Learning how to troubleshoot electronic circuits is one of the handiest skills anyone can have if they are building or modifying amplifiers or effect pedals. At some point you’ll power-up a circuit and find that it doesn’t work, and then what? For this reason we write “The Repair Bench” section of Guitar Kit Builder about our own troubleshooting of amplifiers and other devices, to pass along to the reader the thought process, tips and techniques of troubleshooting electronic equipment.
THE CARVIN SX200 GUITAR AMPLIFIER
In this edition of The Repair Bench we troubleshoot a Carvin SX200 amplifier. We purchased this amplifier in partially non-working condition for $100. The SX200 is a 100 watt combo amplifier with two 12″ speakers (2×12) and two switchable channels, with effects produced through a digital signal processing (DSP) chip.
We think our amplifier was made around 2001. In terms of appearance it matches the model shown in a photo (below) from Carvin’s 2001 catalog. It contains a circuit board labeled “Copyright 1997” so it’s unlikely to be older than that, and a number of the boards have codes on them ending in “-01” so perhaps those are date codes. We don’t know for sure about that. We do know that later versions of the amp changed from designating the two channels as “A” and “B”, as they are in our amp, to “1” and 2″ in later years.
Another indicator of age are the speakers used. Fortunately for us our amplifier has the earlier British Series speakers that were made for Carvin by Eminence and are quite good. According to Eminence they were a custom design, original equipment manufactured (OEM) for Carvin in the 1990s. They are similar to the Eminence Red Coat series, but are not identical to any other Eminence product. Carvin later offered speakers also labeled “British Series” however these are reported to have been made in China and are not rated well.
One identifier of the Eminence-made British Series speaker is a thin label with the model number “381208” as shown in the photo.
Repairing electronic equipment requires gaining access to the chassis of electronic equipment, where potentially lethal voltage levels can be easily touched, leading to shock, injury or death. If you don’t know how to safely conduct electronics repair work, please do not open up an amplifier or other gear. Have someone experienced show you how to keep yourself safe, or refer the work to an experienced technician.
Our initial tryout of the amp confirmed what the seller had told us. Channel A, the drive or lead channel, operated correctly but Channel B, the clean channel, gave off a significant hum and almost no signal level. Further, rotating the volume control caused a very loud scratching sound with a lot of presence. In our experience a volume control with these symptoms often indicates there is an improper level of DC voltage on the volume potentiometer. This was not your basic “dirty pot” problem, with a low volume of scratchy sound.
As described in previous columns, we always begin our repair work with a close visual inspection of all components. We’re looking for clues of what might be wrong, or what’s happened in the past. Our first look inside the chassis, from the top of the printed circuit boards, showed a very clean amplifier with no signs of prior repair, abuse or neglect.
Since the circuitry for the bad channel was likely to be on the preamplifier board we needed to remove this board from the chassis. To do this required removing the knobs, potentiometer nuts, and jack nuts. When we pulled this board away from the front panel to view the underside we found evidence that someone had attempted a repair of the volume pot for the B channel. This evidence, shown in the photo, is of a post-factory soldering job.
You can see the difference in the soldering of the two adjacent pots, which were wave soldered in a factory, versus the hand soldered pot on the right. This is not to say that there is anything wrong with the soldering job on the right, it’s just evidence that a repair was attempted at some point. Given that two other potentiometers, not shown, had similar solder indications, we surmised that at some point someone had attempted to repair the noisy volume control by swapping potentiometers.
With our initial visual inspection completed it was time to locate a schematic diagram for this amplifier.
CARVIN SX200 SCHEMATIC
A bit of internet searching quickly found a couple of sources for Carvin schematics. The site www.carvinservice.com showed several revisions of the SX200 schematic, but after looking through all of them we realized that their designations did not fit with our amplifier. The first clue was that the channels were labeled “1” and “2” instead of “A” and “B” as in our amp. Beyond that, the labels on the op amps on the schematics did not fit with what we saw on the actual circuit boards. Our conclusion was that these schematics were all for later versions of the amp.
The other site for Carvin schematics is www.carvinmuseum.com, but we found that their SX200 schematic was also for later versions of the amp. Our next thought was that maybe the preamp board was the same as used in other Carvin models, and we were aware that the SX100 was a similar amp. When we grabbed the SX100 schematic from www.carvinservice.com we were pleasantly surprised to see that the actual drawing was labeled SX100/SX200, and that the preamp fit what we observed on the circuit board.
Given our initial suspicions about DC voltage on the B-channel volume pot, we focused on the preamp input section of the schematic, shown here:
DC VOLTAGE MEASUREMENT
Our plan to diagnose the amp was to begin by measuring the DC voltages in the front end of the preamp. To do this we needed access to the preamp circuit board while it was powered up. We had already removed the board for inspection. To measure voltage on it we needed to make sure that the board wouldn’t be touching the chassis while powered up. This can be tricky in some amps, because you’re limited by the slack in whatever connecting wires and cables are connected to the board. In this case there was just enough slack to flip the board over, so we could have access to both the component side and traces on the underside. We placed some cloth from a T-shirt between the circuit board and the chassis (see Photo), to prevent anything from touching or shorting out. We find that this approach works well, but different solutions need to be used for different situations.
With our digital multimeter set for DC voltage we began to measure, beginning with the volume pot, and as suspected found about 13 volts DC on the pot terminals that connect with resistor R19. This voltage is pretty close to the power supply rail voltage, and in our experience should be no higher than a few tenths of a volt. Measuring further we found similar voltages on the output and inverting input (-) pins of op amp A1A. We found -14.2 volts on R17 at input of op amp A1A. Likewise the DC voltages on these pins would normally be no higher than a few tenths of a volt.
At this point we couldn’t be sure whether the high voltage was coming from the op amp A1A, or from transistor Q2. To isolate the source of the voltage we de-soldered and lifted one lead of resistor R19. We found that the high DC voltage was now gone from the volume pot and transistor Q2, but still present on the op amp terminals. Our conclusion was that the op amp contained an internal short circuit, that was connecting the power supply voltage directly to the output and inverting input pins.
One verification of our diagnosis was that with resistor R19 lifted, we could now use the B-channel through input jack A/B without any hum or scratchiness.
We used our de-soldering tool to remove the A1 op amp, an NJM4558D chip, and ordered a replacement from Digi-Key. When the new part arrived it was soldered in place and the B-channel was found to work perfectly. Amplifier fixed, case closed.
Sites That Link to this Post
- Guitar Amp Repair Kits | Learning Guitar | September 24, 2015