indianajo, all the output emitter resistors measure 0.000V across them. Each channel has two potentiometers adjacent to the opamp for setting DC balance. At 0 gain you use one pot and at 1/2 gain you use a second pot to set DC balance to less than .05V. I used R105(pot) to adjust DC balance to 0 and when I checked offset at 1/2 gain no adjustment was required.
djk, I will be replacing all the high fail items that you have mentioned. No I don't want to disassemble the amp again. I also plan on putting a load on the amp and let it run for 24hrs at 1/2 gain just to minimize any immediate failures.
djk, I will be replacing all the high fail items that you have mentioned. No I don't want to disassemble the amp again. I also plan on putting a load on the amp and let it run for 24hrs at 1/2 gain just to minimize any immediate failures.
1n5240 is a 0.5 W zener with 20 ma test current.
I'm not going to download 1N961 datasheet I'll leave that to you. i mostly get datasheets from Datasheet catalog for integrated circuits, diodes, triacs, and other semiconductors, view. If the 1n961 is a 0.5 watt or smaller that should be fine. The b is 5% tolerance, Don't forget the electrolytic capacitors, the ones with a plus on one end, that are aluminum cans filled with slime water sealed with cheap rubber. The rubber age cracks, running or sitting on the shelf. If an amp has problems I'll replace them all at 15 years, although I do it proactively at twenty. I replace two at a time and test, to make sure I didn't make a bad solder joint. If I did, if the amp performs worse after, I know right where the problem is.
I've never had a zener go bad that wasn't blown by a blown up transistor, but I suppose 1n961 are early technology and djk knows something I don't about their life. I have heard other techs say a zener diode is a poor quality diode waiting to fail. I had lots of blown zeners in my latest amp, but everything around them was blown, too.
I use 10 ohm sliding tap 225 watt ohmmite resistors to test, set to 8 ohms. For my bigger amps I use five ohm 225 watt Dale resistors I got at a surplus house for $5 each + freight. the 10 ohm sliding tap resistors were $8 when I bought them, but are about $30 now. You can also assemble parallel collections of surplus resistors, say twenty watt 100 ohms or something. I put these on a metal plate with metal tabs hooked up into the logs, to get them up off the coffee table so they don't burn it.
Congratulations on 0 speaker voltage at idle.
I'm not going to download 1N961 datasheet I'll leave that to you. i mostly get datasheets from Datasheet catalog for integrated circuits, diodes, triacs, and other semiconductors, view. If the 1n961 is a 0.5 watt or smaller that should be fine. The b is 5% tolerance, Don't forget the electrolytic capacitors, the ones with a plus on one end, that are aluminum cans filled with slime water sealed with cheap rubber. The rubber age cracks, running or sitting on the shelf. If an amp has problems I'll replace them all at 15 years, although I do it proactively at twenty. I replace two at a time and test, to make sure I didn't make a bad solder joint. If I did, if the amp performs worse after, I know right where the problem is.
I've never had a zener go bad that wasn't blown by a blown up transistor, but I suppose 1n961 are early technology and djk knows something I don't about their life. I have heard other techs say a zener diode is a poor quality diode waiting to fail. I had lots of blown zeners in my latest amp, but everything around them was blown, too.
I use 10 ohm sliding tap 225 watt ohmmite resistors to test, set to 8 ohms. For my bigger amps I use five ohm 225 watt Dale resistors I got at a surplus house for $5 each + freight. the 10 ohm sliding tap resistors were $8 when I bought them, but are about $30 now. You can also assemble parallel collections of surplus resistors, say twenty watt 100 ohms or something. I put these on a metal plate with metal tabs hooked up into the logs, to get them up off the coffee table so they don't burn it.
Congratulations on 0 speaker voltage at idle.
Usually when I buy a capacitor, the distributor has 200 to 2000, not 2. You are not buying electrolytic capacitors off e-bay are you? What is the manufacture date? You realize these modern capacitors, a 2001 capacitor has the same year code as a 2011 capacitor, ie "1"?
Remember, rubber deteriorates sitting on the shelf or in circuit. Unless they are 10000 hour service life capacitors which probably has something better as a sealant than gum rubber.
The top US distributors that sell new things direct from the manufacturer that I know of are farnell.com, mouser.com, digikey.com, partsexpress.com, avnetdirect.com, alliedelectronics.com.
They do charge for shipping and the minimum is usually about $8.
