Sansui G-5700 No sound - Clean pots or take bias measurements

Hi Guys,

First time post here on AK.

Purchased a G-5700 with power but no sound ( knew this while buying since I want to get into some vintage stereo repair). Started on a DBT and it power on but no clicks, no FM Stereo light, no light on protection.Opened the top and saw everything looked fine except C612 looks like it is bloated on top. Will plan replace it. Checked some power at Output transistors and they seem to be fine in-circuit. II have ordered tools like soldering iron and a real DMM and waiting on parts.

So here's my question
1) While I am waiting, does it make sense to take out the front and clean the pots. Will this make it come out of protection.
2) Wait for parts to remove output transitor and C612, replace, check bias etc and check to see if it comes out of protection.

Which steps would you recommend

btw, I must have studied the schematics 20 times and I read most posts here on AK so I am hoping to follow advise and some logic.

Any thoughts!.

Thanks again!

S

Ahh...the wonderful F-2980 board. There has been some truly epic threads dealing with this board in various models. Look it up.

I've had my own struggle with one in a G-7500. Far greater minds than mine have been tested trying to sort it out. It seems to defy diagnostics. My advise to you would be to practically gut it and replace all of the E.C.s, diodes, TRs, trim pots, and "N.I.R." type resistors. Install good subs correctly and it pretty much has to work. DON'T trust was you see in the service manual, go with what's on the board.

Thansk Sriskie. I will have to wait for some tools before gutting and replacing the parts.

Being a veteran of several F2980 boards I can tell you this....

If you are not electronic savvy, if you do not have a lot of experience, you will have a lot of trouble getting the problem sorted....
I am not saying its impossible, but this driver stage has stumped a lot of experienced people...
You fiddle with one thing, then something else goes wrong, you fix one part, another part fails....it can be very frustrating working on these amps...
Its not for the noob (no offence).....

However if you want to carry on there are a few people here who can help...

AK member Gazzer has been meticulously working through these boards often, and to the point where he made his own schematic....

There are also a couple of different versions of the board too.....

So, the amplifier will be in protection because there is excess DC on the output, so the protection circuit had opened the speaker relay to protect your speakers from the DC....

The bulging cap will just be the plastic, if you push down on it you'll see.....the alloy can will be fine.....

It won't be dirty controls or anything like that......but they will most likely need cleaning, but I'd wait until you get the thing running first before you do that...

Why is it in protection?
Blown driver transistors.
Blown output transistors.
Open/intermittent circuit bias trimmers causing the bias to run away...
Wildly drifting DC offset caused by intermittent DC offset trimmers...
Values of Fusible resistors drifting wildly off spec...

There's a few possible causes......
You will need to at a minimum replace all the fusible resistors, the DC offset, and Bias trimmers. Also, you'll want to get rid of that glue holding the caps on the circuit board....

But you need to diagnose the problem first, don't just start replacing things and hoping for the best.....

Good luck......

Thanks kevzep. Very good insights..I am reading your and ghazzer's posts as I write.. I will look at taking some readings and post back for more advise.

I think I am ready for some frustration..time will tell!

Cheers!

Welcome to AK!!!

Kevzep's first advice to me was to replace the four 100Ω trimmer pots. I opted for Borne style multi turn trimmers, rather than the single turn style that were originally installed.

Check the 7A main fuse on the 2980 bd. It is probably open if the OP TRs are gone.

Kev is right about this series not being for the faint of heart, or those of us with limited experience. You might be much better off to shelf the 5700 and find a simpler amp/receiver to work on, just to learn and build your confidence. I have an ancient (1967) degree in EE and 40+years of electronics experience (mostly Radar & other sensors), but have had to re-learn most of the fundamentals needed for this kind of work.

Don't be discouraged, just have a plan, learn the fundamentals, get some good tools & learn how to use them. Most of all: learn how to respect electricity.

Good Luck - - -

So finally got my soldering iron and got a chance to take some measurements. Pulled out the Output Transistors and checked the reading.

Both 2SA1105 (PNP)
B-C -> .468v 8.08 M Ohm 8.54 M Ohm
B-E -> .496v 11.33 M Ohm 11.09 M Ohm
C-E -> OL
E-C -> OL

Both 2SC2580 (NPN)
B-C -> .513v 7.91 M Ohm 7.49 M Ohm
B-E -> .517v 7.96 M Ohm, 8.11 M Ohm
C-E -> OL
E-C -> OL

Based on what I have read this seems like they are good. Right.?

