02 July 2014

Laney VC30 212 Tube Amplifier: No Output and Falling Apart

Note: This amp was a pain to get back together. I don't mention it in the narrative below, but before I put everything back together (with any amp), I drain the filter caps. When you're trying to position the chassis just right with one hand while fumbling with the bolts and screwdriver in the other, it's best to be safe.

Anyway, this Laney VC30 212 (PCB 9072-3)— was beat. It was missing most of the bolts holding the chassis in the cabinet, and there was no output.
Fig. 1. Yes, that is duct tape on the handle.
The high temp fuse on the back was fine, but the fuse on the board was blown (at the far right on the board in Fig. 6). I replaced that and it worked fine. I spent quite a while in a car parts store, then the hardware store trying to find matching bolts and speed nuts (see Fig. 2). Both places had one part but not the other matching part. Frustrating. And if you're looking for replacement bolts, these were metric: M5 .80.

Fig. 2. What are speed nuts? These thingies. 

After messing around all morning hunting down parts, I finally was able to put it all back together (no easy task) and there was no output again. Because it worked when it was apart, I thought maybe one of the bolts was touching (and therefore grounding) one of the parts inside the chassis. I removed the bolts and stuck a plastic rod into each hole to see if it bumped anything. As I expected, I bumped something in one of the holes. The right hole holding the board on back of the chassis had very little clearance. The original bolt must have been shorter than the one that was being used when I got the amp. The bolt was grinding away at the backside of the PCB every time it was screwed in. I got a M5 .80 20mm bolt (roughly 3/4"), and it was just right. Long enough to get some purchase in the threads, but not so long that it touched anything inside the chassis.

I took the amp apart again and removed the board to check the back for damage. To remove the board, by the way, you will need to desolder the wires connected to the Bright and Drive switches. Other than that, it was pretty easy—remove the nuts and washers from the knobs and inputs and two screws holding the board to the chassis. On the back, the damage from the bolt was apparent. It gouged two separate copper areas (Fig. 3). I scraped between them to ensure that there was no continuity and put a piece of tape on it in case the wrong bolt was ever used again.

Fig. 3. Damage from the bolt that was too long.

Fig. 4. I ensured that there was no untoward continuity among the damaged traces and covered the area with electrical tape in case someone ever uses a long bolt in that spot again.

As I mentioned, getting the amp back together was no mean feat. The speed nuts tend to move around when you slide the chassis back into the cabinet, so some of the holes lined up the the holes in the cabinet, but some did not. The top bolts were easy, but the four side bolts were not. There is room to reach into the cabinet and up over the front lip of the chassis with your hand and move the nuts that are closer to the front of the cabinet (see note at the top of the page), but not the ones toward the back. To do that, I slid a little ruler up into the gap and wiggled the nut into place (Fig. 5).

Fig. 5. Blindly moving the speed nut into position with a small ruler.
It's hard to do this with any precision, but you only need to get it close enough to be able to stick something in the hole and finish lining it up. I used my plastic rod again for this.

I got it all zipped up the second time and it sounds fine. Here are some board pics, in case they might be of any interest:
Fig. 6. As you can see, this amp has been modded previously. Some of the components have been soldered to the clipped legs of components that have been removed. I found a resistor that looked okay, but when I poked it, it moved. One of the legs was broken and you couldn't tell by looking at it. I replaced it.

Fig. 7. There are a lot more surface mounted components on the back of the board than I would have expected. The covered bolt damage is on the right.