Hey there! I bought this Peavey Express 112 to leave it at my music school as a students amplifier, it cost me 35eu, so nice deal there, the clean seems to sound mostly good. I cleaned all the pots and reverb connectors and now reverb sounds good, also the seller told me the amplifier wouldn't switch to drive channel, and yes it doesn't, with the tester the switch seems fine. I found that there is a resistor burnt, R101 is completely fried, so i removed it but I don't have a replacement yet, I'm going to install a 2W resistor instead of 1W.
I'm not sure if the noise is going to go down once i fix this resistor, but it seem to have quite a bit of humm, I didn't expect it, is not too bad or too loud, but if i have to work with it for 5h straight at student volume can be annoying, since its a transistor amp and also had other peaveys didn't expect that standby hum, doesn't seem to change when you push the clean channel, it stays the same, at 0 or at 10.
schematich:
https://elektrotanya.com/peavey_express ... nload.html
So maybe it might be overdoing it but i was thinking to replace at least the filtering caps, that would be: C70, C56, C67, C61, i measured the 2200uF, they are on the lower side at around 1900uF and 2000uF, maybe I can install 4700uF (could that help with humming?) Since those caps are from the 90s, maybe they work just fine but maybe fresh caps help with filtering, I'm not sure, my idea is to spend a max of 15eu in components if necessary.
Then my other idea would be to try and modify the powersupply filtering i attach an image, maybe its a dumb idea, if anybody has as better idea, maybe I should replace the bad components first and see how it works then. So my idea to keep it simple was to add a 1mH inductor and add another stage of filtering, maybe a 150k Rsistor in paralel to those 2200uF capacitors to ground?
any help will be appreciated!
Thank you!
Peavey Express 112 Hum Noise
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Manfred
- Tube Twister
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The circuit diagram shows two different ground symbols, one is the power supply ground and the other is the housing ground.
This has been done to prevent ground loops.
Check whether you have a good connection to the housing at the point marked by me.
This has been done to prevent ground loops.
Check whether you have a good connection to the housing at the point marked by me.
I wouldn't do anything until you fit R101. Since it is burnt it is passing a lot of current and looks like a filter with C73 for noise on the output. It might fix the noise problem but it might have been overloaded due to some other component failure.
I checked it and its screwed to the heatsink of power section transistors and the heatsink is screwed to the chasis, so yeah it should be 100% grounded to the chasi.Manfred wrote: ↑29 Aug 2024, 21:12 The circuit diagram shows two different ground symbols, one is the power supply ground and the other is the housing ground.
This has been done to prevent ground loops.
Check whether you have a good connection to the housing at the point marked by me.
Housing Connection.jpg
On the other hand i found that D25 between R93 and R91 had a cold soldering, the leg was moving through the hole, so maybe it made contact but wasn't properly soldered.
About buying or not buying stuff, im asking what are the best options because i don't have any electronics store nearby and I'll have to order online anyway, so id rather upgrade a few things as an excuse. Would it be worth to put 4700uF capacitors instead of 2200uF? And swap the 5W resistors by new ones, maybe even 8W resistors.
Thank you
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phatt
- Transistor Tuner
Better minds here will know more but if CR25 went open circuit due to cold solder joint then the output would probably be frying and likely the reason R101 got fried.
A word of advice; Don't replace parts willyNilly until you establish what is wrong.
CR25 is part of the bias string with C23,C24 for the output TR's. the Hum will likely be a faulty bias causing a DC offset on the speaker out.
First re-solder CR25 and then set your DMM to DC and check how much DC voltage is on the speaker output. It should be very close to Zero, i.e. a few mV. Don't connect a speaker until you check.
Also note Normally CR25 should be in contact with the heat sink as it helps the diodes to track the bias as the output heats up.
And it is shown on the layout as being under the heat sink.
you could be lucky,,It maybe all that is wrong.
Phil.
A word of advice; Don't replace parts willyNilly until you establish what is wrong.
CR25 is part of the bias string with C23,C24 for the output TR's. the Hum will likely be a faulty bias causing a DC offset on the speaker out.
First re-solder CR25 and then set your DMM to DC and check how much DC voltage is on the speaker output. It should be very close to Zero, i.e. a few mV. Don't connect a speaker until you check.
Also note Normally CR25 should be in contact with the heat sink as it helps the diodes to track the bias as the output heats up.
And it is shown on the layout as being under the heat sink.
you could be lucky,,It maybe all that is wrong.
Phil.
Okay guys! I changed the damaged resistors and it looks like it solved the problem of the main function of the amplifier, clean works, distortion works, I'm surprised of how usable it is on the dirt channel lol and the speaker actually sounds pretty decent too, what a bargain!
