Peavey Classic vtx series- Troubleshooting repair help needed!!!

[marley]

Good to know. I will get the gator clips/m.m and inspect the standby switch more closely.
Thank you for the manual! It is the same amp.

I have extra el34 tubes here, but from what I read, they are lower voltage tube. I am going to call a few places later today and see about tubes and testing.

I will check the voltage on leg 5. Thanks again for the help, Brian

[oc disorder]

I think the fault is more likely in the solid state componentry in the power amp.
I could be wrong....but usually tubes work or not - they don't usually fade away after 30 seconds.
Measuring the voltages will tell a lot.
If you notice in the small print on the right of he schematic
Vi (Idle) = 40mv max 20mv min ..10mv MAX difference between sides
This is measured across firstly R126 and then R 130 those 2 big wire wound resistors
on the left of the transistor heatsink. (often referred to as cement resistors).
Comparing one reading to the other there should not be a difference of more than 10 mv .
These are the emitter resistors and this gives us a clue as to how hard the transistors are working
ie how much current is flowing through them as by ohms law current flowing through the
resistor produces a voltage across it.
Current = voltage divided by resistance if you want to do the math's for fun !
Negative probe to the outside edge which is ground.
The other important small print voltages (Vk) are easily measured between ground and tube
pins 1 and 8 (the respective junctions of the collectors of Q6,7 and the cathodes of the 6L6's ).
These two separate readings should not have a total difference of more than 10 volts.
By the way this is where the signal enters into the cathode ...unusual design ..same as
Music Man amps.
Vk (idle) = 95v max 75v min 10v MAX difference between sides

Another measurement should be taken from the bases of Q6 and 7.
Facing the amp with the transistor tab away from you and the legs towards you
its the leg to the left.
You should get about half a volt there .560 volts (notice they left a decimal point off the diagram).
The other measurements they provide are 1.90v at junction of R119 and diode CR39
and 1.24v at junction of R121 and diode CR32 (which is also junction of R122 & R127).
1.10v should be found at junction of R116 and CR28 and also junction of R136 and CR37.
See attached diagram.
When you said "I did take it apart" I presume you removed all the knobs and pot nuts
and flipped the main board over and inspected all the solder joins ?

Again Q6 and Q7 and the two emitter resistors R126 and R127 are the main suspects as
they are prone to get hot.
Good luck on your misson !!!

[marley]

Hello again. I have not had time to open up the preamp yet. In the mean time, I have left it plugged in and have periodically powered it on to see how often the status light would illumate when the standby switch was flipped, it was at least 30% of the time. I did this to see how long it would stay on if I did not strum the guitar. It still remains on for 15-20 seconds. Does this help to point out the standby switch as the problem?

oc disorder: I plan on following your advice and I will let you know how it turns out. Thanks again, Brian

[oc disorder]

I doubt the s/b switch is the problem..but need to eliminate these silly obvious things now rather than later.
Still need to do what Bill52 suggested in post 9.Measure the voltages particuarly the high voltage.
The status light is illuminated by Q1 after its base senses the HT via 2 x 220k 1 watt resistors.
Its possible that the components here could be faulty (i.e. faulty indication) which is why we really need to know
what is at the HT terminal.
What happens when you remove the tubes and switch it on. Does the status light stay on ?
It could be just something on the supply board. Faulty capacitors - diode breaking down under load..Ht fuse not making
proper connection loose connection.
Without any meaningful readings is impossible to figure out whats going on from where I'm looking.. can only make educated or otherwise... guesses. CR38 and CR 31 could also be suspects ..maybe breakdown under load but then the 500mA fuse should blow.

[marley]

Ok, I have the preamp out and I am getting all my ducks in a row. I am printing out the instructions to read over tonight and then start measuring tomorrow. I am sure I will have some pretty basic questions about what to do so please bear with me. I really do appreciate the time and help I have received from members of this board. Oc disorder especially! Brian

[marley]

I have just removed the tubes (preamp is out) and powered the amp on then switched the standby to high power. Status light is on and is staying on. When I flip standby to neutral, light fades out. When switched to low power, the light will not come on.

Where should I begin to measure? The tube sockets? Brian

[oc disorder]

I was hopeing someone else would chime in with some objective comments but they seem to be letting us go...??
It seems that there is definitely a problem with the HT supply.
I hope there is not a problem with the output transformer .. as it is the initial recipient
of the HT before it (the HT) goes to the anode of the 6L6's.
Its possible we wont get meaningful readings until we have a known 500 or so volts on the tubes.
Have a close look at the ends of C3 and C4 the two main capacitors on the power board. They usually have a point at one end that bubbles/bulges when the cap is under stress sometimes leaking electrolyte through that hole.
If that is the case I would order 2 either sprague or F&T 100 microfarad 350 volt axial
(leads out either end) electrolytic capacitors and fit them immediately.

