Fender Hot Rod Deluxe problems

[gibson17]

Hello All, I am new to the forum, I have parused through this site to see if there was a thread for some help, but alas there is not.

I am repairing a Fender Hot Rod Deluxe that has seen better days. I figured a simple tube replacement / bias job would be all this bad boy needed but this is not the case. It seems there has been someone inside this amp before as there are some what appear to be mods, and it also looks like some components burned up at some point along the way. Specifically R78 and R79 and Cr13. I removed both R78 and 79 and tested them, their resistance is what is shown on the body of the component. I also went through and fixed all solder connections for power tubes and pre amp tubes as well as the poor soldering job on 78/79.

Here is what the amp is doing, turn on the amp, all is well, heaters light up on tubes. Come off of standby and a rediculious loud buzz coupled with the pilot lamp dimming, if left that way for more than 30 seconds hi temp fuse blows, along with R78 and R79 getting scorching hot. Any hints on where to go next, I ask because there is no way for me to keep the amp off of standby to test it with my DMM. Thank you for taking the time to read this and hopefully some time to help. I also have a copy of the schematic.

[Jazz P Bass]

The pilot lamp dimming may be a clue.
That is your 6.3Vac heater winding.
Pull all of the tubes & measure across the lamp.
If it is a steady 6.3Vac, then you possibly have one or more bad tubes.
The power tubes are the most likely culprit.

[gibson17]

Thanks for the speedy reply. I had already replaced the tubes but it does the same thing with old and new. The amp makes the same noise with no tubes in when i come off of standby. I tried what you said, i pulled the tubes and tested across the pilot and got 6.8v on standby and when i come off of standby the pilot dims, same noise ensues and i get 4.3v. Any thoughts?

[Enzo]

Your power transformer is being very seriously loaded down. The standby doesn't affect the heater circuit, so I suspect the high voltage circuit is doing the loading. It drags the whole transformer down.

Pull the power tubes. Very briefly verify loud buzz when standby switch goes on. Back off. I will assume it does. Look for D4 and D5. They are on the schematic connected to the power tube plates. In the amp they are on the power tube socket board next to each socket. Is either shorted? Easy way to test, just check pin 3 of each power tube socket to ground/chassis.

If one is shorted, remove it. Now check the output transformer wires. Is either tube socket pin 3 still shorted to ground?

The center tap of teh output transformer primary winding is connected to B+ at P18. That should be a red wire, pushed onto a terminal just beneath the row of filter caps. If we still have the problem at this poibnt, pull that red wire off P18, and power up.

With no power tubes and no shorted diodes, if the amp still loads down, and pulling the red wire off P18 stops it, then the output transformer is shot.

[Enzo]

And those two resistors? They are dropping resistors for the two 15v zener supplies. They normally get real hot, in fact they have been known to unsolder themselves. This is a design weakness. SOme guys mount 25w resistors - the nice aluminum ones - on the chassis somewhere, and run wires back to the board where the resistors were.

Of course, if one of the 15v zeners is shorted, its resistor will get even hotter. And your ICs will not have their operating voltages.

[gibson17]

With tubes pulled, turn amp on... quiet, switch off standby... loud buzz, back off. I have already left off P18 inadvertantly and it was noise free, sound free as well. I am looking on my schematic and there is no D4 / D5. There is a CR4 and CR5 located right near the power tube sockets which is next to R61 and R62... is this what I should be looking for? Also should I be looking for continuity from pin 3 to chassis. The only way I know how to check a resistor is to unmount it and then check it. If it is shorted I will have no value almost always looking burnt. P18 is indeed a red wire located beneath two large caps which are C31 and C33... they have P18 labeled on the schematic CP18. There is a diode loacted right next to those dropping resistors that also looks to have taken a hit, probably from heat as you are explaining to me about this amp. It is labeled CR13. Thank you very much for your help, i will check this stuff tomorrow and report back!

[Enzo]

There are two drawing conventions for diodes: D for diode, or CR for Crystal Rectifier. They mean the exact same thing. All my schematics and pictorials use D for diodes in these amps. If your boards are marked CR instead, just adapt. Yes, those are what you want to check. They are shorted or not.

And P for post or CP for connection post. Different draftsmen drew the prints.

What schematic are you using? I am looking at Hot Rod Delixe/DeVille drawing #0057251000 Rev A 21-JUN-01, and the circuit board says Fender 2001 on it near the jacks.