Any advise of next steps.?

Thanks
S

a good start would have been checking the dc offset before the relay .
and the power supply voltages too .

Thanks..Can i still check it with the OP TR off circuit?

its best to replace them as they test ok . well at least they aren't shorted . a meter test will only tell you if shorted or open . full power test is only sure way way with transistors . lets assume they are good for now .
i don't know enough about this amp whether its ok to power up without outputs fitted or not .
i do know if there is high dc offset it wont let the relay engage .
another thing i know is if the power supply isn't up to snuff with any any equipment it wont work correctly so its always worth checking first . it can tell us things like if voltages are low something could be drawing too much current . or it could just be a bad supply sending stuff out of balance .
do you have an oscilloscope ? or at least a signal tracer ?

Ok replaced the OP Transistors and started to check to bias current on the R-CH.

On the F-3093 board, checked between #3 and #4 ( both are blue wires ), I am getting varying mV. Started with 1.2mV and as I held the mini grabbers it kept on increasing all the way up to 40mV. Not sure if this was expected. I did not touch VR 04 .

the other mistake I made is that I did not connect the L-CH board (F-2981) to the heatsink. As I was measuring R-CH, I saw smoke & a small spark on the left. I immediately shut it down. Not sure if I fryed the OP TR's. Will have to check again.

So my quesion:
On F-3093 are the 2 blue wires the place to check DC offset.?
Should it stay stable rather than just increasing?
Any way I can disable L-CH while I figure out R-CH?

thanks
S

Read EchoWar's Sticky in the "Solid State" forum to learn about DC Offset.

The correct place to measure is at the speaker output terminals, with the appropriate conditions set up.

Did you consider replacing the single turn trimmer pots? The original ones do not age gracefully.

Thanks. Will read the post first

Have you seen this?

http://www.audiokarma.org/forums/showthread.php?t=510606&highlight=g-7700

Thank you Sriske, That is very good information.

btw, I just swtiched the system back on a DBT and noticed that the 6A fuse is blown. Not sure of this was a result of overheating of OP TR or short..Will buy some fuses and come back with results.

Thanks

seemesh said:

Ok replaced the OP Transistors and started to check to bias current on the R-CH.

On the F-3093 board, checked between #3 and #4 ( both are blue wires ), I am getting varying mV. Started with 1.2mV and as I held the mini grabbers it kept on increasing all the way up to 40mV. Not sure if this was expected. I did not touch VR 04 .

the other mistake I made is that I did not connect the L-CH board (F-2981) to the heatsink. As I was measuring R-CH, I saw smoke & a small spark on the left. I immediately shut it down. Not sure if I fryed the OP TR's. Will have to check again.

So my quesion:
On F-3093 are the 2 blue wires the place to check DC offset.?
Should it stay stable rather than just increasing?
Any way I can disable L-CH while I figure out R-CH?

thanks
S

Click to expand...

You've blown up some output transistors, so you need to identify these and replace them.....

You need to measure the DC offset before the speaker/protection relay, no point in measuring it at the speaker outputs if the unit is in protection...

But thats all irrelevent at the moment because you have some other more important issues to deal with now.....

You probably also have damaged driver transistors as well - these all need testing and replacing if needed. You'll just blow up new output transistors if you don't check these and replace if damaged.

Bias increasing way beyond spec' when you are not adjusting anything is a dead giveaway for unresolved issues, you should power off immediately if this happens again.

kevzep is correct. The fuse blew for a reason, and most likely NOT a momentary one. Replacing the fuse and trying again will only cost you another fuse and PROBABLY more damage to your components or circuitry. You will need to search for the problem(s) before you apply power.

Take good notes on what you try, what you find and what you fix. Bring it up on a DBT if you think you have fixed a problem. This series is very touchy, and unforgiving. I have been working on a 6700 for over two years (off & on) and just when I think I cleared all the hurdles, BLAM, and I have to try again.

Thanks Guy! Looks like I got my work cut out.

So if I have to look at issue with power off, does it mean I have to take out parts and measure it outside of cricuit.?

btw, another dumb question. I was looking to get some parts from mouser and/or digikey specially the fuse. When I look for a 6A 120V 1.25 in X .25 inch I am not able to find anything in that size. Every thing that I find is 5mm X 20 mm. I am looking for a 31.75mm X 6.3mm

As always thanks for the great advice.!

S

yes parts out of circuit .
fuse is 32mm ..its safe to use higher voltage .