Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: smackoj on January 21, 2017, 10:25:53 am
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hello gents; I picked up an Ampeg BA 115 chassis advertised as 'untested'. I did a thorough visual inspec. and see or smell nothing burnt or disconnected. I put it on my current limiter and get the led power light on but no juice (light) appears thru the limiter bulb. I pulled the 2 B+ wires off the brd and got 27 vac on each. I pulled the J22 ground wire off the board and got 7 vac. The schemo says it should have 40 vdc plus and minus on the rails and it gets divided by resistors to make 16 vdc plus and minus for the preamp. I can't really find a good place to check for 40 vdc with the brd still installed because there is so much hot glue goop on the power resistors.
Should I be thinking that the PT is bad because of the 7 vac on the ground wire? Thanks
http://www.guitarkitbuilder.com/img/BA115_schematic.pdf (http://www.guitarkitbuilder.com/img/BA115_schematic.pdf)
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the led power light on but no juice
That's a basic indicator at least the + half is good
7 vac on the ground wire
That might send you down the wrong path.
I can't really find a good place to check for 40 vdc
check on d10/D13 directly and look for the +/-16, if that's good, the 40 is probably good
have your meter all clipped in and set up, THEN powerup long enough to get reading, 30sec?
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> 27 vac on each
27V rms AC gives 38V DC. This is OK.
This DC should appear at the rectifier +/- pins and at the big caps. Also at the S pins of the power output devices(be careful not to short!). These are ALL glopped?
> I pulled the J22 ground wire off the board and got 7 vac.
Probably a bad idea (can kill stuff). All it tells you is that the two loads are not exactly balanced. We know that: the LED hangs on just one rail(+). The voltage you will see in this condition depends on too much stuff to work out.
You do not say what is WRONG. It doesn't blaze the lamp-limit. The LED comes on. You have good-enough AC voltages off the PT. I would put back that J22, then check for DC at the speaker outputs. Over 1V, do not connect speaker. Under 0.2V suggests the power stage has no gross fault at no-load condition.
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Thanks guys; 'what is wrong' I just didn't see any light thru the limiter bulb so I didn't want to try anything else accept testing voltages until I asked for help.
I will test the source on the chips and the DC at the spkr jack asap and report back.
thanks again
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A happy solid-state amp may not pull enough current to even dim-glow a lamp.
The lamp is just-in-case the amplifier is a near/dead-short on the wall plug. You passed that test. Move on.
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I finally got my limiter lamp back from my wife and the results are:
.21 vdc at output to spkr
Q2 +24 vdc
Q5 -36 vdc
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Q2 +24 vdc Q5 -36 vdc
That's at the (S)ource on each?
can you get on the +/- 16 by Q3/Q4. as long as you're probin, what's the bias?
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I did as you said Shooter and found the following:
bias voltage at the trim pot is neg. 1.35
bias voltage at R41 is +1.35 vdc
R34 should have +40 vdc but I only found 24 v
At R50 there should be a plus 16 v but I found only 13 v
and I did find -16.5 at R42 but it's open because there is 0.00 vdc on the other side of it. there is some slight discoloration on the brd where it looks like the resistor smoked.
thanks again
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Does it hum?
Get hot?
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Yes it does hum quite a bit thru the spkr
No it is not getting hot
thx
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I think the BIG caps C6 C14 4700u 50V should be replaced.
The voltages are consistent with a failed C6. (You get a "DC" near AC RMS rather than 1.414 times RMS.)
The amp is over a decade old and not built to last forever.
C6 and C14 each do the same thing for their side of the power. If one has failed, the other one is likely to die soon.
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replace the big caps. got it. thanks
did you see where I found R42 open? 3 comments up from your last post.
thanks
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WAG for R42, Q1, Q3 C20. a base to emitter short might draw enough current to take out a 1K, c20 would most likely short then open, but usually you find the results all over the amp.