The next thing i'd like to improve, IDK if its a design issue or the amp itself is like that, but there's a lot of hiss, and when the reverb is at 0 there's the humm, it sort of goes away and turns into more hiss, when you start turning up the reverb... I would say at rehearsal levels it wouldn't bother too much, but as I say my intention is to use this amp at music school as a room amplifier, so hiss will be heard for 6h straight… so reducing it is good if possible hahaha
here goes a couple of videos to show how loud the noise is. The hiss and hum is the same for both channels, all volumes and pots at 0, doesn't change at all.
noise
https://photos.app.goo.gl/GZJ4r9h29f3jM8c88
sound demo
https://photos.app.goo.gl/Ysz57zCCj4P6W4r19
(at the other school I use a Peavey Studio Pro 112 and its silent)
PS: I read that J174 Q1 is the mute transistor that silence Distortion channel when clean is on, I'll try to check that out, it could drop the hiss at least on one channel, but it still a bit on the loud side anyways. There must be something else
PS2: Also note when I try to press the channel button it makes this sort of noises, but that coul be because there are no screws holding anything, so not to worried about that.
The next thing i'd like to improve, IDK if its a design issue or the amp itself is like that, but there's a lot of hiss, and when the reverb is at 0 there's the humm, it sort of goes away and turns into more hiss, when you start turning up the reverb... I would say at rehearsal levels it wouldn't bother too much, but as I say my intention is to use this amp at music school as a room amplifier, so hiss will be heard for 6h straight… so reducing it is good if possible hahaha
here goes a couple of videos to show how loud the noise is. The hiss and hum is the same for both channels, all volumes and pots at 0, doesn't change at all.
noise
https://photos.app.goo.gl/GZJ4r9h29f3jM8c88
sound demo
https://photos.app.goo.gl/Ysz57zCCj4P6W4r19
(at the other school I use a Peavey Studio Pro 112 and its silent)
PS: I read that J174 Q1 is the mute transistor that silence Distortion channel when clean is on, I'll try to check that out, it could drop the hiss at least on one channel, but it still a bit on the loud side anyways. There must be something else
PS2: Also note when I try to press the channel button it makes this sort of noises, but that coul be because there are no screws holding anything, so not to worried about that.
Check the temperature of the output filter resistor you changed because any high frequency noise it was designed to knock out is when everything else is working properly. The fact you have hiss or some noise is after the resistor network has already tried to cut it out and failed. The noise source needs to be found to prevent heating the resistor and wasting power on a signal that should not exist. As you know without the resistor it was bad but it shouldn't be and is the reason the resistor failed in the first place. The output resistor filter is suppose to stop resonances in the amplifier when signals are amplified not kill noise when no signal is present. You have not fixed the original problem yet I think.
How can I narrow it down? I just have a tester, not an oscilloscope, trouble shooting when is not burnt is the hard part for meRuptorGB wrote: ↑07 Sep 2024, 13:43Check the temperature of the output filter resistor you changed because any high frequency noise it was designed to knock out is when everything else is working properly. The fact you have hiss or some noise is after the resistor network has already tried to cut it out and failed. The noise source needs to be found to prevent heating the resistor and wasting power on a signal that should not exist. As you know without the resistor it was bad but it shouldn't be and is the reason the resistor failed in the first place. The output resistor filter is suppose to stop resonances in the amplifier when signals are amplified not kill noise when no signal is present. You have not fixed the original problem yet I think.
(I've built many pedals and some amps but haven't fixed that many)Looking at the schematic i think the J174 can contribute to the noise factor, but im not 100% sure, it measures 44ohm from leg 1 to 3 (SGD) configuration, the data sheet says it should be 82ohm from source to drain, so it might not be dead but faulty?
I can't see the schematic but it says it is a service manual one so must have bias voltages you should check noting any that are off pointing to a problem.
J174? 1 to 3 (SGD)?
Perhaps you could touch a capacitor across different stages of the amp to ground discharging it between moves of course. It might find which point reduces the noise then you can isolate the area but it is difficult without instruments and feedback circuits with interaction.
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phatt
- Transistor Tuner
If R101 burns then a sure sign you have Hi Freq instability somewhere.
You noted that the hum increases when you turn up the reverb. Well I'd suggest you start by disconnecting the Reverb circuit, done by lifting one end of R75-22k. (U4A mixes the Reverb back with the Dry signal, I'm not sure of the purpose of J174 )
If that resolves the hum then likely the pickup section around U3B is oscillating way above audio freq.
As RuptorGB already mentioned you replaced R101 but the issue that burnt out R101 has not been fixed and R101 will likely fail again.
If no change then move onto mixer U4A or U4B has problems, U4B forms part of the Poweramp.
You can also lift R73 that will disconnect the whole preamp from the poweramp and if the hum is still there then sure bet that the issue is in U4A or the poweramp itself.
If the amp is running to spec then R101 would never get very hot, let alone burn. Ouch.
Phil.
