fender hot rod deluxe low volume & distorted output

[Djeans]

Sounds like you have the power amp fixed now, and now need to check the preamp. For a first test, plug your guitar into the power amp in jack. Do you get a good clean strong signal from the speaker?

Check the low voltage supplies for both + and - 16 volts. Also check the preamp tube sockets for cold solder joints. I don't remember did you change the preamp tubes?

With a guitar plugged into the power-amp in jack, I get a clean signal, but it is not a nice loud signal - it is fairly quiet. I did not change the preamp tubes yet. I'll check the preamp tubes for bad solder joints, but could you dumb down the first part of that sentence just a little? Sorry, I'm new to this type of repair.

[Djeans]

Ok, I have started checking all the voltages and haven't found anything outside a 10% variance yet . . . but haven't got through all of them. Not sure where the test point is for the low voltage supply. There is +16 and -16 on one side of R78 & R79 and + & - 33 on the other side. I have sound coming from both clean and drive channels now, but still low volume, and when I turn it up, I noticed the tubes glowing more than usual. Used the thermocouple on my meter and temped the right tube at 390ºF before I decided to turn it off.

I really appreciate all the suggestions.

Thanks

[52 Bill]

With a guitar plugged into the power-amp in jack, I get a clean signal, but it is not a nice loud signal - it is fairly quiet. I did not change the preamp tubes yet. I'll check the preamp tubes for bad solder joints, but could you dumb down the first part of that sentence just a little? Sorry, I'm new to this type of repair.

Sorry, hard to know the level of anybody's abilities. If the signal from the power amp in jack is clean at least, then one can assume that the power amp is working and the problem is coming from the preamp. Try plugging in a spare cable from the preamp out jack to the power amp in jack, any help?

How old is this amp? There is a history of problems with the low voltage power supplies in this series of amp. Newer amps don't fail as often as the older ones did, but there are still a few problems. The two power resistors get hot enough to unsolder themselves from the pc board. Try reading the 16 volts whlie pushing on the resistors with a stick (R78 and R79). If the voltage drops out, then the resistor will need to be resoldered.

The other things to check, is the speaker plugged into the main speaker jack and not the extension jack? Does the speaker get warm? Touch the metal frame of the speaker where the basket meets the magnet structure. Are there any loose items on the board? Try tapping around the pc board with the handle of a screwdriver. Any response from the amp like noise or crackle?

[Djeans]

Thanks so much for the reply.

The amp is from 2002 - not sure if that classifies it as older or newer with these amps. I'm at work right now, and won't be home for another 8 hours or so (I live in Hawaii) so I won't be able to test any more of these things until tomorrow.

Will check on the power resistors tonight and let you know.

The speaker is plugged into the main (internal) speaker jack - have not tested to see if speaker gets warm. Will test that later tonight also. I have tapped around on the board with a chopstick and not heard anything. The only time I have heard anything from the speaker is when touching tube pin joints with the multimeter probe, or when touching TP28 and TP27 with probe (blue and brown wires).


Thanks again for all the help. This amp is been a real challenge.

[Djeans]

Ok,

Plugged a cord into the pre-amp out and power amp in - No change same low volume output. Tested the power resistors while nudging them with a chopstick - read a constant 16 (+ and -) no fluctuation. Turned it on and let it sit for awhile - speakers did not get hot. Wasn't able to play it much, as everyone else is asleep right now, so not sure if they would have heated up if played more. Noticed something that I hadn't noticed before today, and that is on the left tube, when I would strike a chord, I could see a blue glow on the side of the tube in addition to the orange glow in the center of the tube. It reminded me of those novelty plasma balls that looked like lightning inside a glass ball, if that makes sense. Not sure if that is a problem, or just something I never noticed before.

[J M Fahey]

Dear djeans.
Fender provides an excellent schematic with, not only DC voltages, but AC signal voltages at different points along the path, from input jack to speaker terminals.
Inject the signal shown, set the controls as suggested and measure where indicated.
Somewhere along the path, you will find *much less* than expected, and be real close to the actual failure.
You will need a reasonably good multitester which can read AC volts without being confused by DC present there, so those cheap $10 multimeters, or the ones with just , say, 200 and 700VAC scales won't do.
Get a $35 up one, which has an AC voltage scale starting at 200 or 400mVAC.
Worst case, carry a 9V battery to the shop, and "measure" it with the scale set to 200mVAC or 2000mVAC.
It should "blink" for a second and then show nothing (or a *very* small value)
If it shows , say, 9V or more, it's being fooled by the DC and is not suitable.
For test signal you can download a 400Hz or 1KHz MP3 from various sites and play it with an MP3 player.
You'll need to make or buy a stereo 1/8" plug (earphones size) to mono 1/4" plug (guitar size) .
Post results.
EDIT:google the schematic and post it here, so we all talk about the same.

[Djeans]

I have a Fluke 77III meter that I have been using. I also have a Extech 430 True RMS meter as well if that would be more suitable.

