Problem With McGuinn E1 Compressor Circuit

[pacoderm]

On my 1989 new-to-me 370/12 RM LE there is a problem when I switch on the E1 compression/treble boost circuit. Rather than getting the clear, sparkling "The Byrds" tone, the tone is muted and distorted. At times the tone quality and the volume wander up and down by themselves, but most of the time the tone just muted and distorted. Additionally, many times when plucking a string, particularly the lower strings, the initial pluck is missing, then the volume comes back as the note rings. When the pick hits the string, it's more like Elmer Fudd saying "puddytat", than the sharp stinging chime we all love.

Also at times, and for no reason there will be the loud buzz/pop that you get when plugging a guitar cord in with the amp on. It's not like I'm jiggling the plug in the jack or anything, it just happens out of the blue. I'm inclined to think that these two problems may possible be related, because the jack is also the power switch for the battery.

I've put a brand new lithium 9 VDC battery in, and cleaned all the pots with Deoxit D5, including the trimmer pot for the compressor gain. Incidentally, after I did that, I had several minutes of the correct jangle tone, then it went back to muted and distorted. At both the extreme ends of the trimmer pot's range, the output of the guitar is lost/goes away. I've set it at the recommended 1.5 VDC, but that resulted in an even worse tone, because that's near the lower extreme of the pot's adjustment, just before the output goes away entirely. Accordingly, I dialed it up to 2.5 VDC, which is more in the middle of the pot's range, and the tone and volume get better but still bad. The measured voltage at the trimmer, however, does not waver.

Any suggestions? Thanks in advance!

[libratune]

This sounds a bit complicated. You might try posting your question in the Rickenbacker.com official forum: http://www.rickenbacker.com/forum/index ... 203d64ce39

[pacoderm]

Thanks, I did submit my registration for the Rickenbacker Forum several days ago, but I haven't received confirmation from them yet, and therefore cannot post to that site. I did email the write-up here to "service@rickenbacker.com" and have had no response. I also asked in that email if they could check into to my forum registration status. I would love to have this posted there but can't do it at the moment. Thanks again.

[jps]

Thanks, I did submit my registration for the Rickenbacker Forum several days ago, but I haven't received confirmation from them yet, and therefore cannot post to that site. I did email the write-up here to "service@rickenbacker.com" and have had no response. I also asked in that email if they could check into to my forum registration status. I would love to have this posted there but can't do it at the moment. Thanks again.

RIC is closed for the holidays. They should be back after the New Year, but also keep in mind, their full attention for the first few weeks will be preparing for the NAMM Show. Be patient.

[Jubal81]

What you're describing, combined with knowing it's got some years on it make me suspicious of dead capacitors.

Do you have pictures or a schematic?

[pacoderm]

Here’s a thread that has the schematic, pictures and lots of analysis:

viewtopic.php?f=8&t=384616

My first thought was capacitors too, especially with the circuit not being able to produce the initial pluck of the note or chord. Might you be able to tell from the schematic where to start checking? Thanks!

[Jubal81]

Looks like the only electrolytic cap is the 22uF (C11) on the power rail. It's the one that looks like a soda can. If it's conducting DC (bad/fail), the supply voltage will drop to near nothing.

Do you have a digital multimeter to check voltages? Should be 9V on one side of that cap and 0V (GND) on the other.

[pacoderm]

I do, and will check later today. It's the light blue component next to the 3-way switch, correct? If that's it, I'm guessing that I'll be able to find one that will fit -- are they readily available?

[Jubal81]

I do, and will check later today. It's the light blue component next to the 3-way switch, correct? If that's it, I'm guessing that I'll be able to find one that will fit -- are they readily available?

Yes, the blue can and yes, easy to replace.

This one looks about right (72 cents):
https://www.mouser.com/productdetail/ni ... 9%2Fu1A%3D

These are polarized, so you'd need to make sure you orient it correctly.

[pacoderm]

OK, I checked the capacitor and it checked out fine. Now here's the part that I don't understand. While I had it apart, I readjusted the trimmer pot that controls the gain to the compressor circuit according to Rickenbacker's spec, which is 1.5 VDC. I rotated the pot several times full on in each direction, and different from when I last did this, I would loose the signal only on the low end of the range. I had the guitar plugged in while I did this, strumming the strings to see how it sounded. Well, it sounded great after setting level to 1.5 from the 2.5 VDC it had been at. I couldn't get it to miss a note. I put it all back together (slowly, checking the sound quality at each step of reassembly) and it still sounds great. I then swapped out my guitar cord for another one. I've been playing it for several hours yesterday and today, and it's fine. Every once in a while it will miss a note, but even that seems to go away after about 20 minutes of playing. So, I'm going to live with it for a while and see how it goes. I may even adjust the gain down slightly to see if that makes the problem go away completely, but am reluctant to play with it now that it's working. Perhaps this all points to a dirty trimmer pot, that after manipulating it some more with the Deoxit D5 in there, cleaned itself up? Who knows...

[8mileshigh]

I had a dead McGuinn circuit board and asked JH for help. He said that 99% of the time, any problems with the circuit were down to the black chip shown in the attached photo. I replaced it and bingo, all working again. I've seen a few of these chips on Ebay. Hope that helps?

[jdogric12]

Is the chip's designation on the schematic?

[8mileshigh]

I'm no good with schematics I'm afraid

[cjj]

If the chip is actually in a socket, as it appears to be in the picture, try carefully unplugging it and reinserting it a few times. IC sockets are notoriously bad and sometimes this can clean up the connections. Same goes for any connector. It's amazing how often re-seating connectors will fix electronic problems.
The next big problem is pots, especially trim pots. There's a physical wiper that contacts the resistive material - yep another connector of sorts. Little bits of dirt and corrosion get in there and mess things up...

[pacoderm]

Thanks everyone for your ideas. As for the schematic, there's a thread to it in a post from me earlier, there's no designation for the chip. Searching this forum and the Rickenbacker forum, I've found a quote from JH along the lines of the chip was specially made by a company that was subsequently bought by another company that then went out of business. Bottom line is that a replacement chip is not currently being manufactured (and can't be remade), so the only way they may be available is as 8mileshigh suggests on EBay. I did a search and none are currently available, I'll keep checking regularly though, thanks for the suggestion!

I hit the trimmer pot with more Deoxit D5, along with the tone selector switch, switching both back and forth many times to clean them up. I reset the chip in its socket several times, again using Deoxit to clean the connections. I then (as I alluded to earlier) changed the gain on the trimmer pot to the compression circuit down to 1.1 V DC from 1.5V DC. That sounded better to me, and without the dropping of the initial pluck of the string/chord. I set it up to 2.5 V DC and had lots of missing plucks. I then equated the missing plucks/notes with the gain setting, and set the gain back to 1.1V and all was good. Until I played it again later, the missing notes were still there. The tone is great, but missing notes are not good. Imagine Mr. Tambourine Man with the very first open G note not there. You hit it, and it doesn't come out of the amp. And Petty's Listen to Her Heart, with the initial open A not there. Kinda spoils the overall effect. And this happens many times during the song. I'll note that his symptom occurs at some times and not at others, which usually points to a hardware problem.

Thanks again for all your suggestions, I'm trying everything! I'll add that in all my guitar playing, I've never used a compressor, so maybe this is normal compressor performance (?). But I also never heard Roger McGuinn miss a note, and I miss many (and it can't always be the operator!).