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SA-7800 - relay won't click.

RHSquared

RHSquared

Member
Hi guys,

I bought a SA-7800 amp from e-bay some time ago. Advertised as blah-blah, gwo, etc. and the magic relay won't click.
So, where do I start. I have a DMM, a tiny experience in electronics and nothing to lose - it's not working anyway and never has (since I have, that is).

Cheers,
Rad
 
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There are many causes, but here are the main four:

(1) One channel is offsetting and triggering the protection circuit.
(2) A power supply failure in the section that feeds the differential amplifier transistors is causing both channels to offset, triggering the protection circuit.
(3) There is a problem in the special power supply that runs the rapid turn-off portion of the protection circuit. This failure is usually a small capacitor.
(4) The relay itself is bad. Usually this is due to damaged contacts. Coil failures are rare.

Fred

* * * * *
 
RHSquared said:
Hi guys,

I bought a SA-7800 amp from e-bay some time ago. Advertised as blah-blah, gwo, etc. and the magic relay won't click.
So, where do I start. I have a DMM, a tiny experience in electronics and nothing to lose - it's not working anyway and never has (since I have, that is).

Cheers,
Rad
Click to expand...

You didn't say if you had downloaded the .pdf SM from the database.

Fred Longworth said:
There are many causes, but here are the main four:

(1) One channel is offsetting and triggering the protection circuit.
(2) A power supply failure in the section that feeds the differential amplifier transistors is causing both channels to offset, triggering the protection circuit.
(3) There is a problem in the special power supply that runs the rapid turn-off portion of the protection circuit. This failure is usually a small capacitor.
(4) The relay itself is bad. Usually this is due to damaged contacts. Coil failures are rare.

Fred

* * * * *
Click to expand...

V(gr) is ground referenced voltage reading, the black dmm lead goes to a bare metal chassis ground, and probe with the red dmm lead, insulate all but the very tip of the probe in case of slips.

(2) (3): check the power supply first , GWR112 board
pins 15, 16, 17: V(gr) +48vdc regulated
pins 19, 20, 21: V(gr) -48vdc regulated
pins 29, 30: V(gr) +46vdc unregulated, this can vary somewhat
pins 31, 32: V(gr) -46vdc unregulated, this can vary somewhat
pin 22 V(gr) -?? vdc thisis the turn-on/turn-off signal for the protection circuit

(1): The protection circuit is ON the power amplifier circuit board GWH118
and we need to see if it is justified in keeping the speakers disconnected.

V(gr) R71 either side
V(gr) R72 either side

These two resistors are larger than the rest, 2 watts, on either side of the protection relay.

That's enough checking for now.

I REALLY hope the amplifiers are ok, these are the NSA circuit, and are a bear to troubleshoot remotely, especially with a troubleshooting beginner.
 
Last edited:
Fred, Mark, thanks for the quick reply guys, much appreciated.

OK. I have the SM and I took the measurements.

First of all, will it work without a load, that is, without loudspeakers attached?
I guess it should, as the protection circuit has the terminals disconnected anyway.

GWR112,

pin 15,16,17 0.2vdc

pin 19,20,21 -1.1vdc

pin 29,30 +48vdc

pin 31,32 -48.5vdc

So, aparently, we have a problem on the power supply board.
What next?

Cheers,
Rad

pin 22 -3vdc
 
OK, first we look for AC voltages coming into the board:

Pin 40 V(gr) 49v AC this is the green wire from the transformer to one side of the fuse
pin 41 V(gr) 49v AC this is the blue wire from the transformer to one side of the fuse

Then check that the other sides of both fuses have the same voltage.

After that, there is a line of four diodes D27, D28, D29, D30 (and one disk cap C45) on the other side of the transformer mounted next to the fuses on the power supply board. It will be easier to measure C29 & C30's voltages there, because the cap leads are hidden underneath them.
On the diodes, read the DC V(gr) on all four leads that are on the side of the diodes furthest from the transformer. That will tell us if the regulators are getting power.

An interesting thing is that the -48v regulator is a "tracking" regulator, designed to mirror the +48v regulator, so IT could be working....

Next, there is a zener diode, D31 which is the closest diode to pin 15 on the board, check the V(gr) of the banded (the end with the stripe) side.

Then in a line between THAT diode D31 and the 4 others (D27-D30) is a 1/2 inch square power transistor marked "D712", read the V(gr) on all three leads left to right (b.c.e) with the lettering facing you, be careful, it should have a heatsink, and that heatsink should have +63 volts on it. Remember about insulating your probe tip? It's EASY to slip and short things there...
 