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I am not sure what 'WAG' means shooter.
thanks, jack
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in this context, Wild Ass Guess :laugh:
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> I found R42 open?
Is that dead-nuts-dead-ZERO? Or just very small?
The way that too-clever scheme works, a just-right wrong voltage between rails will put that point real near zero. Get the big stuff fixed, then look at the small stuff.
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dead nuts zero on the meter
the power caps are ordered
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Not to hijack or get into the weeds, but the "circled" circuit is really hurting my face trying to understand it, any ideas or wisdom :dontknow:
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Looks like a variable NFB circuit used as an automatic gain control. Probably used as a compressor. The LDR part of the opto-coupler is parallel to the feedback resistor for U3-A. The output of U3-A is sampled from the right side of C10. As the signal increases, the opto-coupler shines brighter and the LDR resistance goes down. This reduces the feedback resistance which decreases the gain of U3-A. Just the opposite happens when the signal at C10 decreases. So, weak signals get amplified more, and strong signals get amplified less.
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Thanks for the high tech info SL
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Thank you!, I kept getting hung up on the steering diodes n missing the fbk line from c10 :BangHead:
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The actual feedback path is directly from pin 1 thru the LDR and back to pin 2.
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> the "circled" circuit is really hurting
Gain-control, not instant NFB.
When not limiting, gain is set by NFB at 47K:10K.
The resistors set 3/4 power rail references. If the Output passes 3/4 of the rail, the opto's LED lights. This shines on the LDR across the 47K and reduces gain. The LDR is selected for faster attack than decay. It is an Audio Limiter, though the time constants are pretty arbitrary. It is not a "hard" limiter which is why the threshold is 3/4 of the rail (perhaps 80% of clipping). If you push hard, it will clip, but not easy.
Below that, the JFET appears to be a crude power-on mute. The JFET is normally "on" (when no voltage anywhere). The Gate resistor pulls it to "off" when -16V appears. But not right away. This gives time for the rest of the system to thump and stabilize before signal is passed to the output stage.
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The actual feedback path
Ya I got that, normal op amp closed loop, the rest above, that messed me up.
the JFET appears to be a crude power-on mute
like that part :laugh:
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A "crude power-on mute" works fine for me because I'm a crude bass player. IOW any note will do as long as it's somewhere close to one on the page. :icon_biggrin:
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well gents she's got new filter caps and a new 1k in the R42 slot. hit the switch and no hum and a very faint hiss so I'm real happy with the
end result. Couldn't have done it without you guys so muchas gracias amigos. here are a few pics of the amp just for general interest.
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I'm real happy with the end result
so the rail volts are good? Did you get the bias set per schematic?
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Shooter; I haven't tested voltage or bias yet. I will report back asap.
thx, jack
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I would want to see near 40V both sides ASAP.
For bias, I would be happy if at idle, the heatsink was somewhat warm. The bias scheme here is dubious, MOSFETs are not as fussy as BJTs, there's no exact optimum. If there is "some" current flowing, enough to warm up, not enough to be hot, that's good enough.
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Is the bias set by just the one trim pot?
The second pg of the schemo has a note stating to set the bias at 5mv across R61. Does that sound right?
thanks
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Looks right, it's a .1ohm R, can't see much else for it but convenient measuring of current
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Yes, 0.005V/0.1r is 50mA and a fine bias. And will throw 4 Watts of heat, about 1/10th the heat of FULL ROAR, so "warm" not "HOT".
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hi; I tested voltage at the source of the output chips and got -37.5 and +37.5
BUT I screwed up trying to change the bias with the trim pot and blew a fuse. I replaced the fuse and put it on the current limiter and found a short. I looked closely and R42 was burnt open again. I will replace it and try again. I didn't realize the trim pot was so sensitive. My bad
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I looked closely and R42 was burnt open again
Just center the pot for now without power on should be 250ohms either side of wiper.
with a new R42, measure your +/- 16VDC and see what you have.
