Great vid Brad. Lot's of talk about opamps and IP stage noise here. The best opamp for IP stage is (jfet) OPA1642. Biggest noise generator is opamp current noise (5532 = 0.7pA/Rt Hz) reacting with large input resistances, R16 / 39k being worst offender. Why they chose such high R16 / R18 values is beyond me, 2k / 6k2 would be fine here. OPA1642 has Rail 2 rail OP stage so a little extra head room for free, it also does not have crazy BW so is pretty old-amp layout tolerant. I've upgraded quite a few pro mix consoles with these and they sound really good. Lastly they have very low CM distortion when fed from high or unbalanced impedances. This is an area where other opamps (553x / TL07x) fail badly and the TI designers put a lot of effort into. In plain English means they will work well in a wide variety of (high impedance) circuits like EQ's, tone stacks etc etc. Nice to see TI are still catering for us audio folks. :)
Just saying, as the owner of amp #2 (yes the one Brad actually worked on), he!s mad an amazing difference to my much loved JC77. It’s now soooo much better. Usable volume pot and doesn’t hiss me to death when using. Greta stuff Brad. Worth every cent.
Glad to hear it suits your needs, Ken. Word must have spread as I have another 2 in the queue now for the same treatment. Give me a yell if you need anything else, mate.
@@BradsGuitarGarage hey Brad. Thanks. I sure will. It was worth the trip across the city. But only for the bigger stuff. Also I’m still amazed at how reasonable your pricing was on that repair/upgrade. Thanks heaps.
I liked this video a lot. I own a JC-77 that I have had since I bought it new in the late 80's and have loved it. The only complaints I've ever had about it are the hiss and the fact that it gets so loud so early in the rotation of the volume control. To say the volume control is not very intuitive was a very accurate statement. After finding this video I made similar changes to my JC-77. The 1M audio taper pot makes the volume control significantly better. I also reduced the gain on op-amps IC-2 & IC-3, also going with 75K feedback resistors. I used different op-amps though. I went with an OPA2134PA in IC-2 and an LM4562 in IC-3 and replaced all the carbon resistors in the signal path with metal film. Though the OPA2134PA is not as quiet as the NE5532, the OPA2134PA with its true FET input sounds better and was an acceptable trade off to me. None of the jacks on my amp were broken but the amp is very old now and plastic does gets brittle as it gets old. So I replaced all my jacks with metal Switchcraft jacks. You can duplicate all the switching, grounding and insulating functions of the original plastic jacks with four 12As and three 11s. I used black shoulder washers and nuts on the jacks to make them look almost factory. It's nice to be able to play my JC-77 at home now without rattling the windows with the volume only at 1 or 2. LOL
Yes, 75k is where I landed after some experimentation. From memory, this bloke told me he never took it above 2 anyway. If you go too low with that value though, guitarists who ride the vol knob on the guitar won't have enough make-up gain from the amp, so it pays to talk to the customer about their playing techniques before starting work.
@@BradsGuitarGarage Oh it still gets more than loud enough to gig with. I also observed that the distortion on my JC-77 is more usable after these changes. I didn't mention it before but my JC-77 had a problem with the reverb so I ended up going back in this morning after receiving some parts during the weak. While rebuilding the reverb drive circuit I went ahead and replaced IC-1 & IC-4 with LM4562NAs since I have a lot of those on hand. The reverb issue turned out to be Q3 & Q4 transistors. They are supposed to be a complimentary pair but they were not complimenting each other much at all.
Cool man! I have one I picked up for free. I like it and I don’t like it lol. So it was dead and turned out the owner twisted the fuse holder, till the inside wire broke. I fixed that and it came back to life. But it does have the hiss at times, without chorus on even 1 it has a severe drop in volume to where it’s shrill and no bass or eq seems to work. It sounds beautiful on its own with chorus at times. But with an OD pedal or anything it sounds extremely boxy, no matter what I do to the eq. I’ve cleaned on pots with Deoxit and it helped a little. But I’m not sure what’s going on as it just has a horrible bass sound. Mids are just boxy and not very present. Either mine is really broken, or it’s only good to play clean with chorus as a one trick pony. I’m keeping it, and hopefully it’s just something fixable. Bud def will have to buy a fender amp or something more suitable for my OD tones.
For some reason the combination of listening to you play through the amp, combined with your Australian accent, led me to listen to Midnight Oils first album for the first time in nearly 30 years. Totally underrated album.
