Peavey T-Max Head output cuts out

[Bjorn218]

My bass player owns a Peavey T-Max bass head that has stopped properly working. When playing at rehearsal volumes the light above the power switch will flash, and as it does this the output will sound as if it is clipping, then stop full output and only sound like it is barely at bedroom volume.

I do all the tech work for my own guitar amplifiers, but I really do not know how to start troubleshooting a solid state output amp.

He believes that the problem lies with the new preamp tube we installed in the amp a few weeks ago, but this is happening to the power section and affects the solid state preamp channel as well.

Would any of you please help walk me through troubleshooting for a possible DIY repair? I do not yet own a scope so am limited to using a DMM at this time only.

[soundguruman]

Seems like it may be a bad connection on the incoming power connector.
Re-solder the power connector, and test again.

[Bjorn218]

Thanks for the rapid response. I am at work now and will not be at my work bench until after 6pm CST. I will do as prescribed then report back at that time.

[Bjorn218]

Seems like it may be a bad connection on the incoming power connector.
Re-solder the power connector, and test again.

Where the power comes in to the amp, The neutral terminates at a Spade and is connected to an additional wire that moves on into the amp(I think this is a manufacturing design so as to not have power cord wires snaking through the amp itself). No loose connections on neutral.

The hot wire from the power cable terminates at a spade and is connected to a reset switch. The other leg of the reset switch connected further in on the amp(same with neutral). Which is a small daughter board. No loose connections, but the reset switch will not reset.

It also looks like the reset switch was siliconed to the chassis, but has since pulled away from the chassis. Could the reset switch be bad, and would connecting the two Hots to bypass the reset switch to test if amp output still fails a good idea?

There are no visual signs of component failure on any of the amp's multiple boards. All fuses are good have positive continuity.

[Enzo]

The circuit breaker will only reset if it is tripped.

[Bjorn218]

Since the button will not reset I take it the breaker is bad and needs replacing then.

[soundguruman]

Where the power comes in to the amp, The neutral terminates at a Spade and is connected to an additional wire that moves on into the amp(I think this is a manufacturing design so as to not have power cord wires snaking through the amp itself). No loose connections on neutral.

The hot wire from the power cable terminates at a spade and is connected to a reset switch. The other leg of the reset switch connected further in on the amp(same with neutral). Which is a small daughter board. No loose connections, but the reset switch will not reset.

It also looks like the reset switch was siliconed to the chassis, but has since pulled away from the chassis. Could the reset switch be bad, and would connecting the two Hots to bypass the reset switch to test if amp output still fails a good idea?

There are no visual signs of component failure on any of the amp's multiple boards. All fuses are good have positive continuity.

wiggle the wires around with a wooden chopstick
or tap on the boards and connections, one small area at a time, with a wooden chopstick.
it still sounds like a bad connection somewhere

I think you said the power light was flickering.
often taping the board or connections, or components can help find an intermittent connection
look for the power light flickering off and on, when you tap a certain area or wiggle a certain connection.

try to narrow it down to a specific small area

But use an insulated tool to do your tapping and don't shock yourself.

Often it can be a bad connection from what you are saying. It is not unusual for an amp like that to have a cracked solder joint...

Often you will have to take the circuit board out to re-solder a bad connection...

[Enzo]

Take the wires off the breaker and check it for continuity with an ohm meter, if it has continuity then it is working.

My earlier suggestion meant the thing might not be tripped. In its set condition, the little button is there springy, but there is no resetting to do if it is not tripped. Have you determined it is tripped or are you assuming it has because the amp won;t work?

[Bjorn218]

@ Soundguruman - When the amp is turned on, and any signal was inputted into the amp, it would go into "protection mode"? The green/red DDT compression light would turn red and amp output would go from normal volume level whatever that might have been to something like you could hear a signal, but knew it was wrong. I likened it to how you can hear a guitar signal through a tube amp transformer when volume is turned all the way down, but without the buzzing of the transformer. This amp is pretty wild for me. I thought my Mesa Dual Rectifier had a lot going on in its circuit. There are 2 preamp boards back to back, the poweramp board across the bottom of the chassis, a board on the back of the amp containing the circuitry for the footswitch, bi-amping, line out etc.., and the mains board. Since you mentioned a bad power input connection I pulled the neutral block under the power cable and resoldered those spade connections, removed the mains board and re soldered all spade connections and the four capacitors and power resistor on that. I did not fire the amp back up afterwards do to what I will now explain.

@ Enzo - I understand what you are saying about the circuit breaker now Sir. I am getting continuity across the circuit breaker, and yes the button sticks out a bit and is springy. The button sticking out almost a 1/4" is what was messing with my head. Here is the rub on this circuit breaker though. One of the spade connections on the bottom of the switch is discolored close the the plastic body as if it was hit by some fair amount of heat. it has that rainbow of blue, green, and purple that metal gets once its heated. The same leg is pushed up against the body and is twisted somewhat at an angle. The other leg shows no signs of heat damage. It is pushed against the opposite side of the plastic body, but is not twisted. In other words, my gut tells me this circuit breaker, while showing on a meter that it is good, when the amp is on, and current is running through it, gets hot and send the amp into protection mode. If I remember correctly that the DDT compression is to keep the poweramp from clipping, and isn't pleasant to hear.