Here's the next measurments sets.

So, the green and blue wires actually go into the transformer noted T2 on the schematics.


Pin 40 reads 50.8 vac (and the same on the other side of the fuse)
Pin 41 reads 50.4 vac (and the same on the other side on the fuse)


The diodes:

D27 - 69.4 vdc
D28 - 69.7 vdc
D29 - -69 vdc
D30 - -69.2 vdc

On the zener's cathode (that's the striped side, isn't it), it reads anything between 0.01 and 0.3 vdc.


And finally on the Q34( 2SD712)

b. 48.5 vdc
c. 65.6 vdc
e. 47.8 vdc

Amazingly while measuring the transistors, the relay clicked and it's clicking now every time I turn the amp on. Wow, that a powerful thing - the DMM ;)))

I just thought that it would be good if I re-take the first set of measurments:

pins 15,16,17 - 47.7vdc
pins 19,20,21 - -47.4vdc

pin22 -1.88vdc

pins 29,30 48.2vdc

pins 31,32 -48.2vdc

GWR118

R12 -47.3vdc -46.99vdc
R11 same as above.

Hmmmmm, I wonder, what's going on. I bet it's tricky to fix a component that's "playing up".

Rad
 
Last edited:
The green, brown and blue wires come from T1 and run the four diodes, the power indicator lamp and the protection's power sensing circuit. T2 runs off of the green and brown wires.

The erratic results could be coming from a bad solder joint. Also if your ground lead connection for the dmm is not solid, there will be strange results.Look around the power transistors on the power supply board - they tend to crack as they heat and cool over the years - and you need a magnifier to see the crack, so just re-solder the joints and try again.

BUT the R11 & R12 wrong!! R71 & R72voltage readings for BOTH amps going to -47v or so says the problem is the same for both channels, probably the power supply as you move around inside the unit.

There are 4 voltages that must be present at the amplifier card for it to work:
pin 1 +48v regulated
pin 2 -48v regulated
pins 9 & 14 +46v unregulated
pins 10 & 13 -46v unregulated

Then we need to watch the output voltage.
markthefixer said:
(1): The protection circuit is ON the power amplifier circuit board GWH118
and we need to see if it is justified in keeping the speakers disconnected.

V(gr) R11 either side R71
V(gr) R12 either side R72

These two resistors are larger than the rest, 2 watts, on either side of the protection relay.
Click to expand...
I had the wrong R numbers... How, I don't know.

but the readings indicate a power problem.
 
OK. So, I started measuring the GWH-118.

pin1 +47.7vdc
pin2 -47.4vdc

pin 9 +48.5vdc
pin 14 +47.9vdc

pin 10 -47.5 vdc
pin13 -47.8 vdc


R71 -36.2 vdc
R72 -17 (not stable)


And the last measurments were taken after I managed (despite your warning) to make a short between pin 10 and R63 (I was measuring on the bottom side of the board). The result was a spark and the relay won't click again.
I guess I'm not meant to be a troubleshooter :(
 
markthefixer said:
<snip>
.Look around the power transistors on the power supply board - they tend to crack as they heat and cool over the years - and you need a magnifier to see the crack, so just re-solder the joints and try again.

<snip>
but the readings indicate a power problem.
Click to expand...

Perhaps you missed the above^^^^^^^

At least fix the power supply, and get ONE of the channels back operating correctly, then we'll see just what type of damage you did when you shorted the emitter of a NSA circuit power output transistor to it's collector voltage.

BUT, if you cause MORE damage, then we are at the point of diminishing returns.

As the situation has progressed, I have lost almost all hope of getting that amplifier channel going - as it is the MOST DIFFICULT amplifier circuitry type to remote troubleshoot there is.

There is an AK'er (inductor) over there with a shop - it is his repair business - that this may be better off at.

I'm not even sure of your readings, the r71 etc didn't make sense with "the relay clicking". Perhaps if you took a picture and marked it where you were probing, or I could remark it and email it back (pm the email address to keep it out of the clutches of spambot harvesters)
 
Regarding the power supply board - it's giving stable readings now. I suppose it is worth re-soldering the transistors in case there are any cracks, but as I said the power supply board gives steady +47.6vdc on pins 15,16,17 and -47.4vdc on pins 19,20,21.