I have a schematic that I have been using to check things out with:

HRDV-Sch.pdf

I'll set about trying to get the sine wave signal going.

[J M Fahey]

Excellent.
A couple details:
1) they ask for 4mV at the input.
Doubt an MP3 player can go that low, and even if it did, signal will be *very* noisy (digital stuff).
I'd set it to full output, which often is around 200mV, and build a 50:1 (or whetever's required) attenuator, such as a 47K/1K resistive divider.
Then proceed as suggested by Fender.
2) I would also, for testing, bypass the preamp, inject around 90mV ainto the power amp in and test just this section.
Beware that this signal's value is unfortunately not stated by Fender , but I guess its value by what's present at TP16.
Beware also that it gets amplified by U1B (about 20X) before being applied to the power amp proper (V3A grid) so any +/-15V related problem will make the *Tube* section seem to malfunction.
Probably it won't be necessary, butif you want to test it "raw", you can apply it to TP23 without lifting anything.
Of course, in this case your generator must be able to supply almost 1V RMS.
Good luck.

[Djeans]

Started working my way through testing the voltages, and look what I found.



WTF? Looks almost like it was cut. Weird. Gonna see if I cant solder that lead back together and see what kind of difference it makes.

[J M Fahey]

Well, definitely it can't be a *good* thing, that's for sure.
Anyway, signal tracing is the way to go for your kind of trouble ... and I would notbe surprised *at all* to find that perfectly good signal reaches that cap ... and dies there, leaving only fuzzy dirty weak crosstalk to go on.

[52 Bill]

That cap is the first stage cathode cap. It was cut to either reduce the gain of the amp or because it was suspected as being bad.

Let's start from the beginning on this amp. It looks like there has been a bunch of work done on this amp before you started working on it. What is the history of the amp? Did it ever work correctly for you? When you first posted it, was because the output was distorted and you replaced the 82K plate resistor in the driver stage, right? Did it work correctly after you replaced the resistor? Was the volume level normal at that time?

[Djeans]

That cap is the first stage cathode cap. It was cut to either reduce the gain of the amp or because it was suspected as being bad.

Let's start from the beginning on this amp. It looks like there has been a bunch of work done on this amp before you started working on it. What is the history of the amp? Did it ever work correctly for you? When you first posted it, was because the output was distorted and you replaced the 82K plate resistor in the driver stage, right? Did it work correctly after you replaced the resistor? Was the volume level normal at that time?

This amp was purchased used as backline for the club I work in - I am responsible for the upkeep of our audio gear. The amp worked fine for about a year, then started exhibiting the low volume / distorted output problem about a month ago. - I did replace the 82k plate resistor, and it worked fine on my bench, then when it was back at the club, same low output problem.

Yesterday, I reconnected the broken lead on that cap, but it did not have any noticable effect on the sound / output of the amp.

My next plan was to test the voltage at each test point on the schematic and see if I find anything outside the 20% variance. For a signal, I was planning on using a 1k sine wave signal generated from an app on my iPhone - hopefully that should satisfy the requirement. . . Am I correct in assuming that the voltages on the test points in ovals are for AC , and the ones in rectangles are for DC ?

Thanks again.

[Jazz P Bass]

Am I correct in assuming that the voltages on the test points in ovals are for AC , and the ones in rectangles are for DC ?

Yes.
The figures in the oval are Vac.
The figures in the rectangle are Vdc.

[Djeans]

Ok,

Here are the results of going through and testing each test point.

I started with a 1k sine wave generated from the app "Audio Tool" on my iPhone, all knobs on the amp at 50%, except for the reverb at 100%CCW, Normal/Bright out, Drive Mode Yellow, Drive Select In.

Everything seems normal until TP5 (R11), then most everything goes haywire after that. Looks like it is a 100k resistor, and I tested it at 99.3k so not sure what's failing there.

Some of the Test Points don't list a value (16-18, and 30-36) so I'm not sure what the reading should be there.

Fender Deville Test Points.pdf

[Enzo]

TP16-18 are specified, look on schematic just left of U1b and below Q1 Q2.

TP30-36 are specified in great detaqil in fact, look at the notations to the left of them for a whole listing of conditions.


Don't sweat the 4mv thing. I just plug a line from my bench stereo tuned to a music station into the amp input. I have no idea its level other than a lot more than 4mv. Instead of looking for some exact signal voltage, I look at the relationship the schematic offers. If it wants 100mv at a grid and 10v at a plate, that is about 100 fold increase. If I see the stage input has maybe a volt and a half and so does the cathode, then whatever voltage I have coming in, I expect about the same leaving. I try to put it all in a context.

Whoever mentioned signal tracers up above was on the right path, in my opinion. Throw some mussic in the front and follow it along stage by stage and see where it goes haywire.

If i get an amp that sounds just awful or not at all, I go down the row of tubes looking for reasonable plate voltage (not zero, and not all the way up to the B+) And the cathodes. (all should have a volt or two or whatever a milliamp or so wouold provide. And of course power tubes - check their DCs. After that, I just follow signal through the stages.