g-7700

tim in pa

Active Member
decided to try to get my receiver back into working condition - "operate safely" led not on. really miss it. have a photo copied manual, somewhat difficult to read.

able to perform the dc offset adjustment. but when attempting the bias current adjust, the test points for 87 and 88 are empty holes on my 2980 ckt board. see that it actually a reading across R601, TR601 emitter resistor. is that also on the 2980 or elsewhere. i have read that the manual was incorrect and that 3.3mv is the goal.

any help with the bias adjustment points would be greatly appreciated. thanks in advance!
 
found the R601 on the 3058 amp ckt board, and was able to decipher the test point for the bias adjustment. bad thing is after the dc offset and the bias adjustments, the safely operate led/speaker relay still don't energize. found 45.4 vdc on the control circuit for the relay, all the way, r601, r604, & zener, no voltage drops happening. Im guessing there is no sinking ground (pin 1 of the IC601??).
 
If you got your manual at www.HiFiEngine.com it should be "current".

Bias current should be set at ~10mA. That is reading 3.3mv across the 0.33Ω emitter resistor (½ of R601) between TP87 &TP89 in the left channel and TP88 & TP90 in the right channel.

The F-2980 is a very finicky board, with a hair trigger. The bias circuit can be off to the races in a heartbeat. Most people who have dealt with this board will suggest that the first thing you should do is to replace the single turn 100Ω trimmer pots (VR01-04) with Bourne type multi-turn trimmers. The originals don't age gracefully and you can lose a bank of output transistors if the bias trimmer opens.

My 94 year old Mother is in better shape than the tracks on the 3058 boards, and that is being generous. You will save some aggravation if you can avoid dealing with either of these boards. That said, I have considered having modern replacement boards made with double-sided copper clad boards and a new layout, so let me know if you are interested. The same PCBs are used in the G-6700 & G-7700.

There are many pics, hints and drawings scattered thru my thread at:
http://www.audiokarma.org/forums/in...t-receiver-restoration-attempt-g-6700.480777/
They are focused on the G-6700 so some of the components are different. The "X-ray" pics should be useful for locating components from the back side.

Good Luck - - -
 
thanks for the info. found the bias 3.3mv to be relatively easy to dial in, but the dc offset 0v seemed to be very difficult, and drifted somewhat when i come back to check it. guess the bias and offset adjustments affect each other. so i see where the new trimmers could help for sure.

i haven't layed the soldering tip to the board yet, but thanks for the heads up, i will hold it down to 25w.

honestly don't know where i downloaded the schematics from, it was a number of years back.

seems that with the 2 adjustments made, the relay will still not energize. i measured applied voltage (45.5vdc) is all along the circuit to the HAI2002 input pin 1, so no voltage drop across the zener. i think that suggest an open at pin 1?? or bad IC? will verify these readings today. thanks again.


fyi... my rig bought in japan in 1980
sansui G-7700
sansui SE7
Pioneer CTF-950
Pioneer PL-630
Bose 901 Ser IV
 
A link is a good idea. I'll include it in my reminders. Better yet, I'll put it in my signature.

- Pete
 
found my first physical signs of a problem, brown under a host of capacitors. looks like the sm parts list for the 2980 board doesn't list all of the caps. any recommendations for cap replacement process? other than mark polarization, desolder, read pn and order?
 
I can send you a parts list - I'll look for it tomorrow. I don't punch a clock anymore, and I'm not very good at keeping one either.
 
found my first physical signs of a problem, brown under a host of capacitors. looks like the sm parts list for the 2980 board doesn't list all of the caps. any recommendations for cap replacement process? other than mark polarization, desolder, read pn and order?
. The brown stains under the caps is probably the notorious Sansui glue. It is not an indication of a failed cap.

Don't take anything off the board before having the new parts on hand. Make a parts list by reading the values on the actual caps while they are still on the board. Take a picture or make a sketch of the board and mark the polarity of each of the caps. Then remove and replace the caps one at a time. it's the best way to ensure you get everything back in the right position with the correct polarity.

- Pete
 
The F-2980 bd can be worked on without removing any jumpers or other boards. If the receiver is working when you start, is it advisable to check operation after every replacement?

Just wondering.
 
hello. resurrecting old thread. am now retired and have time to fix this g-7700.

find myself now without access to an o-scope. will i need one to recap?
i see several recap/restore kits on-line for like $185, is this a recommended way to go?
suggestions?
tia, tim
will be moving on to re-belt my CTF-950 and PL-630 next
 
hello. resurrecting old thread. am now retired and have time to fix this g-7700.

find myself now without access to an o-scope. will i need one to recap?
i see several recap/restore kits on-line for like $185, is this a recommended way to go?
suggestions?
tia, tim
will be moving on to re-belt my CTF-950 and PL-630 next
Get your parts from a trusted supplier like Mouser, Digikey and so forth, don't buy that rubbish of ebay, they wont have everything you need and there is no way to guarantee they will be of a decent brand and so forth.
You also need more than just a few caps there may be other components you need and so forth...

The G7700 is not something I'd recommend to a beginner, but if it is running you might be okay.
There are a number of issues these amps have which are not easy to resolve, so if you take it on, take your time and ask questions, they can go unstable and fry themselves very very easily, so beware.

Good luck and let us know how you get on....
 
Be careful of the eBay capacitor kits. The G-7700 had a major production change during it's production run. Early and late production units will have different capacitors. Be sure any kit that you buy applies to your actual unit.

- Pete
 
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