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May 23, 2024, 01:34:24 AM

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#1
The Newcomer's Forum / Re: 80's JMF Spectra 125 SS Co...
Last post by Tassieviking - May 22, 2024, 07:23:14 PM
Do you have access to another Amp?
If you do try connecting the send to the other amps return to see if the pre-amp is good, and then connect the other amps send into this amps return to see if the problem is in the power amp.
You could try the guitar into the return jack but it would be very low output, that would test the power amp by itself.
The best place to start checking might be the power rails, the +-15V and the +-37V.
Sometimes the main filter capacitors can dry out when not used for a long time.
#2
The Newcomer's Forum / Re: 80's JMF Spectra 125 SS Co...
Last post by DrGonz78 - May 22, 2024, 07:05:59 PM
Just a quick reply but perhaps study up on crossover distortion in a solid state amp. The symptoms you describe are similar. Play soft and it sounds fine. A hard attack signal will reveal the crossover distortion. It might be biased cold but that might not be what is going on. Just something to consider.
#3
Tubes and Hybrids / Re: Mesa Dual Rectifier
Last post by Miyagi_83 - May 22, 2024, 06:58:30 PM
Hey, Psabin8951,
I'm no expert, but a couple of things popped in my mind.
Which fuses does it blow - primary or secondary?
As I understand Mesa Dual Rectifiers, it should have a rectifier selector switch. Does the amp do that with solid-state rectifier selected?
Could it be a short on the primary side of the power transformer? 🤔
Like DrGonz78 said, a schematic and voltage readings would help.

You need to be super careful and you must know what you're doing if you ever decide to tinker with it yourself. As you probably know, there are lethal voltages inside that thing. If uncertain, take it to a tech.
#4
Amplifier Discussion / Re: Peavey Bandit 65 transform...
Last post by DrGonz78 - May 22, 2024, 06:58:07 PM
Quote from: mr.death on May 22, 2024, 04:11:09 PM
Quote from: DrGonz78 on May 19, 2024, 12:19:47 AMWhat method did you use to make the conclusion that the transformer is shorted?
I watched a dlab vid. He shows how to check the transformer. I did his test and it failed. Also when I turned the amp on I could hear the transformer frying, bubling sound. Its toast.

Okay just had to ask or we might have let someone trash a power transformer. I like D-Lab a lot and he is a solid provider of good services. I never heard a bad transformer make bubbling noises. I mean I have heard a buzzing sound from old worn out transformers. But most bad transformers and we're talking fuses blowing when turned on.
#5
Preamps and Effects / Re: Joe Davisson's Vulcan prea...
Last post by Miyagi_83 - May 22, 2024, 06:34:09 PM
Quote from: Umlaut on May 22, 2024, 04:18:37 PMOh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.

The 337's I tried today were very consistent, or maybe that's just my limited experience. Out of the 15-ish pieces I tested, one biased at 4.44 V, the five I selected are between 4.51 and 4.55 V, and the rest gave a reading of around 4.6 V. All that with 1k emitter resistor, 22k collector resistor, and the 4.7M / 1N4148 / 1M network connected to the base, as per Joe Davisson's schematic.
#6
Preamps and Effects / Re: Joe Davisson's Vulcan prea...
Last post by Miyagi_83 - May 22, 2024, 06:24:29 PM
Yeah, I meant the last stage, the emitter follower. I'll leave the load resistor at 10k then.
Speaking of the 3-stage BJT version, I kinda know how that thing sounds because there is a video of it on YouTube and, indeed, it does display the quality you described earlier. Still, I'd like to (and am going to) try the 4-stage preamp because I'm curious what it can do. There are no clips that I was able to find so I might become the first one to upload some :D
I started putting the circuit together on my breadboard, but I needed to take it apart halfway through because a lot of the connectors were overly loose, so I figured I'd retension them. Should have checked that first...
I might have some time to get back to working on it this weekend.
#7
Just joined here, total noob, and mechanical engineer, so electronics is an art that I'm just trying to begin to learn.  I've done rudimentary soldering and should be able to replace any component that is available today.  I'm at a loss now after some troubleshooting.  Looks like a lot of good minds on this forum, so here goes.

I have a 1980's JMF Spectra 125 SS Combo Amp (pre-Dean Markley) that is no longer producing clean sound and volume (on the Clean or Distortion channels).  It sat in the basement for years and years (probably corroding) up until now.  I've cleaned the pots (no crackling), replaced two corroded ceramic resistors in the power amp PCB (.33 Ohm and 33 Ohm in between the Power Amp board 3792 and 3716 RCA transistors), brushed the boards somewhat clean, and reflowed one solder joint.  The amplifier produces some lower volume, distorted, flat sound (midrange-e only) but only when I strum kind of hard.  Otherwise, the amp doesn't pick up the sound from the guitar pickups on light strumming.  Also, the volume knobs basically don't do much no matter what volume/master volume I turn. The reverb knob changes the shape of the sound slightly.  Using an extension speaker gives same sound, so it's not a blown speaker.  It also starts screeching/rumbling after 30 seconds or so of being switched on, but hard strumming overpowers the screeching while playing.  Plugging guitar into the Line In/Power Amp In also gives same result.  Also, plugging a short cable into the Line-In and Line-Out together does nothing.  I have attempted to check resistor resistances (the two ceramics were bad), capacitors, diodes, transistors with a decent Fluke multimeter, but can't see anything apparent.  Note, I did not remove all components from the board.

I don't have an oscilloscope, nor do I know how to really use one anymore, but I don't really want to have to hand it over to an electronics repair shop (repair will likely cost way more than it's worth).  Wonder if anyone has any suggestions/questions/comments?  Hope the links work.

Power Amp Board

Power Amp Schematic

Preamp Schematic
#8
Preamps and Effects / Re: Joe Davisson's Vulcan prea...
Last post by Umlaut - May 22, 2024, 04:18:37 PM
If you are talking about the last bjt, emitter follower indeed, I wouldn't go above 10k unless I wanted to purposefully introduce "cathode follower"-ish distortion.
FWIW I breadboarded the 3-stage version of this circuit, not the 4-stage one, and there was plenty of gain and still some wiggle room to push it with a TS without turning into white noise. YMMV :)
Oh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.
#9
Amplifier Discussion / Re: Peavey Bandit 65 transform...
Last post by mr.death - May 22, 2024, 04:11:55 PM
Located in Toronto Canada. Thanks thus far...
#10
Amplifier Discussion / Re: Peavey Bandit 65 transform...
Last post by mr.death - May 22, 2024, 04:11:09 PM
Quote from: DrGonz78 on May 19, 2024, 12:19:47 AMWhat method did you use to make the conclusion that the transformer is shorted?
I watched a dlab vid. He shows how to check the transformer. I did his test and it failed. Also when I turned the amp on I could hear the transformer frying, bubling sound. Its toast.