zenith2134
Addicted Member
I've said it before, Hitachi is such an underdog in the hifi game. One of my favorite secondary integrateds is the HA-6 from '83. Amazingly low-noise phono section and a ballsy output stage. Can be found CHEAP too.
Hitachi SR-903 Restoration
- Thread starter Fredrik 1977
- Start date
CoolOhm
Cool Ohm
Update...
Replaced those diodes and then still the same failure to come out of protection. Good news though, the rapid heating of the output transistors Q711 and Q712 on the right channel is no longer an issue.
So I started probing around further, and found two more suspect transistors. I think these are drivers, TO-220 package. They are identified in purple in the attached updated diagram.
The unfortunate part is that these too appear obsolete. I can find suitable replacements, but not in complementary pairs.
Can anyone suggest a suitable modern alternate to the NEC B628/D608? Also known as 2SB628/2SD608...
Also, Happy Near Year everyone! Enjoy the music in 2018!!!
Replaced those diodes and then still the same failure to come out of protection. Good news though, the rapid heating of the output transistors Q711 and Q712 on the right channel is no longer an issue.
So I started probing around further, and found two more suspect transistors. I think these are drivers, TO-220 package. They are identified in purple in the attached updated diagram.
The unfortunate part is that these too appear obsolete. I can find suitable replacements, but not in complementary pairs.
Can anyone suggest a suitable modern alternate to the NEC B628/D608? Also known as 2SB628/2SD608...
Also, Happy Near Year everyone! Enjoy the music in 2018!!!
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CoolOhm
Cool Ohm
CoolOhm
Cool Ohm
I noticed that NTE, the original manufacturer lists some replacements and these are available directly from them.
The ON SEMI alternate suggested by redk9258 does seem to match more closely (hfe), whereas the NTE material seems beefier (higher ratings). Which is more important? BTW I can also get supposed original NTE parts from China via ebay at about the same price...
I made the attached comparison table showing the original specs, followed by the NTE and ON SEMI replacements...
The ON SEMI alternate suggested by redk9258 does seem to match more closely (hfe), whereas the NTE material seems beefier (higher ratings). Which is more important? BTW I can also get supposed original NTE parts from China via ebay at about the same price...
I made the attached comparison table showing the original specs, followed by the NTE and ON SEMI replacements...
CoolOhm
Cool Ohm
Here is an update. Got those Fairchild KSA940/KSC2073 replacements and installed them. Not much change. The unit came out of protection but took very long to do so (about one minute, wheras it would normally come out in about 3 sec), then pretty quickly went back into protection.
At this point, the only thing I can see as suspect are a couple of resistors, only one of which was in the path of the short (R733 and R712L) that appear to be reading low when tested in circuit. I thought damaged resistors would read high or infinite resistance, but not low. So I am wondering if what I am seeing are actully typos in the circuit diagram (already have spotted a few).
So I can play around with those a bit, but I am more inclined to play with the protection circuit itself. Maybe be thermal switch is flaky? Regardless, I would love to bypass it temporarily to be able to set the idle current - which must need doing with virtually all transistors in the amplifier replaced with modern equivalents.
Best regards,
D
At this point, the only thing I can see as suspect are a couple of resistors, only one of which was in the path of the short (R733 and R712L) that appear to be reading low when tested in circuit. I thought damaged resistors would read high or infinite resistance, but not low. So I am wondering if what I am seeing are actully typos in the circuit diagram (already have spotted a few).
So I can play around with those a bit, but I am more inclined to play with the protection circuit itself. Maybe be thermal switch is flaky? Regardless, I would love to bypass it temporarily to be able to set the idle current - which must need doing with virtually all transistors in the amplifier replaced with modern equivalents.
Best regards,
D
CoolOhm
Cool Ohm
It's alive! I assembled this beast and tested it tonight through my JBL Decade 16's. It sounds amazing and is very loud. Here is a picture so show off the LEDs. Every single incandescent bulb for illumination and function indication was burnt except the AM one. Really pleased with the look.
The right channel overheating as it turns out was caused by the trim pot wiper not making contact. I spent over a day trying to find what might be wrong, then finally gave up and tried adjusting the pot even though I was getting 240 mV with the pot turned right down. As soon as I turned it up a bit the current plumetted and the output trannies cooled right off.
To all those on this thread who contributed advice and suggestions - a huge thank you!!! I would have been lost without your help, and I learned a great deal fixing that blown left channel.
The right channel overheating as it turns out was caused by the trim pot wiper not making contact. I spent over a day trying to find what might be wrong, then finally gave up and tried adjusting the pot even though I was getting 240 mV with the pot turned right down. As soon as I turned it up a bit the current plumetted and the output trannies cooled right off.
To all those on this thread who contributed advice and suggestions - a huge thank you!!! I would have been lost without your help, and I learned a great deal fixing that blown left channel.
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The Fuxtor
Addicted Member
That's a beauty!! Nice job!
Fredrik 1977
Super Member
woah! great to see someone had somesort of help of my old thread!
thats the point of all my receiver threads. my 903 was traded away for other HiFi stuff that needs attention in some way.
the dude using it now is super happy. best receiver ever! he says.
thats the point of all my receiver threads. my 903 was traded away for other HiFi stuff that needs attention in some way.
the dude using it now is super happy. best receiver ever! he says.
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CoolOhm
Cool Ohm
Oh Man. Your thread was a total lifesaver. I just found it because this whole ordeal started out as a simple LED conversion... Then zap! oops! I felt cheap hijacking your thread but it sure worked out for me. Thanks so much! That old beast is working great and my brother is totally happy, and blasting music again like when he was a kid!