The Oils were basically a punk band. All the songs chosen for the radio do not reflect this. I really wished I had a chance to see them live in their heyday. Born in the wrong decade, unfortunately.
@@BradsGuitarGarage Yeah, me too. First heard them with 'Beds Are Burning' when it came out, then investigated their back catalogue. My mind was blown by their self titled debut album. I think I was 15 or something, into music and production and up to that point hadn't heard anything as raw and direct as that album. It's fair to say it changed my entire view of recorded music and politics. They remind me a lot of early REM in that sense. I've just gone out and bought the CD after having it on vinyl, tape and CD previously haha.
@@BradsGuitarGarage Coming from the UK, I only ever got to really hear Midnight Oil and INXS. What struck me about the oils and the first album, was their guitar tone and general sound was different to what I was used to hearing on commercial records. It reminded me of the tones me and my mates were getting using beaten up Peavey Bandits and a basically broken Roland amp - hence what sparked my memories from your Roland amp repair video. It made me feel like music was somehow attainable and exciting.
This was great watching as I have a mix of tube and SS amps! On the solid-state side of things, these 70’s-80’s era always seemed to have fairly poor distortion. My 1983ish Yamaha JX Series 55 is similar. I’ve always wondered (without any real knowledge of the circuitry), could this era of amps have the distortion circuit modified to have some improved chip, there are so many great overdrive pedal circuits these days (Klon, Blues Driver, Plexi Drive, etc…), could one of these chips/circuits actually be incorporated into the existing amp? Or it is just not electronically possible. Thanks.
I kinda wonder if there is also some hiss coming from the chorus circuit. This seems like the same era as the Roland Juno60-106 which had a noisey chorus when turned on.
Is that a power supply located directly behind the input jack's, for the IC's perhaps? That's not very smart. Is there any chance that some of the hiss is actually Johnson noise from carbon comp or carbon film resistors?
Looks like you have a small splinter right below the base of your little finger on your left hand when you're holding the custom IC circuit board. That would drive me nuts within about 20 minutes, it'd be slightly inflamed and itchy or painful almost immediately andI'd have to get a pin or a razor blade and dig it out ASAP. PS, nice job on the boards!
Requesting that you could assemble a kit with the stand-up PCB adaptors, ICs, pot and MF resistor. I didn't see anything on the website..Thanks for the demo.
I just got a JC-77 and love it, it seems to work well but has an unusual odor kind of like a somewhat sweet warm epoxy or resin smell. Are you familiar with this smell? Should I be worried? I can't yet tell if it always smells like this or only when it's on. I'm wondering if it could be a sign of bad capacitors or other components but since it seems to work well I'm not sure. I'm not sure I want to pull it apart but maybe this would be a good first step? The previous owner said it was in good shape the only issue he had was sudden drop in output after the amp has been on a few hours that is remedied by adjusting the volume pot he said this only happened a few times. Some JC manuals mention servicing for if "unusual odor" occurs.
If you have an unusual odor coming from any electrical device you should get the device checked out. With that said, JC-77s can be 30 to 38 years old now. It would not be a bad idea to have a pro take a look.
Is this typical for these or this one more noisy than normal? What vol pot did you use? Looks like 1M audio taper, any chance of a part number or link??
I've got same amp, and have the noise floor issue, and when playing a popping crackle, haven't opened it yet...going to go through it, by any chance are your sip adapters on any websites for purchase?
Rail to rail decoupling to counter any instability due to the extra trace length between the IC die and the pins. No ground is present on the little PCB so that's all we can do on it. Rail to ground decoupling should be present on the main PCB in a well designed amp.
Great vid Brad. Lot's of talk about opamps and IP stage noise here. The best opamp for IP stage is (jfet) OPA1642. Biggest noise generator is opamp current noise (5532 = 0.7pA/Rt Hz) reacting with large input resistances, R16 / 39k being worst offender. Why they chose such high R16 / R18 values is beyond me, 2k / 6k2 would be fine here. OPA1642 has Rail 2 rail OP stage so a little extra head room for free, it also does not have crazy BW so is pretty old-amp layout tolerant. I've upgraded quite a few pro mix consoles with these and they sound really good. Lastly they have very low CM distortion when fed from high or unbalanced impedances. This is an area where other opamps (553x / TL07x) fail badly and the TI designers put a lot of effort into. In plain English means they will work well in a wide variety of (high impedance) circuits like EQ's, tone stacks etc etc. Nice to see TI are still catering for us audio folks. :)
plus the 5532 has surprisingly low input impedance, so there's also that for 1st stage opamp consideration
Just saying, as the owner of amp #2 (yes the one Brad actually worked on), he!s mad an amazing difference to my much loved JC77. It’s now soooo much better. Usable volume pot and doesn’t hiss me to death when using. Greta stuff Brad. Worth every cent.
