Ages and ages ago, a coworker brought me his Fender Frontman Reverb amp saying it didn’t work. He had a second one that did work, so there wasn’t any rush to get it working, so I took the unit home, and went to work on it. Disassembly was easy enough, and finding the schematics showed that the unit was identical to the Fender Frontman 15R, which has schematics (in the service manual) here. The US Fender site SAYS it has schematics for the 15G-15R, but only the 15G is shown, meaning the whole reverb section is left out. =/
Anyway, after pulling the unit apart, and doing a little reading online, I figured the issue was probably the op-amps, or the main amp IC (TDA2050). So, I pulled all the op-amps, installed sockets, and replaced them with some nice BB OPA2134’s that I had around. But, that just caused horrible oscillation. I dropped the stock op-amps back in, and that problem resolved, but still couldn’t find the issue. All the solder pads looked good, and I touched up everything I could find. I also hot-glued anything that would not enjoy a bouncy ride (the main filter caps, etc). Checking the voltage rails everything looked good as well. The schematics do give information on testing. They say set all knobs to 12 o’clock, the gain to 9 o-clock (careful on this one, I had mine set to 3 o’clock for the longest time while troubleshooting and couldn’t figure out the issue), and the reverb on, but turned all the way down to 0. I had previously made a BNC to 1/4″ TRS adapter, but fearing that was an issue, I purchased a good Pomona 1297, which is made for telephone switchboards, I guess. Anyway, neither resulted in any signal coming through. Finally, I grabbed a BNC to test clips lead, and hooked directly to the jack side of R1, and got a signal! So, the input jack was the issue. Triple checking the solder pads, I finally removed the jack, and pried the lid off it. Ah ha! The little “tab” that comes in contact with the “knee” of the tip had broken off completely. So while you could put pressure on the 1/4″ jack and get SOME contact, it wouldn’t be very good.
Searching eBay, I did find you can purchase replacement jacks for a reasonable amount (the part is a Fender 0990912000, 5-pin mono amp jack), but given the issues with the current one, and the multitude of issues online with the traces/pads breaking on the jack, I decided it order some “real” jacks, the Switchcraft 12A jacks, and then solder wires directly to the PCB. You want the 12A because it’s the shunting/shorting style, so when nothing is plugged in you don’t get a loud ground hum. You also get a nice strong, metal, jack. It’ll outlive the rest of the amp. Interestingly, Mouser was half the price of Digi-key for these jacks, but I just ordered a 4-pack off eBay.
Installing the jack is obviously easy, just wire the ground and shunt pins to ground, and the tip pin to the input of the amp. I did wind the ground and tip lead together to help with any signal pickup, but that’s unlikely in such a short run. Install back into the case is easy enough, and the end result looks good (only downside being the mis-match between the jacks). All and all, an easy repair if it wasn’t for the fact I spent 10x longer on it than I should have. =/ Remember kids, the OSI model doesn’t only apply to networking. Always check your physical stuff before bothering with the rest of the rabbit hole.