I am attempting to repair a Peavey Standard 130 watt power module and would appreciate some advice. I am not experienced in amplifier or electronics repair but I have taken all the electronics courses that the local Jr College (Mesa Jr. College, San Diego) had to offer. I have a lot of theory but not any practical experience.
The amplifier will pass a music signal to the speaker when I patch an audio signal directly into it, but the music is cracked and fuzzy, very distorted. Here we have bypassed the pre-amp front end completely. This unit is in a Standard Power Pak, which is a guitar setup.
I have the schematics and have been poking around with a computer sound card oscilloscope (Daqarta). I have also constructed a simple tone generator so that I can use a stable signal to trace.
What problems might allow a simple amplifier like this to still work even though it is distorting the signal? I have checked the main power transistors in circuit using a diode checker on my Beckman DMM.
I recently successfully repaired a Peavey XR600E using this same diode checker to identify the 3 blown transistors in the left side. I also had the right channel to compare readings to, so that was a great help.
Any help you can provide would be greatly appreciated.
Scope the output where the speaker would connect while feeding a tone into the amp and see what the distortion looks like. That can give you valuable clues.
The most common failure in a power amp is the output stages or the power supply. There will be a transistor or several transistors in parallel that reproduce the positive half cycle, and one or more to reproduce the negative half cycle. If one side goes out , you still have sound but only one phase of the sine wave, thus, big distortion. Check all of the output transistors with an ohmeter. Check the driver transistors too. Check the low valued emitter resistors (usually less than an ohm). bg
Also, be sure that your 'tone generator' is not over-driving the input of whatever stage you are connecting to, and is completely AC coupled, and is not loading or otherwise shifting any bias supply that may be present at whatever point you are connecting in at, which is a little unclear from your post. I am assuming that you are going in at the input to the power amp when you say that the preamp is "bypassed"? Does this amp have an effects send / return jack pair ? This is usually a good place to 'break in' post preamp / pre power amp. Also, as James suggested, looking at the shape of the distorted output aginst what you are putting in, will give a very good clue as to what is going wrong. Just as a matter of interest, what shape wave is your home-built tone generator producing ? It must be a sine wave for general testing. Most audio stages do not like having a square wave thrust up them ...
All measurements taken with the tone generator built from a circuit posted on
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Tone generator outputs 800 Hz at 6.1 mV AC. (Original circuit runs around 1 kHz at 10 mV AC. I had to substitute some resistors/caps.) Tone generator is connected directly to the input of the Peavey 260 power amp module, which seems to be a generic power unit for several different Peavey products.
Trace measurements were taken with the DOS version of DAQARTA, a sound card oscilloscope program from
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DC ON THE OUTPUT - NOISES
Generator off, amplifier energized - Initially quiet, 0.1 mV DC on output.
20 seconds -2.1 mV DC, low buzzing/humming noise begins, remains constant. No other noise present at any time.
The DC present on the speaker output changes from around -25 mV at idle to +25 mV upon shutdown, at which time it rapidly declines, over
10 to 20 seconds, to 0.
EXCESSIVE AC CURRENT DRAW -
It has not blown the 5 amp fuse that is standard for this unit.
I did not check for hot caps. I will run the unit for 30 minutes, unplug, bleed down the caps, and then feel for unusual temperatures, just as an exercize.
DEATH CAP -
I see in the schematic and on the board that there is a .022 uF 600 V cap connected to the primary side of the power transformer input lead and to ground. A dpdt switch allows one to select which of the incoming power leads will connect to this cap. This sounds like the death cap mentioned by Mike Schway in the AGA General FAQ, item 70. The amp does have a 3-wire power supply cord factory connected and the green does go directly to ground. Should I remove this cap? There is also a .022 uF 125 VAC cap across the two power lines which I know is necessary.
TRACES -
I am including the traces in a zip file. I hope it is alright to post here. If the system won't allow me to post a binary I will be right back with a hyperlink to the file. Is a hyperlink the preferred way to do this?
EXPANDED means that the time base displayed has been shortened from 25 ms to 6 ms to show more detail in the individual waveform.
TONE GENERATOR OUTPUT * what it is. The waveform is a little crumpled at the bottom. Still, the amplifier should be able to reproduce this without a problem. The wave is consistent, and that is what matters.
