Check the DC voltage at P1(+ SPKR WHT). If it is more than 50mVdc, do not switch on without the current limiter.
I would suggest cold test for all the diodes and zeners. Put the DMM in diode mode and check all the diodes with the power removed. If the amp was on, wait for the capacitors to discharge.
A forward biased diode or zener will show 0.4 to 0.7 on the diode range, if these do not have low resistance across.
Is there any tantalum capacitor on the pcb?
Gajanan Phadte
I would suggest cold test for all the diodes and zeners. Put the DMM in diode mode and check all the diodes with the power removed. If the amp was on, wait for the capacitors to discharge.
A forward biased diode or zener will show 0.4 to 0.7 on the diode range, if these do not have low resistance across.
Is there any tantalum capacitor on the pcb?
Gajanan Phadte
Diodes
I'm not sure what changed, other than the fact that I turned the amp on a few minutes ago without the current limiter but the P1 showed 110 mV the other day and now it is 12 mV and it was 12 mV yesterday when I checked it and posted that the sound is distorted.
I don't know all the variables for checking diodes. Some are zener and when I check them using the diode or ohm setting I get nothing, its dead. But a zenser doesn't work until it gets a certain voltage and then it allows power through. D18 is a good example of a zenner and the result I get is an open. either way I hook up the test leads.
I have black barrel shaped diodes with a grey stripe on one end which are rectifier diodes. for instance: D53,54,54 and 56 which are the AC rectifiers.
Diode setting:
D53= Red to cathode - 1.49/ black to cathode - .49 all four are almost identical
D57 and D58 are zeners both show R to cathode - 1.41 B to cathode - .61
So there is a problem with D18 and several other zeners because they are showing open.
.
I won't say there aren't any tantalum capacitors I will say I do not recognize any. But I don't think there are any on this board.
BR
I'm not sure what changed, other than the fact that I turned the amp on a few minutes ago without the current limiter but the P1 showed 110 mV the other day and now it is 12 mV and it was 12 mV yesterday when I checked it and posted that the sound is distorted.
I don't know all the variables for checking diodes. Some are zener and when I check them using the diode or ohm setting I get nothing, its dead. But a zenser doesn't work until it gets a certain voltage and then it allows power through. D18 is a good example of a zenner and the result I get is an open. either way I hook up the test leads.
I have black barrel shaped diodes with a grey stripe on one end which are rectifier diodes. for instance: D53,54,54 and 56 which are the AC rectifiers.
Diode setting:
D53= Red to cathode - 1.49/ black to cathode - .49 all four are almost identical
D57 and D58 are zeners both show R to cathode - 1.41 B to cathode - .61
So there is a problem with D18 and several other zeners because they are showing open.
.
I won't say there aren't any tantalum capacitors I will say I do not recognize any. But I don't think there are any on this board.
BR
I replaced q18 and q19 transistors with (Q18) TIP31C RF Transistor NPN and
(Q19) with TIP32C Bipolar Transistor PNP
Could my distortion on the clean channel and the overheating to the point of the amp shutting down be a result of the two different transistor descriptions? I'm talking about one is an RF transistor and the other is Bipolar.
BR
(Q19) with TIP32C Bipolar Transistor PNP
Could my distortion on the clean channel and the overheating to the point of the amp shutting down be a result of the two different transistor descriptions? I'm talking about one is an RF transistor and the other is Bipolar.
BR
All the diodes and zeners that show .4x to .6x in one direction are good.
All the diodes that u suspect, pull out one of its lead from the board while heating the solder point and then test using diode function on the DMM. Be careful do not force pull. Use a forcep on the lead side or use a desoldering pump.
google images for tantalum capacitor
Gajanan Phadte
All the diodes that u suspect, pull out one of its lead from the board while heating the solder point and then test using diode function on the DMM. Be careful do not force pull. Use a forcep on the lead side or use a desoldering pump.
google images for tantalum capacitor
Gajanan Phadte
Last edited:
OK, got 30 minutes free.
Let's catch up from Jan. 14 on.
Sorry if I repeat an answer that somebody else did, I'm going post by post or it will take too long.
