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Messages - solderdripper

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The Newcomer's Forum / Re: Fender 85 Issues - No sound sort of
« on: November 13, 2017, 03:18:06 PM »
Hi Ed, I don't think I am very far ahead of you in skill, but I think I can offer you some starting advice.

Worked before he shipped it, huh?  :o  Well if you say so...  :)
The only things I can think of that would be affected by a simple shipment, or things that can be temperamental, are wire connectors, jacks, and pots.  From the diagram, it looks like those input jacks have a lot of switch going on. I would try some continuity tests or resistance checks through a guitar cable - e.g. one probe on the tip of the free end of the cable, the other to the jack's output pin (#2) - and also through the closed contacts on the input you're not using.

From where you are now, #1 question would be, Is it even the right fuse?  Hopefully there's a label on the board or chassis, make sure you get correct replacements, and buy a few.

The bottom left of the diagram seems to show the power supply circuit, it has some test points with values marked "VDC".  After you get a new fuse, you can check the supply. Connect multimeter ground to chassis and check the DC voltage at these points to see if your supply is.. supplying the proper voltages.

If those look ok, you can try to isolate the problem to preamp or power amp.
Connect the ipod to one input, then run a cord from the PREAMP OUT over to the POWER AMP IN on your Yamaha amplifier. Power them on and see if the preamp in the Fender is passing the signal.   I think this is similar to what you did already? Did you use a 'preamp out' signal from your Yamaha into the "POWER AMP IN" on the Fender when you did that?  One thing I've learned, jacks and pots are the most suspicious things on a used amp. Wiggle everything to try to make sure it isn't a scratchy pot not letting the signal through, or a stubborn jack.

If you don't get anything through the preamp, you can check the voltages at the supply pins on the ICs in the preamp path; if those look good, you can set up an audio probe and see if you can follow the signal through the circuit to find how far it gets.

That's probably long enough for starters, and will get some more information for others to consider.   Get the right fuse; check proper connectivity on all of the pins on the input jacks before trying the amp again.

Hope this helps!

I bought this kit I built it and connect guitar to input and speaker to output and it works well enough.  Now I have a base to build and test preamps against. I was working on a Phabbtone front end, hopefully I can get back to that soon.

Was wondering why you said you wanted 30w and then I saw the kit you linked. Isn't that a stereo amplifier? 30w might mean 15 on each channel.

Sent from my SM-G935P using Tapatalk

Tubes and Hybrids / Fixed my Marshall AVT20 combo
« on: September 09, 2017, 06:57:36 PM »
Not much of a question, just wanted to tell a little story:
I have a Marshall AVT20 "Valvestate 2000" combo, it has a 12AX7 preamp and TDA2050 IC power section, 10" speaker in a sealed enclosure. I think its really great, I love Marshall tone and this brings it in a small package. Replaced Celestion speaker with a Red Fang and liked that, too.

One day a few weeks ago I was goofing around and tried to run headphone-out to the input of my L5, and it killed the little AVT.  My guess is that using the guitar cord in the stereo headphone jack shorted the ring to ground, which fried the TDA2050 power amp.  I'm not sure exactly why; the headphone jack does have some components between it and the chip, but I guess they did not provide much of a load when the output was connected straight to ground. Error condition was a loud humming noise unaffected by any knob. The amp doesn't have any effects loop jacks, so I wasn't able to isolate preamp/power amp issues.  Then I discovered 22vDC at the speaker connectors. Checked some cap and resistor values in the headphone and power circuits, but I found no other damage. I ran out of ideas on what else to test so I went out on a limb and ordered some TDA2050s, couldn't get them from Digikey or Mouser but I actually bought ten for $8.61 on the big 'A' site. Ten was actually cheaper than buying two... They just arrived by slow boat, and the first one out of the package was good and fixed the amp. It was very easy to replace, its held on to a nice big heatsink by a metal clip. I put a sil-pad underneath it, its wide enough that it wraps up the sides of the chip, laying in the channel it fits in on the heatsink.

Now I have another amp online, and some TDAs to play with. Was thinking of making a modular amp, but that's another thread  :tu:

Hi guys, I'm a LAB Series nut and enough of a lurker on here that I registered to chime in with this barely-relevant nugget -- I found this earlier today, actually. 
Interview from 2011, Tabor says that he has two pre-amps specially built for him by Egnator, "We call it the Gretchen Module because it was designed to emulate the Lab Series L5 as best as possible, and they did a really good job at getting very close on that."

I still don't know if it is the compressor or the multifilter that he likes best.  :)

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