I just "saved" one of these amps from the electronics recycling trailer at our dump. When I took it home and tested it, I found that the preamp and effects sections all work with clear output to the headphones. However, there is no sound coming out of the speaker, other than a faint hiss coming from the tweeter. This is only my second repair of a salvage amp, and I think that I just got lucky on the first one (a Fender Princeton 85), so I'm in a little over my head with my new hobby. Does anyone have any experience with the power amp section on these things? I don't see anything obviously burnt. Any advice would be much appreciated.
Update - I removed the power amp board and looked at it closely. Three of the caps are leaking and presumed failed. Also one of the power transistors is burnt, along with two adjacent resistors. I'm planning to replace everything that is noticeably bad, and the mate to the burnt transistor and see what happens.
Two possibly useful thoughts:
1) Have you tested the woofer to make sure it survived the death of the output transistor? Sometimes the speaker dies along with the amp. If it's dead, that might have you re-thinking the repair.
You can use your DMM set to a low ohms range and verify a reasonable DC resistance across the speaker terminals (usually DCR is about 3/4 of the speaker impedance, i.e. about 6 ohms for an 8 ohm speaker, 3 ohms for a 4 ohm speaker, etc.)
If the woofer has fried, you have a decision to make as to whether to splurge on a new one, or not.
2) The amp board seems to have suffered considerable damage. Rather than repair it, it might work out cheaper and easier to buy one of those cheap class-D audio power amp boards off Ebay or Amazon or your favourite Chinese supplier, and just replace the stock power amp board with it.
A starting point to know what class-D board will work is to measure the power supply voltage going to the (fried) stock power amp board. If you know that voltage, and the stock speaker impedance, we can work out the wattage of the amplifier needed to replace the stock one. (It would really help if you can locate a schematic for the amp, but Behringer is notoriously unhelpful with schematics.)
As an aside, Google says Behringer rates the K1800FX at 180 watts. Knowing Behringer's reputation for truth in advertising (cough, cough), my guess is that the K1800FX will probably turn out to produce maybe 45 watts RMS in reality.
-Gnobuddy
Thanks for responding. I looked at the 4 ohm woofer with the DMM and it is reading like an open circuit. I'm going to hook it up to a load just to make sure. I have the circuit diagram from another posting on this board. It looks like 40 volts with a dual Mosfet output stage. It also looks like the caps and voltage regulators for 5V, +/- 15V are on the board with the power amp, which is fed directly from the transformer. I think this would make subbing in a class D power amp board difficult unless I also add a power supply.
If I can get sound out of the speaker, I might try to fix the board by substituting the blown parts. The burnt transistor is NPN power transistor from an earlier gain stage. The final stage mosfets, which are attached to a large heat sink look okay. If the speaker is blown, it's going back to the dump.
If I can get sound out of the speaker, I might try to fix the board by substituting the blown parts. The burnt transistor is NPN power transistor from an earlier gain stage. The final stage mosfets, which are attached to a large heat sink look okay. If the speaker is blown, it's going back to the dump.
That's not good. 🙁
Understandable. Mebbe someone might find the cab, chassis, transformer, knobs, and tweeter useful - why not put it on the "Freebies" section of Craigslist?
-Gnobuddy
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