Post #92
There's nothing "dead" there.
As Enzo noted, that's supposed to live within millivolts of ground.
*If* you measured, say, 5 to 30 or 40V there, we'd be in trouble.
So let's discard this non problem and move on.
Post #94
Don't worry.
You'll have them there *if* you feed the amp the Fender indicated signal, or 91mV AC at the Power amp input.
Why 91 mV?
Because that's what's present at the preamp out (TP15) under same conditions.
But if you input "0", the expected output will be "0" too.
Which you got.
Fine.
Well, *that* may be a problem.
Post #97
Well, we are getting closer.
The louder the volume the brighter the light on the current limiter. And the more the amp cuts out.
Normal. That's the current limiting lamp doing its job.
No, you hear distortion because the power amp is distorting because it's still not fully repaired.
You'll be there soon.
You were unlucky to start by a complex (and in my opinion misguided) design.
That same design, in MEF , repaired by an experienced tech and helped by a couple heavyweights (even Teemuk chimed in) took about 100 posts, so now you know what you're up against.
No, that's a very poor and desperate repair technique.
You add new unknowns to an already complex problem.
OK, my Doctor mandated raw vegetables and boiled chicken lunch is over, we'll continue later.
Let's catch up from Jan. 14 on.
Sorry if I repeat an answer that somebody else did, I'm going post by post or it will take too long.
Post #92
There's nothing "dead" there.
As Enzo noted, that's supposed to live within millivolts of ground.
*If* you measured, say, 5 to 30 or 40V there, we'd be in trouble.
So let's discard this non problem and move on.
Post #94
Don't worry.
You'll have them there *if* you feed the amp the Fender indicated signal, or 91mV AC at the Power amp input.
Why 91 mV?
Because that's what's present at the preamp out (TP15) under same conditions.
But if you input "0", the expected output will be "0" too.
Which you got.
Fine.
Well, *that* may be a problem.
Post #97
Well, we are getting closer.
The louder the volume the brighter the light on the current limiter. And the more the amp cuts out.
Normal. That's the current limiting lamp doing its job.
No, you hear distortion because the power amp is distorting because it's still not fully repaired.
You'll be there soon.
You were unlucky to start by a complex (and in my opinion misguided) design.
That same design, in MEF , repaired by an experienced tech and helped by a couple heavyweights (even Teemuk chimed in) took about 100 posts, so now you know what you're up against.
No, that's a very poor and desperate repair technique.
You add new unknowns to an already complex problem.
OK, my Doctor mandated raw vegetables and boiled chicken lunch is over, we'll continue later.
How a SS amp works
JMFahey
Thanks for the reply. I figured out some of what you told me in your post. Thank you and Enzo and other for correcting me and sharing your insights.
BR
For the rest of your.
I bought a book recently titled: How to Hotrod your Fender amp. It pertains to tube amps but I bought it hoping to learn more about "how" amps work. And because I want a Fender Deluxe Tweed. Its a very informative book and does explain the systems of tube amps and how different people approach the wiring from different angles to accomplish "Better Sound". But of course I don't have a tube amp I have a Solid State amp. So I am having a hard time applying much of the info in this book over to my SS amp. So I went in search of...
It took some looking but I came across an article on How SS guitar amps work. Now this is an interesting article. This fella explains from a simplified circuit how the power is setup and how its regulated etc... I can't say I understand it all yet but this is a heck of a head start for me. This fella explains the math, setup, regulation, how the amp controls the speaker (I found that part really interesting)
I am attaching the file for those of you who wish to read and hopefully get a better understanding of SS amps. Good luck!
BR
JMFahey
Thanks for the reply. I figured out some of what you told me in your post. Thank you and Enzo and other for correcting me and sharing your insights.
BR
For the rest of your.
I bought a book recently titled: How to Hotrod your Fender amp. It pertains to tube amps but I bought it hoping to learn more about "how" amps work. And because I want a Fender Deluxe Tweed. Its a very informative book and does explain the systems of tube amps and how different people approach the wiring from different angles to accomplish "Better Sound". But of course I don't have a tube amp I have a Solid State amp. So I am having a hard time applying much of the info in this book over to my SS amp. So I went in search of...
