Troubleshooting SS amp...30vdc on OUTPUT!

[Prattacaster]

Bingo, this could be a major turn in events here, I did as R.G. said and sure enough the negative side of the Output cap is NOT connected to ground at all. I have two schematics that I've been working from because they both matched so close EXCEPT that my amp does NOT have the 10ohm resistor to ground off of the REC OUT. The schematic ive been working from has a 560 3w resistor from the negative of Output Cap to the REC OUT, my amp DOES have that resistor just not the resistor to ground. So of course my meter show OPEN from Output cap (-) to power supply cap(-).

Does this then go back to Jazz P, in that I need a speaker load to reference ground and pull down the voltage. My guess is yes, my need for a dummy load is smacking me in the face.

Unrelated question here... the speaker cap has 4-10" speakers, each of the speakers' rings are tied together at a point in the cab and then grounded at the chassis, the resistance is OPEN line to the negative speaker terminals. I was wondering what this is all about.

[R.G.]

I suppose making a dummy load for a 100 watt amp isnt something cheap,

How about four of these in series:
2 Ohms 25 Watt Resistor

2 ohm, 25W, $0.50 each. Their minimum order is $10, so you could load up on 20 of them and have many, many series/parallel loads at quite high power; or you could buy other goodies like test clip terminals and plugs to mount your high power resistors, etc.

[Prattacaster]

2 ohm, 25W, $0.50 each. Their minimum order is $10, so you could load up on 20 of them and have many, many series/parallel loads at quite high power; or you could buy other goodies like test clip terminals and plugs to mount your high power resistors, etc.

I've already checked it out, its not as expensive as i thought especially going with 25w instead of 50w resistors.

I have an 8 ohm high power speaker that i could use as a test subject, time is of the essence right now. Any concerns of hooking the speaker up an seeing what happens?
I do NOT want to risk any components, if i have to assemble a dummy load box not a big deal, just need to know all options.

[Jazz P Bass]

Any low value resistor at this point will show whether or not the 30Vdc is going to drop.
At idle, if the amp is working properly & the offset voltage drops, there will not be very much current.
If the offset settles then you can safely use a speaker.
You want to set the center voltage & the bias.
The Service Manual states :"Use an 8 ohm load".

[R.G.]

Any low value resistor at this point will show whether or not the 30Vdc is going to drop.
At idle, if the amp is working properly & the offset voltage drops, there will not be very much current.
If the offset settles then you can safely use a speaker.
You want to set the center voltage & the bias.
The Service Manual states :"Use an 8 ohm load".

Yep: what he said, one more time, with emphasis. That's why I was after the use of the pre-existing 820 and 10 ohm resistors. They ought to pull any leakage down, if they're connected. But any low value resistor you have will do it.

[J M Fahey]

each of the speakers' rings are tied together at a point in the cab and then grounded at the chassis, the resistance is OPEN line to the negative speaker terminals. I was wondering what this is all about.

They are grounding the speaker frames.
Very common on "speakers everywhere" distributed PA systems (hotels, gyms, airports, etc.) to avoid static buildup which could damage speakers or amplifiers.
Not really needed in close placed speakers, but doesn't hurt either.
Just shows how thorough those Japanese Engineers were.

[Prattacaster]

Big thanks to R.G. and Jazz P Bass, I did as you both said and waahhlaa 0.00 vDC offset with a 20uF 450v Atom, I wired back to the original ouput cap and it seemed to swing from 45mV to -35mV, not a consistant swing, sometimes it would settle to nearly 0.00. This was all done with the bulb limiter. I got the center voltage dialed in to exactly half the supply.

Then to the bias... It doesnt seem to be in the ballpark at 100mV. I'm guessing this is off because the bulb limiter. Any help is much appreciated.

Thank you J M Fahey for the explaination.

[Prattacaster]

Can anyone tell me why the bias is so high, the lowest I can get it is 90mV, the manual says to set it at 10mV. This is with a bulb in series between wall and amp.

[Jazz P Bass]

Is the lamp glowing brightly (please indicate the lamp wattage)?
If not, then it is time to remove the limiter.
Then see if you still have a bias issue.

[Prattacaster]

soft orange glo, 100w bulb, 100w amp, I was afraid to remove the bulb limiter in fear of the bias rising more. A higher wattage bulb would be cool but I dont have one.

[Jazz P Bass]

The soft glow indicates that the amp is NOT drawing 100 watts.
The 100 watt rating is the amps output power into a speaker.
An amp of this type should not pull more than 30 watts at idle.
How to test that.
With an ammeter.
(Or a Kill-A-Watt) Link: P3 - Kill A Watt

[Prattacaster]

Thanks Jazz P Bass,
Alright, I hooked up an 8ohm speaker and plugged staight into the wall.
Output transistor B+ = 122.5vdc
Center Voltage = 62.2vdc
Bias voltage= .138v or 138mV

This is across a .39 ohm resistor which calculates to be 354mA. This is with the trimmer set to minimum, any slight turn clockwise and the bias voltage rises. How do I get the bias cooler?

[Jazz P Bass]

I cannot download the schematic from post #1.
Where are you getting the test point info from.
The Service Manual that I have indicates 30Vdc for thr center voltage.

[Prattacaster]

G100 Service Manual.pdf
This is the manual I am using, it is a pretty close match. This one says center voltage 45vdc but with a B+ of 93vdc. I have a B+ of 122 so I need to be half that like R.G. mentioned.

Bias at 10mV at TP which is also pin 4. I substituted the STV-3h with 3 1n4004 in series. And I substituted the 2sc1586 output transistors with a cross reference match(MJ 15022 from Bdent) that were much cheaper. I really need to get the bias down quite a bit, I replaced the bias trimmer originally and I know it tested to 220ohms. I would guess maybe lower R18 value a little?

[Jazz P Bass]

Did you do TP 1-3?