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Messages - aruption

Amplifier Discussion / Re: Rickenbacker TR25 Noise
March 31, 2020, 06:40:40 PM
I ended up tracing the noise to the transistors near the inputs, replaced them with new ones, and the squealing noises seem to be gone. i might end up replacing the rest of the transistors anyway just to keep everything fresh. As far as the distortion from the speaker, I took the speaker out and manipulated the cone a bit until the distortion went away. Seems to be holding up, but I think I will look into getting a new speaker anyway.
Amplifier Discussion / Re: Rickenbacker TR25 Noise
February 09, 2020, 04:36:48 PM
Any thoughts? Bad transistor?
Amplifier Discussion / Re: Rickenbacker TR25 Noise
January 21, 2020, 06:00:20 PM
So the original slight distortion definitely is the speaker. I hooked the amp to a different speaker and that distortion did not come through the new speaker, but I could still hear the unhooked original speaker making that noise!

The squealing is there no matter what speaker the amp is going through, whether or not any instruments are plugged in, and even when a speaker is plugged into the external speaker jack.
Cleaned the boards and all of the connectors in the amp, to no avail.

This is a recording of the noise:
Amplifier Discussion / Re: Rickenbacker TR25 Noise
January 21, 2020, 10:49:08 AM
I'll pop it open later and try some cleaning.

The squealing responds to the tremolo controls, just like it does to the volume control.
Amplifier Discussion / Rickenbacker TR25 Noise
January 21, 2020, 09:14:11 AM
Bought an old Rickenbacker TR25 guitar amp about a year ago. After a few months I noticed that there was a quiet distortion audible in addition to the clean signal. More noticeable at lower volumes. Someone suggested replacing the power supply caps, which I did, but that little bit of distortion didn't go away. Not worse, but not better either.

Left the amp alone for a few months, when I came back to it, it now makes an awful squealing and static noise. The noise increases as the volume is turned up and is silent when the volume is at zero. I haven't opened it up yet to check where I worked on it before, but I figured that should be the first thing I check. If not, any ideas what else it could be?
Amplifier Discussion / Re: Ampeg SVT450
June 27, 2019, 08:09:46 AM
Finally got around to replacing the op amps after dealing with a divorce lol. Unfortunately this did not fix anything (with the amp). What's the next thing I should take a look at?
Amplifier Discussion / Re: Ampeg SVT450
February 23, 2019, 08:19:07 AM
Bad op amps then?
Amplifier Discussion / Re: Ampeg SVT450
February 21, 2019, 05:43:30 PM
Both 16v rails are right in the 15-16v range.

If I am reading the pinouts correct, then on IC1 there's -14v on output 2/pin 7 (0v on the other three outputs), on IC2 there's -14v on output 1/pin1 (0v on the other three outputs), and no voltage on the output pins of the IC3 and IC4.

At the test point near D4, I am reading 14v on one side of D4 and 0v on the other side. Is that correct? Because it looks like the side that is reading 14v is the side that the test point should read 0v, because that is the side that connects to R13.
Amplifier Discussion / Re: Ampeg SVT450
February 19, 2019, 07:48:35 PM
Sorry, that was unclear. I was saying that the problem is definitely in the preamp, because I did the tests as you described, and had a clear undistorted signal from the power amp input. The rest was still super distorted. There's no issue with hum, just distortion of any signal being input.
Amplifier Discussion / Re: Ampeg SVT450
February 19, 2019, 11:48:58 AM
In that case, it's definitely the preamp. Changing the EQ settings or limiter/pad switches don't do anything to make the sound better or worse (although the pad will still reduce the overall volume). Engaging the other EQ section doesn't change anything either.
Amplifier Discussion / Re: Ampeg SVT450
February 16, 2019, 10:53:31 PM
Thanks for the schematic. My testing setup is limited to a DMM. Do you think the issue is likely to be in the preamp or power amp section?
Amplifier Discussion / Ampeg SVT450
February 14, 2019, 07:21:55 AM
My friend just picked up an Ampeg SVT450 that's definitely got some issues. Plugging either speaker output into any speaker, any notes played on any instrument are super distorted, and do not sustain. Using the line out post, and note played is distorted and sustains infinitely. Only the line out pre sounds normal, but that of course is because it's bypassing pretty much everything in the amp. What should we first take a look at under the hood?
I ended up taking it to a vintage keys specialist in NYC. Turns out there were three things wrong with it. First, I installed the e-caps backwards  :-X Fortunately this did not kill anything else! Second, the bridge rectifier symbol on the original schematic was backwards. Third, it needs a working vibrato lightbulb to produce any sound, even with the vibrato off. So it works, aside from the usual hum and hiss. The main problem now is what sound like a loose connection in the cheekblock where the mains switch is. Playing the highest octave of the keyboard will set off these noises. So that's what I'll be looking into next!
When he finally got around to it, he found that the bridge rectifier was indeed dead, and replacing it got what he though are appropriate voltages to the entire board...except for the pickup, which still has 0 volts. He is pretty sure that the oscillator circuit is working properly, but is stumped about why there's no voltage on the pickup (and this guy's a savant with amp electronics). Any ideas about what I should look at next? I'm probably gonna go pick it up from him later this week.
Hi there!

I actually just learned that from another forum, and it makes sense now about the continuity that I'm seeing. Thanks! I brought it over to a guy who's much more knowledgeable that me. He hasn't had a chance to actually fix it yet, but he is pretty sure that the bridge rectifier is dead, and that the entire preamp is not getting the 12V it needs from the transformer. He's out of town for a couple weeks, but when he figures it out, I will report his findings back here!