Glad to hear it suits your needs, Ken.
Word must have spread as I have another 2 in the queue now for the same treatment.
Give me a yell if you need anything else, mate.
@@BradsGuitarGarage hey Brad. Thanks. I sure will. It was worth the trip across the city. But only for the bigger stuff. Also I’m still amazed at how reasonable your pricing was on that repair/upgrade. Thanks heaps.
@@BradsGuitarGarage It's a good mod that makes a great vintage amp even better. So I am not surprised that others would want it done to theirs.
Thanks man, I just learned so much from this repair vid. Top work bloke!
I liked this video a lot. I own a JC-77 that I have had since I bought it new in the late 80's and have loved it. The only complaints I've ever had about it are the hiss and the fact that it gets so loud so early in the rotation of the volume control. To say the volume control is not very intuitive was a very accurate statement.
After finding this video I made similar changes to my JC-77. The 1M audio taper pot makes the volume control significantly better. I also reduced the gain on op-amps IC-2 & IC-3, also going with 75K feedback resistors. I used different op-amps though. I went with an OPA2134PA in IC-2 and an LM4562 in IC-3 and replaced all the carbon resistors in the signal path with metal film. Though the OPA2134PA is not as quiet as the NE5532, the OPA2134PA with its true FET input sounds better and was an acceptable trade off to me.
None of the jacks on my amp were broken but the amp is very old now and plastic does gets brittle as it gets old. So I replaced all my jacks with metal Switchcraft jacks. You can duplicate all the switching, grounding and insulating functions of the original plastic jacks with four 12As and three 11s. I used black shoulder washers and nuts on the jacks to make them look almost factory. It's nice to be able to play my JC-77 at home now without rattling the windows with the volume only at 1 or 2. LOL
Yes, 75k is where I landed after some experimentation.
From memory, this bloke told me he never took it above 2 anyway.
If you go too low with that value though, guitarists who ride the vol knob on the guitar won't have enough make-up gain from the amp, so it pays to talk to the customer about their playing techniques before starting work.
@@BradsGuitarGarage Oh it still gets more than loud enough to gig with. I also observed that the distortion on my JC-77 is more usable after these changes.
I didn't mention it before but my JC-77 had a problem with the reverb so I ended up going back in this morning after receiving some parts during the weak. While rebuilding the reverb drive circuit I went ahead and replaced IC-1 & IC-4 with LM4562NAs since I have a lot of those on hand.
The reverb issue turned out to be Q3 & Q4 transistors. They are supposed to be a complimentary pair but they were not complimenting each other much at all.
You're definitely the master of cool PCBs.
So awesome work man.
Nice fix..Great sound...That would knock out a gig or two, kinda' versatile sounds..! Ed..😀😀
Wow Brad you are quite accomplished at beautifying some dodgy amps, that sounded way better than it had a right to!
Never heard of a JC77, only the JC120. Oh, yeah, I nearly forgot......*FIRST*!
Cool man! I have one I picked up for free. I like it and I don’t like it lol. So it was dead and turned out the owner twisted the fuse holder, till the inside wire broke. I fixed that and it came back to life. But it does have the hiss at times, without chorus on even 1 it has a severe drop in volume to where it’s shrill and no bass or eq seems to work. It sounds beautiful on its own with chorus at times. But with an OD pedal or anything it sounds extremely boxy, no matter what I do to the eq. I’ve cleaned on pots with Deoxit and it helped a little. But I’m not sure what’s going on as it just has a horrible bass sound. Mids are just boxy and not very present.
Either mine is really broken, or it’s only good to play clean with chorus as a one trick pony. I’m keeping it, and hopefully it’s just something fixable. Bud def will have to buy a fender amp or something more suitable for my OD tones.
For some reason the combination of listening to you play through the amp, combined with your Australian accent, led me to listen to Midnight Oils first album for the first time in nearly 30 years. Totally underrated album.
The Oils were basically a punk band.
All the songs chosen for the radio do not reflect this.
I really wished I had a chance to see them live in their heyday.
