Time to play with this one
Had to borrow a fuse holder cap and a fuse and put it on the big ugly DBT with 60w bulb
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Next input signal 1Vp-p and hooked scope probes to input and (16 ohm) output
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Input channel one and output channel two
Amplifier power off (ch 1 at the bottom)
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Next amplifier power on and had aux volume at half and master volume at half
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Not much amplification going on :grr
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Next I tried to max out both volumes and see the result but it started flattening out the top of the output
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The most output I could get is about four Vp-p
IMG_20251026_200022872.jpg
xP
Now...granted there are a bunch of switches on the back so who knows
And...idk what I'm doing :'(
So there's that but it looks like it works to me idk :lmao:
One really wierd thing that happened...I could hear a very faint hum with the power off. I leaned in to listen better and thought wow is that my 1 khz signal ?! So I reached up and changed the frequency and yes it was my signal I could hear it with the amp power off :loco
I'm gonna propose that the above is probably not SOP for amp testing :tu: :loco
I'm gonna connect a speaker and see if I can get ear bleeding 1 khz :tu: :lmao:
Alright well either
-im not doing this correctly
-or this amp is putting out one watt :lmao:
So I -connected an 8 ohm speaker
-Scope leads across speaker terminals
-Maxed out aux and master volume
-increased signal until output started clipping (about 2.8 Vp-p)
Did math
IMG_20251027_193140650.jpg
:grr
So I guess I have a sick one here xP
Meanwhile it cut off a few times I could hear a distinct click whenever it did. It was not intermittent it wouldn't come back until I turned it off and turned it back on after s moment.
And the fuse is good.
I guess that explains why all the cover bolts were missing
A couple things I noticed about this amp is there is no relay click maybe it's not supposed to have one because it "works" with out it.
Also I noticed the bottom panel gets warm
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but I saw a video pointing out that the output transistors are attached to the bottom panel
IMG_20251027_192957130 (1).jpg
So right off the bat I suspect a bad voltage regulator :lmao:
I was becoming suspicious of this guy
IMG_20251027_220732397.jpg
albeit for no reason I just figured it looks like it gets hot and may be failing. But then I figured it's probably not a voltage regulator since it's marked ECB and TR 201
:'(
Might be time for some freeze 🥶 spray since it seems like something is getting hot and shutting down.
Hmmm
Seems like some more stuff to learn
https://www.ssguitar.com/index.php?topic=4594.30 (https://www.ssguitar.com/index.php?topic=4594.30)
Specifically (per G1)
"I'm not sure your test set-up, but it's critical that you do not ground the speaker (-) with any test gear, grounded load, etc."
(I was clipping my scope ground to the negative speaker terminal). :grr
The thing about not grounding the load is pretty much specific to amps that use current feedback and amps that operate in 'bridged' configuration.
This one is probably not either, but no harm in moving the scope ground to chassis, that is usually the best place for it anyway.
Any time you see a heatsink or a high power resistor, expect heat. So I expect that TR201 to get hot.
In older stuff, they did not have dedicated voltage regulators so they used regular power transistors in a regulation circuit (often 'pass transistor' topology).
Check the DC readings on it's pins, that may give an idea of what it is doing.
You may be in for a bit of a rodeo with this one, having no schematic. ;)
I have located a schematic for the MPA-201.
As it is a dual channel amplifier you will have to take anything on it with a grain of salt.
The power amp looks as it may be the one that you have.
Although the component designators will most probably be different.
The regulator circuit appears to be a simple 15 volt zener diode.
Thanks guys :tu:
I know it's not much but in the meantime I still need to do stuff so I think I will try to flip that board over and reflow TR 201 (or remove and test and attempt to identify then reflow )
Also I can remove that green capacitor that looks cracked :grr and try to see if it has any markings on it so I can replace it.
I left it on with signal last night for about an hour and it would modulate for a while then cut out then after being quiet for a while it would come back on then repeat.
So I'm guessing that something is heating up and shutting down then cooling off and doing it all over again.
Also I noticed one of the binding posts somebody unraveled it but put it back.
xP
So I'm in a pickle trying to invert this board to desolder TR 201 and that cracked green (poly film?) capacitor
These two sets of wires are attached to the chassis
IMG_20251104_185612460 (1).jpg
The bundle on the left looks doable since it had a nut and bolt 🔩
But the bundle on the right was adhered with silicon ? and it started to come loose and I'm wondering what this is :grr :loco
IMG_20251104_185702892_HDR (1).jpg
But wait there is a clue
I need to find out what this symbol indicates
IMG_20251104_190431048.jpg
:grr
The symbol is a thermistor.
J5 & J6 on the output section of the schematic.
Ok I got these guys out
IMG_20251104_193910419 (1).jpg
IMG_20251104_193644002 (1).jpg
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Pinout same as PCB markings
IMG_20251104_194046603.jpg
Idk if this means it's bad or weak
IMG_20251104_194054229.jpg
Not sure what to make of this or how to identify
IMG_20251104_194150536.jpg
:'(
Most Japanese transistors like that omit the '2S' prefix. If you see a transistor with just A,B,C, or D in front of a number, that is usually the case.
