CRATE GX-15R Help

[Mick Bailey]

Exactly the same as your guitar cable. I opened the PDF fine from the link in the other thread. Enzo mentioned the insulator between the IC and the heatsink; if it had one originally you need to replace it. You cannot use thermal grease on its own if it needs insulating. The thermal grease just helps to make contact between two surfaces by filling any voids. Note that if there's an insulator, there's usually an insulating bush so that the screw is insulated from the device tab, otherwise it will short out. Best to check between the tab and heatsink with your DMM set to resistance when you reassemble it (if it needs to be insulated). Do this before you switch on.

[jagora]

Mick, Thanks for that information, On mine the I.C. Chip looks like it marry's the Heat Sink with a small circular indentation that clips onto the Heat Sink and then a small metal clip secures the two together. I never noticed any insulator or adhesive grease. I appreciate all of the caution notes and warnings as well.

Joe

[jagora]

Mick,
I tried it this afternoon and It worked. I hooked up a known working Speaker from another amp and did exactly what you described as a disturbance test. The speaker made a crackling sound (In a Good Way) So now I am Gung Ho to remove and replace the TDA 2030A chip. Wish me good luck and I'll let you know how it goes.

Thanks Again
Joe

[Mick Bailey]

That check tells you that the amp IC is amplifying the noise and is probably good, and shouldn't be replaced. You say that you amp is not making any sound. If the preamp was getting a signal up to that power amp chip then it would make a sound - just like touching the pin makes a sound. We don't know what the quality would be like, but it would produce something.

The fault lies further back.

Did you check the voltages are present on TP10/TP11? (+/-15v) ?

[jagora]

Mick,
Yes I did check the test points 10 and 11 with the (power on) and the negative probe of the ohm meter grounded to the chassis and (without) a speaker connected. It did produce Voltage at those test points. What's the next culprit?

Joe

[Mick Bailey]

Simplest way is to diagnose the amp in clean mode - this way you cut down the signal path by half. Firstly, make sure the switch is operating correctly, as a faulty switch means the amp will not work. In Clean mode the signal gets amplified by 2 sections of IC1 which is a quad opamp and each section is shown as a triangular functional block. So the signal is fed into the first opamp and appears on the output pin 1. It then goes via C1 and gets picked off the wiper of volume pot P3 and is routed via the slider switch to the second opamp (actually the 4th one in the package if you look at the pinout for a TL074) and outputs to the tone stack on pin 14 and at the same point splits off to the reverb section.

In the absence of a scope and signal generator you can use the 'disturbance test' to introduce noise into the amp at various points. With the controls turned up and a plug inserted into the input socket to open the grounding contacts, carefully touch a probe onto IC1 pin 3. Anything? then try pin 1, pin 13 and pin 14. You should get a loud hum through the speaker at every point.

Edit; Or you could use your DMM set to AC volts and use a sine wave source plugged into the input. 400Hz/1KHz will do it. You should 'see' the signal on the opamp outputs. Also worthwhile checking the outputs for DC to see if the opamp is shorted, or there's unwanted DC on the input.