Hot Rod Deluxe...low volume, weird voltages

[Brad1]

Got my friend's Hot Rod Deluxe back with the other ceramic resistor (R78) loose, so it didn't switch channels. I fixed R79 last year. (This thing gets a LOT of use, and moving).

The clean channel was working fine last Tuesday. Now, the amp has very low volume, gets compressed and distorted, and causes the output tubes to do quite a bit more of a "blue-light show" when playing.

All I did was lift the two ceramic resistors, and apply a bit of high-temperature (650 degree) silicone under the ends to help hold them in place, and resolder them, running a solid-core wire from the leg of R78 where the pad had lifted to the next component on the trace. The switching now works fine.

I'm getting crazy voltage readings starting at pin 1 of V1, according to the schematics.

TP2 there shows it should be 180mV. I read 245V.
TP3 is OK, TP4 shows 13 mV, I get 0.
TP5, TP6 and TP8 are close, but a few volts low.
TP7 shows 33mV, I get 0. TP9 close.
TP10 shows 580mV, I get 0.
TP11 shows 13.3V, I get 230V.

And on, and on.

Is it possible I broke a ground or something while removing this thing from the chassis? Does it sound like something is shorted? Bad component that blew out after the switching got fixed?

Nothing looks burned, and I've inspected all the solder joints, and poked around and tapped on everything. I tube-tested, and then swapped out ALL tubes, to no avail. I did continuity from the tube sockets to the main board. I've changed guitars, cables and speakers. I've measured resistors, and did quite a lot of circuit continuity.

Any ideas? It's driving me crazy that a relatively simple fix (I thought) turned into this.

Thanks,

Brad1

[Brad1]

BTW...

I also ran a POD, and also an ART Tube MP into the Power Amp In, and it's still futzy.

Thanks,

Brad1

[acorkos]

I'm getting crazy voltage readings starting at pin 1 of V1, according to the schematics.

TP2 there shows it should be 180mV. I read 245V.
TP3 is OK, TP4 shows 13 mV, I get 0.
TP5, TP6 and TP8 are close, but a few volts low.
TP7 shows 33mV, I get 0. TP9 close.
TP10 shows 580mV, I get 0.
TP11 shows 13.3V, I get 230V.

you are measuring DC voltage. most of those the test points are AC measurements. set your DMM for AC volts and measure again. the voltages on the schematic with a + or - in front of the number are DC. those without are AC

[acorkos]

I also ran a POD, and also an ART Tube MP into the Power Amp In, and it's still futzy.

check power tube screen resistors R61 and R62 and
PI screen resistors R57 and R58

[Brad1]

"you are measuring DC voltage."

D'oh! Now I feel stupid. I really do need to enlarge this schematic from a 8x11 so I can see it, anyway. Back to redo those readings...

"check power tube screen resistors R61 and R62 and
PI screen resistors R57 and R58"

Checked all those already, and they measure proper resistance, and continuity is good.

Thanks,

Brad1

[Enzo]

Look CLOSELY at the solder on the two power tube sockets where they are mounted to their circuit board. See if any of the solder joints are cracked. Or just resolder all of them, make sure to discharge any power supply voltages.

[Brad1]

OK, now after getting my head on more straight, blowing up the schem so I could actually read it, and taking advice, I did all those and continued. (Kind of overlook obvious stuff and do stupid things when you only mess with these once or twice a year).

Resoldered the tube sockets. No change.

I DID run both a guitar signal, and a 1KHz sine wave (at about 4 mV at input) through the Preamp Out, and into a mixer, and the Amp In input of an old Peavey Musician head...and it seems to have a pretty good, clean signal coming from that. With that input, now I'm actually seeing closer to the voltages listed in the preamp section on a lot of it.

ALL of the voltages in the power supply section are fairly close. The BIAS setting is set to exactly +60mV.

I guess this is pointing me more toward the output section? I know I didn't short anything while disassembling, and I discharge everything with a bleeder before doing anything after it has been on. I've done continuity beyond the ribbon cable ends, and those seem good.

I'm only getting about 2.25 VAC at TP27 and TP28.
TP24 and TP25 is about 1.5 VAC, each.
TP23 is about 395mV. TP22 about .018V.
TP21 is 0 V.
TP20 is about...1V...wait....looks like a very small "pit" in the top to the right of the pin1 circle. Magnifying glass....hmmmm....looks like maybe...kind rough with an xacto tip. Would a bad TL072 (U2) cause this?

Thanks,

Brad

[acorkos]

make sure you've plugged the speaker cable plug into the correct jack (not the extension speaker jack....check the tube chart to be sure)

[lowell]

Also check PI plate voltages/resistors, I've seen many-a plate resistor bad on the PI in those amps.

[Brad1]

Fixed...

Bad solder joint in the output section. Works fine, except wonding about one thing. Didn't LOOK bad...it just was.

Left 6L6 (as viewed from back) has quite a bit of blue, while the right doesn't.

Need new tubes? I think these are fairly old. I have bias set at exactly 60mV. Maybe a resistor drifted out of spec, as compared to the other?

I think I'll pop in another set this evening to see what they do.

Thanks,

Brad1

[lowell]

if there isn't an audible hum I wouldn't worry about it. also do you mean bias is set at 60 milli "amps."?

[Enzo]

On these Fender amps there is a test point on the corner of the board. Set it to 60mv. Ther is a 1 ohm resistor somewhere nearby that the test point samples across. So you set it to millivolts resulting in milliamps elsewhere.

[lowell]

ok makes sense. just wanted to be sure.