MAIN OUTPUT - NO INPUT * shows the speaker out with no input. There is what appears to be a 60 cycle signal peaking between +20 mV and -20 mV. Sharp spikes of released energy occur whenever the trace crosses the zero line.
MAIN OUTPUT - SIGNAL * is the speaker output waveform with the input signal energized. The tops of the waveforms are rounded, and peak around 50 to 100 mV. The bottoms are narrow V-shaped spikes that extend down to -150 to -200 mV.
COLLECTOR 6530 WITH INPUT * displays an 800 Hz waveform with a distinct 60 cycle spike. The mate to this transistor,
COLLECTOR 6533 WITH INPUT * shows the same waveform minus the 60 Hz spike.
431C BASE WITH INPUT EXPANDED * for whatever it is worth. Is this some kind of crossover point for the positive and negative cycles?
A RECAP - (In more ways than one....)
According to AGA, Technical FAQ, article 21, item 7, line 5, a Mr. Zenon Holtz advises us that, (regarding noisy amps and electrolytics)
"Usually, in really old gear (over 25 years), just replace them at the first sign of problems - they have a finite life."
There is a date stamped on the inside rear of the metal mounting plate that reads "Aug 18, 1977". It doesn't appear that the amp has ever been worked on.
So I guess the first order of the day is to do the recap job. Should I replace the mylar and ceramic disc caps, too? I am a total newbie here, so any and all advice is welcome. I have the time to do it right. Is there any difference between cap makers? Are there any favorite suppliers?
I have ordered a few parts from Peavey in the past, they are very fast and do not charge for shipping. Perhaps they would be the place.
Do the 'scope traces show any signs of other trouble? Or do all the glitches relate to crappy cappys?
All the diode junctions I could test in circuit seemed to show logical results, considering they are often paralleled with other components. However, when I take the board apart to do the caps I intend to check what components that I can.
Thanks for any and all help,
d0ct0r rAtstAr
Apparently I can't post binaries to this newsgroup. Here is a link to the 'scope traces if anyone is interested.
How would you go about checking the 10 watt .33 ohm resistors? I am not sure my DMM will measure anything that low.
I can check the diode PN junctions with the diode checker on the DMM.
The distortion is not real big, but it is present. As per my latest post here, I believe the culprit is leaky old electrolytic capacitors, but it may not be the only trouble.
Do you commonly unsolder different components to measure them? I am new to this game.
The generator is producing a sine wave at 800Hz, 6.1 mV AC. I am injecting it directly into the power amp input, which is where the preamp was plugged into.
The Standard 130 watt power amp module is used in different Peavey products as a back end. In this configuration it uses a guitar amp front end. The two metal chassis plates are the front (preamp) and back (power amp) of a wooden box that contains them.
This amp module is built with a PA front end with 4 inputs, or it can be built into it's own self-contained unit with a simple front end as a power booster, which is really just a power amp.
I've had a look at your results, and the first thing that I feel that I must say, is that the output waveform from your test circuit is simply not good enough to do any serious evaluations of what is going on. It really does need to be a sine wave. How can I put it ? What you are doing, is a bit like trying to find a steering fault on your car, with one of the tyres flat. You either need to have a proper signal generator, where you can properly control the output level and frequency, or use a second sound card with a signal generator program running. You can easily build yourself a generator using what's generically called a 'function generator' IC or a less sophisticated one using a phase shift oscillator.
Looking at your results, it's very hard to say if what you are seeing is fundamentally 'wrong'. All amps will have a degree of residual hum, albeit usually very small. However, when you start looking at output stages with the vertical scale cranked up into the mV / cm range, the stuff on there can start to look horrendous. What you see in "Main output no input" is the residual hum, and at +/- 30mV or so, I would not have immediately declared this excessive. Some crossover distortion is apparent, but at such a low drive level from the hum, it may be perfectly natural for the output stage to produce such.
It's hard to say exactly what we are seeing on "main out signal", because the output level is still extremely low, compared to what you would have here for even a modest 'in use' output level. On that point, I am surprised that you think that the input level for the power amp is likely to be 10mV. I can buy that for the maximum input level to the preamp, but the input to the power amp is likely to be volts. Do you have some documentation that suggests such a low input level ?
