Sansui G-7500 Fried Resistors F2980 Board

Good info & documentation!

Are the 100W resistors in the right channel still getting hot?

I am a little concerned that your 72W DBT is knocking your rail voltage down from ±63vdc to ±13.8, although that 22% of nominal is in line with the ~25% I see on other voltages. You currently have 100Ω resistors installed for the 0.33Ω emitter resistors, so it should be safe to back off the DBT a bit. Do you have a 100W incandescent? Anyone else have a comment?

Before you do that I'd like to consider the high voltages around the OP TRs in the right (lower) channel. There is 13.0vdc on the base of both OP TRs, as opposed to the 1.16vdc and 0.605vdc in the left channel. you can see that the L ch OP TRs are just barely conducting because the voltage at the emitters is 0.875vdc and 0.873vdc (I think that the 0.873 should be negative). The R ch OP TRs are conducting with a gain of 1.0 or they both have a collector to emitter short or ???? because they both have the same voltage on the C & E. This is probably causing the high voltage across the 100Ω emitter resistors. Please clarify if the 27.4vdc was read across R72, or across R72 & R78.

The voltage on the base of the R ch OP TRs is way too high, and probably related to TR 16 & 18. It looks like whatever voltage is on their collectors is getting passed on to the bases of the OP TRs.

Sorry, but I am not good enough at this to draw better conclusions without being "hands-on". I'm hoping that someone else can point out something that I am missing.
 
ghazzer said:
Sorry, but I am not good enough at this to draw better conclusions without being "hands-on". I'm hoping that someone else can point out something that I am missing.
Click to expand...

I'd like to see the OP get rid of that bulb he is using and use a standard incandescent one for a start. The helpers are having to constantly guess at things because of that unknown variable.

And likewise, I could very likely fix it if was on my bench, but it isn't. I am really not so good at guiding people through repairs in this medium. :)
 
Hyperion said:
I'd like to see the OP get rid of that bulb he is using and use a standard incandescent one for a start. The helpers are having to constantly guess at things because of that unknown variable.

And likewise, I could very likely fix it if was on my bench, but it isn't. I am really not so good at guiding people through repairs in this medium. :)
Click to expand...

I would love to get another bulb and I look everywhere I go. The only filament bulb I have been able to find is a 200W. Is that too much? I was out again this morning and the only filament bulbs they had were 200W. I measured the primary voltage, with a 100W halogen in my DBT and it was 36.3vac. The 100W halogen uses 72W. The primary voltages measured 27.34vac (2 red wires), 25.91vac (2yellow wires), and 2.4vac (2 purple wires).

The 100W emitter resistors still get a little hot but not nearly as much when I replaced R51 on the right channel.

The 27.4vdc was read across R72 & R78. The voltage across R72 was 13.7vdc and across R78 13.6vdc.

I will verify the readings at pins 59 and 60 (+ or -).

Any ideas on where I can get a bulb?
Thanks!
 
You wont get the Green safely operate indicator to come on when on the DBT.....It might come on with a 200watt bulb though...
 
I know this is a kludge, but you could string two 60W or three 40W in series. Seems like we always had more of the smaller bulbs on the shelf.
 
Thank you ghazzer for all the help you have provided. This will be my last post on this thread. I threw in the towel on this receiver. It was the first piece of equipment that I failed to bring back to life even though I am just a novice. With the help of ak members who have, and work on Pioneer equipment I have been successful at restoring a SX-636, and SX-737. I know I’m probably going to get flack from my parting comments but I have to say that (ghazzer being the exception) that I feel I didn’t get the help from the Sansui techs that I got from the Pioneer techs. All the Pioneer techs and novices were all super nice and seemed to enjoy helping. The Sansui techs acted (at least it seemed to me) like I was trying to take food from their mouths. I apologize for taking up so much of their precious time.
 
