Hi. Not our mainstream line of work - we work with digital mostly. Will study the schematic found on the internet. The output darlington transistors appear to be ok. Even with no input, the output is extremely loud with static / crackle / pop. Any ideas? Will post more details and fix as we continue the review. Have not checked the main power supply rails yet (+-40V and +-16V).
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What model number? Got a schematic to boot with it? -
Thanks so much for replying. No, could not find one..yet. Will share the full details on Monday (or sooner if we run back to the office). Still fixing a Fender 212R which is slightly different in the schematic than the public ones on the internet. Has noise even on clean input. Static / distortion with a mind of its own. The power rails are stable. Amp appears to be ok and loud. Replaced most of the pots and did replace each connector with little improvement. Working with a 1 khz tone from our scope as input and tracing as we go.Comment
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Attached.
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Found the following on internet which may be the scematic - will confirm it on Monday.
Symptoms are:
with or without input, blasting loud random static / snap / crackle pop. Volume control has no impact on the output noise. Cut the darlington output transistors to measure in diode mode and they appear to test out fine and without a short. Have not yet injected a 1khz tone into the input to monitor with a scope.
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Check with the oscilloscope what goes into the PA. Would be my first step.Comment
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can you split the power amp from the pre-amp to seeif it still generates noise?
btw, the 16v regulation circuit is taking the piss - i hope these arent marketed as "high end"
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Power supply rails appear to be stable. +-40 is ~38v. +-16v is ~16v. After assorted permutations to open this beast, our 1khz injected tone appears to be clean at the output. Soldered back the darlington output transistors and mated the speakers - still clean. When brought in, the noise was unbearable out of the speakers. Loose connections? Will keep testing.Comment
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maybe cracked solder joints.
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Indeed on the DSP board. See attached. Soldered them back but still no activity on the LEDs.
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Plugged in a guitar which works fine with our attempt to play music but there is a background high pitch tone (much like the 1khz we were injecting from our scope). This is when the guitar is plugged in and sitting idle. If the 1/4" plug is disconnected, then there is still this tone (corrected update).
Will post a new audio of the tone.
Is this normal to have this background (annoying) tone? Still trying to validate if the DSP is working or not - all of the LEDs on this DSP board are dark.
Last edited by mon2; 07-22-2025, 10:39 AM.Comment
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A fresh recording of this tone - no inputs are mated with this amp.
The probe is going to our portable speaker with a 0.1uf cap in series - the amp speakers are disconnected while we experiment to avoid risk of damage to the true speakers.
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focus on the dsp board i think
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Yeah - the DSP board appears to be dead and was purchased this way. Will search for the DSP component but not sure if there is firmware involved or not. A royal pain to probe at the awkward angles for this vertical mounted PCB that is like an oreo sandwich inside of the cut slot. No connectors - just raw PCB pads being soldered to mate with the 90 degree main board.Comment
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I guess they use the DSP board for the delay, reverb etc. If the reverb isn’t working the DSP is dead. Verify if it is getting any power.
These kind of speakers have a hard life. Being tossed and kicked around, stored in a wet basement, spillage, etc… perfect recipe for cracked solders and other damages.Comment
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Have verified that the DSP is indeed receiving 3v3 via the +10v rail then a local LDO. However, no led is being driven. Will investigate the 2N2907 that is onboard the add-on board with the bank of leds.
Yes, found about 4 screws that are rusted but otherwise, very clean solder joints under the PCBA. The unit is working much better than when it was brought in but a bit baffled as to how it 'cured itself'. Perhaps the handling of the assorted cabling / pcbs. So far, unable to see any bad solder joints other than on the DSP board (which is still not working).Comment
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The DSP remains dark but believe it may need a foot pedal to trigger. The client will bring in those pieces next week. The unit is 99.9% better than how it was brought in. Still have that high tone I can still hear but may not matter when the volume is cranked a bit which drowns out this tone. Puzzled on the source of this tone - disconnected by surrounding UPS and test equipment used to isolate myself from the mains but the tone still remains. The DSP schematics are not public from what I could find on the internet.Comment
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Well…you suppose I got a Fender Aoucustisomething 100 in. It has also a DSP, no schematics to be found for it. My DSP works, I got some problem with one channel no audio, than popping in the speaker and the it works like nobodies business…
I am sure the DSP is running something… upgradable firmware or not.Comment
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