It took some looking but I came across an article on How SS guitar amps work. Now this is an interesting article. This fella explains from a simplified circuit how the power is setup and how its regulated etc... I can't say I understand it all yet but this is a heck of a head start for me. This fella explains the math, setup, regulation, how the amp controls the speaker (I found that part really interesting)
I am attaching the file for those of you who wish to read and hopefully get a better understanding of SS amps. Good luck!
BR
There are some perfectly good solid reliable transistor power amps like the Peavey 400BH. When they do go down, they are not difficult to repair.
This amp is a convoluted nightmare. best I can figure is they did every trick they could think of to squeeze a few extra watts out of a marginal amp design. SO now there is an overly complicated circuit, and it can be clumsy enough for the experienced tech, let alone for a novice.
Tubes or solid state, the lesson to learn is that troubleshooting is a universal approach. yes there are detail differences between transistors and tubes, but the essence of troubleshooting is to isolate the problem. We use a systematic approach to narrow down where the problem can be. Eventually we find the spot where the problem occurs and at that point it is usually pretty straigtforward to repair.
For example, when an amp "blows fuses," I tell someone to first check the output devices for shorts, then I suggest they check the power supply rectifiers, and third, look for shorted filter caps. Note I didn't specify tubes or transistors. A 6L6 and a big transistor are both ouytput devices in their respective amps. A 5U4 tube or a silicon diode are both rectifiers, whichever you have. And a cap is a cap.
Simple techniques like plugging a signal into the power amp jack or the FX return to test just the back half of an amp are powerful methods.
And tube or transistor or ICs, you can apply a signal to the input and follow it with a scope or signal tracer through the amp stage by stage, looking for where it disappears or gets distorted or does whatever evil thing it does.
Learn how to troubleshoot, and you can fix anything. If you bump into a technology you never saw before, all you need to do is find a few facts about how it does things, and your basic troubleshooting takes over from there. I started in electronics working on short wave radios, then some audio, then some pro audio. But where I really learned to troubleshoot well was working in relay logic systems. No tubes, no transistors, just relay coils and the contacts they move.
This amp is a convoluted nightmare. best I can figure is they did every trick they could think of to squeeze a few extra watts out of a marginal amp design. SO now there is an overly complicated circuit, and it can be clumsy enough for the experienced tech, let alone for a novice.
Tubes or solid state, the lesson to learn is that troubleshooting is a universal approach. yes there are detail differences between transistors and tubes, but the essence of troubleshooting is to isolate the problem. We use a systematic approach to narrow down where the problem can be. Eventually we find the spot where the problem occurs and at that point it is usually pretty straigtforward to repair.
For example, when an amp "blows fuses," I tell someone to first check the output devices for shorts, then I suggest they check the power supply rectifiers, and third, look for shorted filter caps. Note I didn't specify tubes or transistors. A 6L6 and a big transistor are both ouytput devices in their respective amps. A 5U4 tube or a silicon diode are both rectifiers, whichever you have. And a cap is a cap.
Simple techniques like plugging a signal into the power amp jack or the FX return to test just the back half of an amp are powerful methods.
And tube or transistor or ICs, you can apply a signal to the input and follow it with a scope or signal tracer through the amp stage by stage, looking for where it disappears or gets distorted or does whatever evil thing it does.
Learn how to troubleshoot, and you can fix anything. If you bump into a technology you never saw before, all you need to do is find a few facts about how it does things, and your basic troubleshooting takes over from there. I started in electronics working on short wave radios, then some audio, then some pro audio. But where I really learned to troubleshoot well was working in relay logic systems. No tubes, no transistors, just relay coils and the contacts they move.
There is a resistor (R106) between between Q18 and Q19 (Section 2,B) It shows fu47 for a designation. I did a search for it and am not finding any info other than other people asking about it relating to the many FM212R that are giving people problems, including a fella on a Bronco forum claiming it is a capacitor not a resistor. If it were a cap I don't imagine it would carry a designation beginning with an "R", that would cause nothing but confusion. Does anyone have any intel' on this item?
