Hi.
I have a Randall RG1003H on my desk for troubleshooting.
It makes an awful lot of noise about 2 secounds after I play a note and then mute the string, on every channel, especially channel 2 and 3.
It does not develope the 100W that it is supposed to do with a 4 ohm cabinet.
Reverb won't work with the control, only when I touch the reverb unit that is located inside.
The user manual is cut off in the middle of the explaination on how the footswitch works...
There is a schematic on the internet but on the actual pcb are a few mods that don't mach whith the schematic
"Developed by Randall, assemblied un China"...
Any ideas what to look for?
I have a Randall RG1003H on my desk for troubleshooting.
It makes an awful lot of noise about 2 secounds after I play a note and then mute the string, on every channel, especially channel 2 and 3.
It does not develope the 100W that it is supposed to do with a 4 ohm cabinet.
Reverb won't work with the control, only when I touch the reverb unit that is located inside.
The user manual is cut off in the middle of the explaination on how the footswitch works...
There is a schematic on the internet but on the actual pcb are a few mods that don't mach whith the schematic
"Developed by Randall, assemblied un China"...
Any ideas what to look for?
Hi
I think you will need to look for this down in the instruments section, where they move guitar amp threads.
First decide if it is mechanical at all. When you play a note, ther is speaker vibration, and that can trigger symptoms. So with the amp on, but not playing anything, hit the top of the amp with your fist. Turn the reverb down first. If the amp reacts, you have a loose connection inside.
And have you tried it with different speaker cabs?
Low output sounds like the signal is impeded somewhere, and your 2 seconds of noise could be distorted signal peaks as it fades out, with some severe DC offset along the signal path. I'd go through the thing looking forst at power supplies to make sure both the high and low voltage supplies are present and up to voltage. Then if the ICs are getting proper power, look at the output pins on each for DC offset.
Is ther anything digital in this model? A missing digital bit can turn music into noise.
I think you will need to look for this down in the instruments section, where they move guitar amp threads.
First decide if it is mechanical at all. When you play a note, ther is speaker vibration, and that can trigger symptoms. So with the amp on, but not playing anything, hit the top of the amp with your fist. Turn the reverb down first. If the amp reacts, you have a loose connection inside.
And have you tried it with different speaker cabs?
Low output sounds like the signal is impeded somewhere, and your 2 seconds of noise could be distorted signal peaks as it fades out, with some severe DC offset along the signal path. I'd go through the thing looking forst at power supplies to make sure both the high and low voltage supplies are present and up to voltage. Then if the ICs are getting proper power, look at the output pins on each for DC offset.
Is ther anything digital in this model? A missing digital bit can turn music into noise.
Its not a mechanical issue. I hit the amp a couple of times and no reaction.
Also tried different cabs: 6 ohm modified JBL speaker (really old...), and 4 ohm 2x15 Peavey bass cabinet. Same result, although different bass responses.
And yes, there is something digital in this amp: an 8-Bit MCU (HT48R06A).
Funny thing: when I touch the chassis and move my other hand over the open end of the cable (one end plugged into the amp, other end unplugged) without actually touching it, the noise almost disappears. Maybe its a grounding issue...
Also tried different cabs: 6 ohm modified JBL speaker (really old...), and 4 ohm 2x15 Peavey bass cabinet. Same result, although different bass responses.
And yes, there is something digital in this amp: an 8-Bit MCU (HT48R06A).
Funny thing: when I touch the chassis and move my other hand over the open end of the cable (one end plugged into the amp, other end unplugged) without actually touching it, the noise almost disappears. Maybe its a grounding issue...
All opamps (4558) have +14.90VDC and -15.60VDC from the psu. That's less than a volt in difference and should be OK.
I havn't checked the power transistors, yet...
I havn't checked the power transistors, yet...
Sounds more like a high frequency oscillation then. get a scope on it and see if the circuit is making RF.
I'm no expert with "'scopes" of any kind.
But I have a "Parallax USB Oscilloscpe" (2 channel, 20Vpp max) that I can connect to my laptop. If could tell me how to use this 'scope on the amp I'll sure give it a shot...
But I have a "Parallax USB Oscilloscpe" (2 channel, 20Vpp max) that I can connect to my laptop. If could tell me how to use this 'scope on the amp I'll sure give it a shot...
Hi PhantomBox,
I moved your thread for you. It's under the "Instruments and Amps" section now.
-Chris
I moved your thread for you. It's under the "Instruments and Amps" section now.
-Chris
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