Making the best out of my butchered G7700

So here we go.....................
This is the first 7700 I bought which had the dime-store kitset poweramp fitted.
After months of pondering and doing a major on the second one I bought I have decided to get this thing going.
All of the power transistor heatsink assembly and boards were long gone, the only thing left was the external heatsink.
I sourced the correct outputs, found another driver board which I have had to completely re-transistor, its from the later model so I have had to add extra Base resistors, Ive got 10 turn pots for the DC offset, and ive made up a VBE circuit to regulate the bias current.
I have had to do "the best out of a bad situation" with the OP's, Ive done tag board for the emitter resistors and OP's, I dont think it looks too bad, not original but at the end of the day "electronically" it will be a Sansui again..........

Looks like a good bit of fabricating there :yes: Considering what you had to work with!
Just out of interest where did you get those output transistors from?

I think your fabrication work looks fantastic. You should be rightly proud of it. Hope all sounds well once it's together.

Scott T.

kevzep said:

So here we go.....................
This is the first 7700 I bought which had the dime-store kitset poweramp fitted.
After months of pondering and doing a major on the second one I bought I have decided to get this thing going.
All of the power transistor heatsink assembly and boards were long gone, the only thing left was the external heatsink.
I sourced the correct outputs, found another driver board which I have had to completely re-transistor, its from the later model so I have had to add extra Base resistors, Ive got 10 turn pots for the DC offset, and ive made up a VBE circuit to regulate the bias current.
I have had to do "the best out of a bad situation" with the OP's, Ive done tag board for the emitter resistors and OP's, I dont think it looks too bad, not original but at the end of the day "electronically" it will be a Sansui again..........

Click to expand...

Don't forget to update your schematic with the changes....:thmbsp: looks good.

Very nice. Did you attach it all to the back of the heat sink?

- Pete

Glenz75 said:

Looks like a good bit of fabricating there :yes: Considering what you had to work with!
Just out of interest where did you get those output transistors from?

Click to expand...

Got them from the Sansui Parts shop in Denmark.........NOS sankens and for a good price too, but I notice his prices are going up as his stock is depleted.

I will be updating my schematic, its amazing what dosent stay in your head.

Thanks for the encouragment, I was worried it looked a bit "home built" but I think it will do the job..........................
Yes I did attach it to the back of the Heatsink, I had some mylar insulators for some TO3's which I modified to work with OP's Im using.........and then I used some heat resistant washers to space the tag board off the heatsink...

It wasnt long after that I managed to fit into the chassis, had to drill a few holes but it worked out OK I think............
It all looks quite normal from the back, and there might even be a bit more room to move on the inside..............

Still waiting for some transistors to turn up............apart from that, its all wired up and looking good to go.
I will update more as soon as its running.......................

Looking good.

In the first pics, the left output looks pretty close to the heat sink.
Maybe it just looks that way in the pic.

Rob

Robisme said:

Looking good.

In the first pics, the left output looks pretty close to the heat sink.
Maybe it just looks that way in the pic.

Rob

Click to expand...

Yes they are quite close but its safe.................I hope, I will find out soon............

Well bit of a story.......................
So I got it all together, I measured the OP's and supplys and made sure I was in good shape.
Double checked my wiring, and on the way found some discrepencies in the schematic, anyway I was good to go.
So I started to wind the variac up..................now remember, the driver board I have in this amp came from the later version 7700. I looked long and hard at the powersupply section because I was a bit worried I might take something out in the tuner section. It says in the bulletin that the digital boards from the tuner section are not interchangeable...........the first thing that happened was a capactitor on the tuner board exploded, so I stopped and unplugged the tuner board and the digital display board. Ive got some work to do finding out about the tuner supply voltages I think. They must have changed something when they changed the tuner section in the later version............Thats fine I can handle that.
Back to the amp. So I made a VBE multiplier to regulate the bias current. Now in theory I had to wind the trimmer fully clockwise to obtain minimum bias..........so what did I do? exactly the opposite of what I should have.
So as I wound up the variac everything was looking good and I had two meters on each channel looking at the Bias current........all good I kept winding it up........and then when I got to about 80% line voltage, the bias took off and ran away very very quickly, the OP's got very hot, I couldnt understand what was going on, and before I had time to turn it off I heard the powersupply growl as the OP's shorted out.
Great!! The last ones in the world well vitually you know what I mean....
So while I was at work today I was looking in our old stock of spare parts for amps we do not own any more, and I found two bags of Toshiba transistors. They are 2SC3281 and 2SA1302 (complimentary black NPN/ green PNP).........so I downloaded a datasheet and discovered they are very similar to the 2SC2581/2SA1106 used in the Sansui. The Toshibas are higher voltage higher current and faster devices.
So I grabbed the bags of devices 20 in each bag, Great score!!!
They were originally for the Carver PT1250 amplifiers we had 15 years ago, the amps are long gone now so I was able to score these OP's.
I got home and loaded them in, I then discovered my error with my trimmers, I corrected that and measured everything up and started to wind up the variac.................same thing monitoring the bias on both channels, this time we had success, it ran. I set the bias at 6mv for starters, then I realised the relay haddnt turned on, so I looked at the DC, oooppps!! 15VDC on the left channel, the right channel I managed to get down to 0-2mv, but the left channel, I quickly traced it back to the JFET, I took it out and put another one in, and we were off, I got that one down to 0-2mv as well.
I need to modify the VBE circuit a bit to get a better adjustment sensitivity for the bias........but its in the ball park, I could afford to run it a bit higher with these OPs, they can handlw 30% more current than the sankens........
It sounds great on my test speakers, I have to run it next to the other one on the JBLs to compare, it was throwing out 42V from the output before I could see any clipping on the scope.........................Awesome, might end up eclipsing the other one!!
So I have to find out what is going on with the tuner section, and there is a bit of a hum in the right channel, originating from the preamp. Could be caps. This poor amp has had such a hard time with its heart being ripped out all those years ago, but its coming back to life bit by bit.
To add to everything the digital display actually works on this amp!!
I also want to LED the panel lamps, nice blue ones or something like that, but we will see, Ive got to get that tuner working..............
I didnt take pictures but I will see what I can do in the morning...............
Its all good!! You guys over there in America should be able to hear it if you put your head out the window...............