However refit the tubes and measure at the tube socket.
You should get 525v at pin 3 ..if you are at all apprehensive about touching probes onto
such lethal voltages connect one end to a clip you can connect to the chassis and only use one hand to touch with the other probe with the other hand in your pocket !
That way you can't absentmindedly rest one hand on the chassis while the other is
poking around 500v.
Maybe get your neighbour to come over while you do it and he can pull the plug if you light up !
You will only get the 95 volts on pin 1&8 if these is 525v on the tube at pin 3 .
Thats why the "big one" is the place to start.
The other smaller voltages are independant of this, so give or take a few millivolts should read ok.
If you have a diode test function let the amp discharge for a few minutes (if the transformer unplugs from the power board unplug it or remove the HT fuse) switch the s/b to centre position and measure each diode Cr1,2,3&4.
The diode test function should show a reading one way (.5?? to .8?? approx)and no reading the other.
Have fun !!!

[marley]

Just to be sure, I need to check and see if the tube is getting its 525 volts? I can do this.

Short story:
I bought a chinese tube amp (el34 tubes) and within 1 month, one of the tubes went t.u. and took out the resistor. I guess I got lucky it wasnt worse. When I was trying to find the problem, it was suggested to check the high voltage at the tube socket (450 volts). I had planned on waiting for a friend but went ahead and checked it myself, no problems.

Yes, it does get the pucker factor going but thats ok. I will use a gator for the ground and only work with one hand. I may not do this today ( health problems=insomnia) and I want my head straight for this.

I inspected the 2 caps and they look good, no bulging or sign of fluid. I really appreciate the time you are putting into this, Brian

[marley]

Hello again. I took some measurements at the tube socket and this is what I got:
Pin 5 had 15 volts
Pin 3 (on either tube socket) was showing 2 volts! Does this mean it is the power transformer?

I also measured the resistance of r126 and r130 and they measured at 5.6 and 5.7 ohms.

Tips for the next step? Brian

[oc disorder]

I guess it's time to send out a search party to find the missing 523 volts !
Don't think it could be the power transformer as it's unlikely to be intermittent.
"I also measured the resistance of r126 and r130 and they measured at 5.6 and 5.7 ohms."
Well thats good we now know that those emitter resistors are ok.

Well it definitely has a problem with the HT supply.
I've attached a simplified circuit of the area you need to concentrate on.
Some times a simplified circuit makes it eaiser to see whats going on

I have also attached a diagram of the connections from the transformer
to make sure there is not something silly there.
Check that fuse is 0.5A or half an amp slow blow (T 0.5A). (T=timed or time lag or slow blow)
I'm not sure if one of the wires is grey or green that leads to the fuse.
Make sure these correspond to what you have.
Try that 2 volt measurement again check it on pin 3 again than check it where the wire from the tubes goes onto the power board*. I marked it on the diagram.
It's where CR3,CR4 and C3 meet so any of those exposed wire "ends" will do
to touch the probe on.

I presume you are measuring while the switch is on the high power setting.
Do the same again on the low power setting- measuring at pin 3 and at * (above).
While it is measuring 2 volts DC , set your meter to measure high AC volts and you can check the AC voltages coming from the transformer where they join to the power board G , BK & R. Between G & R will be the max ac voltage (400 and something)and between BK and G and also BK and R should be approx half of the reading of G & R.

My guess is when you have 2 volts DC on Pin 3, the transformer will still be putting out its required high AC voltage.


Now remove the tubes and do all 4 DC measurements again.
Hi pwr 1.Pin3 tube
2.terminal on pwr board (to tube BD)
LoPwr 1.Pin3 tube
2.terminal on pwr board (to tube BD)

Check the AC again if you want to.

Normally here I would suggest new tubes but.....!!!!! if there was a serious short shorting out 523 volts , I would expect the fuse to go.

Seems like an intermittent connection or one of the bridge rectifier diodes intermittent CR1,2,3&4.
Looking at the simplified diagram apart from the transformers and tubes there are only about $20 worth of bits that could go wrong.

Must confess starting to run out of ideas.

[marley]

I have not finished with all testing yet but I did re-check the high voltage on pin 3. This time it reached 34 volts before it stopped and dropped back to 2 volts. This was with the standby switch in the high power position. I didn't know if this would help point to something specific but I did want to check and make sure I had measured the right pin correctly. Thanks, Brian

[Jazz P Bass]

Have we ruled out a faulty standby switch?
Jumper it.

[marley]

Not to sound like a complete idiot, but how do I jumper it. I have blue, red and black wires (in that order from top to bottom). What 2 do I use to jumper? Thanks for your input, Brian

[oc disorder]

In electronic test equipment, a jumper cable is used to make a temporary contact between two points for the purpose of testing a circuit.

[marley]

Ok, I jumped the standby switch both ways, high power and lower power and I am still having the same problems.

When I measure out of the power board into the tube socket, I get the same rise and fall of voltage(anywhere from 2 up to 34 volts) as I was measuring at pin 3.( I have not checked the ac power in yet)

I did measure pin 5 again and noticed only 1 tube was receiving 15 volts, the other was 5.