Blues Junior Reverb Hum

[woodrow f call]

Thanks! I was just realizing today that I thought I might be hearing both and it was confusing me. The 60hz is pretty quiet and is present when the reverb is turned down. I could still hear it when the reverb was turned up. I think that was screwing me up because I wasn't picking both out.

Cool trick with the tuner!

So it might be time to replace the filter caps..... at least for the reverb power supply. I'll let a few others chime in.

[maxkracht]

From my experience, replacing filters in a blues jr is always a good idea. I've seen plenty of failures in fairly new amps.

[nuke]

Ok, I shorted the RCA cables as directed. As predicted there was no noise.
Checked the voltages on the IC pins (4 and 8 ).... One read 15.01VDC and the other was 15.00VDC.
I did not see any AC using AC or mV settings on my Fluke 117.

OK, the problem is not the electronics on the board.

We're down to the tank and the cables.

Or maybe someone did a mod somewhere, like adding a reverb footswitch connection.

[nuke]

From my experience, replacing filters in a blues jr is always a good idea. I've seen plenty of failures in fairly new amps.

Yeah, all the big ones that say, "ICC" on them, especially the grey ones. I usually get the pair of low-voltage filters too at a bare minimum, though those seem to hold up better.

[woodrow f call]

Ok, I shorted the RCA cables as directed. As predicted there was no noise.
Checked the voltages on the IC pins (4 and 8 ).... One read 15.01VDC and the other was 15.00VDC.
I did not see any AC using AC or mV settings on my Fluke 117.

OK, the problem is not the electronics on the board.

We're down to the tank and the cables.

Or maybe someone did a mod somewhere, like adding a reverb footswitch connection.

Thinking about this..... I shorted both send and return cables at the same time...... doesn't this just send any signal in the reverb circuit to ground? Meaning I shouldn't get any noise unless it's in the downstream of the return circuit?

I'm thinking it could be anything upstream of the return cable.



Also, I haven't jumpered the two amps yet.

[Stevem]

You should not short the send cable nor did I say to do that.

However if you read any DC voltage out of the send cable then that IC chip has a problem.

[woodrow f call]

You should not short the send cable nor did I say to do that.

However if you read any DC voltage out of the send cable then that IC chip has a problem.

Sorry, I got yours and Nuke's posts mixed up.

When I short the Return cable, I get a much louder hum, but it does go away with the reverb knob turned down.

No DC voltage on the send cable.

[woodrow f call]

If you have clip leads and another amp then pump the signal from the send rca into that other amp, does that amp hum?

Just to make sure I understand this setup correctly.... Can I take the send RCA and plug it into the send of amp 2..... Turn both amps on, turn up the reverb and see if amp 2 hums? Basically checking the upstream side of the reverb tank?

[nuke]

Close up photo of the opamp and surrounding area of the board?

Any signs of prior work?

[nuke]

You should not short the send cable nor did I say to do that.

However if you read any DC voltage out of the send cable then that IC chip has a problem.

I was the one who said that. It won't hurt anything with either the TL072 or 4560.

Here's a snippet of the reverb circuit from the 1995 schematic, which I think is the only USA made version. (not 100% certain). The second is a 2001 schematic for comparison.

reverb-schem.jpg

reverb-schem-2001.jpg

[woodrow f call]

Pictures of the reverb part of the circuit as requested.

[woodrow f call]

Pictures of the reverb part of the circuit as requested.

[nuke]

Ok your board is a 1998 Rev. C, which is on page 10-11 of the "blues junior service manual" you can score on the 'net.

Few interesting things:

The op-amp is in a socket, which I don't think is factory work. It is also a TL082 vs. the called out TL072. That's not necessarily wrong. Back in the day, the TL082 and TL072 came off the same wafer fabrication process, and the 072 were binned out as low-noise, while the higher noise ones were stamped 082. Today, they're all the same chip, the TL082 meets the same spec as the TL072. TI still provides them under both numbers, but they state they're exactly the same today.

The socket kinda says, someone has been there before. I don't see anything odd there otherwise. The back side of the board will reveal prior work most of the time.

C23 is a weird value, and I've encountered a bad one before. It is a 0.47 uf electrolytic. The other weird thing is that cap is not subjected to any DC voltage, which oddly enough is necessary to keep an electrolytic healthy over time. Just for grins, replace it. I'd prefer a film capacitor in that spot, but whatever you can get easily will work, even two 1.0 uf in series will work.

The other thing I notice, the jack has been replaced with metal, which is good. It should have a shoulder bushing in the hole, and an insulating washer to isolate it from the chassis panel hole. The jack should only ground to one place and not the chassis, that helps hum too.

Your four main filter caps should just be replaced. They're ICC brand and look original. They fail a lot, just not very good parts. I like the Kemet and Vishay caps you can get from Digikey or Mouser. They're intended for hardcore long-life applications like office lighting and will outlive all of us.

You might want to go over the back of the board carefully and look for prior work or cracks and stuff. Use a magnifying glass and take your time.

What reverb pan did you use?

[woodrow f call]

Ok your board is a 1998 Rev. C, which is on page 10-11 of the "blues junior service manual" you can score on the 'net.

Few interesting things:

The op-amp is in a socket, which I don't think is factory work. It is also a TL082 vs. the called out TL072. That's not necessarily wrong. Back in the day, the TL082 and TL072 came off the same wafer fabrication process, and the 072 were binned out as low-noise, while the higher noise ones were stamped 082. Today, they're all the same chip, the TL082 meets the same spec as the TL072. TI still provides them under both numbers, but they state they're exactly the same today.

The socket kinda says, someone has been there before. I don't see anything odd there otherwise. The back side of the board will reveal prior work most of the time.

C23 is a weird value, and I've encountered a bad one before. It is a 0.47 uf electrolytic. The other weird thing is that cap is not subjected to any DC voltage, which oddly enough is necessary to keep an electrolytic healthy over time. Just for grins, replace it. I'd prefer a film capacitor in that spot, but whatever you can get easily will work, even two 1.0 uf in series will work.

The other thing I notice, the jack has been replaced with metal, which is good. It should have a shoulder bushing in the hole, and an insulating washer to isolate it from the chassis panel hole. The jack should only ground to one place and not the chassis, that helps hum too.

Your four main filter caps should just be replaced. They're ICC brand and look original. They fail a lot, just not very good parts. I like the Kemet and Vishay caps you can get from Digikey or Mouser. They're intended for hardcore long-life applications like office lighting and will outlive all of us.

You might want to go over the back of the board carefully and look for prior work or cracks and stuff. Use a magnifying glass and take your time.

What reverb pan did you use?

I've made a list of parts to order (Mostly capacitors) and just recap the amp.

I put the switchcraft jack in. The input jack was smashed.

I don't think the amp has been worked on before. Except the Blue Dog speaker. It doesn't look like it, but if that socket isn't original, I don't know. It could be that the socket was supplied to fender with the chip and it's one part. Looking in the 1995 service manual, it isn't called out.

[nuke]

For the jack, get S1029 shoulder washer and S1028 insulating washer.

Tubesandmore have a good diagram on how to stack them up in the panel, and you can get them there too.