Well, things are a bit better than I thought because the 3005 schematic matches poorly to the 5005.  Post 26 here has a redlined schematic marking the changes: https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/50328-marshall-lead-12-5005-combo-needs-help/page2

The 5005 seems a lot closer to the layout and schematic designations of the 5002 20 watt combo: https://www.drtube.nl/schematics/marshall/5x02.gif  Some definite tweaks in the preamp circuit, so maybe the idea that 3005=5005 needs an asterisk next to it.

So my IC1 voltages are from C15 (=17.51) and C16 (=-13.76) in the amp and the redlined schematic.  So it's off balance, too high positive and too low negative.  And unless that conked out the chip, that doesn't seem to explain the silence.  When I have time in a couple months I'll have to get a scope and just trace it out.

Phil, it's at .006v across the speaker out terminals with the speaker still hardwired in. Seems mostly harmless.

JPB, indeed.  I just need to figure where it's going wrong.

Gonna piggyback on this old thread since there was discussion of bad voltages.  My Lead 12 crapped out.  No fatal short fuse blower stuff and the output is good enough to put hum through the speaker.  But some voltages aren't getting where they need to be.

C2 = -7.82 (supposed to be -16)
C6 = +0.30 (supposed to be +16)

C14 = zero  (supposed to be 10)

C17 = -19.45 (supposed to be -19)
C18 = +19.52 (supposed to be +19)

My wall voltage hovers at 123-5, so I'm not worried about the slightly high voltage to the power transistors, but the rest is obviously off.

What am I looking for as culprits?

(BTW, just incidentally. C11 in the tone stack was omitted at the factory.  Weird.  Didn't notice a lack of effect when the amp was working.)

$40, so I got a deal. :loco

I figured it was worth a chance for that price.  If the PT isn't shot.  I'll get some pictures up next weekend if I'm not swamped.

The Roland is a rust bucket, including the power transformer, chassis, and some of the on-PCB connectors.  Bugger has seen water.  So this might be a more specific rehab process, though I still could use a general approach.

I picked up a Roland Jazz Chorus that is as DOA as was advertised.  My last buy-it-broken experiment had just needed deoxit, but this one is no lights/power/output, so I want a methodology to proceed on.

Timely thread for me.  I'm looking at some pt iron I got when my local RadioShack liquidated, and trying to figure out how to use it.

The big two for current are a 25.2v ct 2A and a 12.6v ct 3A.  The 25.2v should be plenty good for a home-use chip amp power amp. 

The lower-volt 12.6v doesn't obviously serve any project I can find, though the VA math is just fine for similar low power use thanks to the current. Ideas?

I'll have to work through that info.  I can't get your host site to play nice, though--is that one of the mini sized pans in your project, or the more conventional short format?

Anyone have experience with the Belton "mini" sized reverb pans?  Like BMN2 or BMN3 series?  A few inches shorter than the normal short pan configuration seems like it could enable a tight squeeze or two, but I wanted to see if anyone had gone there before trying to procure one--U.S. distributors seem to all pass on the minis.  As good? Nearly? No good?

(I'd think of the Digilog modules for new SS design.  Shamefully, I'm looking for a tube design I'm sketching out, but this forum seems the most likely to have experience with non-traditional pans, so I'm asking here first.)

Quote from: J M Fahey on October 15, 2015, 03:22:01 PM
SICA makes killer speakers for a reasonable price, that one must not be an exception.
Looks very good, a mini EVM speaker in a very compact (and light)  frame.

I would love some mini Thiele cabinets built around something like that if they had 95% of the sonic size of my TL-806 with EVM-12L.  If Sica put out plans like EV did back when (and got that model into the U.S. supply chain), there are enough people with space constraints and/or bad backs . . .

I picked one up for a song and don't need the schematic yet, but better to be ready.  The JX-20 and -50 can be found online, but not this guy for some reason.  I presume I can extrapolate from those and visual inspection, but figured there was a chance someone here might have it.

Great little amp, BTW.  I bought it expecting it would give the blackface/silverface feel that it does have in spades, but I was surprised at how decent the pull-boost overdrive is.

So how would one go about designing for a more bullet-proof chipamp?  My noob reading and data sheet studies are making me think shooting very low on the voltage range for the opamp in question would help reduce the on-die thermal stress.  So, say the infamous Vox Pathfinder/Cambridge TDA-series chip upgrade "now it's louder!" Mod that does not increase volume--well, might going from a 20 watt part to a 50 watt part reduce the failure rate of that component?

I guess single-railing the supply when it enables non-insulated heat sinking can help heat transfer, but it won't really help that on-die issue.

Thanks, Juan!  I have been following those links and beyond.  The tda2822 is interesting--can't believe I'd never seen that one before.

Reading up on mixed-mode feedback, something I wanted to play with, I've gathered that fixed gain power opamps don't work ideally for that circuit type.  But I can't find an explanation of why.  Any explanation or reading anyone can point me to?

Thanks!

I've got some 386's on hand to breadboard some beginner poweramp stuff with, but I get that they're limited beyond their available power output.  The appeal of low voltage components for experimenting is high, though--a 9v is easier than doing a bipolar supply off AC mains.

So I'm looking for other components worth experimenting with.  The LM380 is supposed to be the non-crippled parent version of the LM386, and it looks like it could run quite capably off a wall wart with decent current capacity, simplifying life greatly.  Or am I off base on that?  Is the heatsinking via the pins in the 14 DIP as easy as advertised?

Any other low power chip amps of note that I could similarly outsource the power supply to batteries or wall warts or such?  Any decent "cookbook" references for any of them?

Quote from: Bill Moore on August 13, 2014, 08:26:23 PM
Bear, I picked up one of these a while back, and had to wedge toothpicks under the knobs to get it to play. Do to the age, I replaced all the pots, and the jacks, (it's already apart, why not). I also replaced the 30 yo filter  electrolytics, and added an aux output jack. The switch was also intermittant, so it too was replaced. I enclosed the rear to gain a little more bottom from the 10" speaker. All parts are availlable from CE, or AES.
What a great little amp!

Bill,

Was the aux output a speaker out or an alternate line out?  I was gonna move from the hardwired speaker to a jack and plug anyway--I suppose it's no more trouble to add an aux speaker jack, too.

And definitely liking it already.  In fact I've been scouring craigslist for more cheap ones.  Makes sense because I want to be able to use the individual delay tap outputs in my Akai Headrush.