SHorted diodes won't look any different, they just won;t be diodes inside anymore.

You DON'T want continuity to ground from pins 3 of the power tubes. If you find it there, it means they ar shorted to ground, causing your problem. We take the measurement to determine that.

If the diode, not resistor, measures shorted, THEN remove it and recheck. If it does not check shorted, then it isn;t. There is nothing a part of circuit can do to make a shorted part look not shorted. The reverse of that is not true, of course.

I have never seen a shorted rsistor.

[gibson17]

Enzo, thanks for the help, i really appreciate it. i am using Hot Rod Deluxe drawing #050399 Revision A, May 7th 1996. It has Fender Logo near the input jacks dated 1996. I dont know why I said shorted, I meant open, must've been past my bedtime. I had some continuity between Pin 3 to ground on XV4. I then tested the Diode in circuit and it appeared shorted, I then removed it and did not have continuity on pin 3, Upon testing the diode out of circuit it appears fine on the diode test of my DMM it reads 1.631. I then powered up the amp with not tubes to see if it solved the issue, same loud buzz, at this point I decided to remove the CR from XV5 and it tested the same on my DMM, Powered up the amp once again and got the same noise. After all that, I removed CP18 and the noise is gone. Per your post above I assume that means I have a bad OT? How much does an OT run nowadays and where would you recommend I look for one, I usually use AES. Thank you again for all of your help!

[Enzo]

If lifting those two diodes doesn;t cure it, but disconnecting the center tap does, you have only one thing between yiou and a bad transformer. Reconnect teh red wire, but pull the brown and blue wires off the power tube board. If it STILL draws current, then the transformer is the only problem left. SOunds bad to me. If pulling the brown and blue wires makes a difference, then explore the sockets, one of them may have an arc to ground from pin 3.

[gibson17]

Ok, so I reconnected CP18 and Removed the brown and blue wires. Pretty much the same result, dimming pilot but this time no noise, I am assuming this is because of disconnecting the Power Tube board. So that leaves me with a blown OT. I guess the silver lining here is I looked up the OT at AES and it was pretty reasonably priced at $45. hopefully somewhere down the road this will help the next guy with a blown OT. I wonder if it is possible to add that to the title after the fact that this thread has already been created to help the next guy even further. Thank you again, I'll have to add you to my Christmas card list

[gibson17]

I recieved the Output Transformer over the weekend, and got around do wiring it in on Sunday. The amp now works. I also replaced CR4 and 5 with the R3000 diodes that it asks for. There is a bit of a strange buzzing that sounds like it is coming from the Power Caps right above CP18. I put a fresh set of tubes all around and cleaned all sockets once again. I am using a Bias Probe and set V4 to 30mv, when I went and checked V5 it was at 20mv. Shouldn't the power tubes on this amp be together as far as voltages go? I am used to working on Marshall 100 watt amps and I am used to two trim pots for the bias, each side of the 100 watt amp is always together. Something Else i should be looking for or is this just the way these amps behave?

[g1]

The bias readings will depend on whether you used matched 6L6's. If you did you should get closer readings than that. The Marshall circuit allows you to compensate for unmatched tubes with the 2 bias pots, the Fender circuit does not.

[gibson17]

G-one, I bought two slightly used Groove Tube 6L6GE's, they are matched together. I own a few tube testers and I have a nicely calibrated TV-7 D/U. I test all tubes new and used on it for shorts and Gm readings. They were close to the same readings, perhaps I'll switch sides and see what happens, I also will run them through the test again, and perhaps i'll also try throwing in the old tubes and see if that changes my bias readings. If it doesn't is there anything i should be looking for. I can tell you that the dropping resistors have scorched the board where they sit. I did remove and reflow, but someone else has been in here a few times before me. Like i said earlier, it seems as though the large power caps right near those dropping resistors are buzzing? This amp is turning out to be a real PIA. Thank you for your reply, i'll go get on it now.

[gibson17]

It turns out to be power tubes, I always try and find a deal on tubes hence the reason for testers, but it usually winds up biting me in the arse, there really is no better test for a tube than in the amplifier However, when I do score a great deal it makes it that much sweeter! I guess the power caps buzzing is a normal sound with these? Thank you G-one, Enzo and Jazz P Bass for your help.

[zkat]

Hate to re-open an old wound but did you check the PT output without load? Red and Brown leads disconnected and all tubes removed I'm only getting 340v on Red. Seems awfully low and curious if you measured unloaded PT output.