As far as your output tests go, a drift in the DC conditions of a few mV either side of zero over a period of a half hour, is quite normal, and would not usually indicate that there is any problem in the output stage worth looking for. The drive waveforms for the bases of the outputs do look pretty poor, but actually look more like an overdrive condition than anything else.
All in all, it's very hard to say what is going on with the results you have shown. First you need to get a proper sine wave with an adjustable output. You need to load the output stage, either with a proper impedance speaker (not very friendly to the neighbours) or a dummy load of some description. For the purposes of getting proper test results, power resistors stacked up in parallel to get to 8 ohms or whatever, will do. You then need to drive the output module to a point where it is delivering some power - that is
*volts* of output across a load, not mV. If you don't have enough output from your generator to do this, then make use of the amp's guitar front end. Drive it with 5mV and 'scope the output of the preamp to make sure that you have got everything set such that it is delivering a clean, but large drive signal to the power amp. The amp's "master", being normally on the end of the final mixdown from the preamp, will then serve as your level control.
Once you have a clean easily identifiable waveform going in (that's a sine wave ... ) and some watts being developed across a load, you will be in a position to make a realistic evaluation of what - if anything - is actually going wrong.
As far as doing a blanket replacement of the electrolytics is concerned, I would strongly advise against that, if you are not an experienced engineer. I would agree that electrolytics are a source of problems in most electronic equipment, and that engineers working on this stuff will sometimes use the shotgun approach where these caps are in a known troublesome environment, such as a switch mode power supply, but with kit like this, you are far better off to actually find the fault, if any, rather than risk creating another which, with the best will in the world, is easily done, particularly by someone inexperienced. The slightest slip-up in a DC coupled output stage such as this, can be disatrous, resulting in the magic smoke escaping from many of the components ... If you really want to learn proper fault finding, it's not the way to go.
I'm sure that there are others on here who have also been mending PA amps for many years, who would agree with most if not all of what I've said, so I hope you will take it as valid observations and advice.
Agree 100%. For a period of time I quit servicing all other pro audio gear except for PA gear. I had stacks of Soundcraftsman amps to the ceiling. Worked on Peavey CS series almost as much. Did weekly repairs on Crown, BGW so I am reasonably qualified to comment. I think what's happened here is the quality of testing and measurement on the OP's side isn't good enough to provide help. The OP needs to step up to the plate. These amps aren't rocket science and some fundamental troubleshooting steps much more often then not will reveal the problem(s) provided you have the skill and experience to perform them. All these dynamic tests the OP is doing will do little in the absence of a quality static workover with just a simple DVM.
The emitter resistors tend to open up completely or have high resistance. As long as you get continuity below 1 ohm you should be OK. I would desolder the power transistors which lifts one leg of the emitter resistors, seeing as you want to test the power transistors out of circuit anyway. Your DMM diode checker can be used to test the power transistors for opens and shorts which is a very common failure. bg
Thank you Meat Plow for sharing your time and experience. I'm quite willing to admit I am inexperienced and appreciate any and all advice I can get. As you can see from today's posts I agree with you and Arfa totally.
problem(s)
That is the problem. I don't have those skills. I am learning them now. Can you recommend some good books?
I am interested to learn more about how to do a static workover with my DMM.
If this amp (plays, but fuzzy and somewhat distorted) came to you would you listen to music through it or scope it or do a static to it first?
I moved out here to the Oregon backwoods after working in construction for 35 years in Southern California, so I am out of touch with many resources as far as on-the-job training.
There seem to be a number of these old amps here that need a little resurrecting. I enjoy this and may make some money and friends pursuing my hobby. I can take these amps home and work on them at my leisure, so it is a good deal for me.
If I may ask, where and how did you learn your trade? If you had amplifier repair jobs stacked to the ceiling, I am guessing that you worked in a major city.
Thank you for this information, BG. They test around .9 ohms, which is out of the 10% tolerance, but this reading may be due to my meter and lead connections. I will call it continuity below 1 ohm.
The main power transistors read OK as measured across the diode setting on my Beckman DMM. I read 500 - 600 mV drop across the appropriate pn junctions. These are out-of-circuit readings. I plan on pulling and checking the smaller transistors today.