Born in the wrong decade, unfortunately.
@@BradsGuitarGarage Yeah, me too. First heard them with 'Beds Are Burning' when it came out, then investigated their back catalogue. My mind was blown by their self titled debut album. I think I was 15 or something, into music and production and up to that point hadn't heard anything as raw and direct as that album. It's fair to say it changed my entire view of recorded music and politics. They remind me a lot of early REM in that sense. I've just gone out and bought the CD after having it on vinyl, tape and CD previously haha.
Australia was the epicenter of raw music in the 80-90's.
Everything was so pure and from the heart.
@@BradsGuitarGarage Coming from the UK, I only ever got to really hear Midnight Oil and INXS. What struck me about the oils and the first album, was their guitar tone and general sound was different to what I was used to hearing on commercial records. It reminded me of the tones me and my mates were getting using beaten up Peavey Bandits and a basically broken Roland amp - hence what sparked my memories from your Roland amp repair video. It made me feel like music was somehow attainable and exciting.
We all thought it. You said it. 🍻
This was great watching as I have a mix of tube and SS amps! On the solid-state side of things, these 70’s-80’s era always seemed to have fairly poor distortion. My 1983ish Yamaha JX Series 55 is similar. I’ve always wondered (without any real knowledge of the circuitry), could this era of amps have the distortion circuit modified to have some improved chip, there are so many great overdrive pedal circuits these days (Klon, Blues Driver, Plexi Drive, etc…), could one of these chips/circuits actually be incorporated into the existing amp? Or it is just not electronically possible. Thanks.
Who’s this SHAZEE girl ya keep taking bout ? 😊
ua-cam.com/video/_zm--OT8P3U/v-deo.html
Stellar job on an outdated junky amp.
With the best chorus circuit ever made. Fender has some real shit amps too
I kinda wonder if there is also some hiss coming from the chorus circuit. This seems like the same era as the Roland Juno60-106 which had a noisey chorus when turned on.
Is that a power supply located directly behind the input jack's, for the IC's perhaps? That's not very smart. Is there any chance that some of the hiss is actually Johnson noise from carbon comp or carbon film resistors?
Looks like you have a small splinter right below the base of your little finger on your left hand when you're holding the custom IC circuit board. That would drive me nuts within about 20 minutes, it'd be slightly inflamed and itchy or painful almost immediately andI'd have to get a pin or a razor blade and dig it out ASAP. PS, nice job on the boards!
Requesting that you could assemble a kit with the stand-up PCB adaptors, ICs, pot and MF resistor. I didn't see anything on the website..Thanks for the demo.
Not a bad idea, Ryan. Find my email online and I can sort a kit out for you.
I just got a JC-77 and love it, it seems to work well but has an unusual odor kind of like a somewhat sweet warm epoxy or resin smell. Are you familiar with this smell? Should I be worried? I can't yet tell if it always smells like this or only when it's on. I'm wondering if it could be a sign of bad capacitors or other components but since it seems to work well I'm not sure. I'm not sure I want to pull it apart but maybe this would be a good first step? The previous owner said it was in good shape the only issue he had was sudden drop in output after the amp has been on a few hours that is remedied by adjusting the volume pot he said this only happened a few times. Some JC manuals mention servicing for if "unusual odor" occurs.
If you have an unusual odor coming from any electrical device you should get the device checked out. With that said, JC-77s can be 30 to 38 years old now. It would not be a bad idea to have a pro take a look.
Is this typical for these or this one more noisy than normal? What vol pot did you use? Looks like 1M audio taper, any chance of a part number or link??
It was an Alpha A1M from memory.
And yes, all of these models are particularly noisy.
@@BradsGuitarGarage Thanks so much Brad, ill listen to before buying
I've got same amp, and have the noise floor issue, and when playing a popping crackle, haven't opened it yet...going to go through it, by any chance are your sip adapters on any websites for purchase?
Shoot me an email and we may be able to work something out, mate.
@@BradsGuitarGarage great thanks for that will do
Curious, what's the cap(?) on the PCB for?
Rail to rail decoupling to counter any instability due to the extra trace length between the IC die and the pins. No ground is present on the little PCB so that's all we can do on it. Rail to ground decoupling should be present on the main PCB in a well designed amp.
Where can I get the adaptive pcb ?
Drop me an email if you're in Australia, otherwise postage costs may be prohibitive.
Interesting, as usual, but I hate solid state amps.