20 Most Recent Fender Hot Rod Deluxe Guitar Combo Amplifier - Page 5 Questions & Answers

You say the "tubes are fine" I take it that you have either replaced them with new or used a very sophisticated tester on them. A heater-cathode short might cause the symptom. There are a bunch of tests I would want you to run alnd get back to me with the results.

Heat up the amp but DO NOT play anything into it for the usual failure time or more and they start to play i... did hum occur WITHOUT playing? With all volume controls down does the hum occur?

The right tools is an oscilloscpe to fing out if the filter caps are good. These are C31, C33, C35, C36.

With hum present in failed stater, plug in a grounded plug into the power amp in jack... if hum is stiil there, check the +/- 16 volts regulating Zener for ripple. get schematic here:

http://elektrotanya.com/fender_hot-rod-deluxe_sch.pdf/download.html

It would help to know which cap you replaced. I see a lot of relays in the unit that are swtched "dry" that should a contact be oxidized might cause the problem. Such intermittents are hard to find the root cause. If the replaced cap happened to be one on the +/-48 volt supply, best to check that you have DC across it with not very much ripple using an oscilloscope. Even brand new caps can be bad... and those of course are polarity sensitive... hope you got them in right if those are the ones.

Check loudspeaker is not seized, by gently moving cone to and fro. If it is very stiff, chances are that it has been overloaded and seized. Test this by substituting it with a known good speaker.

You may have a switch that may be bad. For the sound level high and low may need new tubes or possibly may not be seated fully into position. These tubes due get quite hot and do get loose over a period of time.Hope this helps.

Realize that the number on the volume control is only relative... it means nothing regarding what the amp can produce. With a high level output guitar you can easily drive the amp into distortion at very low volume control level settings... what counts is the actual output sound volume and specifically the voltage output going to the speaker. Once the amp reaches saturation, any further input just pushes it into distortion or "flat topping".
An oscilloscope on the output will quickly show when amp starts to flat top.

If it worked before you changed the tubes, it is possible one of the replacements (probably a small tube) was bad... it happens... Another possibility is that a socket pin has gone bad or a pin has broken or bent off... again probably one of the small ones... 12AX7/12AT7 or equivalent.
Swap old ones back in one at a time.

It is not clear if the volume knob you are talking about is in the amp or the guitar... SINCE you said you verified the amp is OK I have to assume the volume control you are talking about is in the guitar and my answer is based on that...

I SUSPECT that the resistance element and/or the wiper in the volume pot is damaged. One other possibility is the soldering of a wire to the volume pot may be bad. If you are competent with a soldering iron you could replace the pot yourself or take it to a competent guitar shop for repair. I would myself suggest you open the electronics and look for anything obviously wrong. I have ALSO seen one other problem where one lead of a pot is touching the CONDUCTIVE paint in the cavity where the controls are. If the shaft nut becomes loose and the pot slightly rotates this can happen, grounding out the signal.

Is it the amp for sure? You need to try these things first :

Plug your guitar and lead into another amp. Does it buzz?
If so, Change the guitar lead with a NEW ONE.
Still Buzzing? Your guitar needs attention.

Ok so if it's not noisy in another amp, turn on your fender with nothing plugged in. It should be silent? Is it? If not, check to make sure no ones messed with the earth lift switch.
Next, you should try it off a different ring main. If you have upstairs sockets and downstairs sockets, they should be (but not always) on a different ring main. Try it? If you live in a flat for example, or are unsure, try it round a mates house. If it's noisy off one ring and not on the other, you have some mains interference from something else plugged into the ring main and it's not your Amp.

Is it still noisy?

Well so far, I'm hoping you have been using it all Clean and direct with NO FX Pedals and such like. These thigs can add numerous buzzes and hums to any rig. Again break it down, try each one by one until you get the rid of the buzz.

If after all this, your Fender is still being noisy, you can safely take it to your nearest music store to get it repaired without feeling like a plonker :-)

I suspect the power supply has an issue. Most likely bad filter capacitors. This could cause "motorboating" where power is drawn and supply voltage collapses and then recovers repeatedly.

Time for the repair shop. The parts cost should be nominal... labor... well...

The unit will require repair and if you are not adept at electronics, you will need to take it to a shop. Likely there is failure in the power supply or amplifier that has blown the fuse. Replacing the fuse WITHOUT repairing the cause MAY do additional damage.

