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Here's an interesting one:
Amp: Carvin 2x12 Belair Combo
Carvin_Belair_Schematic.pdf
Issue: Customer complains of distortion and low output in the clean channel.
After looking over the service order, I set the amp up on the bench for a play test and establish a base line. I confirm the clean channel has an overall unpleasant distortion on all level settings, and proceed to check the output on the scope.
I can get a clean sine wave at the lowest volume settings, but the signal begins to clip hard very "early". The overdrive channel will starts out with clipping at the lowest gain settings. I normally would inject signal into the output stage to eliminate or confirm where the problem was, but I didn't this time (but I probably should have, it's good practice).
Interestingly, the signal output did not show the problem as drastically as the audio test playing the guitar. One thing I noticed, was that the volume control on the guitar was loud and scratchy like there was DC on the pot. Using a dummy plug in the input, I measured around -700mV to ground at the input jack, and 0V when V1 was pulled. So, first things first, labeled his preamp tubes and installed known good bench tubes in the pre and output stages to eliminate bad tubes. The scratchy pot and bad distortion was still there, so I moved on to the next phase of checking voltages and signal tracing.
Following the schematic, all the voltages at the relevant nodes in the clean channel checked out. But one thing I did notice was the voltage at pin 1 on V1 was +38V (indicating that the 220k resistor had gone open *measured around 9.1MΩ). Even though this stage is in the overdrive channel, and the plate is AC grounded in the clean channel, I had a suspicion that the low adjacent plate voltage was coupling into the clean channel input and causing our problems.
My suspicion was confirmed after replacing the 1/4W 220k resistor with a 1W, and playing the clean channel again to find the high frequency response and clean headroom returned for it's normal function. We double checked that this was the cause and fix by opening up the plate again, just to be sure.
All said and done, we were fortunate that we were able to find this one pretty quick. But I could have easily seen this one having us chase this ghost around for a while.
Amp: Carvin 2x12 Belair Combo
Carvin_Belair_Schematic.pdf
Issue: Customer complains of distortion and low output in the clean channel.
After looking over the service order, I set the amp up on the bench for a play test and establish a base line. I confirm the clean channel has an overall unpleasant distortion on all level settings, and proceed to check the output on the scope.
I can get a clean sine wave at the lowest volume settings, but the signal begins to clip hard very "early". The overdrive channel will starts out with clipping at the lowest gain settings. I normally would inject signal into the output stage to eliminate or confirm where the problem was, but I didn't this time (but I probably should have, it's good practice).
Interestingly, the signal output did not show the problem as drastically as the audio test playing the guitar. One thing I noticed, was that the volume control on the guitar was loud and scratchy like there was DC on the pot. Using a dummy plug in the input, I measured around -700mV to ground at the input jack, and 0V when V1 was pulled. So, first things first, labeled his preamp tubes and installed known good bench tubes in the pre and output stages to eliminate bad tubes. The scratchy pot and bad distortion was still there, so I moved on to the next phase of checking voltages and signal tracing.
Following the schematic, all the voltages at the relevant nodes in the clean channel checked out. But one thing I did notice was the voltage at pin 1 on V1 was +38V (indicating that the 220k resistor had gone open *measured around 9.1MΩ). Even though this stage is in the overdrive channel, and the plate is AC grounded in the clean channel, I had a suspicion that the low adjacent plate voltage was coupling into the clean channel input and causing our problems.
My suspicion was confirmed after replacing the 1/4W 220k resistor with a 1W, and playing the clean channel again to find the high frequency response and clean headroom returned for it's normal function. We double checked that this was the cause and fix by opening up the plate again, just to be sure.
All said and done, we were fortunate that we were able to find this one pretty quick. But I could have easily seen this one having us chase this ghost around for a while.
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