Hey all,
Trying to repair my Musician 400 series head. Some background, I bought the amp used, gigged it for about 8 months until after a long practice, 2 or 3 hours the night before a gig, the amp started smelling really strongly of the burnt resistor smell. I thought/hoped that it was just too hot and turned it off. When I turned it back on the next day everything seemed okay, until the show when it totally crapped out started sounding all crunchy and weak.
Flash forward a few months later I actually get the time to open the sucker up I can see a few fried caps in the power amp section and some burn marks on the old cement resistors. I change out the some caps and things sound pretty good, there is still a bit of hum on the effects section of the pre amp, but the normal section sounds great, all the output levels look good, no DC going to the speaker or anything.
I turn it up and start pushing it hard and it still sounds good, but the light bulb limiter I have the amp plugged into starts to light up. The louder I play the brighter the light bulb gets, but when the amp is just on, or I am playing with the volume at 50% or less it's fine. What could be going on? is this normal? This is my first repair and I can't find anything about something like this on the web.
The lamp is doing it's job just as it should and the amp may well be fine as you *May have* fixed the issue.
As there is no DC at speaker then I'd bypass the lamp but keep an eye out for overheating parts as you don't really know what caused the fault and it may just return again.
Meantime check the bias, if it's a small trim pot it may have drifted over time causing things to run hot.
Phil.
Idle current is the state the amp is at when you test it using the light bulb limiter. When the amp is at idle the light bulb should be either dim or not lit up at all. As you increase the volume and push the amp it will draw more current so the light bulb lights up. So as Phatt said the lamp is doing it's job.
That makes sense, just wanted to make sure.
Some more questions for you fine folks, if you're kind enough to indulge me.
Since there is a hum coming from the preamp section I figured might as well change out those caps too while I have the head open. Took a look and they seem to be a different value from what the schematic shows. Schematic shows 100uf/25v caps, where the amp has 150uf caps. I'm thinking maybe these caps have been replaced before and someone used different caps. Which leads me to wonder, could that be the cause of the problem? Or is it a sign of a preexisting issue, something is causing the caps to go bad. Do you know if these ever came out of the factory with different caps, like would peavey just use what was lying around if the value was close?
The caps may have been replaced with better ones. 100uF @ 25V sound like smoothing caps on the preamp section, the Voltage is no where near high enough for the power amp sections.
Can you post the schematic and I can tell you for sure.
Quotethere is still a bit of hum on the effects section of the pre amp
How have you determined this ?
Is it a low frequency hum ?
If it is low frequency and it only happens when you increase the volume, then it is more than likely mains hum.
Any amp with a PSU that is inadequately smoothed will have hum. There is also the fact that poor earthing can also be a cause, so make sure that you have not, inadvertently, pulled off any wires.
As an aside the resistors you are calling cement are ceramic heat dissipating resistors, with a wattage rating that can go from 1W up to 100W or more. Browning of the ceramic is quite normal as these are used in the output section of the power amp.
Hi LateDev,
I figured the problem is with the preamp side, specifically with the effects input (this amp has a normal input and an effects input) because when I have just the power section of the amp plugged into a speaker, disconnected from the preamp section there is no hum. Also, the hum goes up and down when if I turn the volume/gain knob on the effects side. If I turn it all the way down, and play through the normal side of the amp there is no hum. The other effects knobs effect the hum too, like if I turn up the depth and rate, the hum oscillates. The equalizer in the preamp also affects the hum.
Here is the schematic and some photos, I already changed out the 50/50 caps from the power section, they were visible damaged. One of the big 6000uf caps attached to the transformer looked like it had a little bulging, but they both tested good.
No, having 150uf caps in place of 100uf caps will not cause hum.
Since the other preamp channel works without hum, the issue is not power supply ripple or filter caps. The first suspect would be the input jacks to that channel, specifically the grounding of them.
As you have stated that turning the effect volume up increases the hum I really need to see a good cct diagram of that section in particular. However I do believe that the level is at the beginning of the effects section so it shows that the hum is coming in from the pre-amp section.