The first thing i would do is not leave it on in this way for too long. You could blow out a very expensive Transformer if its a faulty speaker. The worst thing you could do to a TUBE amp is run it without a speaker . I would test the speaker first. Just remove the wires going to it. And connect the speaker to either a car stereo or house system at low volume very low. And see what you get. if it works ? Good! Now check your guitar chord/wire. Try different ones try different guitars. If that doesn't work . Look into the input jack on the instrument input hack on the amp.. They do come loose off the board.Either get a new input jack or resolder the old one. if you only have one guitar look at the guitar output jack. Loose wiring on the guitar could cause this also. Dirty In and out put jacks could cause this. A pencil with Very fine sand paper wrapped around it will clean a dirty input/output jack. Try these things. Just remember poking around your amp is Dangerous. The electrical components hold a charge even unplugged that can kill you. No joke..

TRY EQUALIZING THE SOUND.

I need you to run a few tests... one at a time.

Plug a set of headphones into the preamp output and simultaneously a set or a guitar or other low impedance device into the power amp input jack (this disconnects the power amp from the preamp)

Now, does the squealing noise appear in the headphone when in OD?

I also want you to test if the normal/bright switch has any effect when in normal configuration in OD?

If the normal bright switch has little effect on the squealing that helps isolate which stage is allowing the feedback.

I suspect that the source of the feedback is either a bad filter capacitor in the power to the preamp OR one of the contacts in the two realys that are for OD are making poor contact. These relays are switched "dry" which means there is little current to clean oxides off the contacts so they can make poor or no contact. An open contact would leave a grid floating as a high impedance "antenna" to pickup and feedback.

The first thing I would do if you had an oscilloscoppe would be to see that C36, a 22mfd 500 volt cap does not have high frequency ripple across it. One could try paralleling a new 22mfd cap across it to see if problem goes away... you will GUARANTEED have trouble finding a 500 volt one of these and have to settle for a 450 volt one which SHOULD be OK judging from the circuit. You can probably get one from Antique Radio supply... use Google to find them. Remember this amp has lethal voltages in it so if you are not adept at working on electronics of this type, best to take it to a shop that is.

The thing you can try is to swap tubes V1 and V2 which are 12AX7's... if major change occurs including loss of any signal, replace the one that WAS in V2 position.

More than this gets into the electronics and if you don't have test equipment it is not a DIY thing so you would need to take it to a shop. There are several components including FET's Q1 and Q2 and what looks like BAD engineering design around these particular parts that could cause the problem.
The bad engineering looks like tinkerers designed the circuit rather than an engineer. Gates of these FET's are left floating on the end of a 1N4148 diode according to my schematic... I have seen MUCH poor engineering or lack of in a lot of the audio equipment.

colearthur16, Only a slight chance of problem with your speaker. You will need to prove that all stages of amplification of the audio signal from input to output are not being driven into distortion (clipping) because of a bad DC power supply rail to the low level amplifiers or the high power "final" output amp! To prove this, you will need the help of an electronics technician who has access to a variable audio signal generator (amplitude and frequency) and a cheap 5 to 20 megahertz oscilloscope to view the amplified waveforms of the amplifier stages in the Fender Amp. Remember the following statement for the rest of your life!!! "All musicians who use electronic amplification to play musicial notes through speakers should have between 4 and 8 hours of classroom instruction on how typical amplifiers (tube and solid state) operate and how to troubleshoot them when they fail to perform to rated specifications!!!"
Cole, have the Fender amplifier (not the speaker) tested by a qualified technician to prove amp is not at fault!!! 12fixlouie

This is PROBABLY due to a bad volume control pot or wiring to the pot. Sometimes people have used DeOxit or Blue shower in the pots and it wrecks them. The resistance element may be cracked in he pot. The break would be on the side of the pot that connects to ground.

CHECK VOLUME CONTROL BUTTON.

There are many components in the system. Electrolytic capacitors are most suspect. C49, C50, C45, C51

There are many others but these are top suspects. You MAY be able to detect a change by cooling these components with circuit cooler spray.

ALSO check the +/- 16 volt power sources in the unit.

It depends on the pedals and the rest peripherals. A typically value is about 100-200mVolts. Could I help you further? What is your problem?

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First check if the reverb "tank cables" are on tighly where they attach.