Eden WTB 300V mute circuit

[Randall]

Can I get some help with this mute circuit? Amp has very low out put, with almost nothing at the SEND jack. Signal passes inserted at RETURN jack. I find when I lift R162 where it connects to the send jack, and the jumper that same point to the send jack, I get signal. So this leads me over to the left side of the schematic to the mute circuit, which I will assume has failed. D3 and D4 test good in circuit, and the input jack ground switching to R47 and 48 is good. I see where if either Q3 or Q8 were shorted it would mute. But that's where I run into trouble. I don't see what Q2 is doing, and I don't see where the mute signal or voltage feeding D4 is coming from.

The JFETs Q3 and Q8 are J108, which is obsolete, would a J111 work? Same deal with the 2SA1015, pretty slim pickings. I find a ECG290A as a cross, would that be OK?

WTB_300V.zip

[nevetslab]

The Idss current on the J108 is considerably higher than the J111, and its' ON resistance is lower than the J111, but, I'd give it try...both are N-Ch FET's. Q2 turns on Q3 muting FET when there's nothing plugged into the input jack. Q8 mutes when the MUTE switch is engaged. I too don't see the MUTE switch, but do see the source....it's control voltage comes from Q7 via foot switch.

[g1]

I find when I lift R162 where it connects to the send jack, and the jumper that same point to the send jack, I get signal.

I'm a bit confused how this would disconnect the mute Fet's from the send jack? From your description they are still in circuit so I'm more suspicious of a bad connection.
Or did you do something else to disconnect the Fet's from the send jack?

[Randall]

"Q8 mutes when the MUTE switch is engaged. I too don't see the MUTE switch, but do see the source....it's control voltage comes from Q7 via foot switch."

Ah, there is no mute switch, so I bet it is only a foot switch function, which it did not come to me with.

"I'm a bit confused how this would disconnect the mute Fet's from the send jack?"

Yes, on second look, that is a very good point. The mute section is still connected. Hmmm.. I will try to recreate that.

[nickb]

"Q8 mutes when the MUTE switch is engaged. I too don't see the MUTE switch, but do see the source....it's control voltage comes from Q7 via foot switch."

Ah, there is no mute switch, so I bet it is only a foot switch function, which it did not come to me with.

"I'm a bit confused how this would disconnect the mute Fet's from the send jack?"

Yes, on second look, that is a very good point. The mute section is still connected. Hmmm.. I will try to recreate that.

In this jumpered situation, V2B has a finite but low output impedance. A J108 that is ON will also have a finite but low resistance. So you will get signal thru but it will also be attenuated.

Measure the Vgs of both FETs to see what they are doing. They should be be around -11V for normal operation. If that is OK then lift the drain of one FET at a time to find the bad one.

[Randall]

I replaced Q3 and Q8, and now it works. It was faster to do them both while the board was out. I put in J111, anyone have a problem with leaving them in or should I get a better sub?

[nevetslab]

I replaced Q3 and Q8, and now it works. It was faster to do them both while the board was out. I put in J111, anyone have a problem with leaving them in or should I get a better sub?

Same foot print, same sex. If it mutes and un-mutes, and no longer drags down the signal applied.....you're on the air! Not like it's a critical console channel where leakage signal would let a muted signal thru to mess up a mix for not having as low of the original ON resistance as the J108.

[J M Fahey]

Doubtlessly FET sales must have been sinking for decades, so low Vp "Audio" types became stupid expensive or even plain disappeared.

Now low quality high Vp switching type ones are still used, so are the only ones still available, and for a reasonable price.

Point is that building a full FET preamp such as classic Randall, old Roland and Yamaha guitar amps, etc. , is hard/expensive to impossible, but ... muting? .... still very doable even by the cheesiest ones.

Most will switch ON/OFF reliably with about -5V applied to gates, and all with, say, -7 to -15V ; not sure what´s available on Eden, too lazy to unzip it, specially because of the abysmal quality of Eden drawings.
But you check it for me

[Jazz P Bass]

"specially because of the abysmal quality of Eden drawings. "

You got that right!

A 3rd grader could do better.

[Randall]

Welp, this head came back. It runs for about 10 - 15 mins, and then it goes silent. I think the mute circuit is activating as it warms up. I have not opened it again yet, but I wonder what could maybe do that? Subbing J111 for the original J108, could that be possible?

[Enzo]

Let's not assume the mute is activating, it could be failing, which is different. Or it might not be the mute itself anyway. FInd out.

Q8 mutes when the MUTE switch is engaged. I too don't see the MUTE switch, but do see the source....it's control voltage comes from Q7 via foot switch."

So the mute switch is on the footswitch.

You were looking at R162 earlier. Look at it again. With the amp running, and the low level condition happening, measure resistance across R163. It is parallel the mute JFETs. Does it measure a low resistance? If so, either the JFETs are shorted/leaky, or they are not being turned off.

In that case look at the gate of each JFET, is only one missing the gate voltage, or both? Note Q2 and Q7 use the same circuit.

Before we worry if 108 or 111 is better, we need to determine that the parts are actually where the problem lies.

[Randall]

Thank you Enzo.

R163 out of circuit = 110K ohms. In circuit with amp powered down = 11 ohms, this I don't get. In circuit, amp powered up in stand by mode = 108K ohms. In circuit powered up passing signal = 83K. With no input plug inserted = 20 ohms. With amp passing signal gates to Q3 and Q8 = -10.7v.

AAAAAnd.... now it isn't failing. It ran for about 20 minutes, making a few pops every once in a while, then popped and faded in and out, the ohmeter across R163 bounced around, and did go to 22 ohms for a couple of seconds with no sound, But then it came back and has run for a solid hour with no trouble.

I even tried waving a heat gun around for a bit, but it made no difference. It did fail on me a few times yesterday while it was in the cab.

[Enzo]

JFETs are ON until turned OFF by a gate voltage. When the amp is powered down, then there is of course no gate voltage, so the JFETs measure low resistance.

OK, so it seems like teh JFETs are turning off and on. There are two. WHen it goes down, check to see if gate voltage is still present at EACH. It only takes one to mute the amp. We might find only one is really doing this. And then we can determine why.

[Randall]

"JFETs are ON until turned OFF by a gate voltage."

Oh, my mistake. It makes sense now. I'm dying to see what the gates are doing, but so far it has not failed again.

[g1]

Got any freeze spray? I know you tried a heat gun, but sometimes one will work but not the other. Also, any caps related to the gate voltage may also be suspect.