My first receiver restoration attempt: G-6700.

Well, I finally got back to the 6700 this weekend.

I followed Overundr1’s advice and put OP TRs A&C in the Right Channel and B&D in the left. I used new mica washers and applied just a smidgen of white thermal paste to both sides, then pressed them to the back of the TRs.
xP1030846.jpg


xP1030847.jpg

My new 3058 bds have a slightly different pin layout and I moved the emitter resistors to the track side. All designed to make it easier to get the boards into their mounting positions, and it helped a lot. I used a dental pick to align the holes in the mica washers with the holes in the TRs and the mounting holes.
xP1030848.jpg

Once everything was secure I went back and rechecked the wiring continuity between every OP TR pin and the lead of a component on the F-2980 bd that it should be wired to. Don’t want to have to go back in later to correct something I had overlooked.

Checked out the switch on the F-2853 bd that I cleaned earlier and it seems OK.
 
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Bias Current:

The adjustment instructions say to set it for 5V ±1mv, but that is wrong. The schematic says that there should be 10 ma flowing through a 0.33Ω resistor, or 3.3mv. I have replaced the 0.33Ω emitter resistors with 0.47Ω resistors, so I need to eventually adjust for ~4.7mv.

I set up the DBT with a 75W bulb and connected the DMMs to jumper holes 87&89 and 88&90, both reading across the emitter resistor tied to the emitter of the NPN output transistor of that channel. This table shows the elapsed time in minutes after power on and the Bias Voltage in ma for the L/R channels.

0:01____0.3/0.3
0:30____0.4/0.4
1:00____0.5/0.5
1:30____0.6/0.6
2:00____0.6/0.6
2:45___11.6/0.6
3:45___11.7/0.7
5:00___11.2/9.4
6:00___11.2/9.4
8:00___11.3/9.4
10:00__11.4/9.5
12:00__11.5/9.6
14:00__11.6/9.6
15:30__11.7/9.6
18:00__11.7/9.7
19:00__11.8/9.7
20:30__11.9/9.8
22:00__12.0/9.8
26:00__12.1/9.8
27:30__12.0/9.9
30:00__12.1/9.9
22:00__12.1/10.0

I was surprised when the readings took a sudden jump of ~10mv, and that there was about a two minute lag between the left & right channels.

B+ was ~ 44.8vdc with the 75W DBT.

I increased the DBT to 135W and the readings stabilized at 11.8 & 9.7 mv. B+ was 53.2vdc.

I removed the DBT and the readings stabilized at 11.5 & 9.3 mv. B+ was 59.4vdc.

The G-6700 seems to play FM OK through the Minimus 7s in the workshop with only a wire for an antenna, but it is a little scratchy. Tomorrow I’ll clean the tuner following Dr*Audio’s sticky in the tuner forum, and finished that off with a drying/cleansing blow of low pressure compressed air and a soft brush. Then I’ll let it warm up for about an hour and dial the bias voltages back to 4.7mv measured across one of the 0.47Ω emitter resistors that will give me the 10ma of bias current called for in the schematic.
 
I cleaned the tuner with CRC contact cleaner – the kind that leaves no residue. Tilted it over a little and blew off the last few drops of the cleaner. All the boards and components I could get to easily were brushed lightly and blown off with air at ~ 20psia. Then I did a close up inspection to make sure I had not stirred up any solder whiskers.

When I hooked the FM back up I noticed that the right channel is a little stronger than the left – same on all stations. I plugged in an MP3 player and the left is still weaker. Reversed the inputs and replayed the same song. Left is definitely weaker.

Hmmmmm???
 
I cleaned the tuner with CRC contact cleaner – the kind that leaves no residue. Tilted it over a little and blew off the last few drops of the cleaner. All the boards and components I could get to easily were brushed lightly and blown off with air at ~ 20psia. Then I did a close up inspection to make sure I had not stirred up any solder whiskers.

When I hooked the FM back up I noticed that the right channel is a little stronger than the left – same on all stations. I plugged in an MP3 player and the left is still weaker. Reversed the inputs and replayed the same song. Left is definitely weaker.

Hmmmmm???

So why didn't you adjust the bias down to 4.7mV where it should be? Yeah huge mistake in the manual, 5V!!! That's had a few "newbies" scratching their heads.....

One of the most unreliable things about these receivers is the switches and the rotary input selector is no exception....
Tape monitor, the -20db muting, tone bypass (defeat), and as I mentioned earlier, the rotary input selector switch, can all make it drop level or cut out....

I cleaned the crap out of the switches on my G7700 and every time I use it, which is whenever I am working in my tech room, there is some kind of issue with one side or the other being low level....I just work all the switches and it seems to be fine.....But I have noticed these receivers are prone to being slightly uneven between Left and Right....