Young46 said:
Thank you ghazzer for all the help you have provided. This will be my last post on this thread. I threw in the towel on this receiver. It was the first piece of equipment that I failed to bring back to life even though I am just a novice. With the help of ak members who have, and work on Pioneer equipment I have been successful at restoring a SX-636, and SX-737. I know I’m probably going to get flack from my parting comments but I have to say that (ghazzer being the exception) that I feel I didn’t get the help from the Sansui techs that I got from the Pioneer techs. All the Pioneer techs and novices were all super nice and seemed to enjoy helping. The Sansui techs acted (at least it seemed to me) like I was trying to take food from their mouths. I apologize for taking up so much of their precious time.
Click to expand...

The problem with the G7500 and particularly that board you were working on, is they are very very very vey difficult to repair, even very seasoned techs have trouble with them, and if you talk to ghazzer he will tell you the trouble he had, and in fact never got to the bottom of.
I help where I can, but I actually have a full time, job and run a sideline business repairing and restoring this gear, so I can't spend as much time here helping as I would like to.
I always discourage people from having high hopes about repairing these amplifiers, I have had success a couple of times with them, but the amplifier design is so unstable that everything might look good, then you look away for a second and FIIIZZZZZZZZZZZ, its gone up in smoke again...
For hobbyists, this sort of thing can get very frustrating, and disappointing.

With the Pioneers, which I also work on a lot, is they are all very stable designs, and the way they are arranged in the chassis makes them a lot easier to work on, the service information is a lot more accurate than the Sansui service manuals so its easier to help people work on them...

I would say not that many people actually end up fixing those F2980 boards, there are so many threads here with people trying to fix G7500's and G4700,5700,6700,7700 that run cold after a few posts because as I explained earlier, those particular amplifier are so unstable and difficult to repair....

Speaking of which, I have a G7700 here to repair now!!

Nobody is going to give you flak, but at the same time, what is it that you want?? Its an open free forum where all the people here selflessly give their time to help people such as yourself.
There is usually a reason why the help is not there, and when its regarding, for example, a Sansui with an F2980 board in it, not a lot of people have had any luck repairing them.....

Hope that explains it a little for you....
 
Young46 said:
that I feel I didn’t get the help from the Sansui techs that I got from the Pioneer techs.
Click to expand...
Well, is easier to work on Pioneer gear, I have worked on a few, remotely guided a couple of folks to fix them...
As Kevin said, you have one of those cases that you need to be really really good to make it work.
So as you can see. sometimes one need to accept the fact that not always will be someone available to fix your gear remotely, you'll to figure out by yourself or just accept the fact that is beyond your limits, and this will happen some gear, no matter what forum.

Sansui sub forum is one of the most collaborative forums that I've seen ever.

Kevin... stop praising Pioneer gear, that's the virus talking and we need to address that.:p
 
The G7500 amp, aside from bias trimmers going sketchy after 30-40 years, is quite stable and reliable, and really not that hard to fix. However, walking someone through the repair over the internet isn't very easy, and is much more time consuming than one might realize. If I had it here, I could repair it just fine.
However, I don't think I have the time and focus to walk the OP through it step by step.
 
I had my own adventure with the F-2980 on a G-7500 a few years ago. I too attempted to solicit this forum to find the magic bullet to fix it to no avail. After reading through many long threads on this board by people way smarter than me, It was obvious there was no easy way for me to sort that board out. Ghazzer did what he could for me too.

Eventually I gave up trying to figure it out and just fixed it. I took the philosophy of work harder, not smarter. If all the components are good and installed correctly, it has to work. I shotgunned the hell out of that board. Everything I couldn't be absolutely confirm was good I replaced. I have no way of testing transistors at working voltage so they all went. Some subs may not have been ideal but It was stable and sounded significantly better than my unrestored G-7700. I did recap the F-2852 Equalizer board though.

I know this way of doing things is frowned upon around here, and I threw away a bunch of perfectly fine parts, but it's fixed. My labor and the parts were cheap. I'm sure it would have cost me quite a bit more to enlist a professional.