So what you have there is a 2SD569.
For what it's worth, here is a partial schematic I made the last time I saw this model.
MPA-100 32-2023.gif
Thanks guys I appreciate I have to put forward that since I so greatly appreciate your help I have to let you know this is for all essential purposes a cadaver that is being worked on and not something I desperately need like working for my son on Christmas day or something :dbtu:
With that said let's carry on
I may have found a replacement for TR-201 but I'm in a little over my head with the ratings
Idk if it ( Toshiba TT015B) will be able to endure the rigors of the environment
IMG_20251105_115614370 (1).jpg
:'(
Meanwhile here is what I think for the capacitor
Found what seem to be some suitable replacements but not sure what voltage rating I need
Might be some info in the schematics kindly provided so I'll take a look
Btw wasn't sure if 334K means 334,000 picos or not
IMG_20251105_121855381 (1).jpg
And with that I'm gonna try to get out of y'all's hair I appreciate the help I reckon y'all have much more important stuff going on :dbtu:
The cap has the 4 as a multiplier, not as a digit. So 33 with 4 zeroes, in pF.
For the transistor sub, power and current & Hfe look ok but I don't think the Vceo of the TTC015B is sufficient. I would look for the TTC004B.
Yes sir thanks also I think I learned 1H means 50 v DC xP
Ok
Got my transistors in yesterday so today didn't have a lot of time but hey onward we go
Anyhow wife gave me too much coffee so I had some hella jitters trying to get this in
Also I verified pinout etc
IMG_20251112_115000866 (1).jpg
And the ugly truth
Oh yeah that nut that was glued to the board and fell off added to the entertainment
IMG_20251112_114933339.jpg
I did not have tons of confidence in this install so I checked for continuity but fingers crossed apparently no solder bridges
So now just waiting on my capaci-TATERS :'( :'(
Too much fun
Just had a cow looking at the collector which appears to be touching the heatsink :grr
Anyhow I did some more continuity tests and I think it's ok but yeah gonna have to pay attention to stuff like that
IMG_20251112_121324986.jpg
EDIT: note to self. Next time you need to bend the legs 90 degrees approximate the horizontal distance needed then find a nice firm sharp 90 degree edge to use as a die
Because it also messes up trying to solder when legs are coming through the hole at an angle SMH
EDIT 2: Maybe instead of nice sharp 90 degree edge I should have said nice CRISP edge. Like when I used to be in Alberta and when I got cold ❄️🥶 the locals would say "She's getting CRISP"
That 'heatsink' looks to me to be a ceramic pad (for heatsinking purposes).
It should be nonconductive.
Well I got my capaci-TATERS in
IMG_20251114_044529921 (1).jpg
Got home from work and installed
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Clipped the leads
IMG_20251114_045431954.jpg
Reattached the board which was a major pita getting that little spade fork under the ground screw
Powered it up and its about the same it doesn't get loud with both volumes maxed.
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Im declaring a marginal "success" because I've had it on for a while and it's not shutting off like it was before.
Also I noticed that the bottom chassis is not getting warm 🥵 anymore.
I just felt the little "heatsink" and it's pretty warm but not hot.
So kinda inconclusive since I wouldn't even know how to operate this amp if it was brand new. Might be time to get a 🎤 microphone 🎙�
When I first powered it up to test it I got no sound but forgot about this already molested wire wrap so I attached it and had the same low sound.
IMG_20251114_053329975.jpg
There are two posts maybe I should try the other one but I don't want to short something out. :grr
Or maybe I need to study the schematic kindly provided and try to figure it out.
Or maybe they both need to be connected to a wire :grr :'(
xP
Ok nm yes it does get pretty loud I had the knobs facing away from me I thought the two on the end were aux/phono volume and master volume.
Turns out the tone knob is on the end so when I turned aux/phono it started to get loud. :tu:
I want to crank it back up and measure the collector emitter voltage Vce man that's crazy it can get really high.
I told the owner of the junk store where I got it we need to try it out with a mic and maybe he can use it to talk to his shoppers xP
EDIT:
I attached the mini grabbers to the collector and emitter and cranked it up all to max
I didn't really see any crazy high voltage
Strange I had about 50-55 volts DC and AC at "idle" and when I increased volume(s) it bounced around sometimes hitting 80-90 but settling back down
EDIT 2 : ok just dropped it off at my friend's store he got a mic and we tested it out it works really good except it feeds back any time the mic is near it.
So that's a good feeling thanks to all for the help. I'm glad to pass it on maybe he'll sell it or keep it but I have a little more space 🚀 now :lmao:
Only problem is I let him have my screw 🪛 in fuse cap :'( and also my test speaker but it was way too big anyway it was like an (at least) 12 inch big woofer xP