I think whats happened is when you went from Aux to Tuner, you have aggravated the switch.....

So get in there and clean clean clean!!!:thmbsp:
 
Besides what Kevin suggested---
Using your scope ( or dvm) probe the inputs to the amp board l and r with a constant tone input from your signal generator and compare thereadings between the channels. If they are different values and I suspect they will be work backwards through the switches and boards til you find the spot where the signal levels out. I also suspect switches first and balance control second.
 
Yes, that is my plan. I have been down this road before (see page 15). Now I have a little more experience, and have already mapped out many of the points that will need to be checked.
 
I zapped something again!

I set up the 1KHz signal generators and attached them to the L&R coax lines coming into the bottom of the F-2980 bd. The function selector was in Phono 1. Then I monitored the junctions of R05&R07 and R06&R08 with the o'scope and adjusted the sig gens until the two signal were equal. Then I moved the probes to the junctions of the emitter resistors in each channel and noted that the left channel was about 25% less than the right, confirming my suspicions that the problem is on the 2980 bd. I only wrote down that the scope showed 3.8 divisions on the right and 2.9 divisions on the left, and failed to record (or remember) the v/div settings, other than that I feel sure they were the same.

So, I went back to the FET and started measuring signal or voltage levels at every point, working my way toward the OP TRs.

When I got to C18 it only read 1.06V. C17 read nothing, but I noticed that my auto ranging DMM was flashing different icons, like it was searching for the right function or range. Then there was a POOOOF and a flash of light and all was dead.

The main 7A fuse is blown, but beyond that I have not looked at anything……..
 
Kevzep has been saying that there may be a cracked track on this board, so I'll have to take a closer look at that. I was using a test probe, not a grabber, and was applying slight pressure to keep the probe from shifting.

I am just hoping that the fuse blew before it took out any of the OP TRs. Was so disgusted that I haven't been back to the bench yet. Taking controlled readings at all these points should give me a better idea of how these driver circuits work.
 
Looks like it took out the Left channel OP TRs & the main fuse. I'll have to pull the F-3058 bd and do some further testing.

This puts me back to square one, right where I started in post #1, 590 days ago. Except that I have built new F-3058 bds & replaced almost all the parts on the F-2980 & 3058 boards. I haven't worked on this every day, so I have been taking your advice on relaxing and taking breaks.

And I'm too stubborn to quit!
 
Well actually your MUCH further ahead....you have a MUCH better idea what your doing.. Your bench is set up... I blow shit up all the time... it's just part of the FUN...
 
Yes Mike, you are correct. I have swept a few of the cobwebs out of my feeble brain and more of this is making sense now. More, but not enough.

Both of the OP TRs in the left channel were shorted. I haven't checked any other components yet.

Looking at the 7A fuse I could see that the inside was blackened and the filament was opened and curled to one end. Just to be thorough I checked it with the DMM and was very surprised to see a dead short – just like there was a good fuse installed. I pulled the fuse and the DMM confirmed what my eyes could see: It was open.

Then I read across the empty fuse holder and still saw a short. Same with the power switch ON or OFF. When I pulled the power cord it showed open. I also pulled the power conversion plug. The fuse holder reads short with both plugs in, and open with either or both of them removed.

Observation: One DMM has a flashing display of 000.0Ω when I read across the fuse holder. The other gets a flickering display of 0.L, and has to be reset. It finally dawned on me that both DMMs were alerting me that I was trying to read continuity back to the wall socket, a known NO-NO. But I didn’t see the path in the schematic.

This extract from the schematic in the service manual does not show me any path that should show a short (assuming it is correct). I am currently trying to backtrack all the wiring and create my own schematic of this area (with the receiver unplugged!).

XFMR & Fuse-A.jpg

Can anyone explain this for me?
 
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Odd.. I assume the unit is unplugged from the wall....
remove the power jumper plug and try again...
 
The empty fuse holder ends read 'short' if the power cord is plugged in AND the power jumper plug is in. Remove either and it reads open.

How could it do that unless one of the small caps is shorted?
 
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Nope, a quick comment to this one at a late hour.

I can't explain what you are seeing, and it doesn't make sense. But I think you'll find something that ties it up later. I suggest plugging it into a dim bulb tester and seeing what you've got.

You could spend a lot of time scratching your head over this, and keep you from finding out what's wrong with it. So move on.
Check the pc side of the board below the fuse holder. It might've carbonized.
<<Throws darts at wall.>>
 
Ummm Steve , NEVER do a resistance measurement with the unit on AC mains ( attached to wall ) . I know your OCD gene MUST KNOW the answer to this mystery ( I have the same gene ) ..I'll make this easy.. assume your plugged into a ISO transformer or variac you would draw a coil across the 2 pins of the AC cord… ( waiting for the light bulb to go on in your head )
 
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