Don't give up on it, but be very careful not to do anything to cause further damage. A DBT can be your best friend.
 
This just goes to show you that it is hard to beat luck, be it good or bad!

My experiences with the F-2980 board have been limited - mostly failures. I think that Sriskie was very fortunate to get his G-7500 working, and I wonder how many F-2980 bds Tarior has worked on that were easy to fix. No difficult ones??? You guys must have busted the curve.

My experiences have been like Kevzep's description: Go thru everything carefully; get it working; then an hour later POOOOOFFF!. There just seems to be a narrow line in it that runs a crooked path between stability and catastrophe. Overundr1 has theorized that some of the small "pumpkin" caps may not be the optimum value to keep the circuits from oscillating. After some unspecified time they start oscillating, and the next point on that crooked line is when you get the POOOOOFFF.

I wish I had more faith in the Sansui schematics and other documentation, but I have seen and read about too many GOTCHAs.

One of these days I'll reopen that box, and see if Pandora has left the building . . . . .
 
a full careful rebuild should in theory be first thing to do . testing every part before replacement including the replacements .
 
Sriskie said:
I had my own adventure with the F-2980 on a G-7500 a few years ago. I too attempted to solicit this forum to find the magic bullet to fix it to no avail. After reading through many long threads on this board by people way smarter than me, It was obvious there was no easy way for me to sort that board out. Ghazzer did what he could for me too.

Eventually I gave up trying to figure it out and just fixed it. I took the philosophy of work harder, not smarter. If all the components are good and installed correctly, it has to work. I shotgunned the hell out of that board. Everything I couldn't be absolutely confirm was good I replaced. I have no way of testing transistors at working voltage so they all went. Some subs may not have been ideal but It was stable and sounded significantly better than my unrestored G-7700. I did recap the F-2852 Equalizer board though.

I know this way of doing things is frowned upon around here, and I threw away a bunch of perfectly fine parts, but it's fixed. My labor and the parts were cheap. I'm sure it would have cost me quite a bit more to enlist a professional.

Don't give up on it, but be very careful not to do anything to cause further damage. A DBT can be your best friend.
Click to expand...
That is one way, and you got lucky, one thing that does help, if your replacements near the output section, particularly the drivers are lower bandwidth than the originals, this will help with stability.

tarior said:
The G7500 amp, aside from bias trimmers going sketchy after 30-40 years, is quite stable and reliable, and really not that hard to fix. However, walking someone through the repair over the internet isn't very easy, and is much more time consuming than one might realize. If I had it here, I could repair it just fine.
However, I don't think I have the time and focus to walk the OP through it step by step.
Click to expand...

I disagree with this, I have worked on enough of these now to draw my conclusions from experiences with these, several 7700's and a few 6700's, and they are all the same. I wouldn't call them reliable.....
The design is really pushing the limits, 20mhz output transistors are not easy to keep stable....100mhz drivers..
Most are all unstable and waiting to Chernobyl themselves. The trimmers aren't any worse than any other 35 year old trimmers, it matters on these amps because when they do go intermittent, they will fry the trimmer and anything in their way between the trimmer and the output transistors.
But for an experienced tech, I agree they are not that difficult to fix, given the difficulty of helping someone over the internet, and the level at which you need to be to repair one of these, well, that's why we've ended up here...
I also do not have the time to help, I got a little but involved but its hard when you don't really know how much the person you are helping does know...
 
petehall347 said:
a full careful rebuild should in theory be first thing to do . testing every part before replacement including the replacements .
Click to expand...
You should have a go at one of these amps, and then come back and see if you want to say the same thing.....:D
 
kevzep said:
You should have a go at one of these amps, and then come back and see if you want to say the same thing.....:D
Click to expand...
i have never seen one and unlikely to see one in the future . i dont really want to .i have micro electronics to deal with at the moment . 0201 ..
 
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