You noted that the hum increases when you turn up the reverb. Well I'd suggest you start by disconnecting the Reverb circuit, done by lifting one end of R75-22k. (U4A mixes the Reverb back with the Dry signal, I'm not sure of the purpose of J174
) If that resolves the hum then likely the pickup section around U3B is oscillating way above audio freq.
As RuptorGB already mentioned you replaced R101 but the issue that burnt out R101 has not been fixed and R101 will likely fail again.
If no change then move onto mixer U4A or U4B has problems, U4B forms part of the Poweramp.
You can also lift R73 that will disconnect the whole preamp from the poweramp and if the hum is still there then sure bet that the issue is in U4A or the poweramp itself.
If the amp is running to spec then R101 would never get very hot, let alone burn. Ouch.
Phil.
I've been running the amp for about 15 min playing around and it looks fine, R101 is not getting hot at all at room volumes at least, also Poweramp TRs are at an ok hot temperature at fingertouch so that's also a good sign.
Connecting the guitar directly to the Return, reduces slightly the hiss, but not the hum, I'll try to isolate and narrow down it a little more disconnecting the reverb first, and then the preamp and let's see what difference bring each.
I'm starting to assume that maybe the noise floor on this amp is not as low as I expected? Could that be true? or maybe those old 5W resistors can get noisier over the years? This one was built at the 97.
I think what could have been the issue that blew that resistor on the first place was that diode CR25 that wasn't soldered properly, the leg was surrounded by Solder but it was just surrounding it, not making a good contact, and it was a little brownish on the leg, the diode looks good. So maybe over the years of use and abuse that small faky contact ended burning the resistor? I fixed that solder while also replacing R101, and it runs perfectly cold.
So... in case nothing is wrong, I'll still do the preamp and reverb test, but if it all works just as it should and the noise is at "what's supposed to be" would there be any way to reduce it to a degree?
Thank you guys! Your help is invaluable!
PS: on the schematic there's also that suggestion to reduce reverb feeback, I guess to make the reverb control less sensitive, that could reduce a little bit of the noise coming from the reverb circuit, so maybe that's also a good idea.
PS2: call me crazy, I was thinking to replace most sound signal resistors to metal film … but only after testing what's been proposed here!
Connecting the guitar directly to the Return, reduces slightly the hiss, but not the hum, I'll try to isolate and narrow down it a little more disconnecting the reverb first, and then the preamp and let's see what difference bring each.
I'm starting to assume that maybe the noise floor on this amp is not as low as I expected? Could that be true? or maybe those old 5W resistors can get noisier over the years? This one was built at the 97.
I think what could have been the issue that blew that resistor on the first place was that diode CR25 that wasn't soldered properly, the leg was surrounded by Solder but it was just surrounding it, not making a good contact, and it was a little brownish on the leg, the diode looks good. So maybe over the years of use and abuse that small faky contact ended burning the resistor? I fixed that solder while also replacing R101, and it runs perfectly cold.
So... in case nothing is wrong, I'll still do the preamp and reverb test, but if it all works just as it should and the noise is at "what's supposed to be" would there be any way to reduce it to a degree?
Thank you guys! Your help is invaluable!
PS: on the schematic there's also that suggestion to reduce reverb feeback, I guess to make the reverb control less sensitive, that could reduce a little bit of the noise coming from the reverb circuit, so maybe that's also a good idea.
PS2: call me crazy, I was thinking to replace most sound signal resistors to metal film … but only after testing what's been proposed here!
You need a comparison so you know what the amp should be like as it is only a cheap unit when new. If the hiss is irrelevant when playing then switch it off when not in use. Save electricity, increase lifetime & no annoyance. It is not clear if you have hiss or mains hum. Hum is a mains smoothing fault and hiss is a circuit fault, likely effects, that are complicated but is it normal for the amp? Is it really a fault or are you being pedantic?
Still a bargain. 
-
phatt
- Transistor Tuner
Reverb tanks are prone to pickup mains hum from power Tx.
Disconnect the RCA cord at the tank and short the terminals and see if the the hum goes away.
As for Hiss then you can increase the value of C53-100pF try 250pF or even up to 470pF, that will reduce the HiFreq that can pass. Most of these amps have way too much Bandwidth anyway. (most of the 112 combo peavey amps I've used are harsh/brittle due to wide bandwidth
) ED; To test, just tack on a larger value cap will Add to the 100pF, when you're happy make it permanent.
Phil.
Disconnect the RCA cord at the tank and short the terminals and see if the the hum goes away.
As for Hiss then you can increase the value of C53-100pF try 250pF or even up to 470pF, that will reduce the HiFreq that can pass. Most of these amps have way too much Bandwidth anyway. (most of the 112 combo peavey amps I've used are harsh/brittle due to wide bandwidth
) ED; To test, just tack on a larger value cap will Add to the 100pF, when you're happy make it permanent. Phil.