I'm impressed with your dogged determination Kev! OT.....I would love to hear more about your impressions of NZ perhaps by PM.:yes:

I took a shot showing the Toshiba outputs crammed in there, I still have to finish mounting them properly, Ive made an alloy bracket to go over the top to clamp them down, bit more thermal goo and its good to go...........
Im going to work on the tuner today after I have played around with the resistor values in the bias circuit..........
Okay this might sound over the top, but I ran it to 42V from the out puts with an 8ohm load, that seems to work out to be 220watts, now I could not see any distortion at all in fact I backed it off to be sure it wasnt distorting.....wish I had a distortion analyser...................
I know when the HiFi magazine reviewed the G7700 back in the eighties they claimed it was throwing out 160watts, I dont know any details, I just know I read it somewhere............
Now with these Toshiba outputs it seems to go a step further.........
Im having a bit of fun here...........I just hope I have another triumpant day with the tuner!!

This is what we do in New Zealand, we make stuff from other stuff not ment to go with that stuff, but we do it anyway and make it better!!

Here it is working.............the Tuner Lives!! Including the dreaded unreliable digital display.................
I had to change a cap on the tuner board to a higher value and also change the digital powersupply to different voltage rails on the Driver/supply board, they have changed a couple of things in there on the revised version of the F2980, but against the advice in the service bulletin you can ineed use the revised F2980 board in the earlier amps, you have to add Base resistors for the extra output transistors, as well as the above mentioned changes the the voltage rails.
The cap is C28 on the F2988 tuner board which goes from 100uf 16V to a 220uf 16V.
I think thats it.
I have been monitoring the bias current all morning and I have had a look on the scope for crossover distortion, I cant see any evidence of any so Im going to leave it the way it is, I think it sounds better than my other one!!
To boot the OP's run fairly cool, the heatsink only gets lukewarm when its getting a thrashing.............man Im gona stick the lid on it and give it heaps!!!

Congratulations!! That is a bit of dogged determination. It must feel great to get it working.

- Pete

LBPete said:

Congratulations!! That is a bit of dogged determination. It must feel great to get it working.

- Pete

Click to expand...

Ive never felt so victorious in my life.............I thought it was all over when I fried the sankens (what a shame) but such is life when one enters uncharted territories with a new circuit idea (the vbe multiplier), so I remained philisophical..........and then found those toshiba outputs.............
Im suprised you havent heard it over there, Ive got it on my JBL ES90's at the moment................ cranking!!

I'm impressed! You've got some determination there...:yes: I think I would have given up if it was mine, but then if its something special I would keep at it, like my AU20000 now that a right 'prick' of a fault which literally took me six months to finally fix, it was a intermittent which would happen occasionally and it took outputs with it if you weren't holding your mouth right.... Here is the link to the post, its a very condensed version of what actually happened... http://www.audiokarma.org/forums/showthread.php?t=172151

Congrats on that G7700 resurrection I take my hat off to you

Cheers
Glen

Glenz75 said:

I'm impressed! You've got some determination there...:yes: I think I would have given up if it was mine, but then if its something special I would keep at it, like my AU20000 now that a right 'prick' of a fault which literally took me six months to finally fix, it was a intermittent which would happen occasionally and it took outputs with it if you weren't holding your mouth right.... Here is the link to the post, its a very condensed version of what actually happened... http://www.audiokarma.org/forums/showthread.php?t=172151

Congrats on that G7700 resurrection I take my hat off to you

Cheers
Glen

Click to expand...

Cheers Glen, I never thought I would see this one live again, but after the other one I looked at what I had left over and thought well it looks like ive got enough here to reserrect this one, and after enjoying the sound of the first one I did, I thought it was worth having a go at.........they really are very precise sounding amps..........and with my Bias circuit my mate got me onto it was always going to be risky with the first fire up........
But then to keep cool and remain philisophical when I fried those Sankens.........took me on that search at work through our old spare parts for amps we dont own any more brought me those awesome toshiba OP's.....I think they may be obsolete now but Ive got 10 pairs so Im all good for spares.........
This one has more authority in the sound than the other one.......must be the toshibas I think..............

I will have a look at your link..........I think its good to post these long drawen out projects, I hope it gives people hope, and to keep going and never give up!!..............
Im listening to that amp now, and its become my favourite......The Pioneer SX-980 is with my daughter playing death metal now, and wierd thing is she loves the pioneer, she thinks it looks "cool"...............
Everyone is happy!!

Wow...Kevin....just read upon your 7700 thread. VERY nice work! If you can get your'un goin', then I'll certainly give my 7500 a go.

If I get desperate............got any of those outputs left?

Hadn't seen this thread yet, Kev.

Good on ya for makin it live again!