And regarding for example R71 - I've attached two photos.


Cheers,
Rad
 

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:D I now have confidence in your "map reading".

We need to concentrate on R70 & R71 readings, they are the output of the amplifier. Both were pulled to the negative voltage rail, where they should have balanced themselves near zero volts (the "dc offset").

Since both were pulled low there is most likely a common fault. Once the channel that doesn't have R63 (odd numbers are one channel even numbers are the other channel) balances itself near zero, that channel has a reasonable chance of working. That channel is read at R70, so we need to watch the resistor's voltages, while confirming the presence of those power supply voltages AT the amplifier card.

So what are the current r70 & R71 readings?
And whenever you say the power supply voltages are ok, I will assume you mean that :
RHSquared said:
pin1 +47.7vdc
pin2 -47.4vdc

pin 9 +48.5vdc
pin 14 +47.9vdc

pin 10 -47.5 vdc
pin13 -47.8 vdc
Click to expand...
 
Thanks for the quick reply!

Happy New Year!

I'll take the readings tomorrow as a already had a few and we don't want to burn whatever's left from the amplifier, do we ;)))
 
Some New Year's development.
I switched it on this morning and the amp smoked two resistors in a quite spectacular way - R57 & R59 (I'm not sure about the 59 - it's barely readable on the SM) - 150Ohm / 0.5W.
As a result the relay clicks again.
R71 is at -0.014vdc
R72 is at -0.017vdc.
I will have to replace the resistors, I guess, before we can continue with anything.
 
No MORE, STOP. That's IT. It blew the driver stage. The fact that the protection clicks on is because the smoking ruin of the left channel just happened to disconnect itself from the output terminals. Whatever is going on internally isn't making it out to trigger the protection.

That means that the bias stage turned on both drivers at the same time. The bias stage is the nightmarishly complex portion to troubleshoot - everything has to be PERFFECT there before the power is turned back on.

markthefixer said:
<snip>then we'll see just what type of damage you did when you shorted the emitter of a NSA circuit power output transistor to it's collector voltage.

BUT, if you cause MORE damage, then we are at the point of diminishing returns.

As the situation has progressed, I have lost almost all hope of getting that amplifier channel going - as it is the MOST DIFFICULT amplifier circuitry type to remote troubleshoot there is.

There is an AK'er (inductor) over there (UK) with a shop - it is his repair business - that this may be better off at.
Click to expand...
 
OK, so does that mean that it's well beyond a beginners capabilities to fix whatever is going on?
If that's the case I'll just have to bin it. I've already fixed an amp using the help of Mr. Inductor and it's not cheap.
So, sadly, the amp most probably is going to the junkyard.
 
RHSquared said:
OK, so does that mean that it's well beyond a beginners capabilities to fix whatever is going on?
If that's the case I'll just have to bin it. I've already fixed an amp using the help of Mr. Inductor and it's not cheap.
So, sadly, the amp most probably is going to the junkyard.
Click to expand...

Sadly, probably. But if you do fix it, you will shed the mantle of beginner - call it a baptism by fire.

Here's what I would do if it were sitting in front of me:
I would remove EVERY transistor for that channel one at a time and test them. That also involves identifying which lead is emitter, base and collector for each transistor.
I would test every resistor around the removed transistor for correct values.
Then I would reinstall that transistor (if good) and move on to the next one.
I would remove and test every signal diode and at $0.05 ea, probably just replace them.
I would carefully check the 0.22 ohm emitter resistors for damage, I suspect at least one is zapped. THAT comes from a manufacturer in the USA (just a few miles away from me no less..) unfortunately.
Then I would go over the back side of the board with a magnifier, looking for bad solder joints or cracks in the conductors that could have induced your original problem.

That's a long afternoon for me, ........ for you ... ??? :dunno:

In addition to apply this example to your situation - I would also have to find out what parts suppliers there are in the UK and what transistors they have so I can find replacements.

You could sell it on ebay for parts, or contact AK'er Adrock (he's in the UK iiac) by PM, he has a sa-7800 that has a possible arcing in the flouroscan display tube, so you may be able to at least get some of the parts back into use.

btw: the total parts replacement is what SolutionRoom did on his sx-3900 thread (you don't have to do ALL the resistors like he did) to fix a similar (NSA) type amp. That comes down to finding suitable replacements in the UK, or $$ shipping them from the USA (from mouser.com). The shipping costs will probably hurt more than the parts costs.
 
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