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Fun and Games with a Fender 75

3/18/2004 7:33 PM
Fun and Games with a Fender 75
I have posted about this a couple other places too so ...... I picked this baby up last weekend. I have sorta been looking for one because it seemed like they might have a lot of potential. OK, it was cheap, but it also has some things (effects loop, line out, reverb, channel switching, those things) I wanted to screw around with so I bought it.  
I went back and read all the threads at various bbs's about the 75's. Pretty much love/hate and not much in between. The schemo is a PIA for sure. But chopping at it a piece at a time maybe I'll be OK. That chassis is crammed full, but there is a lota stuff I'm gonna pull and re-arrange just to free up some space before I start messing with it. Just guessing I'll bet I can snip out 8-10 feet of wire by the time I get it cleaned up. Man, I have never seen such loopy lead dress before.  
All the usual BFing for sure. That Lead circuit must have some secret mod that will make it wonderful. Anybody know that would care to share? Well, if any of you have some hot mods that you can suggest, I'd be crazy to hear them. Particularly about the lead channel. ;)  
Just for starters, it seems to me that the "Master Vol" could be eliminated where it is prior to the "Lead Master" by moving one wire (would it still need that 100Kohm load to ground at that point?), and the spot on the face panel used for a gang-pot post-(BF)PI MV.  
And now, a word from our sponsors:

3/31/2004 4:19 PM
TEM OK, I started without you {;-)
Well, I plunged into the thing.  
So far I have finished re-arranging the power amp and rail to make it a bit easier to deal with. All new Atoms in there now. I put a couple 100mu in there instead of the 220mu like was suggested for the general Rivera era amps. Now's the time to tell me if that was a mistake ;) . The low power option got ripped and those two double throw switches came out. Not sure what I can use the lowpower swt hole for. By rearranging the power section I have cut out about 3feet of wire so far. That's net of course so far.  
Re-built the bias section as BF adjustable with one filter and one resistor onto the tab strip where it was. Moved the one cement resistor over to where the death cap used to be so I could put the bias cap right on the strip. Gets that resistor heat away from the phase inverter too.  
Since I ripped the ground swt/death cap and the ext speaker jack I moved the bias pot to where the ext speaker used to be. Going to stuff a output selector in the ground swt hole since the OT has 2/4/8 ohm output leads. Got the bias pot nice and close to where it should be. That removed a foot of wire all by itself.  
The BF PI is in process. Have to hit the electric store and get some more resistors and a couple caps. Then I'll fire it up less tubes to see if any smoke escapes and check voltages. If that works OK then I'll stuff in the tubes and see how it sounds with the new bias and PI.  
Still have to wire the heater circuit properly. Now that I have all the wires off the floor of the chassis at the back I can stuff the heaters wires in that back corner out of everythings' way.  
If that all goes well then it will be preamp time. Looks like another few extra feet of wire in there too.  
This is really time consuming trying to make some order out of it while making quite a few changes and still having to work around existing structures. But fun I guess. It will def be fun if it all works OK when done.  
I could still use some hints about how that Lead channel works. And why it looks like it has four gain stages even with the Lead at zero. Being an ElectricMoron has some real drawbacks.  
Any help on that would be most appreciated.  
4/1/2004 6:56 AM
John G
Charlie did you get the Fender 140 schem I sent you?  
John G
4/2/2004 1:08 PM
TheElectricMoron John, So this is where you came from m
You have mail too.  
The Adventure continues.  
I guess I will cut and paste the tale of the ages and put it up here now and then. I'm sure my bumblings will be of some fine amusement to the readers ;)  
thanks again for your help with this mother of all projects  
4/4/2004 9:26 PM
John G
Thanx for the mail. The f/b line you refer to ?? is in fact the audio mute line activated by the i/p jack sockets and kills the audio after the volume control when no cable is inserted. This trick was one of the first times I saw it implimented. It's now very commom, and makes for a dead quiet amp with no input.  
Sounds like you are enjoying yourself.  
John G
4/5/2004 9:40 PM
Yeah, I was dopey when I wrote last
Thanx for not flaming me on that f/b thing ;) I was drawing lines a couple hrs later and saw what was happening. what a goof - get old get stupid  
Last couple amps I have done over for me ended up with one input jack because I needed another place to stick a pot. These nylon button jacks look as tho they will lend themselves to a single jack if I just put a jumper on it. I have a bag of those things that someone took out of a Fender PA and I never looked at them before.  
I kinda let it rest over the weekend, just did some planning. I kind of like to make notes about sequencing mods and then revisit them a couple times to see if I still feel the same way. Not sure if it is really useful since I hardly ever change the plan, but it does make me think about it before I melt anything. Ripping the low power thing and rearranging the power amp was a PITA. I'm just going to have to get a big iron to melt those chassis solder connections so I can move things around more. I read here some time ago about taking a gun and cutting the very tip off the end of the element and using it like an induction heater by just sticking the ends of the two heater stumps on each side of the wad of solder. Going to have to try that.  
Better get back to work.  
take care and thanks again for your help  
4/6/2004 7:25 AM
John G
Be afraid be very afraid!!!! You scare the hell out of me describing the attack of the demented soldering gun. I once saw a bearded guy light a cigarete will a blow torch, I nearly dropped a trouser bundle with fright, I had visions of the human torch.  
Do take care we don't want your name changed to " The Electrified or(Electrocuted)Moron" (Mormon).  
Incrimental modification is the only way to go with regular backtracking to make sure you're on track.  
All the best.  
John G

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