D
D
Hi
I know this thread is over 1 year old. I picked up a Hitachi SR 803 a few months ago. It needs replacement lamps for the dial area, functions indicators. I saw a seller on eBay selling axial LEDs for the Hitachi receivers
These are the type of LEDs with the mini board and you just solder the terminals. The function indications are mini LEDs with red and white leads. Has anyone tried this out? and what were the results? Thanks
I know this thread is over 1 year old. I picked up a Hitachi SR 803 a few months ago. It needs replacement lamps for the dial area, functions indicators. I saw a seller on eBay selling axial LEDs for the Hitachi receivers
These are the type of LEDs with the mini board and you just solder the terminals. The function indications are mini LEDs with red and white leads. Has anyone tried this out? and what were the results? Thanks
Stefanrsko
New Member
Hello 
I am quite new at AudioKarma but found this topic about SR903 restoration. Great thread anyway! I own same same Hitachi receiver but mine is black ;-)
Bought it about half year ago. Till now i cleaned volume pot cause was making noises and also fixed light bulbs. Simple changed them for LEDs. Did nothing with caps, etc. I was considering some deeper restoration, however I wasn't decided because it was working very well, only some minor noise i can hear(nevermind if volume was up or down) on both channels.
Since few days there is more constant noise coming from left channel unfortunately. I love this gear so decided to make restoration same same as on this topic.
I have few questions i would like to ask But on the beginning need to say that i am not very experienced with electronics, soldering. However i restored minor vintage gear before and i think i can handle this one too.
Ok, so: why replacing Hitachi C1344 transistors with BC546B? Also, why add an 1uf MKT to 3.3k resistors to main caps? Last one: why doubled main capacitors from 6800 to 12000 etc? Is it significant improvement? I am asking because those caps are 1/2 price od restoration parts costs
I am aware that those questions are mainly aimed to the author od this thread however i sent private message (no answers yet) but the author was last seen here in 2019 so I am asking everyone of you dear users
Od course i will start new thread with my own restoration. Now i put questions here because they are connected with authors posts.
Thank you all in advance.
I am quite new at AudioKarma but found this topic about SR903 restoration. Great thread anyway! I own same same Hitachi receiver but mine is black ;-)
Bought it about half year ago. Till now i cleaned volume pot cause was making noises and also fixed light bulbs. Simple changed them for LEDs. Did nothing with caps, etc. I was considering some deeper restoration, however I wasn't decided because it was working very well, only some minor noise i can hear(nevermind if volume was up or down) on both channels.
Since few days there is more constant noise coming from left channel unfortunately. I love this gear so decided to make restoration same same as on this topic.
I have few questions i would like to ask
But on the beginning need to say that i am not very experienced with electronics, soldering. However i restored minor vintage gear before and i think i can handle this one too.Ok, so: why replacing Hitachi C1344 transistors with BC546B? Also, why add an 1uf MKT to 3.3k resistors to main caps? Last one: why doubled main capacitors from 6800 to 12000 etc? Is it significant improvement? I am asking because those caps are 1/2 price od restoration parts costs
I am aware that those questions are mainly aimed to the author od this thread however i sent private message (no answers yet) but the author was last seen here in 2019 so I am asking everyone of you dear users
Od course i will start new thread with my own restoration. Now i put questions here because they are connected with authors posts.
Thank you all in advance.
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Fredrik 1977
Super Member
Ok, so: why replacing Hitachi C1344 transistors with BC546B? Also, why add an 1uf MKT to 3.3k resistors to main caps? Last one: why doubled main capacitors from 6800 to 12000 etc? Is it significant improvement? I am asking because those caps are 1/2 price od restoration parts costs
I am aware that those questions are mainly aimed to the author od this thread however i sent private message (no answers yet) but the author was last seen here in 2019 so I am asking everyone of you dear users
Od course i will start new thread with my own restoration. Now i put questions here because they are connected with authors posts.
Thank you all in advance.
I use "BC" transitors because thats what we use in Europe
the 1uf MKT on the maincaps is called bypass caps. i just put them there for good meashures
its not gonna improve anything...Really.main cap size is determined by physical size. so they fit the clamps. its been 40 years gone by and caps today pack more uf per size than in 1980.
The amp is still in its new home and working perfectly.
The Fuxtor
Addicted Member
Hey ... Where ya been?? Nice to see you posting again!I use "BC" transitors because thats what we use in Europe
the 1uf MKT on the maincaps is called bypass caps. i just put them there for good meashures
main cap size is determined by physical size. so they fit the clamps. its been 40 years gone by and caps today pack more uf per size than in 1980.
The amp is still in its new home and working perfectly.
Fredrik 1977
Super Member
Well i sort of took a break now i have loaded up the batteries and the stack of units in need of repair.
now i have loaded up the batteries and the stack of units in need of repair. 
JB6789
New Member
I own two SR-903's and bought for each the LED lighting kits (7x indicator lights + 6x panel lights) on eBay; for about $90 in total, incl. international delivery charges. Did the install this afternoon on the first unit...piece of cake. Checked polarity for the indicator lights with the multi-meter. Fantastic results. Very happy with this necessary "upgrade".
JB6789
New Member
I do question increasing the capacitance on the filters that significantly. I was of the opinion it is OK to incease a bit, maybe 10-20% depending on parts availability, but to target the factory spec on capacitance. However, it is my understanding that it is OK to increase on the voltage side. I went with 2x 6800uF/100V/20%/105C (DigiKey, 338-381LL682M100N062-ND; Cornell-Dubilier) from the factory 6800uF/80V spec. For the other filter I kept to the 5600uF/40V/20%/105C factory spec (Digi-Key,399-19618-ND, Kemet).
