This 1983 Fender Concert arrived in great condition having been with the same owner from new and obviously well looked after. It hadn't been used for a couple of years as it intermittently blew it's twin T500mA HT fuses. Before applying any power, the cause needed to be found.
I did the usual component tests, filter caps tested great with very low ESR and no signs of bulging or leaking electrolyte. Both ends of the caps under the "dog house" had some brownish stuff on them, probably from the factory. I think it's rosin flux, but why? I've never seen that before. The transformer windings were fine too.
I did the usual component tests, filter caps tested great with very low ESR and no signs of bulging or leaking electrolyte. Both ends of the caps under the "dog house" had some brownish stuff on them, probably from the factory. I think it's rosin flux, but why? I've never seen that before. The transformer windings were fine too.
It took a lot of eye-balling and rummaging through spaghetti before I found a dead short between the screen supply and ground. There were bunches of wires crammed between a pair of tag strips very close to some high power resistors which can get quite hot in normal operation. The plastic insulation on the screen supply (orange wire) had softened in the heat and residual pressure had forced the wire into the sharp edge of the earth tag! You can see carbon deposits from arcing and what appear to be specks of "napalm" splattered on the cement resistor. The wire was re-routed away from trouble, some heat shrink added and soldered back in place.
I was sure the problem was sorted so fired the amp up through a current limiting bulb and observed the usual slight glow of the bulb indicating normal current flow. There was quite a bright flash at the instant of switch-on, but that's normal when heaters are cold and filter caps are discharged.
Confident now, we applied full, unrestricted mains and I played the amp for half an hour, no problem, no hum. Imagine my surprise next morning when the fuses blew again at switch on!
The 83 Fender Concert I believe is a Rivera design and has much higher capacity filtering with 2 x 220uF @ 285V caps rather than the 70 - 80uF found in most Fenders. They are wired in series which doubles the voltage handling and halves the capacitance to 110uF. Because they have 220k bleed resistors to equalise the voltage between them, they will contain absolutely no charge after a few hours turned off. To electrons at power-on, they look like a dead short so there is a large current surge which decays exponentially as the capacitor fills up. If this surge lasts more than a few milliseconds, the total energy (Joules = Amps x Time) through the fuse is enough open it.
After speaking with the owner, he agreed to me replacing all the electrolytic in the amp, which were now 35 years old. This has indeed sorted the fuse blowing problem and has improved the overall sound a little. Its dead quiet.
A bias check revealed the pair of 6L6GC were pulling too much current. That's another rarity as most large fenders are set to run cool. I replaced the 2 bias resistors and they now run at a very acceptable 70% plate dissipation. The handy bias balance pot enables the use of unmatched valves. They both pull the same electrical current but one runs a few degrees hotter. Without the pot the tubes would really need to be replaced with a matched pair.
All in all a very nice amp. I'm not that keen on the distortion channel though there are some good tones to be had if you roll the bass back. The middle pull pot gives a nice edge to the tone too. The spring reverb is lush but I do miss the tremolo on this model.
I was sure the problem was sorted so fired the amp up through a current limiting bulb and observed the usual slight glow of the bulb indicating normal current flow. There was quite a bright flash at the instant of switch-on, but that's normal when heaters are cold and filter caps are discharged.
Confident now, we applied full, unrestricted mains and I played the amp for half an hour, no problem, no hum. Imagine my surprise next morning when the fuses blew again at switch on!
The 83 Fender Concert I believe is a Rivera design and has much higher capacity filtering with 2 x 220uF @ 285V caps rather than the 70 - 80uF found in most Fenders. They are wired in series which doubles the voltage handling and halves the capacitance to 110uF. Because they have 220k bleed resistors to equalise the voltage between them, they will contain absolutely no charge after a few hours turned off. To electrons at power-on, they look like a dead short so there is a large current surge which decays exponentially as the capacitor fills up. If this surge lasts more than a few milliseconds, the total energy (Joules = Amps x Time) through the fuse is enough open it.
After speaking with the owner, he agreed to me replacing all the electrolytic in the amp, which were now 35 years old. This has indeed sorted the fuse blowing problem and has improved the overall sound a little. Its dead quiet.
A bias check revealed the pair of 6L6GC were pulling too much current. That's another rarity as most large fenders are set to run cool. I replaced the 2 bias resistors and they now run at a very acceptable 70% plate dissipation. The handy bias balance pot enables the use of unmatched valves. They both pull the same electrical current but one runs a few degrees hotter. Without the pot the tubes would really need to be replaced with a matched pair.
All in all a very nice amp. I'm not that keen on the distortion channel though there are some good tones to be had if you roll the bass back. The middle pull pot gives a nice edge to the tone too. The spring reverb is lush but I do miss the tremolo on this model.
