GK 400 rb

[catstrat]

Enzo. i think for me more often than not are cracked solder joints, bad pots or jacks as you said. I also need a little more patience and stop trying to rush.

[catstrat]

I do believe i probably get side tracked and start trying to track down problems that don't exist when the problem probably is something easy and simple.

[catstrat]

I just got my limiter built so now maybe i can fix things the right way. Now i need to learn the different failure modes of transistor amps. I wonder what's going to happen when we can't get incandescent bulbs anymore?

[catstrat]

G K 400 RB

Good morning. I had to go back and correct my mistakes and replace damaged parts from my earlier failed troubleshooting attempts. All that's now good. With my limiter now in use, i can turn the amp on with no load and it works as it should. The limiter bulb doesn't even come on. I was able to check voltages at my O/P transistors and that looks good. All of them check within millivolts of each other. Now the problem,as soon as i plug in a speaker load and turn the amp on the bulb flares up about as bright as it can go. Unplug the load and power it up again and everything is normal. The other thing i can just plug a plug into the speaker out and the amp acts normally. It's only when it has a speaker load. I was thinking flyback diodes, but i checked them and they check good both in and out of the circuit. Then i thought maybe C 6 but it didn't show any shorts,in and out of the circuit. Earlier i was testing with my meter and i got a short while checking the filter caps as well across the flyback diodes while in the circuit. Remove from the circuit and no short was shown. Any suggestions? Could i have caused another problem that i need to find? Thanks.

[catstrat]

And of course,the board layout i have doesn't exactly match the one that's in the amp and there is no silk screening on the board either. I'll see if i can find the right one unless someone on here has the right one. Board number is 60041A. Thanks.

[catstrat]

Apparently this amp is old as dirt. A tag on the back says 3-85. Is that the build date ? Now i'm wondering if i even have the right schematic for it. I know the board layout is different. Help !!!!

[J M Fahey]

With my limiter now in use, i can turn the amp on with no load and it works as it should. The limiter bulb doesn't even come on. I was able to check voltages at my O/P transistors and that looks good. All of them check within millivolts of each other. Now the problem,as soon as i plug in a speaker load and turn the amp on the bulb flares up about as bright as it can go. Unplug the load and power it up again and everything is normal. ..... It's only when it has a speaker load.

Don't worry, it's "normal"
MANY respected amps do that.
Fact is, amps when working have lots of "normal" voltages all over it, specially biasing.
Fine, that's what we usually measure to troubleshoot.
But many of those voltages aren't there instantly, there's resistors and caps feeding many , so it takes up to a second for them to stabilize.

That's one reason many amps have a turn on "thump" , voltage at the speaker out will become almost 0V after 1 second, but for the first milliseconds you might have up to one full rail there
LOUD thump, the speaker moves visibly forwards or backwards, you name it.

But it stabilizes very quickly.

Problem is, IF you have a current limiter and a speaker load, the current pulse through the speaker might be more than what the limiter can supply ... and it "stalls" , exact same as a car engine which if engaged is very difficult to start but once running pushes the car easily.
That's why you decouple (shift to Neutral) engine from wheels to start it

Do the same with the amp:
1) disconnect speaker
2) start amp through bulb limiter
3) measure voltages.
a) rail voltages which should be somewhat below normal (because of the bulb limiter)
b) output DC voltage which should be small (<100mV DC)
c) NOTE: abnormally low rail voltages coupled to higher than normal bulb brightness may indicate an overbiased amp, one which will work beautifully ... and suicide later.
If in doubt check bias current through emitter/ballast resistors.
4) only now, if everything checks normal, connect speakers without turning amp off (or you would fall again in the trap).
If it's still fine (voltages), apply some audio, say a couple watts, just to check it sounds good, does not fart or distort, etc.
5) if you raise volume, bulb will pulse with the music, in an extreme case the amp might stall again.
I told you, just a couple watts
6) if everything fine, plug the amp straigtht in the wall.
7) enjoy

[catstrat]

400 RB

Thanks. The question i have using a light bulb limiter, does the bulb light up bright when you first turn the amp on and then dim down within 20 seconds or so after the amp stablilizes. If the bulb stays bright,how long will i have to check voltage before damage occurs? Here are the voltage readings from the o/p transistors with the speaker unplugged and using the limiter. This amp has 60 volt rails.
These are the 15002s collectors on both are negative 56.7 volts. The emitters are negative .07.7 volts and the bases are negative negative ,08.1 volts. The 15001s are an exact match,voltage wise, to the 15002 except they're positive. On the low voltages of the base and emitters i don't remember which meter scale i used so the 0s may be wrong. and they may just be .7 volts instead of .07 on all o/p transistors. Thanks again.

[catstrat]

400 rb

i just tested the voltage per your instructions and everything still in line. The bias voltage should be about 5 millivolts across the two of the emitter resistors according to the schematic. I turned the unit on and let it get stable for about 30 seconds, plugged the speaker in and the bulb was just as bright as before. I'm still missing something somewhere.

[g1]

What is the wattage of your limiter bulb? The idea of it is that the more current the amp draws, the less of the line voltage the amp gets. This should protect it for as long as you need to do some measurements.
You are showing close to proper voltage at the supply rails (57V at collectors) where the schematic shows 60V. To me this says the bulb is not dropping much of the voltage.
What is the DC voltage at the speaker output when you have no load?
There are 4 emitter resistors, one for each output transistor. Measure DCV across each of them. Are they all close to the same reading? What do they measure?

[catstrat]

Hi G-one. Right now the bulb is 100 watts. I have a 250 watt bulb. Should i use that one instead?
The d.c voltage at output is is less than 100 m.v. I measured the bias aross r 26 and r 27 and it was 8 mv. The schematic says 10 mvs. Thanks.

[catstrat]

I just tried the limiter with a 250 watt bulb and after about 10 seconds it lit up with a dim glow. What does that mean?

[catstrat]

I just re checked the voltage at the speaker out to make sure i tested it right and i have 7 volts not less than 100 mv. What does this mean?

[catstrat]

I rechecked the voltage at the output and it's negative 6 volts. I read the meter wrong the first time.What does this mean?

[DrGonz78]

I rechecked the voltage at the output and it's negative 6 volts. I read the meter wrong the first time.What does this mean?

One thing it means is that you should not hook up a speaker to it until there is no substantial DC voltage on the output. Just making sure you don't hook up a speaker anymore or you might damage it.