blues deluxe 1993 not the reissue

[tombo]

Hi,
Im happy to say my fender blues deluxe (1993) is up and running. That said ! I need advice on a couple issues that could cause me a problem down the road. Background ! The last issue was no sound. Like so many others my circuit board was burnt along with resistors R-85 and R-86. I read through the forums and found these were suppose to be 470 ohm resistor. I replaced them only to find R-86 board trace was just to far gone. This is where I need your expert advice..... I bypassed the trace from the cap to the resistor with the new resistor ( didn't use a wire). I had to do the same off the r-86 resistor to the diode. I have the correct voltages reading ( -46Vdc on one end of the resistor (R86) & -16 Vdc on the other.
the same on (R85) only with a positive 46Vdc on one end & +16Vdc on the other.
I checked the diodes ( reading for both) 0.585. I read in the forum these should read above 0.650 ! OK...... Big ? Why are the power resistors so hot ? is it the diodes or the way I bypassed the burnt board traces. Please note : the amp sounds great ! Just concerned about the heat off the resistors. Thanks for any help in advance.
Tom

[Jazz P Bass]

The resistors get hot.

[tombo]

OK.... Thank you.

[Enzo]

They have to dissipate a couple of watts. That doesn;t sound like much but in fact it is a considerable amount of heat. Is that part under-designed? Yes. But It is normal to get hot.

[tombo]

Hi enzo,
Once again thank you for your help. Part of my quest was to find out why this happened . Is there a work around this under-designed part ? I enjoy playing at open mic's and blues jams weekly ! Given the design issue.... Doesn't seem very reliable for live performances. One last thing, Whats your thoughts on replacing R-71 resistor with 100k ? Some owners are replacing it with (Carbon Composition Resistors 1/4W BR BK YW GD) any idea where I could find this resistor ?
Thanks in advance for your services to this forum.
Tom

[52 Bill]

How long have you owned this amp? Had it been working reliably before this problem?

We have discussed the repair of the low voltage power supply on these amps many times here before, and there have been a number of fixes performed and suggested by a lot of people. If you search, I'm sure that you can find out how to change the power resistors to new ones mounted to the chassis, or to use a voltage regulator chip instead of the Zener diodes to get the low voltage supplies, etc. But over all once fixed, the amps seem to work without repeated failures for at least a few years.

R71 is part of the switching circuit. I believe that some of the early models had issues with the relays not switching correctly and changing R71 was the fix for this problem. Does your amp have a different value for R71? Have you been having problems with the channel switching? The type of resistor really doesn't matter in this usage, as it just sets the operating point of the channel switching op amp, so a basic carbon film from Radio Shack or any supplier will work just fine.

[tombo]

Hi 52 bill,

I've had the amp maybe two years. Started having problems with the sound breaking up mostly on the low end maybe six months ago. The amp had mismatched preamp tubes... I contacted fender since the original tubes were no longer available. The recommended tubes for the preamp are GT12AX7R ( which I have now) and the GT6L6R for the power side. I have GT6L6C but they commented that these are typically brighter and thinner sounding. Anyway.... The new tubes didn't really help. I pulled the back and notice the power resistors and board were burnt pretty bad. Thats when I got on this forum and started looking at the problems others were having. I'll seach for the fixes you suggested !
I actually havent had any issues with the switching circuit ! Really helps to know what this resistor is for and what to do should I experence a problem in the near future. Thank you for taking the time to help me out.
Tom

[52 Bill]

Started having problems with the sound breaking up mostly on the low end maybe six months ago.

So the problem with the amp is a low frequency distortion and replacing the tubes did not cure it.

Does it happen at all volume levels or only when played loud? Would you describe it as buzzing distortion or cutting in and out? Have you tried the amp with a different speaker? Have you tried the speaker with a different amp?

The more you tell us the better the directions we can give you to help solve the problem.

[tombo]

I noticed it at louder volumes first but eventually it was cutting in/out at high and low volumes. I Replaced the burnt power resistors which were 330 ohm to 470 ohms and had too by-passed the burnt board traces before i got the correct voltages across R-85 And R-86. As noted, the amp sounds great at this point. I was concerned about how hot the resistors were getting even though I had the correct voltages listed. Received feedback from Jazz p bass and enzo (thanks Guys) that this was normal. After reading your reply...... its sounds like this is really a design issue. I play out alot and need the amp to be more dependable. This means finding the articals related to these design changes you spoke about. As always, Im greatful for this forum and people like yourself for all the help.
Thanks
Tom

[52 Bill]

...the amp sounds great at this point.

I will quote a brilliant man here, "If it ain't broke, stop fixing it." ENZO

The overheating resistors was a problem early on in these amp, but like I said before once they are fixed, they don't seem to fail again.

Fender is still using this power supply design on current models and there has been no major failure rate in the past few years. If there was, you'd see postings everywhere talking about it. It's your amp, so do whatever you want to do with it, but I'd suggest that you play it and enjoy it as is.

[tombo]

Great advise ! I'll take it.
Thanks bill
Tom