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I would just short the dang bias pot. That will set a minimum bias. Bias is not critical in this amp. As you found, it may be hypersensitive. Also pots go bad, which in this case will send the bias HOT, and as you see, blow-up things.
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Well....I should have left the bias pot alone. I replaced R42 with another 1K slapped it back together, put it on the cur limiter and CRAP still a short in the amp. I tested the source pins on the pwr transistors and got something like half a volt dc. So, sorry to say I need more help figuring where to look for the short. Possible one of the transistors before the output or maybe even fried the output trans? None of that stuff smoked but I'm no expert on what it looks like when output trans go pooof.
thank, jack
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what it looks like when output trans go pooof.
Now you do!
I was paid to make equipment work, fast, so with corporate money I would replace a lot of parts in a short time. $30 in parts n a couple hours labor vs $1,000s per hr down.
this would be my "shotgun" list.
Q1, 2, 3, 4, 5 C20, 31, and the 4 zeners.
You can, with power off, measure ohms to ground at both +/- 40, then start pulling parts, when the ohms jump, you're probably close. as long as you're there check the pot, swap or just short the dang bias pot
Take lots of pics, close up so you don't put a good part in backwards.
I would expect the + and - rail to have about the same ohms to ground so if you have many K on one and very few on the other, that's your start.
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Hi; I pulled the 2 output mosfets and tested. Both look gone on the meter so I have ordered replacements via Mouser. I will take a look at the resistance to grd on the rails tomorrow and report back. I'm not really sure what is meant by "short the trim pot"? A little more info would be helpful.
thanks again
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Both look gone on the meter
IIRC, Igfets don't ohm like transistors, but short is short if that's what you're seeing.
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Hi Shooter; I tested the 2 power fets using this "video" method. Let me know if this is not an accurate method? Thanks
https://www.youtube.com/watch?v=RBJGOOTEwfU (https://www.youtube.com/watch?v=RBJGOOTEwfU)
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That looks good to me, See, before you know it you'll be the goto guy for SS repair :icon_biggrin:
In SS you can expect the un-expected, you also usually have a cascading failure, like when 1 transistor fails, it takes others with it, so hang in there. with the FETs pulled and no power, what do you get ohms to ground at both 40 and 16 taps?
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I checked all the points on the schemo where it showed either 16 v plus or minus and 40 v plus or minus. I got apprx 1k at a lot of the spots but back by the power Rs it had close to 2k ohms.
jack
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apprx 1k at a lot of the spots
I just dug out my 60W SS amp, similar to pg2 of yours and I get >100K at my 40v taps, more in line with what I'd expect.
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Hi gents; I have been waiting for the power mosfets (Q2, Q5) to arrive so I can replace them. Should have them tomorrow. I have tested all the other semi conductors and they all seem sound. I ran a voltage check again and I am getting 17 vdc plus and minus at the appropriate spots on the board. I am also getting 36 vdc plus and minus where the schemo shows 40 volts. Possibly the PT doesn't put out quite enough voltage for the higher lines?
I would like to know a little bit more about PRR's suggestion to "short the trim pot". Does that mean run a wire to ground from the trim pot?
Thanks for hanging in there with me. Jack
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I would like to know a little bit more about PRR's suggestion to "short the trim pot". Does that mean run a wire to ground from the trim pot?
NO! That means put a jumper across the outer lugs of the pot. DO NOT CONNECT ANYTHING TO GROUND.
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Thanks SL I rec'd the proper power mosfets (found exact match on the 'bay'). I will report back soon.
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update on the Ampeg BA 115: I have replaced the 2 output mosfets twice.. It is operating without issue except for one BIG issue, no output at the speaker. I have the proper voltages on the pcb and to the source pin of the output mosfets; my best guess is that the solder pads under the output mosfets look to be starting to break down? What should I d o to test if the signal is not getting thru the output fets? should I check for AC and DC at the speaker wires?
thanks
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Did you solder the two mosfets in the correct positions?
