G7500 relay doesnt trip

Would anyone know what could cause a situation in a G-7500 where the audio relay (F-2980 board) doesnt trip and there's 46vdc on junction 44 and pin 3 of the relay transistor?

I traced the 46vdc back to where the junction of the 5W emitter resistors of two power amplifiers (TR601/602). There's 48.1vdc there.... and 48.6vdc on the base. I'm pretty sure there's not supposed to be any dc at that point because it would go to the speakers. Theres also 58.6 vdc on the collector of TR601..

Any suggestions what might be the cause since Ive already replaced the power TR's and the STV diodes. I guess my main concern is where the 46vdc could be coming from?
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Plomaman27 said:

Would anyone know what could cause a situation in a G-7500 where the audio relay (F-2980 board) doesnt trip and there's 46vdc on junction 44 and pin 3 of the relay transistor?

I traced the 46vdc back to where the junction of the 5W emitter resistors of two power amplifiers (TR601/602). There's 48.1vdc there.... and 48.6vdc on the base. I'm pretty sure there's not supposed to be any dc at that point because it would go to the speakers. Theres also 58.6 vdc on the collector of TR601..

Any suggestions what might be the cause since Ive already replaced the power TR's and the STV diodes. I guess my main concern is where the 46vdc could be coming from?
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Shorted output and/or driver transistor(s). Your right its suppose to be around zero volts +/- 10mv. Or something is holding the (+) output at or near rail voltage.

58.6 vdc on the collector of TR601..

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58.6v is rail voltage

and 48.6vdc on the base.

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Definitely a problem..

I guess my main concern is where the 46vdc could be coming from?

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Check the driver transistors TR15 & 17.
Might want to pull and reverify that 601 and 602 is still good as well.

^^^^^^^^^^ 2 nd that 150% !!!!!

just a thought ....fuse resistors ...

Or maybe emitter resistors… best to check, since you have to yank the devices out to check em anyway

Thanks for the inputs! It does look alot like TR15/17 could be where the problem is. I hope the NEW output TR's didnt burn up already! Previously on powerup protection TR19 had fried to a crisp and burned a few reistors in its base network... theyve all been replaced along with the fusible resistors.

Where did you buy the new devices that you installed ( Hint E bay or CHINA is a big red flag )

If the voltage (wrt ground) is ~ the same on both sides of the relay coil, then there is virtually no voltage drop across the coil.

Steve I was wondering when you would chime in !!!

Getting ready to start troubleshooting the TR15/17 circuitry in hopes of finding out where the high voltage is coming from on the emitters of the power amps. I'll be sure to let everyone know what i find. Feel free to chime in anytime!

ghazzer what does 'wrt ground' refer to... im not familiar with the terminology!

vigman i bought the TR's from Mouser. I got the P/N's from Kevzep and they are the correct replacements. Looks like Im going to need to yank them out and check polarities as well as the thermal STV diodes too.

On the oscope I now see what looks like a good audio signal on one of the inputs of the relay. The signal has steady positive and negative rails and the audio signal between the rails increases in frequency as the volume is increased. Finally! Now to find out why the other side isnt working!

"On the oscope I now see what looks like a good audio signal on one of the inputs of the relay. The signal has steady positive and negative rails and the audio signal between the rails increases in frequency as the volume is increased."

The best laid plans as they say:scratch2:... what i thought looked like a good audio signal is anything but! There should be no more than 30 mvdc on that relay input! What Im seeing is +48vdc which fills in from the top downward with increasing audio as I increase the volume knob. The negative audio excursions go to -48vdc negative I would imagine.

Oh well.. back to the drawing board!

You have a shorted device OR are missing a power supply rail on that channel.
Happens to all of us… it's that process called LEARNING and the more banana peels you step on.. you will learn to avoid them in the future.. well then life tosses a bag O marbles at you

wrt = with respect to ground, as opposed to measuring between two points in a circuit.

I feel like it is always better to be precise than to assume that two people are talking about the same thing.

Be warned, these amplifiers are notoriously difficult to troubleshoot and repair.....
I personally really don't like the design from a service perspective....

When they are going they sound great, but when they fail, they really fail, the amp is direct coupled and fairly high speed, so they go POOF!! very easily, especially driving low impedance loads.....

You must have shorted output transistors, and/or fusible resistors which have gone hiZ.
Get your meter out and start measuring voltages......be careful not to slip when you look away at the DMM.....

Thanks to all. Good to see you back Kevzep! I dont know if you remember but I attempted to repair this baby back in 11'... Im getting closer to fixing it after a few years in storage... and finally.....drumroll.....Ive got it to the point where I can get a stereo signal light!!! (real positive moments are few and far between for me on this repair so please excuse my unbridled joy!

Im wondering what Im supposed to be seeing on the audio inputs to the speaker relay on this G7500? Like what the signal will look like at zero signal (knob full CCW) and what happens to that signal as the knob is turned clockwise, increasing the audio signal? Im thinking that the positive and negative DC rails are not supposed to be there? With an FM signal the increase in volume is caused by an increase in frequency of the modulation correct? As compared to AM where more volume means a larger waveform?

It looks like the relay control chip IC601 removes the ground from pin 1 if it detects DC voltage on pin 3 which is tied to the audio inputs of the relay. With no ground on pin 1 there is no ground on one side of the coil so the relay has no voltage across it and doesnt activate, correct Kevzep?

If thats how this thing works, then there can be NO dc on the audio inputs the relay! So it follows that the audio must be purely AC in nature with no dc in it?

As for drivers TR15/16/17/18, all check good with no power applied. Approximately .650 vdc on the diode checker with correct polarity of pnp/npn.

Ok Kevzep, its time to check TR601/602 and STV's. What are the fusible resistors? Are they also called NIR's on the parts list? ...the ones with the little ceramic tubes on their leads?

kevzep said:

Be warned, these amplifiers are notoriously difficult to troubleshoot and repair.....

......be careful not to slip when you look away at the DMM.....

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Kev is 300% correct on this one! It has BMITA more than twice. Now I (almost) always use mini grabbers on component leads to take measurements. Looking at the schematic you can usually find a bare lead that in connected to the point you are trying to measure, and use that rather than a point probe on the track side of the PCA.

The F-2980 bd does have a couple of points that are not easily accessible via a bare lead. I have soldered short jumpers to them for positive connections with mini grabbers, and cover the loose ends with snug shrink tubing when not in use.

Thanks Steve, definately good information from kevzep about taking measurements. Ive been down that road a handful of times over the years and it caused more than a few added hours of troubleshooting...

Sure would be great to play this Sansui for the holidays...

Just got done checking out the final amps/STV-3H diodes. All of them check good. Looks like the high voltage on relay pin 44/emitters common point (pins 57 and 59 of F2980) of the final amps must be coming from somewhere else.

...could be a problem with TR07/TR09 and VR03 and associated resistors/caps?

Will check it out and post results...