Blues Deluxe Reissue-No Sound

[Helmholtz]

I’m calling TP 45 the cathode side of D13 & as mentioned, measured.7vdc....TP 46, the cathode of D14 measured-17.3vdc. I see that TP 45 should be +17. Would this cause low volume? Is this part of the grid bias circuit?

Looks as if D13 is shorted (but could be other reasons as well). What is the voltage at TP42? If you lift one leg of D13 you can test it with an Ohmmeter.

[Perkinsman]

Looks as if D13 is shorted (but could be other reasons as well). What is the voltage at TP42? If you lift one leg of D13 you can test it with an Ohmmeter.

TP42 is 47.8vdc. Pin 8 on U1 is also .7. I pulled D13 leg & in diode mode measured .63, reversed leads & got OL. In Ohm mode, measurements were 385k & OL.

[Enzo]

I think it cute the TP calls it 17v, but the schematic calls it 16v. I generally refer to these as 15v rails anyway. Point is your +15/16/17v is missing. Until that is restored, nothing that uses it will work right either. That includes the channel switching, the FX loop, the reverb, and any other op amp stuff.

So if D13 isn't shorted, then is 470 ohm R78 open? Is D13 parallel cap C40 shorted? C40 is hard to get at, but measure resistance across D13 while it is in circuit. Does it measure low?

A shorted op amp COULD do this, but I tend to expect a shorted op amp to lean on both rails rather than one.

[Perkinsman]

I think it cute the TP calls it 17v, but the schematic calls it 16v. I generally refer to these as 15v rails anyway. Point is your +15/16/17v is missing. Until that is restored, nothing that uses it will work right either. That includes the channel switching, the FX loop, the reverb, and any other op amp stuff.

So if D13 isn't shorted, then is 470 ohm R78 open? Is D13 parallel cap C40 shorted? C40 is hard to get at, but measure resistance across D13 while it is in circuit. Does it measure low?

A shorted op amp COULD do this, but I tend to expect a shorted op amp to lean on both rails rather than one.

R78 measures good. D13 in circuit measures approx .6M ohms, is that expected? Can’t get to c40 unless I pull the board but I will if needed.

[Enzo]

C40 is in parallel with D13. By measuring across D13, you are also measuring across C40. Look at the schematic.

Any chance D13 is in backwards? The cathode - the lined end - is the end NOT grounded.

[Perkinsman]

C40 is in parallel with D13. By measuring across D13, you are also measuring across C40. Look at the schematic.

Any chance D13 is in backwards? The cathode - the lined end - is the end NOT grounded.

D13 appears to installed correctly. The cathode leg is attached to the + side of c40....but so is d14. Is this correct?

Ok, so if d13 is parallel to c40 & it’s measuring high ohm resistance than that would also mean that c40 has a high resistance, which implies that it’s good? I also measured 974uf across d13 on UF mode. So what else can cause .7vdc at pins 8 of U1, U2, U3?

[Enzo]

Sure, use a fingertip. ANy of them getting hot?

[Enzo]

but so is d14. Is this correct?

I don't know what this means. The cathode of D14 is also connected to the + end of C40? The cathode end of D14 should be grounded, which is the NEGATIVE end of C40. And of course the positive end of C41. The anode of D14 connects to the negative end of C41.

You need to find where your +17 went. You have the +48 at TP2, right? That is ultimately the source of +17. Your R78 measures OK. So either the +42 is not getting to R78 , or the downstream end of R78 is shunted to ground. SO is R78 getting HOT? If it is cold, then voltage isn't getting to it. it it is real hot, then something is loading it down.

I am not sure WHERE you measured TP42. I mean it applies to the end of D11, C39, R78. We expect all those to be wired together. But they might not be, so is there the 42v right on one end of R78?

[Perkinsman]

I don't know what this means. The cathode of D14 is also connected to the + end of C40? The cathode end of D14 should be grounded, which is the NEGATIVE end of C40. And of course the positive end of C41. The anode of D14 connects to the negative end of C41.

You need to find where your +17 went. You have the +48 at TP2, right? That is ultimately the source of +17. Your R78 measures OK. So either the +42 is not getting to R78 , or the downstream end of R78 is shunted to ground. SO is R78 getting HOT? If it is cold, then voltage isn't getting to it. it it is real hot, then something is loading it down.

I am not sure WHERE you measured TP42. I mean it applies to the end of D11, C39, R78. We expect all those to be wired together. But they might not be, so is there the 42v right on one end of R78?

Ok, so I turned the amp on this afternoon to start working on it again & it worked!....for 5 mins, then I heard a distinct click & the sound died again. While it was running I grabbed the dmm & measured D13 & pins 8 on the IC’s. Sure enough, all +17vdc so I know they’re fine. They’re back down to .7 again. I see that there are 2 relays, K1 & K2. Would one of these be suspect & how can I test them?

Another observation is that R79 is hot while R78 is not. Is that unusual?

[Enzo]

R78,79 both do identical jobs and should be identical hot. They normally get pretty warm. If no current if flowing through them they stay cold - room temperature. If the 17v end of one is shorted to ground, it gets REAL hot. Could possibly unsolder itself.

Relays? Unlikely. Does the red channel LED come on still or does it stay dark when in failure mode?

Cold R78 means current not getting through it. Check voltage right on the wires of R78. SHOULD have 48v on one end and 17v on the other. If you get zero volts on the 48v end that means the 48v ain't getting there. If you measure 48v on both ends that means the end that should be 17v is not connected to the circuit.

The fact that resistor is cold tells me the circuit is not loading this down. The problem is in the supply.

[Perkinsman]

Regarding the red LED, yes it worked during the 5 minutes the amp worked. I tested each of the functions during that time & everything else worked as well, Reverb, Bright Switch, etc. Now that the amp isn't working I measured 54Vdc at both ends of R78, which is cold R79 will burn if touched and measured 48 & 17. So are you thinking that the cold resistor has a bad solder joint?

[g1]

Yes, it is quite common in this series of Fender. Resolder both those 5W resistors and both those zeners. Check to make sure the pads are not broken from the traces.

[Perkinsman]

I've read alot of threads about these resistors & it was the first thing I checked. I've been under the impression that if they measured in spec & the board wasn't burnt & tapping on them didn't help, they were ok. I'll jump on this first thing tomorrow. Man, it will be great if all it needs is a few solder joints resoldered.

[Perkinsman]

I've read alot of threads about these resistors & it was the first thing I checked. I've been under the impression that if they measured in spec & the board wasn't burnt & tapping on them didn't help, they were ok. I'll jump on this first thing tomorrow. Man, it will be great if all it needs is a few solder joints resoldered.

Success! That’s all it was...just a lousy solder joint on R78! Voltage back up to 48 & 17! Everything works! Much thanks to you guys again! I love this hobby & site!

[Randall]

For the record, that is one of the first things to automatically check for on this series of amps. That, and the solder connections on the tube sockets. Both are very common failures.