I can't see that circuit too well, but if there is a jack input to the effects side then as Enzo has stated, this could be a cause. It could be as simple as Oxidisation of the contacts, which cleaning will sort.
Peavy put all their controls on a front panel which can lead to all sorts of noise problems. You need to make sure that all the leads going from the front panel to the base board are properly connected, paying particular attention to any leads that look like they are local to the effects section.
If you can get a higher resolution circuit diagram, or give a link to one, that would be great.
The preamp board is on the front panel, the controls are mounted on it, they suspend th board off the panel.
When I get to the shop I can post a clean pdf of this amp.
Sorry about that schematic I don't know why it loaded up like that. I am not at home right now but if Enzo doesn't beat me to it I'll get up a better version.
I'll check the inputs, I hadn't thought of that.
Quote from: LateDev on June 30, 2015, 06:01:54 AM
You need to make sure that all the leads going from the front panel to the base board are properly connected, paying particular attention to any leads that look like they are local to the effects section.
What is the base board?
Here is the right schematic.
Its OK I was thinking of another Peavey with a base board, ie the main board screwed to the base of the head unit.:)
Anyhow, back to the amp, as the noise goes when you reduce the gain, then you can rule out the intermediate stage and after that, which includes the EQ.
Arn't cct diagrams wonderful, when drawn by idiots.
There are a number of decoupling capacitors on the rails which could effect hum, however as has been said before, the jacks can be the main culprit.
At this point and after you have checked the jack sockets, you could do the following.
A simple check is to get a 100nF to 1uf cap and attach one end to earth, then use the other end to probe the signal path. This decouples the signal to ground and should helps you pinpoint where the fault is, without having to desolder any components. Don't do this on Valve amps ;)
If you look at the circuit you can see that the signal goes through a small filter gain stage before heading into the effects stage via a 10K resistor. You could just remove one end of the 10K or decouple the signal.
If the hum goes, when the volume is left up then you have ruled out the effects side and shown that the problem lies within the first section of 3 transistors.
The input jacks are grounded solidly to the chassis. The decoupling caps are the 100/50 rated caps in the schematic right? I was going to change those and the 25/25 caps. Hopefully that will work, because I don't really know how to "probe a signal path yet xP.
Decoupling caps are the 100/25v ones on the power supply lines, I don't see any 100/50v on there myself. The 25/25 are not likely causing hum, whether they need replacement or not.
I doubt they are the problem at the moment, but in these ancient amps, the caps I worry about most are the little 2uf/35v electrolytics. Those are often dried out in amps of this era. The schematic is peppered with voltages through the preamp, you might compare yours to the ones on the print.
Any control that affects the hum is AFTER the source of the hum. So whatever stage is offending, it is before the EQ. Since you report the gain controls has an effect, that tends to localize the problem to the first stage - the three transistors just to the right of the input jacks. So do look at the voltages.
Quote from: Enzo on June 30, 2015, 08:49:43 PM
Any control that affects the hum is AFTER the source of the hum. So whatever stage is offending, it is before the EQ. Since you report the gain controls has an effect, that tends to localize the problem to the first stage - the three transistors just to the right of the input jacks. So do look at the voltages.
I do agree however if you look at the fuzz isolation and clipper circuit, there is a path through that section which has not been ruled out, as it is all before the gain control.
Fair enough, the distortion circuit is a sort of feedback around the first stage. It would appear that if we turn the FUZZ and DISTORTION controls to zero, it would ground the paths for each, and leave mostly just the first stage. But no reason not to check them. In any case it would seem narrowed down to this area.
If we can verify the hum is 60Hz rather than 120Hz, it would also help. 120Hz is power supply ripple, and I tend to doubt that given that the other channel and effects channel post gain seem unaffected. I think this board is well enough designed that power supply return ripple currents are not shared by this stage's ground.
I agree and did not see the fuzz control grounding out, so just turning that full anti cw, should be able to rule out if it is the effects side.
Fuzz control "0" gain up = hum then it is from the 3 transistors or jacks.
Fuzz control fully CW and hum its the fx circuit.
switched out the caps and everything appears fine. The amp made it through one practice. Thanks for all your help!