Sansui G7000 tuner problem

[anderslober]

I only have a scope...I dont have all the other fancy stuff........

We have to fix the `bread and water` way....yey!

 

[dr*audio]

Well then the only suggestions I have are:
1. Replace TR01.
2. Replace the ceramic coupling caps.
3. Tighten the screws that hold the tuner / pcb to the chassis.

 

[anderslober]

Awesome....will do. Hope this helps.

I have been chasing the slight hum in left channel.........it seems volume independant......CAN it originate from the PS somehow/somewhere?

 

[anderslober]

All tr`s/caps/diodes on the tone board have been changed.....

 

[anderslober]

Ok fellas......this unit is giving me a hard time......

Unit ON......aux selected...no input......speakers connected to A......volume increased.....hum disappears....disappears fully at 10´o`clock volume.........

Iam fearing this may be a problem with the volume pot..........crud!

I had a similar problem on a Luxman receiver a few months back...........changed the pot and hum in one channel disappeared.

I am waiting for tuner parts to arrive...then hopefully this g7k is saved. The unit is otherwise totally stable.

 

[dr*audio]

Hum is probably bad ground connection. Start with the grounds on the power amp and measure resistance to chassis and power supply ground.

 

[anderslober]

Unit on or off?

 

[Hyperion]

Unit on or off?

Unit off

 

[anderslober]

Ok.....unit OFF.....chassis ground to other grounds..

Left amp....0.7ohm
Right amp..0.69ohm.

L/R volume ground....0.99ohm.
L/R balance ground...0.99ohm....

PS ground.....1.09ohm....

Multiple grounds points......0.95-0.99ohms.

Toneboard ground...0.97ohm.

What WOULD it read if bad?. Some higher value.....but by how much....?

Remeber the hum In left channel disappears at 10 o`clock volume......right channel is fine.
Balance has no effect at all.....fully right...still hum in left.....fully left...still hum in left

 

[pari57]

You cannot measure differences in ground
the difference you measure is in the multi-meter !!!
If there really is a difference in the different ground connections, then make sure all the grounds are equal first.
check for bad solder pads as well !!!

 

[Hyperion]

You cannot measure differences in ground
the difference you measure is in the multi-meter !!!

Easy tiger - I think our friend knows that he needs to short his meter probes together and subtract that reading from the readings he gets.

I can't see much wrong with those readings - but the 10 o'clock business with the hum is interesting, might be a hum loop (circulating hum current in the chassis?) or oscillation - which can manifest itself as hum.

 

[pari57]

Just wait until 10.01 hr to turn on your radio

No seriously now,
might be depending on the value of the pot at 10 o clock
or maybe the difference between the used part and the not used part in the pot section.
maybe 0 to 15 degree is 1 to 2 or 1 to 4 of the total value or something like that.
but a value that can generate a resonance

 

[anderslober]

Hum loop would affect both channels.....wouldn`t it?

 

[Hyperion]

Hum loop would affect both channels.....wouldn`t it?

More often than not - so yes.

 

[anderslober]

Shucks.....shucks.....shucks....I am the end of my ideas for this unit.

I changed the tr01 on the tuner board......didnt help.
I changed the volume control...................didnt help.

I am out of ideas......darn shame.........friggin` Sansui......

 

[rcs16]

I would hope that both of your problems are unrelated, but stranger things have happened.
troubleshooting the RF is not easy even when you have the correct equipment, which we know you do not have, such as a 1Ghz FET probe and high BW scope.
I probably could sell you a old tek fet probe but that is another matter.
Since your finger seem to couple the signal and make the radio quiet, we have to figure out why this is? is it a open cap or bad cap(C09,12?), or something else, open connection?
Need to determine if it is the FM front end or IF/det section, so one method is isolation, need to lift the leg of R22(100ohm) connected to R21, in order to separate FE from IF sections.
Inject a signal from another tuner front end IF o/p (use a short piece of coax) or RF Sg into R22, test to see if the G7000 IF section performs properly.
Can also take the G7000 RF FE IF o/p (R21 hot) and wire it to another tuners IF section to determine the op condition of the g7000 RF FE and IF sections.
I will continue once you perform these tests.

 

[anderslober]

Ok will try. Those ceramic caps hardly ever go bad.............will check`em again........

Could the hum originate from the driver board? Increase in volume/power makes the hum disappear........

 

[rcs16]

Could the hum originate from the driver board? Increase in volume/power makes the hum disappear........

I guess so but I can not be sure. have to figure out if it is the pre-amp or power amp section. On my G-7500 I installed pre-amp o/p, power-amp input jacks. So need to separate the two and test to see where it is being generated.

 

[kevzep]

Ok will try. Those ceramic caps hardly ever go bad.............will check`em again........

Could the hum originate from the driver board? Increase in volume/power makes the hum disappear........

How hard are you touching the FET in the tuner, are you possibly disturbing something to make contact whilst touching the FET?
There really isn't much to the circuit there, just a few caps, resistors, IF transformer, and transistors....

Work on one problem at a time, get the tuner working and then move on to the next thing.

 

[anderslober]

I am only just touching it......not hard or anything.....just a finger on the test point.

I will keep poking around in there.

I will double check and re-test the driver board/tone board......

The tough thing about this is, the hum disappears when the volume goes up.....scratch head!