Soundcraft Gigrac 600

[xtian]

This two-channel amp idles fine when NO LOAD is attached, and all the preamp functions work. Plugging a speaker into Channel 1 gets a hum, shortly (2 seconds) followed by the amp entering fault mode, which can be reset with a simple power cycle. Plugging a load into Channel 2 gets nothing, no sound out.

I have tested all components shown in yellow for shorts, and cannot find any faults. Any suggestions for further troubleshooting?

[Jyvoipabo]

This two-channel amp idles fine when NO LOAD is attached, and all the preamp functions work. Plugging a speaker into Channel 1 gets a hum, shortly (2 seconds) followed by the amp entering fault mode, which can be reset with a simple power cycle. Plugging a load into Channel 2 gets nothing, no sound out.

I have tested all components shown in yellow for shorts, and cannot find any faults. Any suggestions for further troubleshooting?

[ATTACH=CONFIG]51179[/ATTACH]

Hello,

There is only a single protection circuit for the 2 channels.
If one is faulty, the 2 channels enter in safe mode.

I think Channel 1 send DC To the output.
Check all the components of Channel 1 = Resistors transistors and particulary T32 and T24 (HV drivers)

JP

[Jyvoipabo]

Duplicate

[J M Fahey]

This two-channel amp idles fine when NO LOAD is attached, and all the preamp functions work. Plugging a speaker into Channel 1 gets a hum, shortly (2 seconds) followed by the amp entering fault mode, which can be reset with a simple power cycle. Plugging a load into Channel 2 gets nothing, no sound out.

I have tested all components shown in yellow for shorts, and cannot find any faults. Any suggestions for further troubleshooting?

Since you seem to have the whole schematic, can you please post all of it?
I was always intrigued by its interesting concept and would love to see their preamps, master area, etc.

Back to your problem:

1) please post the power supply also, and measure rail voltages, also those feeding the Op Amps on pins 8 and 4

2) always identify which power amp are we talking about at the moment: P1 or P2.

3) P2 seems to be directly fed from (or even sharing same PCB) Power supply, while P1 seems to be fed through flying wires plugged in w3 and w4, so measure voltages there too.

Almost forgot: build and use a bulb limiter, I suggest a 60W to 100W lamp since this is a powerful amplifier, and NO ooad of any kind (speaker or resistor) at output until we find it safe.

3) measure (all DC) voltages at:
* outputs: w2 and w6
*a Net is the track connecting 2 or more component pins, measuring anywhere on it must show thye same voltage, since "all connected points are one", I name nets by components connected to it.
Of course, only one component leg at a time is connected to each Net ... or they would be shorted by it.
"Ground" or power rails are nets by definition.
* Nets Tr12-R26 ; Tr11-R22 ; R25-R19 ; TR26-R52 ; TR25-R55 ; R54-TR28.
I find it very curious that both channels are not the exact same , P2 has R19 on TR9 collector, while TR28 on P1 goes straight down ¿¿¿???

But P2 includes the protection circuit, P1 does not, so not necessarily all voltages will match on both power amps.

Ok, measure and post bthese voltages.
There´s way more to measure, but let´s start with something.

[xtian]

Thank you, JM! I will report back. In the mean time, here is the full set of schematics:

http://schems.com/bmampscom/soundcra...rac_1000st.pdf

[Jyvoipabo]

Hello,

Watch the protection circuit TR36,37,38
R34 is the sense resistor for channel 2
R44 is the sense resistor for channel 1

All that's needed is only one faulty channel for both to get secure.

Remove these resistors will disable the protection and will be necessary to perform live test. (The next step after you have checked all components).

JP

[xtian]

When disconnected from amp PCB, the power PCB is functioning correctly, HT + and - in spec. As a technical enterprise, I'd love to continue chasing the demon, but in practical terms, I've blown my 1-hour bench fee two hours ago, and owner is not interested in dumping money into this amp, which goes for under $200 used.

So, I have to call it quits on this one.

Thank you, friends.