The power amp can play music, but it sounds like an am radio station that is not quite tuned in to the center of the station. I did the chopstick test and it suddenly "tuned in" when I struck a certain transistor, but it relapsed a few minutes later and I haven't been able to do that again. It may have been coincidence.
Someone recommended resoldering all connections of old circuit boards to relieve the accumulated effects of thermal stress. What do you think of this?
Arfa, Thank you for your time. Your information is of great value to me.
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Here is a circuit built around the 8038 that I plan on pursuing. In the meantime I will use a sound card generator.
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is the o'scope program I am using now. It has tone generator capabilities.
My dime-store signal generator is a nifty little device in its own right, but I see that it has severe limitations.
Now I know what residual hum looks like, and how much is acceptable. Very good.
I am 55, retired steamfitter, piecing together a second income out of a fascination for electronics and a love of music and gadgets. I know the theory very well but I have no practical experience whatsoever, especially in the field of trouble-shooting. I am starting out with baling wire and bubble gum and a desire to do more, and operating on a shoe-string budget. I have a friend who knows people with broken music amps.
I built the tone generator from the
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page. I am using an old 8 ohm 50 watt speaker for a load. The generator outputs at 10 mV,
100 mV, and 1 V. I substituted a few R values, and got less output,
6.1 mV instead of 10 mV. This level (6.1 mV) patched directly into the power amplifier input produces an annoying sound level from the speaker.
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I plan on purchasing an 8 ohm 200 watt dummy speaker load from partsexpress.com so I can "pump it up" and see what it be like. I will use my sound card generator/scope to check it out then. I would like to be able to tap off this load to power a speaker at a low volume level so I could hear what is going on, as well as see it.
I'm sure you will agree that starting out with 10 mV for an input is better than using 10 V. Bear in mind that I am shooting in the dark. Now I know that I can safely use several volts of input on the power amp.
It is one thing to take college classes in theory and another to have hands-on experience. We did lots of exercises but they were all laid out for us. I finished all the courses the local junior college had, in 1995. Most of our training was in digital work, with emphasis on the 8086 IBM PC. I built a few neat toys and used the serial port and assembly language to operate them. I would love to tinker with robotics more. It is not possible to make a home business out of fixing robots, not yet anyway.
I would like to apprentice to an experienced tech. I could learn much in a very short while. But that is not an option for me at this point.
Does it seem strange that there is a (possible) overdrive condition when the input is so low? Is there something to be learned there? I understand that the input signal is too low to make much of any of this, except for that I need better equipment.
I'm puzzled why they are around 3 volts pk-pk when the output of the whole circuit is in the millivolt range, but I don't understand the system as a whole yet. I hope to find the time to build these amps in LTSpice, so I can observe the interaction of the various parts in a simulation.
Very good idea to use the preamp (as a preamp.) I will make use of that if I can't get a decent output from my sound card or new 8038 unit that I might build. The preamp needs a more critical look as it is, although I know the power amp is acting bad.
Music injected directly into the power amp input sounds distorted and fuzzy, like an AM station that is not tuned on center. I have done this with other units so I am reasonably sure my source is ok. I got it to come clean for a very short while by doing the chopstick drum solo on the different parts, but it relapsed and I haven't been able to do that again. It was a cool drum solo, though.
I do want to learn proper fault finding. Can you suggest any books, particularly as relates to music amplifiers? I have been reading the faqs for the various electronics newsgroups, including this one.
Would an experienced engineer consider a blanket replacement, especially if the equipment is over 25 years old? I know that electrolytics can dry out over time, but have also been told that there are many variables in that. From what I have read, the replacement may not be as good as the one replaced, in many instances. If someone brought this amp in, would you do a listening test, input and scope, or start measuring with your DMM?
I was able to find 3 shorted transistors in a fuse-eating Peavey by using the series light bulb trick and comparing voltages between channel 1 and 2. Measuring the diode barriers also uncovered the bad units. I repaired a few other units by replacing filament fuses and stroking sticky input and effects return jacks. So I am having a little bit of success with this. But I still have a lot to learn.
Do you have any recommendations on procedures and test gear for evaluating capacitors?
I thank you very much for sharing your time and experience with me. Your observations and advice are very valuable to me. I know more than I did before.
For whatever it is worth, I have posted this question on alt.guitar.amps. Lord Valve has made the very same points that you have made.
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