Author Topic: Tales from the repair bench  (Read 574 times)

Mark Hammer

Tales from the repair bench
« on: August 01, 2022, 04:49:51 PM »
A local buddy brought his Nobels ODR-1 over this afternoon for me to take a look at. He thought perhaps the input jack had gone wonky, because sometimes if he wriggled the plug it would work, while other times it wouldn't. Luckily, I had a spare jack with identical pin spacing, so I unsoldered the existing one, and replaced it (jeez I hate double-sided boards!). We powered it up, and while we got bypass loud and clear, we weren't getting any effect signal.

All controls seemed to function properly, though. Turn up Drive or Volume, and you got more hiss. Adjust the Spectrum control and you changed the timbre of the hiss. So, clearly, all the gain and tone control stages were working properly. There just wasn't any audio signal.

This looked like a job for the audio probe, so I plugged it in. It's just a regular guitar cable, with the plug on one end replaced by an alligator clip for ground, and the hot lead connected to a 0.1uf capacitor, with a nice long lead sticking out to touch various point in the circuit. It's plugged into the amplifier, so that you can hear whatever signal is present at different points prior to the output of the circuit.

Q1 provides both the buffered bypass signal and the signal to the effect circuit, so there wouldn't be anything wrong with that component. But Q5 is part of the switching circuit. When it is turned on, signal is allowed to pass from pin 2 to pin 1, and on to the Drive stage. It wouldn't be the first time in history that an FET failed, so I mimicked it being successfully turned on by temporarily bridging pins 2 and 1. The probe revealed signal at both pin 2 and pin 1, even when they weren't bridged, but still no audio at the output jack. Maybe it's the op-amp chip. I probed the output and then input pins (mislabelled, numerically, in the drawing), but no signal there. Again, all controls are doing what they should. If the chip was fried, none of the controls would work and you wouldn't hear any adjustable hiss at the output.

So, something was happening ahead of the chip. But what, and where did the signal path stop? There's only two things between Q5 and the input to U1A: capacitor C10 (outlined here) and resistor R10. I probed the junction between R10 and the op-amp, and still no signal. I touched the probe to the leads of C10 on both sides of that component, and we had signal. So why is there signal up to that point, but not beyond?

I turned the board over, and looked at the pads for C10. One of them looked suspiciously flat. In other words, there was a "web" of solder covering the hole, when you looked at the copper/solder side of the board, but the lead from that end of the cap was not poking through the solder side the way that all other components did. It looked conspicuously flat. Weird. I thought I'd unsolder the cap to see what was up. I heated up the pad where the lead did poke through, and the cap fell out. It had been soldered to the board on one side but not the other (being the connection from C10 to R10). So, when my buddy had wriggled the plug in the input jack, he had probably made momentary contact between the unsoldered cap lead and the "web" of solder. I soldered the cap in place properly, and the pedal functioned like it was supposed to.

Another one saved. I was none too impressed with some aspects of the manufacture. More than one capacitor was poorly installed. The board was "clean", so it hadn't been modified. This was just a bad factory build. Go figure. Goes to show you, though, that sometimes what stops a pedal from functioning can be something you never thought of.  Also explains why he was able to buy a "legendary" pedal from the seller for $40.



Sparky

Re: Tales from the repair bench
« Reply #1 on: August 01, 2022, 09:43:37 PM »
HHmmm.....well...here's a guitar story....
   Guy brings over a late 90's Hamer Archtop Sunburst model [a Korean model].   I really like Hamer stuff and have some of those Archtops.   He says it won't stay in tune and wants me to do a set-up and check everything out.  As I was adjusting the truss rod and re-tuning it I noticed it kept going a bit flat.  I did all  the usual bridge / nut/ tuning gear /  etc etc. stuff to no avail.  The last thing I checked was the neck joint..........and there was a very slight "creaking"  sound when I pulled on the neck.  I pulled harder and the whole neck came away from the body  [I was pulling the neck parallel to the body].   Amazingly, it looked like there was never any glue in the neck joint from the factory.  I glued the neck and clamped it for a few days.   He ended up selling the guitar to me.  That thing must have lived in it's case for years because nobody experienced any symptoms outside of the occasional tuning problems.   

Keppy

Re: Tales from the repair bench
« Reply #2 on: August 01, 2022, 11:24:26 PM »
I almost bought one of those Hamers in the 90s. I remember endlessly playing one in the local music store at the age of 15 or so. It was a great guitar!
"Electrons go where I tell them to go." - wavley

Sparky

Re: Tales from the repair bench
« Reply #3 on: August 02, 2022, 11:40:13 AM »
......and then there was the guy with a 60's Strat  who wanted to sell it to me.  I asked him if it had the original frets and he said "Yes".     He brings it over and it had and obvious refret.  I reminded him that he told me over the phone it had original frets.  He pulls a small bag out of the case and says "Here they are".   
I've heard similar stories from other people over the years.....

Sparky

Re: Tales from the repair bench
« Reply #4 on: August 07, 2022, 09:52:09 AM »
Hmmm...... I guess the lack of stories may be tied in with the lack of guitar and amp repairs .  Ever since covid things have taken a nose dive.  Sad.