1995 Blues deluxe channel switching problem

[modusmongo]

Hi all. I have the typical random channel switching problem on my blues deluxe. I have opened her up and sure enough, the 470 ohm resistors at R85 and R86 have bad solder joints and show overheating on the PCB. I will replace them and raise them from the PCB to let them breathe. Would someone please tell me exactly what resistors to buy to replace these?

Pics of R85 and R86



Schematic is here.

Should I clean up these solder points and test voltages on other components before replacing the resistors?

It may be worth noting that the problem occurs only when the clean channel is selected, and only when I crank her up. If I select the overdrive channel, the random switching does not (ever) happen.

Thanks!

[tubeswell]

Why - those are 470R 5W wire-wound resistors. Common as dirt. The channel switching problem is more likely a dodgy relay.

[modusmongo]

Hi tubeswell. Thanks for the response. From the little reading I've done, replacing those resistors with higher rated ones is a must, and that they are often linked to channel switching problems. Can you suggest some other troubleshooting I could do? I can read a schematic, but it's VERY slow business for me ;-)

They are clearly overheating and causing cold solder joints...

[tubeswell]

Where is the schematic?

[modusmongo]

Oops! I corrected the link in my original post...

[tubeswell]

Okay, those are supply resistors and if they are running hot, it could be because another component in that part is dodgy and is casuing too much current to be drawn though them. (I would suspect diodes, and/or filter/decoupling caps maybe). You would need to check the idle voltages in that part. If you are in doubt, take it to a tech.

[modusmongo]

According to what I have read, these resistors always run hot and are considered a design flaw in the blues deluxe. Do you see in the schematic, anything that would make you think that the channel switching could be related to the cold solder issue? I suppose that I could just reflow the connections and put the amp back together to test it, but it was a bear to take apart. The PT hase about 20 cables which I had to disconnect from the PCB in order to get it out and inspect it. Thanks for your help!

[tubeswell]

Well you could replace each of the 470R with 2 x 1k 5W wired in parallel (that's 4 x 1k altogether) and that should enhance their heat-handling. But if it were me I'd be checking the idle voltages (/current across each resistor) all the same, just to check that they are in the right ballpark. As to working with PCBs, you have my sympathy. Just be careful when unsoldering components (don't skimp. Always use desoldering braid or something good to suck the old solder completely off when removing components). Twisting and forcing the components away from the board can easily wreck the little trace pads that surround each hole.

[modusmongo]

Do I need to buy wire-wound resistors, or will any old 5W do? I noticed that the ones in the amp are only 10% tolerence. I like your idea of using 2 x 1k for each 470R.

Before I do all this, I will resolder the existing resistors and test the voltages around them. If I understand the schematic (questionable) I need to see 46vdc before R85 and 16 vdc after, the same for R86, but negative voltage. Is this right? Assuming that I verify these voltages, should I do anything else (other than replace the resistors)?

Thanks!

[modusmongo]

Okay, I put her back together (electronically, at least) and did voltage tests as described above with settings as called for in the schematic - all pots at 50% and reverb at 0. I get 49 vdc on the hot end and 17v going to ground afterwards (+ and - voltages as in the schem). Is this too high?

I played it LOUD for 10 minutes without the channel switching problem. I'm leaving it on for an hour and I'll try again.

[tubeswell]

Okay, I put her back together (electronically, at least) and did voltage tests as described above with settings as called for in the schematic - all pots at 50% and reverb at 0. I get 49 vdc on the hot end and 17v going to ground afterwards (+ and - voltages as in the schem). Is this too high?

Those voltages look right (49 =~47, and 17 =~16 for the purpose of tube amps, i.e.: close enough). It means you have 49-17 = 32 V across the 470R* resistor, which is 68mA, which runs out at 2.2W being dissipated in the resistor, so the 5W rating should be sufficient. Wire-wound resistors are usually fairly stable, so it shouldn't be a cause for concern.

* we'll assume its 470R but it is rated at 10% tolerance so it could be something else.

I would start looking in the channel switching circuitry - maybe one of those op-amps

[52 Bill]

What you did, was to put the amp back to the way it was when it left the factory. So based on its age, etc. it will probably work for another 10 years. Fender did have some problems with these resistors as has been well documented on the web. The real problem seems to be the perception that these start to fail as soon as they are turned on. I don't see these amps returning in great numbers once they have been repaired. Maybe it will take another 10 years to see if the solder starts to fail again.

Raising them from the board will probably cause as many problems, as they will be more prone to breaking the solder joints from mechanical vibration. I'd say put it back together and play it.

[tubeswell]

I defer to Bill

[modusmongo]

Thanks guys. I'm going to put it back together and use it this weekend for band practice. If it makes it through the afternoon without switching, I'm going to say good enough and wait the 10 years