I was also informed by my drummer that once at rehearsal, we had put a new preamp tube in this bass amp, and when we powered it up, the power strip the amp was plugged into failed with a nice spark(the power strip, not the amp). We then plugged the amp into another wall recepticle and it ran fine until a few days ago. I had forgotten this, and since the amp was working fine for a couple of months since that event I did not put two and two together. Enzo, I wanted to ask if there is a way to isolate the preamp from the poweramp on a T-max? Can I remove the molex connections from the preamp and plug something into the effects loop return to see if the problem is in fact on the back end of the amp or if it is the preamp itself?

The last thing for right now is the schematic that was sent to me from Peavey did not include any information regarding the poweramp board. I have all the schematics, layouts, part lists for all other sections of the amp. Do either of you have this information? The tech support sent me two pdf files for a total of 17 pages. The first one is blank on pages 3 and 4.

Once again thank you both for the help and I await your responses.

B

[Enzo]

The breaker is in the primary circuit, the mains power. It is all or nothing. If the breaker is open, then the amp remains inert. if the amp powers up at all, even just a light or a fan, then the breaker is conducting. The breaker may well be damaged or need replacing, but it conducts or it does not regardless. You can shsort across the breaker during testing as long as you protect things externally to the amp in some fashion.

You don't need to disconnect the preamp. You might be able to disconnect, but the FX loop stuff is part of the preamp board.

But to isolate the problem, just plug a signal into the power amp in jack. That disconnects the path from the preamp right there.

First, just patch from preamp out to power amp in with a cord, any help? And do the same at the FX loop. Either of those sets of jacks can kill signal if they get dirty.

And not only can you inject signal at the power amp in, you can also input at the regular input and then take a feed from the preamp out or the FX send to some other amp for a listen.

If you are missing files, get back with customer service at the factory, they are there to help. Meanwhile I have the two files that add up to 17 pages. In the file that starts with the preamp on page 1, I see the power amp on page 6 with the board layout on page 8. The preamp tube board is on page 7 between them.

Here:

[Bjorn218]

I could not locally source the circuit breaker and ordered one from Peavey. I will install the new one once it arrives.

Since I had the amp opened up, I pulled the power board to make sure there wasn't any foreign material under the board causing problems.
Put it back together including the circuit breaker I don't feel comfortable with because like you said Enzo, it either works or it doesnt. Since it did in fact work. I was able to power up the amp.

Let it idle for about 15 minutes, then kept the amp at modest volume level, chopstick tapped everything. There wasn't any noises from this.

I then started with the Solid State channel and ran it slowly up to max level. No noises.

Set the master at 3. and repeated the SS preamp volume check. No clipping, no going in to protection mode.

Repeated a third time with guitar plugged in. No noises including both preamp volume and master volume maxed. Stupid loud and would only go into DDT compression when I slapped the string with my thumb, and only until the initial attack faded. Ok working like it should..

Tone knobs swept through their min to max with no problems. Graphic in and out same.

Next, tube pre side. Followed same procedure. The only time the DDT would come on and stay was when pre and post volume was past 3/4 way up and master at around 3:00. But the output would not cut out like before. It was loud, but not unpleasant at all. From my novice perspective, the amp appears to be working properly.

I have it sitting idling right now and will leave it idling for about 45 minutes to see if the amp starts its problem again once its nice and warm.

I cannot point to any one thing that may have caused the original problem except faulty power strip, possible damage to circuit breaker that would cause dirty signal once warm, unknown quality or too hot a rated preamp tube, bad solder joint or bad connection, or any combination of the above.

Thank you guys for the help.

Enzo, I did get back with Peavey about the missing pages. He actually did not have them on their server and had to get them from Engineering.

One thing I did notice in the schematics is any reference to the output jacks, except the spade connections is not there. All other outs are shown.

[Enzo]

Did you open the file I posted just above? It has all the pages, does it have what yours is missing?

The power amp schematic has the two terminals for the speaker out. They simply connect to the jacks.

Count your blessings, your AC mains board is drawn with connections. The same board on a lot of the power amps is left unexplained.

[Bjorn218]

Did you open the file I posted just above? It has all the pages, does it have what yours is missing?

The power amp schematic has the two terminals for the speaker out. They simply connect to the jacks.

Count your blessings, your AC mains board is drawn with connections. The same board on a lot of the power amps is left unexplained.

Yes, the file you posted was exactly what I needed. I actually got the missing pages from Peavey around the same time I was looking at what you had posted.

I saw that about the speaker outs. I'm a bit perplexed on how the output jacks work. Well take that back. One is wired normally, the lower one is only connected at the sleeve. They say that is wired in parallel, I guess thats what its called. Not sure how a jack with only one wire connected to it drives another cabinet, unless the other connection is internal to the cabinet.

Yes I have seen some other schematics for other bass amps. I fully understand about a schematic not being full and the tech having to assume a lot of things of the circuit.

[lowell]

Hey all. Have one here I'm working' on. I don't see the preamp out power amp in jacks in that PDF Enzo. Am I missing 'em or is there a page missing?