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G-7500 DC and Bias Current Questions

gregmcm

gregmcm

Active Member
Have acquired a recent G-7500 project that is now operational. This unit was most likely sitting somewhere for quite some time based on the dirt and dust that was in evidence once I opened it up. Cleaned everything, deoxit the pots and the unit worked fine with the exception of the phono stage which was repaired and now both phono 1 and 2 working. (bad capacitor and resistors)

After running the unit for some hours here is the problem:
- turning the volume to say ~5 on the dial and the speaker relay kicks in and cuts sound output (same for all inputs whether phone, AM/FM or Aux)
- turning the unit off, wait 2-3 minutes - turn unit on and now have output
- listening carefully to the left channel at a reasonable volume and I notice a slight bass distortion. Also might be an even slighter bass distortion on the right channel.

Background:
- I haven't adjusted the Driver Circuit Board for DC Offset and Bias Current on this unit
- I have the schematics and a DMM
- I have adjusted Bias Current on a Sansui G-3500 that I have but this unit doesn't have DC Offset adjustment so I have some knowledge in this area.
- I have mini grabbers and know enough to make sure I'm very careful when making these adjustments.

Questions:
- Does it matter which adjustment I undertake first - DC or Bias
- Does it make sense that either of these adjustments would cause the conditions noted above?
- Anything I should be aware of before undertaking these adjustments.
- am I missing anything?
 
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If the DC offset is high, it could absolutely cause your issue. I generally set DC offset first then Bias Current. It doesn't matter too much which you do first as long as you check both again after an hour or so of mid range operation. If the DC offset won't adjust into range, then you have bigger issues.

- Pete
 
The single turn, 100Ω pots for DC Offset and Bias adjust on the F-2980 board have often succumbed to old age and have failed - often on the first attempt to adjust them after years of sitting. If you can, I'd recommend that all 4 be replaced with multi-turn Bourne style pots before you proceed. Then follow Pete's suggestion.
 
ghazzer said:
The single turn, 100Ω pots for DC Offset and Bias adjust on the F-2980 board have often succumbed to old age and have failed - often on the first attempt to adjust them after years of sitting. If you can, I'd recommend that all 4 be replaced with multi-turn Bourne style pots before you proceed. Then follow Pete's suggestion.
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Steve - Pete, thanks -- makes sense to change out the old trimpots for sure. Would this trimpot be a good replacement?

upload_2016-11-28_9-53-44.png
 
First check bias/offset before you adjust anything. It is finicky ckt because it does not have a ecap in the feedback loop, which means the DC gain is the same as the AC gain, their DC amp with no servo. It can blow the ops at the blink of an eye.
If yo do any adjusting with those old pots, due it on a dbt tester.
Look at the pots in the F-2980, the pin spacing, the wiper pin is offset from the others. can you bend/form those leads to fit into the existing holes? those bourns are on 0.1" centers and leads are meant for a 0.062" pcb.
while you are at it change those o/p devices to NJW1302/3281 ASAP because I got a bag of the old crap they put in there. I'd have to check if a TO-264 pkg would work, it would be even better.

Out of interest and boredom I simulated this ckt yesterday using 2sk117,ksa1845,2sc3502/a1381,MJE15032/15033,NJW1302/3281

Fourier components of V(vout) = 24Vrms/4ohm,20Khz = 144W into 4 ohms, but even a single pair of NJW1302/3281 can't handle that power continuously. This also assume no supply rail drop

DC component:-0.00312897

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 3.397e+01 1.000e+00 -1.02° 0.00°
2 4.000e+04 1.083e-03 3.188e-05 80.79° 81.81°
3 6.000e+04 3.550e-03 1.045e-04 27.31° 28.34°
4 8.000e+04 2.493e-04 7.339e-06 56.64° 57.66°
5 1.000e+05 2.571e-03 7.567e-05 43.89° 44.91°
6 1.200e+05 5.307e-04 1.562e-05 15.88° 16.90°
7 1.400e+05 3.807e-03 1.120e-04 55.58° 56.60°
8 1.600e+05 1.016e-03 2.990e-05 11.23° 12.25°
9 1.800e+05 3.839e-03 1.130e-04 51.50° 52.52°
Total Harmonic Distortion: 0.021017%(0.181786%)
 
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gregmcm said:
Steve - Pete, thanks -- makes sense to change out the old trimpots for sure. Would this trimpot be a good replacement?

View attachment 826213
Click to expand...

The trimmer you have selected is a 100KΩ - you want 100Ω ;)

Note that the picture often is not a picture of the actual part, but an example from the range. Thus not to be relied upon for what you will actually receive - use the data sheet.
 
Hyperion said:
The trimmer you have selected is a 100KΩ - you want 100Ω ;)

Note that the picture often is not a picture of the actual part, but an example from the range. Thus not to be relied upon for what you will actually receive - use the data sheet.
Click to expand...
John, thanks for pointing out my error. This one looks like the correct spec.
upload_2016-11-28_11-14-4.png
 
Terminations LF = 100 % Tin-plated (RoHS compliant) Blank = 90 % Tin / 10 % Lead-plated (Standard)

Up to you, Mouser says the 'non-LF' are more expensive.
 
