having had a few 3H inductors that were reading open on my 1977 220 head(two of them) and '79 320 head (one of them), a few weeks ago Russ Allee was both kind and generous to send me two for my 220 head from his parts bin. they worked perfectly, solved the problem and the amp is now up to spec. I got another 320 head about 2 weeks ago, and wouldn't you know it (?) one of it's 3H (henry) inductors on the graphic eq was also bad (the 70Hz one). so i contacted Russ, and unfortunately he was all out of these pieces of 'unobtanium', with no more on the horizon, and those that have experienced this problem know just how hard it is to find one,
on the 320 amp, instead of it not working at all, it would work right after you turned the amp on, and continue to work fine until after playing it for about 5 minutes, then the 70hz slider would have absolutely no effect. if you cycled the power off and then back on, it would work for a couple of minutes more, until again quickly fading into losing its functionality. to make sure that it was just the inductor, i switched it with the 125hz one (also a 3H), and sure enough, the problem followed the eq band (i.e. the 125Hz slider would work for 5 minutes and then stop working completely). i did a little research on the guitar forums, specifically about varitone circuits that use 3H inductors and found a thread about one guy that uses very small audio signal tranformers that are basically 3H inductors. he mentioned that the Xicon #
42TM018-RC is a 1:1 transformer that measures 3H on both sides, and end to center tap on each side measures 1.5H. i bought two of them (just to have a spare!) from Mouser for $2.47 each + shipping and they arrived today for testing.
here's a couple of pics of the original one next to the Xicon transformer:
on one side of the transformer, it has a DCR of 601 ohms, and the other side measured 502 ohms, which is a little bit higher DCR that the original one, so i decided to use the side with the lower DCR. Russ advised me that since inductors are basically an antenna, that i should leave it's leads long, just in case i had to reposition it (possibly turn it 90 degrees) so that it wouldn't induce any hum in the circuit. so the first way that i tried it was to just mount it standing up in a row with the original ones. i turned the amp on and voila! it works perfectly.
Note: on the 320 schematic, it doesn't have a couple of resistors that my amp (built the 45th week of 1979) has. on my amp there is a resistor feeding the inductors from the slide pot on the two low bands, whereas the schematic has none. feeding the 125Hz band, mine has a 390 ohm resistor and i noticed that with the new inductor the 125Hz seemed a bit less pronounced and a little higher in frequency than it was when the original one was working. the first thing that i tried was to shunt the resistor to make it like the schematic, and although the frequency and gain sounded more like it was originally, at the very top of the slider's travel, it would make a slight clicking sound as you moved it. So, i added a 470 ohm 1/2 watt resistor on the bottom of the PCB in parallel with the 390 ohm one, giving it a total resistance of 213 ohms. i then tried it again and it works and sounds exactly like the original now with NO clicking whatsoever.
here's a pic of it mounted. i used hot glue to secure it to the other ones since these have been known to break loose after many years of use, and the new Xicon one has thin legs/leads that wouldn't support it very well anyway.
note #2: these tranformers can induce hum, so thus far, i can only say that they'll work perfectly without adding any hum to a 320 head (and probably in a 220). i'm sure that they would work in a 370, however due to the different location of them in the amp's layout, i don't know if they would cause any hum problems or not, but you could probably re-orient it physically to work well with no issues.
anyway, just thought that i'd post this for those that wer unable to resolve a non working eq band on their 320/220's graphic eq.
best,
