VTM 60 Burnt 400 ohm/10Watt Resistor, oh my

[GuitaRat]

Nice! Thanks, i will put it back together and check that out.

[GuitaRat]

I am measuring 418 volts on pin 4 on both power tubes

[Enzo]

I think you have a major misunderstanding here. Filter caps do not have an in and out end.

Look at the schematic. Your first filter stage is C3 and C4. They are in series. C4 is the lower one, and it will have half the voltage across it. SO if the top end of C3 is 400v, I would expect 200v at the bottom end of it, which is also the top end of C4. The voltage at that point is evenly split.

The non-rubber end of C4 connects to ground. That is why there is zero volts there.

Same thing with C1 and C2. C2 is the one on the bottom and has half the voltage across it. The top of C1 has 400v? Then the bottom of C1, which is connected to the top of C2, will have about 200 on it.

SInce you have 400v on pins 4 of the power tubes, those caps are not shorted. They may not be in good shape, and may need replacing. I just rebuilt a VTM120 last week. It uses the same board set. But you have 400v on the screens, so while those leaky caps may be making the sound hummy, and may be dropping your average B+ a few volts, at this point they shouldn't be a major upset to the amp working. Bulging ends of cacps, and leaking inner goo are defimite signs the cap needs replacing though.

I am looking at your photo, and at the board layout drawing, and they look the same to me. What do you find to be the difference? Sure looks like the board I revuilt the other day. I had to replace the molex connector for the standby switch. It was burnt. I also had to replace the Molex connector on the little ribbon cable next to the white fuse. It carried the heater viltage to the preamp tubes and was also burnt, so no preamp heaters.

Schematics will almost never "look" like the actual physical parts. The schematic describes teh electrical relationship between the parts, while the layout photo shows the mechanical relationship.

Look in the schematic at the page of the board pictures. If your schematic is the schematic ONLY. then download the link someone put up earlier, it is complete. The filter caps are numbered C1,2,3,4 from top to bottom if you look at it the same way as your photo, 400 ohm resistor at top.

If you have 400 volts or more on all the pins 4 of the power tubes, and also on all the pins 3, and there is more or less -50v on all the pins 5. Then the power amp ought to be making sound.

What EXACTLY is the symptom you are trying to cure at this point? From reading the posts I see your 400 ohm resistor burnt - they do that sometimes. After that, I saw a lot of voltage readings, but I am unclear of a described problem. You will need to replace the bulging caps, but what else are we still looking for?

[GuitaRat]

Enzo,

Thanks for the very thorough explanations, I stopped for awhile to do some reading and brush up, i did notice from the schematic that the cap went to ground, DUH, I used to know that.
Its been well over 20 years since any electronics courses. I am truly a novice but can read schematics, and worked around live elelctricity for 3 years. Just a fan of electronics and want to learn more. What fascinates me is taking an electrical current and manipulating it this way and that way to get the results we do is just fascinating, of course with interaction of chemicals etc..
the layout of the power board was missing the Caps? I saw the resistors.

Schematic looked fine. I will look at it again, maybe i missed somthing.

Anyway, I have a deTuned, slightly distorted sound when i play my guitar through this amp, it has no clean or dirty sound either way you put the volume and gain knobs, I can hear the power in the speaker like the volume is going to be loud but once i pluck a string it sounds weak and detuned.
I have recently discovered that the wall socket it was plugged into was not grounded, dont know if that help this problem occur.

I have replaced the power tubes, and no change to the sound.

I did order the new caps from Hoffman, i would like to figure out if something else is wrong before placing those in.

Thanks, Still reading!

[GuitaRat]

OK your correct the board layout is the same didnt catch the caps listed in between the resistors. I wished i would have seen that would have saved me some time, chalk that up to greeness. LOL

[GuitaRat]

If power board seems OK and voltages leaving it appear fine, post volume seems to work, this leads me to believe my problem is in the preamp section?
How do i check the signal leaving the preamp section?

[GuitaRat]

Gentlemen,

I think i have found a big part of my problem or maybe the whole part, the standby switch seems to be fried or Partially fried. Does anyone know where to get a peavey style rocker switch? When i roll it on slow it definetly breaks up the signal. the signal is loud then quite then loud again as I roll my finger up the rocker switch. Signal out of speaker with guitar does not get any better until i push hard on the switch. Can i bypass this switch and jump the two wires together without frying anything to see if the sound is better? I know its not good for the tubes.

one last question: Could this bad switch cause that screen resistor to burn up? judging by the schem i think it could, in my humble opinion of course.

Thanks for all your help thus far guys, the support here is amazing, even with a newbie!

[stokes]

I would say you definately need to change those filter caps.The voltages may seem fine at idle,but when you put a signal thru the amp,you are asking the power supply for more power and the bad caps cant keep up.If the standby switch appears to be fried,it will need to be changed as well,Enzo may have a source for that,I believe he sees more Peavy amps than anybody else around here.

[GuitaRat]

Thanks Stokes,
I used the link Enzo gave me the other day and ordered the caps, i went ahead and ordered four, not taking chances i will switch out all four.
I found a place for the rocker switch www.tubesandmore.com
price didnt seem bad, 10 dollar minimum on purchases though.
Cant wait to get this thing running again.
Ordered a JTM 45 kit the other day, wife says i have a sickness, too many amps and guitars, so I might as well learn how to fix'em.

Thanks again for all your help!! labeled photo, schematics I really do appreciate it.
I will probably become a regular here.

[Enzo]

IF you think the switch is bad, its wires run down to that Molex next to the 400 ohm resistor. Unplug that and clip the two pins together on the board. Now it is ON all the time. That will tell you.

PV will sell any parts direct at reasonable prices, but Mouser or someone like them will have rocker switches. Just watch the panel hole dimension.

[GuitaRat]

Yahoo!!!
Caps came today, put'em in and she fired right up!!!!
Switch not here but will be soon. detuned sound gone, fricken fantastic!
You guys rock!
Enzo, now another question, how do i add some bottom end to this rig,
can you share any mods i can make?
maybe its just because I have been playing the 100 watter since vtm 60 went down, but it sounds weak.

[GuitaRat]

Never mind the mod. She has warmed up and has full tone and volume now.
Sounding good.
forgot all the tubes were new.

[Enzo]

And if you have not already, play with the little DIP switches behind the cover on the front panel.

[GuitaRat]

Will do!
Appreciate the help!