As I haven't ever replaced potentiometers I would ask what the proper installation is with regard to the pins. Based on the schematic I'm assuming the arrows at VR01/02/03/04 indicate the wiper pin location. With regard to the other two pins does it matter which way they are installed? Secondly -- once these new potentiometers are installed and the receiver is set up using a DBT what position do you adjust the potentiometers to before turning on the receiver?

upload_2016-11-28_12-41-25.png
 
It doesn't matter which way round these particular trimmers are mounted - you can put them on the track side or the component side (if that's what you were asking). Just make sure the track is orientated correctly and the wiper goes to the right place. Don't forget that the trimmer style you install must be accessible with the unit assembled - important !

When adjusting on DBT - what you are doing is making sure there are no nasty surprises, and that DC offset and Bias can actually be adjusted - not 'set' necessarily - only adjusted. Once you are happy there are no concerns with the range of adjustment - Turn The Bias All The Way Down, Then you can try off the DBT on full mains, make sense?
 
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Hyperion said:
It doesn't matter which way round these particular trimmers are mounted - you can put them on the track side or the component side (if that's what you were asking). Just make sure the track is orientated correctly and the wiper goes to the right place.

When adjusting on DBT - what you are doing is making sure there are no nasty surprises, and that DC offset and Bias can actually be adjusted - not 'set' necessarily - only adjusted. Once you are happy there are no concerns with the range of adjustment - Turn The Bias All The Way Down, Then you can try off the DBT on full mains, make sense?
Click to expand...
Makes sense. One other question -- I'm going to assume it might be safer to make these adjustments without speakers attached? I'll likely get the parts later in the week and see how it goes. I'll update once I've completed the work.
 
Yes, no speakers necessary, and you are right - safer. Make darn sure that DC offset is stable before you trust anything other than 'beater' speakers on the unit.

A late addition to my last post
Hyperion said:
Don't forget that the trimmer style you install must be accessible with the unit assembled - important !
Click to expand...
 
Hyperion said:
Yes, no speakers necessary, and you are right - safer. Make darn sure that DC offset is stable before you trust anything other than 'beater' speakers on the unit.
Click to expand...
John, at this point I know enough to know bad things can happen and every effort should be made to minimize collateral damage :). I have seen the smoke and I don't like it!
 
I'm not looking at the service manual right now, but you would be smart to install the trimmer pots in the orientation that agrees with the SM. Such as "turn VR03 CCW to increase bias..."

John ???
 
ghazzer said:
I'm not looking at the service manual right now, but you would be smart to install the trimmer pots in the orientation that agrees with the SM. Such as "turn VR03 CCW to increase bias..."

John ???
Click to expand...

With multi turn trimmers you can't dictate the trimming direction, - they simply don't sell trimmers where you can choose the direction of trimming. You have to turn them to minimum (or maximum) resistance in one direction as a starting point, but you can't dictate which direction that will be - that's a circuit design consideration. However, it might make it clearer if single turn trimmers were used with some means to see where the wiper is. I very seldom use single turn trimmers, as I don't feel I need to know 'where' or 'what' direction the wiper is.

I buy multi-turn trimmers, either 'side adjust' or 'top adjust' always the ones with the legs in a triangle formation, never the ones with legs in a line unless I am forced to. After they are fitted I pay no attention at all to the SM trimmer setting directional instructions. But rather refer to the schematic for the correct position of the wiper for minimum bias, sometimes checking this after setting to minimum bias position with a multimeter just to be sure.
 
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Likely still may have issues with DC offset and or bias but how about this finding. Doing some more investigation on the left speaker bass distoration which seemed to be getting worse. Seems like something is vibrating and gets worse when loudness and or bass tone control is turned up - uhmmm? Check this check that and I discover that the rear section of the speaker (Tannoy C-8) seems to be the cause. Get myself a clamp and low and behold problem largely solved! Before I dig in deeper I'm going to give it a good workout with some vinyl that I can check against another receiver (Marantz 2275) side by side. Temporary solution photo below:
 

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The trimmer replacement will help ensure you can adjust the DC Offset. Before you buy anything, check the DC Offset to get a baseline. If it's more than about a volt, new trimmers won't fix it. The distortion is a separate issue. It's most likely dirty controls. Every knob, switch and push button on the face plate has a switch or a pot behind it. Almost any one of them can cause distortion if it's dirty. Exercise all of them and see if any one affects your distortion. Then clean them all.

- Pete
 
gregmcm said:
and I discover that the rear section of the speaker (Tannoy C-8) seems to be the cause. Get myself a clamp and low and behold problem largely solved!
Click to expand...
and there you go
LBPete said:
Before you buy anything, check the DC Offset to get a baseline.
Click to expand...
exactly and report what you are getting, do not adjust anything yet.
stock up on deoxit and fader lube, read up learn how to use and then see where you are at.
 
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