Fender hot rod--470 ohms 5w resistor, tubes, and problems of sound cutting out

[Damnjoe]

I have a fender hot rod guitar amplifier. I love the amp, it is great and hardly ever has problems, but I am having problems with it cutting out and am having some trouble repairing it. Here is what it does:

 

When the distortion is off, the yellow distortion light shows up (intermittently) and the sound is not right (neither clean nor distortion) When I turn the distortion on, it switches back off (light turns yellow) when I play any amount of sound at all through it. When the clean channel is on, and the yellow distortion light does not turn on, sometimes the red distortion light comes on, but very faintly and intermittently. But when this happens, the sound does not change other than buzzing or loose connection sound. This is not a connection problem with the pedal or cord (i.e. outside of the amp), as it does this when the pedal is not plugged in too.

 

I took the circuit board out, and the board is pretty badly burnt around the 16v supply. The source of the problem seems to be the two 470w resistors. But when I check them with the multimeter, one of them reads about 480 and the other reads about 490. I would have expected them to be less resistance, or no signal, if anything. The problem is not my multimeter. Other than the circuit board, there the resistors don't appear to be damaged.

 

Now, one of the circuits on the actual circuit board may have been burnt out (i am not getting a signal through it). I can repair that, and that may be what is causing the amp to cut out. But then, what is causing the resistors to heat up so much in the first place?

 

I had the same problem probably 5 or 6 years ago, and I took it in to be repaired. The repairman replaced my tubes and did some work on the circuit board. For all I know, the burnt out spot on the board could be from that first time, at least some of it.

 

Should I replace the tubes? Do they wear out that fast? I have read that bad tubes will cause those 16v resistors to get hot.

 

Is there supposed to be that much variance in the 470 ohms 5w resistors, or are they damaged?

 

Now there are 2 zener diodes in the circuit which are right next to those 2 resistors which also appear to be burnt at least on the board. I have not checked those yet.

 

Should I replace the zeners, the resistors, the tubes, and the broken circuit on the board, or is that more than is needed?

 

Any advice or help would be appreciated. Thank you very much.

[SomeoneYouKnew]

 

But when I check them with the multimeter, one of them reads about 480 and the other reads about 490.

480/470 = 1.02 = 102% of specified value. 490/470 = 1.04 = 104% of specified value.

 

Seems quite reasonable to me. I really doubt that an error of less than 5% is gonna cause you any grief.

[Damnjoe]

Okay, that's pretty simple math, I should have been able to figure that out, eh? Thank you very much.

 

I checked the solder connections around those two power supply resistors where the board was burnt, and one of them (R78) was not getting a full connection (what I thought was a burnt out circuit on the board). So I resoldered it and checked the connection, and then tested the amp.

 

Same problem. What would my next step be? Change out all the electrolytic capacitors?

 

Because of the loose connection sound, I would almost think that it is a loose wire or solder joint somewhere else on the board, but I'm not sure how to test for that. I checked the connection in the board on all of the input jacks and pots. I'm assuming then that it wouldn't be the tubes. Could it be the zener diodes? I also read that it is pretty common for the board to get discolored beneath the power supply resistors, so that might not necessarily be the problem area (?)

 

I read a previous post with similar problems (http://acapella.harmony-central.com/showthread.php?p=40487585), and the electrolytic capacitors were the problem, but I have problems in the clean channel also.

 

Any advice on what I should do next?

[SomeoneYouKnew]

Here's a print of your amp. You said the resistors were 470 ohms, so apparently it's a HR Deluxe, not a Deville.

 

http://www.schematicheaven.com/fenderamps/hotrod_deluxe.pdf

 

The zeners are shunt devices. They don't conduct at all until the voltage exceeds 16v. Then they conduct as hard as necessary to keep the voltage from going above 16v.

 

There should be 32v dropped across each of the 470 ohm resistors. (48 - 16 = 32) and 68 mA flowing through them. (32/470 = .068)

 

With no load, the entire 68 mA of current will flow through the zeners to ground. This circuit was meant to be able to supply 68 mA of current before falling below 16v.

 

So measure the voltages at the points marked +48, -48, +16, and -16. Then we'll have a better idea on how to proceed.

[WRGKMC]

The 480 ohm resistors are grid resistors common in many fender amps. They can blow when the power tubes so or vice versa. It can be caused by running athe amp with a higher impediance than the amps designed for or when tubes fatigue. I suspect your burnt board was from a previous repair. It is important the the board didnt turn black ab carbonize. When a board cooks the fiberglass makes it a conductor and you can loose voltage and have future problems. If its just a littel brown then its likely OK.