I bought one extremely correct capacitor from "Stereo Cost cutters" once that lasted all of three powered hours, including fifteen years unpowered on the shelf. Leaked all over the case bottom and blew the fuse.
Remember, rubber deteriorates sitting on the shelf or in circuit. Unless they are 10000 hour service life capacitors which probably has something better as a sealant than gum rubber.
The top US distributors that sell new things direct from the manufacturer that I know of are farnell.com, mouser.com, digikey.com, partsexpress.com, avnetdirect.com, alliedelectronics.com.
They do charge for shipping and the minimum is usually about $8.
I bought one extremely correct capacitor from "Stereo Cost cutters" once that lasted all of three powered hours, including fifteen years unpowered on the shelf. Leaked all over the case bottom and blew the fuse.
I've had the same experience with 'new' parts off the shelf from the local parts jobber, now I only buy from Digi-Key or Mouser. Digi-Key will ship a small package via US mail for about $5.
When buying NOS caps they need to be baked at max temperature for several hours to re-distribute the electrolyte, and then re-formed with a 10mA CCS to the max voltage. Units with excessive electrical leakage will have to be discarded.
Surplus parts may not always be a good deal, I've seen major issues, even with carbon film resistors and film caps.
When buying NOS caps they need to be baked at max temperature for several hours to re-distribute the electrolyte, and then re-formed with a 10mA CCS to the max voltage. Units with excessive electrical leakage will have to be discarded.
Surplus parts may not always be a good deal, I've seen major issues, even with carbon film resistors and film caps.
yes i did think of that as I pushed the buy now button. will take this on the chin and repurchase from Mouser or Digikey. I see specing a capacitor has a few things to not;
temp range? is 85degrees okay or go to 105 degrees?
tolerance ? what should I shoot for? looks like if i spec 10uF 160V there are only 20% tolerace available?
temp range? is 85degrees okay or go to 105 degrees?
tolerance ? what should I shoot for? looks like if i spec 10uF 160V there are only 20% tolerace available?
That part has a high ripple current through it, I would try and get a 105°C part, if available.
You cook to the maximum rated temp only, or just a bit less. I usually do about 150°F, the minimum my oven if stable at.
When that amp was new, tolerance on electros was typically -10%/+100%. I wouldn't go higher than 22µF/200V.
You cook to the maximum rated temp only, or just a bit less. I usually do about 150°F, the minimum my oven if stable at.
When that amp was new, tolerance on electros was typically -10%/+100%. I wouldn't go higher than 22µF/200V.
wanted to hear audio out of the repaired channel 1. Bias setting is still not correct. Hooked up an input and a speaker, cleaned gain pot 1. Turned on amp, light bulb glowed brightly then dimmed, I could hear faint music with gain all the way down. Turned gain pot just a little and music level increaesed. Left it on less than a minute and then powered it down. After power down could still hear faint music for 20 seconds or so. Also realized that I had not turn gain pot back to zero. Noticed the IOC light channel 1 glowing dimly for even longer time. Powered back on and the light bulb glowed and didnt dim and hum from Channel 1...shut it down.
After disconnecting speaker wires it now powers back up with bulb glowing brightly and then quickly going dim. I checked DC offset on channel 1 still measuring 0V.Checked bias setting still the same. Checked driver base to emitter still meauring .6V. Doesnt seem that anythin blew up. Whats going on?
After disconnecting speaker wires it now powers back up with bulb glowing brightly and then quickly going dim. I checked DC offset on channel 1 still measuring 0V.Checked bias setting still the same. Checked driver base to emitter still meauring .6V. Doesnt seem that anythin blew up. Whats going on?
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Good you had no DC out. My test setup includes a pair of charity resale shop car speakers, in series with back to back electrolytic capacitors, placed on the output of the amp. That is capacitor minus to capacitor minus, one capacitor plus to the speaker, one plus to the amp output. That way if the amp whangs to DC output (mine did for weeks) the speakers are not blown up, but you can still hear the music. You can hear a big pop when the output goes into DC voltage when whatever it is fails after it heats up.
I exercise the amp with a battery transistor radio, so that if the input whangs to DC, it won't blow up.