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check your headphone jac also, it provides spker ground with nothing plugged in, can temp a ground also
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According to the schematic I have them correct. The IRFP9140 is the P channel in Q2, and the IRFP140 is the N channel in Q5. The source pin on the P9140 has +37 vdc and the P140 has -37 vdc on the source.
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anything when you plug in headphones?
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I am getting guitar thru it now but the signal is distorted. I guess maybe I need to go back and check all the transistors again?
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getting guitar thru it
speaker?
if so, how does the loudness seem?
you got an old cd player, something you can jack into the CD in and make sure your issue is on page 2 of the schematic? Looks like if you can jack in you want the Master Vol at 0
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Yes, I'm running it thru a speaker. Loudness seems about right to me. I will see if I can find the stuff to do the CD test.
thsnks Shooter
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Ok Shooter; the CD test is done. It was playing so loud that it way over driving the speaker. None of the vol. controls lowered the vol. so the music was good and loud but distorted. therefore, I don't know if the distortion was in the signal or just from the high vol.? My best guess is that it was running all 100 watts thru my 6" car speaker and just well over what the spkr could handle? What's next?
thanks, jack
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just from the high vol.?
whatever input you use also needs it's own volume, sorry, shoulda mentioned that. I can definitely over drive my amp with the CD on max.
EDIT: You also have a line out to plug into, something?
you have a scope?
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my CD player is a real basic one (actually a DVD/CD) with no vol. control. And NO I don't have a scope.
jack
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so is your guitar distorted at low volumes, or only when cranked?
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hi shooter; yes, distorted at low or high. I have a problem with the pcb. the solder pad has deteriorated under te N channel mosfet and I am uncertain how to proceed? I am sure that Loud Technologies probably skimped on the thickness on the pcb to make money off of a low cost amp. So, I'm going to take a few more hours and see if it's repairable? I have my doubts but I'm willing to try.
thanks, jack
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Well pardners I have to call the fight. Throw in the towel and let the fat lady sing. The flimsy pcb has proven too much for me. Ampeg aka Loud Technologies made this amp like all the other low end electronic gadgets on the market. If it breaks, throw it away and buy another one. The N-channel mosfet pads are shot and the mosfet won't make good contact anymore. So, I'm going to count my blessings and be glad that I learned a lot with all the attempts. Thanks everyone for your effort to help me. The learning experience has made me wiser if not smarter. :icon_biggrin:
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I learned a lot
That's what I like best!, glad you didn't get frustrated and gain no knowledge. If you're truly into pain, you can rtv the fet and use 24-26ga solid wire and probably get it, but sometimes as you said;
Throw in the towel and let the fat lady sing
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hey shooter; could you tell me where on the schematic would be a good place to take off the preamp signal? I still might try to use the preamp section to run a nice hefty Allen organ amp I picked up. I'm not sure how I would convert a plus and a minus signal into one wire for the input to the slave amp? This amp doesn't have a "preamp out" jack like most of the more expensive SS amps.
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a good place to take off the preamp signal?
on sheet 2 there is a balanced line out, that is basically "preamp out"
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yes, I see that on the schemo. That should be an easy test to see if the preamp has a nice clean signal.
thanks
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You'll have to do a little surfin on balanced line out, there is no "DC" ground there, just AC ground, not sure if that's a problem. I believe you have 2 inverted signals so you probably want to grab just one unless your PA have a balanced line in.
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senor shooter; hi. I spent another couple hours today making a signal jumper from the 'line out' xlr to the SS Allen amp. It had too much hum at working volume so I took some pictures and put it on ebay as "broken for parts or repair" I thought about taking the signal off of R25 which is right before the 'line out' circuit, but I'll wait a few days and see if anyone else wants to give it a go. I really thought hard about trying to fix the traces and pads but I've spent so much time on this one that I'm leery of investing any more.
thanks, jack
r25