 

From your descripion it sounds like you're having problems with the power relay that switches from drive and clean. It may be the conductors inside are pitted. You can usually pop the cover on it and clean the contacts or reburnish them. I suppose it could be a weak preamp tube too but I dont think that would account for the LED and Channel issues. I'd give the Power relay a go first. The relay is energized by a voltage when switched and makes all the contacts needed for the channel changes. I've also had them blow transistors Q3/Q4 and not change channels properly but theres a bunch of diodes and zenar diodes in that footswitching circuit and it can be any and/or a combination. I'd inspect those transistors first and make sure they arent crackes or show heat damage. The 2N4401 and 2N4403 are very common and can be bought for a buck a piece or less. If they check good then it could be a zenar diode. Be sure to check the 16V and make sure theres no AC leakage.

[Damnjoe]

Sorry, I forgot to say which amp it was. Thank you.

 

Okay, the voltages are about +50, -50, +17, -17 (actually just barely under) This is when I measure it with the power turned on and the standby turned off. Does the voltage need to be exact or is that close enough? I am assuming that at the least, that means the zeners are working, right?

 

To WRGKMC, the board is black on the reverse side in that one spot, but not horribly so. I'm going to try to clean the switches now. I looked over the electrolyte capacitors pretty closely but I'm going to go over the transitors and others closely in that particular circuit now too.

 

Thank you to both of you, this is really helpful.

[SomeoneYouKnew]

Those voltages are definitely close enough. I think you can give the power supply a clean bill of health.

 

The next step would be to do a visual inspection of the connections going from the PS to where these voltages are used. For instance, if you lose continuity in a ground connection, you'll still have about 32~34v between the two 16v supplies, but measuring from the ground buss on the amp to one may be much higher than 16 and to the other much lower than 16. Whichever side is drawing more current will have a lower voltage, in reference to ground.

 

Assuming that's all okay, it would be time to look at specific areas in the amp. Switches, relays, FETs used for switching which mode you're in, etc.

 

Sorry I can't be more helpful here, but troubleshooting online is a clumsy business.

 

Best of luck.

[Damnjoe]

I just found a real basic problem, the ground prong on my PS plug isn't getting any continuity. I fixed it, but I don't think that had anything to do with the problem.

 

My final diagnoses:

 

Sometimes the distortion light stays yellow whether I am on clean, light distortion, or heavy distortion. The sound that comes out of it is mostly clean, but not quite. Sometimes the amp corrects itself--I can switch channels fine and the sound is fine--but I get this quiet buzzing/low feedback sound coming from the amp that starts after I leave it on for about 30 seconds, that comes and goes. If I turn the standby off and then back on, it is quiet for another 30 seconds but then starts buzzing again. Then it will switch back to the yellow light mode out of nowhere, sometimes for a long time.

 

I think this was what it was doing originally (in my first post), but maybe it was switching around so much that I couldn't tell it was doing the two separate things. I did some more searching and it seems this problem is fairly common. Usually it involves a bad part or loose connection in the distortion circuit, like you guys advised. One guy had this problem and said he resoldered the power tube connections (tried it on mine, but it didn't work)

 

This is really difficult for someone who hasn't done this a lot if there isn't visual evidence of a problem. This is the first amp I've ever worked on where I couldn't see the damage.

 

So my next move is to replace the relay switches (T1 and T2--I can't pop the cover on them), then those transistors, then to test the diodes and capacitors in the circuit (or it would probably just be easier to replace them), and whatever else. But I have to order the parts and it will take me forever. I'm kind of on a time constraint right now, so I might just have to take it in.

 

I guess maybe I can resolder all the connections in that circuit tonight as a desperate last attempt to fix this the easy way. I hate letting this damn amp win.

 

Anyway, I will write and say what the problem was when I find out, one way or another. Thanks for all the help.

[Damnjoe]

Sorry this has been a while. I found out the problem for anyone who might need it in the future.

 

I ended up taking my amp in and the problem was simply the connection on the LED light (the one which is yellow, red, or off to tell which channel you are on). They resoldered that and it was fixed.

 

Go figure, that's the one place I did not check. I had even resoldered all of the pots.

 

The problem with the board getting burnt underneath of the power supply resistors (the big 470 ones) can be fixed by replacing those resistors with replacements that have long arms, thereby raising them higher off of the board. They did that as well (after cleaning off the carbon) and my amp should be good for a long time. Despite what some other people have said about this amp, this thing has lasted forever.

 

Thanks again for all the help. Now I know for next time.

 

Best,

 

Joey