Monoprice 15W on the fritz?

[TwoNotesSolo]

I have been super happy with my Monoprice 15W (the original, with the two jack inputs, but with the good reverb tank). I have not taken my Mustang III out of the basement since I have the Monoprice.
The last few times I ran it I had issues where it would drop to very low volume for no apparent reason. at first I thought it was cable problems, but on last week it happened while I was using a wireless system. It was audible but low volume then suddenly started being loud again (it has happened with several guitars and both with cables and wireless so that I really do think it's the amp)

I truly haven't had time to troubleshoot it, especially since it's intermittent, but I was wondering if anyone has experienced that and how to go about troubleshooting it.
- If I can get it to sound low again I should wiggle the cable?
- I should plug into the effect loop to see if it makes a difference
- could it be the tubes? should I remove and reseat them?
- could it be the tubes are bad? I'd hate to spend a bundle on tubes and find out it doesn't help. I can get a "value package" with all 5 tubes here: https://www.thetubestore.com/monoprice-15w-112-guitar-combo-tube-amp-value-package, should I get it so I can swap tubes out? (and do I need matched and balanced tubes for the pre-amp?)
-???

I love that amp and the price was right, I don't want to take my Nace out, and as much as I talk about the Quilter Mach 2, I really don't want to buy another amp, so if I don't fix it, it's back to the Mustang (which I still love in fender emulation mode, but it's a pain to constantly plug and unplug from some of the permanent gear in my music room)

 

[CapnDenny1]

It could be a lot of things.

Don't buy tubes yet. Don't buy JJ tubes ever. (Just kidding, some like them, I don't.) The next set up will sound much better. But prove they are bad first, unless you just want to upgrade the tubes.

You can try wiggling things. Don't touch inside the amp.

I would make sure your volume pot is ok. Are there any spots where it drops out?

You can try plugging a patch cable in the effects loop. Sometimes the return input which has a switch in it to bypass the jack when nothing is plugged in, will be corroded.

If it is a tube or tubes, it's not likely they are all bad. If it's quiet and you plug your guitar directly into the effects loop return, does it get loud again.

Does the volume control or tone or reverb controls work when it is lower volume?

Does it only do it after a while, after it has time to heat up?

Which amp is it? If I can find a schematic for it, I may be able to help more.

 

[CaptainMoto]

You go @CapnDenny1 !

 

[Slofinger]

I had a Carvin Vintage 16 that was doing the same thing. It so happens I live near Demeter Amps who is known for his amp work. It took two trips, but turned to be a capacitor or something to do with one of the tubes. I will try to contact him tomorrow and see if he remembers my amp and what the problem was. It cost me $100 to be fixed and has not done it again for 2 or 3 years.

 

[TwoNotesSolo]

Thanks guys. The thing is, the volume change doesn't seem realted to manipulating anything like the volume pot. When I was on wireless I wasn't even close to the amp when is suddenly started playing a full volume

I'm not home till the end of the week when I will have more time to play with it, but I won't buy tubes yet.

Supposedly this is the schematic for it:



I have a few electronic skills, a multimeter and an oscilloscope if it helps debugging, but I don't really understand tube circuitry.

 

[Danno]

I had a similar problem with an old Blues Deluxe and it turned out to be the power resistors. They didn't have adequate ventilation and when they got hot their resistance would go way up.

 

[PapaRaptor]

I had a similar problem with an old Blues Deluxe and it turned out to be the power resistors. They didn't have adequate ventilation and when they got hot their resistance would go way up.

That seems to be a common issue on the Blues Deluxe, Hot Rod Deluxe and the DeVille. If the resistors aren't replaced and the leads left long enough to keep them away from the PCB, they will eventually burn the PCB. I had a hell of a mess fixing one of mine. It looks like the Monoprice has an almost identical setup powering the ICs for the effects loop and the reverb amp and buffer. The Monoprice resistors (R34 and R46) are 470Ω 3 watt, while the ones in the HRD (R78 and R79) are 470Ω 5 watt.

 

[Danno]

That seems to be a common issue on the Blues Deluxe, Hot Rod Deluxe and the DeVille. If the resistors aren't replaced and the leads left long enough to keep them away from the PCB, they will eventually burn the PCB. I had a hell of a mess fixing one of mine. It looks like the Monoprice has an almost identical setup powering the ICs for the effects loop and the reverb amp and buffer. The Monoprice resistors (R34 and R46) are 470Ω 3 watt, while the ones in the HRD (R78 and R79 are 470Ω 5 watt.

Yep, I'd take a look at those resistors and the board around them. If that's the problem the board is probably showing heat damage. And if that's the problem they'd almost certainly need to be replaced so that they are up off the board and air can get all the way around them.

 

[TwoNotesSolo]

I spent $119 for that amp. but I could spend $350 for the "ultimate" tube replacement set
I'll take it apart this weekend and see what I can see....
I'll watch a few videos on tube safety as a refresher first

 

[OG_Blues]

I wasn't even close to the amp when is suddenly started playing a full volume

When I first read this, I thought it sounded like a "feature" rather than a problem.
Good luck finding the problem. Good advice all around here.

 

[D. R. Miller]

Have you checked the output reduction push button switch (15/5 watt). I have one as well and love it but they are definitely not built with the best of parts. But right now mine is working and my Fender Blues Jr is not so not sure having the Fender name is a plus. I currently playing with the 5 watt monoprice amp connected to a Peavey 12 inch cab with an Eminence Lil Texas speaker installed and absolutely loving the sound I'm getting and it's still too loud for some and I don't even have the volume max'd.

 

[TwoNotesSolo]

Mine is the original with two jacks instead of a selection button and the controls on top

It's been frustrating, I can't get it to screw up at home. I did manage to get it to quit altogether if I pull sideways on the jack, so I may have to replace it, but that's not 1/2 volume.