Fender Blues Deluxe Reissue weak drive channel

[mresistor]

Hi guys, I have a BDRI on the bench which was brought to me with an inop drive channel. It's a 2004 and looks pretty new inside. Upon inspection I found the IC electrolytics to be leaking, so I have replaced the 1 47uf and 3 22uf caps with new F&T caps. That eliminated some of the crackles and pops that were occurring in operation. Initially the drive/master pots had no effect and there was no sound in drive mode.  I have checked TP's on the list that is found on the left side of the schematic in the repair manual. All of them check out fairly close to the specified values. I found K2 to have bad contacts and was not passing the drive signal. I ordered some relays and replaced both K1 and K2. I have sound now in drive mode but it is very weak. With drive/master pots turned all the way up I have sound level equivalent to about 1.5 on the clean channel volume. Something is sucking the drive channel signal down. I have +/- 16.7 v at the 4705W resistors in the ps. All the op amps are getting +/- 16 volts for operation . The mute is functioning with -16v and a positive pulse when channel switching occurs. I can hear the relays switching and have checked TP32 and it reads -16V in and +7v out.
I've checked the coax and it's connections from the drive pot to the PCB  WJ10/11 WJ12/13. It's good. Solder joints good, re-flowed them .
I haven't unsoldered either the drive or master pots as they feel like brand new pots with nice resistance and very smooth operation.
At this point I am about to lift and check R19 and R45 and C53. After that the only thing I can think of is to pull the drive and master pots and check them.
Any and all help is greatly appreciated.
Service manual with schematics and data are here    http://el34world.com/charts/Schematics/files/fender/Fender_blues_deluxe-deville_reissue%20(vintage).pdf


All I have to work with is a Fluke TrueRMS Multimeter.

[mresistor]

Ok  on the Drive pot from the ground side terminal (when switched in Drive mode) to the wiper, the ohms reading goes from 2.1 ohms up to around 246k very nice and smooth throughout the travel.


On the Master pot, from the grounded side terminal to the wiper the ohms readings go from 1k up to around 20k and then back down. Nowhere near 100K. 20% and that would correspond with my assessment of volume level in drive mode.  So, this pot looks bad.

[mresistor]

Pulled the Master pot and checked it.  It checks fine other than the total resistance is only 88K instead of 100K. I don't know what the tolerance is but that is greater than 10% off.  So I was reading through the zener diodes?  I checked D1 and D2 and they read infinite one way and 3.2M the other. Can someone tell me why Fender has two opposed diodes there? From what I understand is they are limiting the excurision of the signal, like clippers? I am supposing they are there to clip spikes when the mode switching and mute is employed.


R19 and R45 are in spec. C53 is good.

[mresistor]

I'm starting to think there may be a problem in the substrate of this PCB.

[mresistor]

Anyone care to put another set of eyes on this. I'm about out of ideas.     The voltages at R78 and R79 and the D13 and D14 are +16.7v and -17.3v respectively. Its the same at all the chips pins 4 and 8.

[mresistor]

Ok thanks to all for the consideration. Guess I am not welcome here anymore.  It's Ok. You know I am am not that great at this stuff, I do my best and try to help other people when I can.
I wish all of you the best of luck in the future,  and thank you all for all the help you have provided to me in the past, I appreciate it immensely whether you realize it or not.

[Shack]

id help if I could bud.....I think Fender put the opposing diodes on the drive channel to clip the signal for distortion, like some pedals.....but other than that, I am lost on PCB tube amps

[Shack]

Be patient mresistor, someone will come on and respond

[PRR]

> I'm starting to think there may be a problem in the substrate of this PCB.

You've done everything I can think of. PCB amps are a royal pain.

[shooter]

You have a scope, or access to one?  tracing *low signal* with a meter is a rough way to go

[mresistor]

PRR   right on about that.These Fender PCBs are pieces of garbage IMO.
I appreciate that you guys responded. There were others that could have that didn't.


I have sent this amp to a local shop, one that has more equipment and resources than I do. I didn't want to do it, but I could go no farther without further investment without any guarantee.  I finally concluded
that there may be something wrong with Op Amp U4 as it wasn't passing a positive voltage pulse.


Thank you again.

[sluckey]

I appreciate that you guys responded. There were others that could have that didn't.

Like who?

I try to read all post, even this thread. Some threads I just skim over. I will post in threads that peak my interest, or that I may add something helpful, or give another point of view. But threads about pcb tube amps that also contain lot's of SS circuitry don't interest me and I usually steer clear. Doesn't mean anything more than I'm not interested. I still like you and have posted in lot's of threads that you've been active in. And will continue to do so.

That's only one reason for not posting in this thread. I'm sure others have valid reasons for not posting as well. I promise you are not being shunned. Stick around. Nobody said you were no longer welcome.

[mresistor]

Sorry Sluckey, I wasn't referring to anyone in particular. I was frustrated and figured there were more people here that may have worked on these Blues Devilles and similar. Looking back though that was a tough problem and may have required PCB replacement. Please don't take it personally.


To Clashnvain   no sir  I never did find the problem with that drive circuit.   

[sluckey]

Sorry Sluckey, I wasn't referring to anyone in particular. I was frustrated and figured there were more people here that may have worked on these Blues Devilles and similar. Looking back though that was a tough problem and may have required PCB replacement. Please don't take it personally.

Never did take it personally. So, you've had exactly two years to get over it. Did you?

[mresistor]

Sure.  I wouldn't have posted in this 2 YO thread but someone hit me up with question about it.. then I read your post,,  just wanted to say what I said.. hope you're having a nice relaxing day, I finally got all the wiring done on a Super Reverb build, and at my age I am also thinking this may be the last one..  I'm not as thrilled now about building amps as I once was - but time will tell.

[shooter]

Op Amp U4

Ic's are pretty cheap n easy, cut all the legs, clean up, solder in a quality socket, new chip, walla.
equipment is pretty essential though, Sluckey's $30 scope, a good meter, but you did cover it, if chasing gremlins isn't fun, they multiply