I thought I had a couple bad diodes. I ordered new ones and they got here today. I pulled the old ones and checked them against the their replacements and apparently they were not bad.
So I started at one corner of the PCB and checked every diode. Any that gave me strange readings or didn't show open one direction and within spec the other way, I pulled one leg and rechecked and all checked out correctly.
I turned the amp on and checked tp1-6, all were what the schematic calls for. Then TP20-28 and all checked out correct except TP24. I am getting strange readings. But the heatsink is blazing hot now so I am going to recheck that point when I get home from work tomorrow and post the results.
The emitters on Q18 -21 always show 42 volts but right now with the heatsink being real hot I am getting crazy readings like 2 and 300 volts. Not sure how that can be. It has be questioning my meter even though it is not a cheap meter. But I know the heat is causing this.
Till tomorrow...
I thought I had a couple bad diodes. I ordered new ones and they got here today. I pulled the old ones and checked them against the their replacements and apparently they were not bad.
So I started at one corner of the PCB and checked every diode. Any that gave me strange readings or didn't show open one direction and within spec the other way, I pulled one leg and rechecked and all checked out correctly.
I turned the amp on and checked tp1-6, all were what the schematic calls for. Then TP20-28 and all checked out correct except TP24. I am getting strange readings. But the heatsink is blazing hot now so I am going to recheck that point when I get home from work tomorrow and post the results.
The emitters on Q18 -21 always show 42 volts but right now with the heatsink being real hot I am getting crazy readings like 2 and 300 volts. Not sure how that can be. It has be questioning my meter even though it is not a cheap meter. But I know the heat is causing this.
Till tomorrow...
After a bit of thought - I would suggest it's a 47 ohm FUsible resistor, although I would expect the value not to be at all critical, and the amp to work fine without it in circuit at all.
Those seem to be FUsible resistors, which behave like standard resistors except when overheated, they crack open.
I suppose the maker leaves an air bubble or something inside the ceramic tube so when heated .... BOOOOM!
The idea is that in the case of a gross short they do not catch fire or burn a hole in the PCB.
Living in Argentina they are not really available for me, so in that case, I replace them with a regular 47 ohms 1/4W resistors but mount them with longer than usual leads, say 1/2" away from the PCB.
In practice, works about the same.
But if yours are cracked open and you can get the real stuff at some supplier, by all means use the originals.
So, what's the last "health bulletin" about your amp?
I suppose the maker leaves an air bubble or something inside the ceramic tube so when heated .... BOOOOM!
The idea is that in the case of a gross short they do not catch fire or burn a hole in the PCB.
Living in Argentina they are not really available for me, so in that case, I replace them with a regular 47 ohms 1/4W resistors but mount them with longer than usual leads, say 1/2" away from the PCB.
In practice, works about the same.
But if yours are cracked open and you can get the real stuff at some supplier, by all means use the originals.
So, what's the last "health bulletin" about your amp?
Update & TP24 results
As I stated in last nights post, I have been testing the Test Points to see if I am getting the values listed on the schematic. Per note 6 TP7 thru 19 are performed with a 3mV signal at the first input. I don't have a signal input yet so I have not performed those points yet. The last remaining one to check was TP24 which does not indicate a value. So I determined which end of R130 and R133 were facing each other by ohming the legs of each. U7 leg 2 and the collector leg of Q23 are both zero resistance to the resistors. The resistor ends facing TP24 show 11.4 v, Q23 C leg shows 11.35v but leg 2 of U7 shows 2.333 v. I don't get that. (Data sheet attached)
I connected the speakers and guitar and made a short video of the meter set to mV which I strummed the guitar on the clean channel. In the video you can hear what it sounds like. Its fuzzy and tinny sounding. The amp was on for about 1.5 to 2 minutes and shut down from overheating just as I turned the recorder off. I walked outside to my carport and grabbed my laser temp meter. The heat sink was 140 deg. The following link is to my server for you to download the video if you wish to see it and listen. The amp shows .5 mV when sitting idle and jumps around when I play the guitar.
fu47 does not appear to be blown so I will leave it alone.
