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Messages - willpirkle

For the caps, set the DVM on AC volts (preferably AC mV) and read the AC voltage *across* the caps, from one plate to the other with power on. The AC voltage should be 0.0 since caps pass AC. If the AC voltage is higher than a few mV then the cap is leaky and should be replaced. You will need a DVM that can measure relatively low voltages (i.e. not the cheapo ones). The absolute DC voltages on one plate won't tell the whole story. There is not a particular voltage threshold above which the cap is considered bad (thus the vague "a few mV") but if it is in the 100's of mV or in the volt range, then it's definitely bad.

The nice thing about the AC voltage test is that you can set the DVM and then very quickly test every cap on the PCB. My old boss would blindly test every cap (even the little 0.1uF bypass caps on op-amps and especially logic chips) as soon as he opened a unit, and very often had the device fixed within a few minutes since this is such a common failure point. For the big electrolytics in the PS, you can usually hear the 60 or 120Hz noise from the speaker as well.

All great points Phil!

If anyone wants to study this concept further, check out the Peavey super-distortion patent (see Fig 3)

You can legally build stuff from patents for your personal education/edification. If you pull out the distortion circuit, it makes a killer distortion pedal. Add pre and/or post distortion filtering of your own design for a personalized distortion box that no one else has!

Happy holidays, and thanks for an awesome website Joe and for all the great contributions from the power-posters. Here's to a rockin 2022!

Speaking of the RCA app book, does anyone have this one in PDF? Ive been trying to track down some of the late 60's and early 70's RCA books, specifically the audio amplifier versions (not the stock power transistor one). Thanks in advance!
The Newcomer's Forum / Re: Orange Crush 12L (No Sound)
September 22, 2021, 07:01:02 PM
Yep, pin 7 is an output, and should be ~0VDC so that IC is definitely toasted (yes, same pin out as the TL072). You could socket it, if you'd like to experiment with other op-amps, or make changing it simple in the future if it fails again. I couldn't find the schematic for the 12L, but I did find the 20L which includes over-voltage protection diodes on the input. Assuming those are also there on the 12L (2 diodes connected to pin 3), then changing the op-amp should do the trick. But Enzo may have something else to check - he's a super-boss around here!
The Newcomer's Forum / Re: Just joined...
August 06, 2021, 08:11:47 AM
There is a TON of knowledge and a great group of people here - welcome, you will definitely have fun!
Amplifier Discussion / Re: My K-20X
July 21, 2021, 08:08:17 AM
In addition, a line out is designed to drive a high impedance load (>10k, such as the unbalanced input to a mixer, FX return, etc...) while the HP out is designed for very low impedances (8-32 ohms, speakers). A more important difference is that line out on an RCA or 1/4" jack, is designed (by spec) to deliver a specific nominal value of -10dBV, or ~316 mVRMS into that load. There is no formal spec for headphone outputs, so you never know what you're going to get, which is why some headphone outs are really loud and others are quiet, when driving the same load. Headphone outs are typically noisier as they usually use headphone (low power) IC amps, rather than op-amps but in many cases you can't really tell the difference (high gain guitar preamps, etc...).
Thanks! I actually had that batch, but noticed the head version was not in there, so I thought I'd ask to see if there was another source. Mine dates to 1985 according to the serial number. Gotta say that through my Marshall 1960, it sounds great. Even the distortion on this model is reasonable (not awesome, but also not horrific like the 120 twins I've tried over the years) and the preamp/distortion are just so simple in the head version.

I need to fix the reverb on this, which is only barely audible with the control on 10, so will open it up this weekend after verifying it isn't the footswitch jack. Thanks for all your help and input on everything. Guaranteed to learn something new and interesting on each visit here.

Its a Roland JC-120H, not a Marshall  :)
I've scored a JC-120H (head) and I have the '84 Schematic as it is called. It is only one page and I'm looking for the rest of it, including disassembly instructions, etc... is there more of the '84 schematic than just the one page? Like a more complete service manual?


Dang!!!!!! Thanks man. Wow - appreciate the heads up. Hopefully, they will price it substantially less than the used ones on Reverb ($250-$300), that kinda reminds me of the TS-9 inflation around 1990 — till they reissued it with the JRC op amp. This is killer.
Wish they'd re-release the Heavy Metal pedal. I left mine in a rehearsal warehouse in 1988. That box in front of my Princeton was amazing (still have the silverface). They are stupid expensive on reverb.
I buy J113's a hundred at a time from Mouser ($0.13 a piece) and use a FET tester I built (the simple one from runoffgroove) then spend a rainy Saturday sorting them. It takes less time than you'd think (and I live in South Florida so we have many rainy days...). The J113 works great for Danyuk's circuit.
Agree with JPB - will likely cause problems and no warmth other than a (potentially) burning ICE board. I'd save that transformer for another project!
Preamps and Effects / Re: Fender STC1 Chorus Pedal
April 23, 2021, 09:05:23 AM
Here's a freebie chapter from an analog electronics book I was working on back around 1997 or so. I do have the copyright on this (the actual piece of paper from the gov't), but am letting it go here because you guys rock!

This is missing a key ingredient for BBD circuits - the compander (NE571) that limits dynamic range into the BBD and then expands it coming out.

I've been pondering the idea of doing a 3-part Udemy class on analog audio electronics... one of these days.

Schematics and Layouts / Re: Roland JC-40 Schematic
April 12, 2021, 07:44:40 PM
Thanks! This is great - I didn't know it was a digital modeling amp.