up having to pay for it from one one of the manual sites. The best site I like the most HiFi Engine only has partial, and Elektrotanya has a copy of HIFI's manual. Looks like a beautiful unit!! Take your time and make measurements in the good and bad channel and let us know what changes. Good luck! You will make it work again!
I'm assuming you have checked both amp channels for cracked connections, heat stressed solder joints etc? Sony is not as good at soldering as they are at designing lol. The fact that it sounds "underpowered" almost hints at a driver or output transistor failure. One half of the "push pull" ckt is working. There should be two low value (3 ohms or less) high wattage resistors in the area of the output transistors. check both to make sure they are not open. Use the good channel as your guide. This can all be done while unit is off. Check each output transistor. If you have a "diode" test on your meter that's good to use. You should read approx .5-.7volts between the legs in two positions of the leads. Anything less or readings in both directions, indicate shorted transistor. If you only have one reading.. maybe open. If you get odd readings, I'd remove transistor from ckt and test, or use good channel as the reference. Check each resistor to make sure they are not open. The fact the DC level increases as you apply more signal also makes me think your missing a voltage on either a driver or output. Does this unit have two of the same transistors mounted on the heat sink, or is it one PNP and one NPN per channel? You can look up the numbers listed on each transistor on line.
What resistor did you wind up changing in supply? Is the resistor before the bridge or after? What does the other side of the resistor feed? A schematic would be an awesome help, and may wind up having to pay for it from one one of the manual sites. The best site I like the most HiFi Engine only has partial, and Elektrotanya has a copy of HIFI's manual. Looks like a beautiful unit!! Take your time and make measurements in the good and bad channel and let us know what changes. Good luck! You will make it work again!
Thanks W2ICA for the info. A schematic would be great as to see exactly where is should be testing. Unfortunately my knowledge is limited, but I think I followed most of what you are saying. I am unsure about the PNP or NPN transistors per channel, but I will do some research. I changed a 1 ohm, 5 watt resistor on the primary side of the power board, before the rectifier. I think it completed the circuit to power the relay in order to turn on the secondary circuits. I found a service manual on servicemanuals.net and they confirmed it included full schematics. I will go ahead and order it.
Well its been a while since I have been able to get back to the Sony. I had a friend of mine help, and with the schematic we identified three transistors on the amplifier board that needed replacing. The good news is that it now runs cool and doesn't trip the speaker protection relay. However it still shows 40 mVDC on the left channel. It sounds much, much better, but I think it still needs some work. I am not getting the warm full sound that I expected, but more of a bright (but not harsh) sound that seems light on bass. Don't get me wrong, its pleasant to listen to, but from what I have read should be warmer and fuller. I don't have anything to compare to, so for now I will keep on listening. Does it need to break in with the new transistors?
SiCo Amp
Member
Hi folks, I am having a problem with my 7800. The right channel is showing between 50 and 58 volts in way too many locations. I found one shorted 2SA678, but need to dig deeper. I pulled the drivers and outputs and despite the high voltages on all of their elements, they look good. Looks like a short is up stream somewhere. As you know, working without a schematic is tedious at best. Is there any chance that you can post of share the schematic that you obtained? Thanks.
petehall347
the brandy coffee man
how are the - voltages comparing ?
SiCo Amp
Member
One channel is normal and the other is shot - some points in the circuit of the bad channel are about 50 volts higher than they should be. Found a schematic buried in the forum last night and will dive into it in the next two days. Will report what I find. One thing that is notable... the schematic lists a 2SA733 in four locations where I have four 2SA678. The 2SA678 is used in the smaller STR-5800SD and STR-6800SD.