BR
As I stated in last nights post, I have been testing the Test Points to see if I am getting the values listed on the schematic. Per note 6 TP7 thru 19 are performed with a 3mV signal at the first input. I don't have a signal input yet so I have not performed those points yet. The last remaining one to check was TP24 which does not indicate a value. So I determined which end of R130 and R133 were facing each other by ohming the legs of each. U7 leg 2 and the collector leg of Q23 are both zero resistance to the resistors. The resistor ends facing TP24 show 11.4 v, Q23 C leg shows 11.35v but leg 2 of U7 shows 2.333 v. I don't get that. (Data sheet attached)
I connected the speakers and guitar and made a short video of the meter set to mV which I strummed the guitar on the clean channel. In the video you can hear what it sounds like. Its fuzzy and tinny sounding. The amp was on for about 1.5 to 2 minutes and shut down from overheating just as I turned the recorder off. I walked outside to my carport and grabbed my laser temp meter. The heat sink was 140 deg. The following link is to my server for you to download the video if you wish to see it and listen. The amp shows .5 mV when sitting idle and jumps around when I play the guitar.
An externally hosted image should be here but it was not working when we last tested it.
This one.mp4 23-Jan-2013 21:29 2.2M So I guess now I purchase or make a signal generator and test the other TP's?fu47 does not appear to be blown so I will leave it alone.
BR
So that's it? Nothing? So insights of what might be the problem here or suggestions on a direction to proceed in? I thought the idea of this board is to solve problems? I am pretty sure I have accomplished all tasks assigned and posted the results each time. Am I missing something here?
I'm reading as fast as I can take in information on the subject of SS guitar amps. ???
I'm reading as fast as I can take in information on the subject of SS guitar amps. ???
A blunderbuss way of fixing it is to remove all semiconductors from the pcb.
Test them all on a DMM for Hfe.
While the transistors are out check all values of discrete components (R's and C's)
Replace all potentiometers. Replace all IC's.
Then put back together and set the pots as per spec sheet.
I had an amp that I used this approach with and by sods law it was the last transistor unsoldered that had an Hfe of 1.
+++++++++++++++++++++++++++++++++++++++++++++++++++++
What I sometimes do is remove the output transistors and feedback the output into the LTP feedback. Wit hthis it is possible to power up the pcb without blowing fuses.
Test them all on a DMM for Hfe.
While the transistors are out check all values of discrete components (R's and C's)
Replace all potentiometers. Replace all IC's.
Then put back together and set the pots as per spec sheet.
I had an amp that I used this approach with and by sods law it was the last transistor unsoldered that had an Hfe of 1.
+++++++++++++++++++++++++++++++++++++++++++++++++++++
What I sometimes do is remove the output transistors and feedback the output into the LTP feedback. Wit hthis it is possible to power up the pcb without blowing fuses.
Last edited:
Nigel
Nigel
I'd like to avoid throwing parts at it hoping to accidentally cure the problem. But as a last resort I will probably do something like that.
When you say pull and test the transistors, are you saying to pull all of them one by one and test them? Or are you talking specific ones? Q18 thru 21 have been replaced and all show correct values until the heatsink overheats.
The issue with the fuse blowing went away when I changed Q18 thru 21.
Thanks!
BR
Nigel
I'd like to avoid throwing parts at it hoping to accidentally cure the problem. But as a last resort I will probably do something like that.
When you say pull and test the transistors, are you saying to pull all of them one by one and test them? Or are you talking specific ones? Q18 thru 21 have been replaced and all show correct values until the heatsink overheats.
The issue with the fuse blowing went away when I changed Q18 thru 21.
Can you explain this further please. I don't understand all the lingo here.
Thanks!
BR
I wasn't suggesting throwing all the transistors away just remove them one by one testing as you go until they are all out. Who knows you might have multiple failures. This leaves few parallel paths for testing RC components.
The LTP is the long tailed pair on the front end but if you are not blowing fuses then you probably wont need to do that.
Its a pig of a job doing it this way but that is usually my last resort method.
I fixed an old Maplin amp that way, turned out someone had put in a wrong transistor !!! They used an npn instead of a pnp.