If the DC voltages are all plausible, then I start AC analysis, where I try to follow the AC through the system. I had dirty input pots, dirty input to amp board connectors, then a bad solder joint where I'd replaced an electrolytic input capacitor. Scopes are nice if you can afford a new one, but I use an "obsolete" analog VOM with a 2 VAC and 20 VAC scales, to trace the music through the amp. I put a 0.47 uf 600 v film cap in series with the negative lead, that I attach with clip lead to speaker return. Note the AC scale of most DVM's produce random numbers on music. They are designed to measure the wall socket voltage, not music. Also you can't see the beats of rock music on a DVM that averages over 4 seconds, and seeing the meter pointer beat time is an important tell tale that you are seeing music, not oscillation of an open high gain point.
Then I test for the music. It will be about 1 VAC coming in, then grows in voltage as you go through the various stages. After the driver transistor, it is all current, you can't see music again until the output. The capacitors I used in series with the speaker were the used power supply ones, if they blow up and leak onto a pad on the coffee table, whoops. Those will be rated at the rail voltage which is what your speaker output could go to worse case.
I'm not sure you have made enough DC measurements. that this wouldn't still be the most productive technique to find problems. Use the speaker to tell you whether there is a problem right now or not, but check DC voltages. The mostly likely problem is a bad solder joint that you yourself made, but I ended up finding a bad solder joint on the input of an op amp that had been there since 1994, occasionally. I also found bad op amp supply filter capacitors, blown capacitors in the VI limiter, vaporized resistors I didn't even know what they were for, just two leads poking up from the output transistor assembly, etc etc.
I was suspicious of your theory that only the driver and output transistor needed replaced. When those blow, high voltage leaks out the base of the driver into other things, all of which need to be checked for proper DC voltage anyway.
For a final test, you can parallel your speaker/capacitor arrangement across part of a load resistor network and cook the amp several hours at near rated wattage. I use two 5 ohm resistors in series on the speaker terminal, and put the 4 ohm speaker capacitor arrangement across one of them. Sixteen ohm or 8 ohm speaker would be better, but for $2 four ohm is what I found. If your solder joints hold up under heat stress, and the output transistors stay isolated from the heat sink when hot, you've done a successful job.
I exercise the amp with a battery transistor radio, so that if the input whangs to DC, it won't blow up.
If the DC voltages are all plausible, then I start AC analysis, where I try to follow the AC through the system. I had dirty input pots, dirty input to amp board connectors, then a bad solder joint where I'd replaced an electrolytic input capacitor. Scopes are nice if you can afford a new one, but I use an "obsolete" analog VOM with a 2 VAC and 20 VAC scales, to trace the music through the amp. I put a 0.47 uf 600 v film cap in series with the negative lead, that I attach with clip lead to speaker return. Note the AC scale of most DVM's produce random numbers on music. They are designed to measure the wall socket voltage, not music. Also you can't see the beats of rock music on a DVM that averages over 4 seconds, and seeing the meter pointer beat time is an important tell tale that you are seeing music, not oscillation of an open high gain point.
Then I test for the music. It will be about 1 VAC coming in, then grows in voltage as you go through the various stages. After the driver transistor, it is all current, you can't see music again until the output. The capacitors I used in series with the speaker were the used power supply ones, if they blow up and leak onto a pad on the coffee table, whoops. Those will be rated at the rail voltage which is what your speaker output could go to worse case.
I'm not sure you have made enough DC measurements. that this wouldn't still be the most productive technique to find problems. Use the speaker to tell you whether there is a problem right now or not, but check DC voltages. The mostly likely problem is a bad solder joint that you yourself made, but I ended up finding a bad solder joint on the input of an op amp that had been there since 1994, occasionally. I also found bad op amp supply filter capacitors, blown capacitors in the VI limiter, vaporized resistors I didn't even know what they were for, just two leads poking up from the output transistor assembly, etc etc.
I was suspicious of your theory that only the driver and output transistor needed replaced. When those blow, high voltage leaks out the base of the driver into other things, all of which need to be checked for proper DC voltage anyway.
For a final test, you can parallel your speaker/capacitor arrangement across part of a load resistor network and cook the amp several hours at near rated wattage. I use two 5 ohm resistors in series on the speaker terminal, and put the 4 ohm speaker capacitor arrangement across one of them. Sixteen ohm or 8 ohm speaker would be better, but for $2 four ohm is what I found. If your solder joints hold up under heat stress, and the output transistors stay isolated from the heat sink when hot, you've done a successful job.
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*A supply voltage detector virtually eliminates this problem
in the DC-30OA. The detector disconnects the regulated
supply voltages to the output stages during these turn-on
and turn-off periods, thus not allowing the DC offsets at the
output to occur.
This may be what I am having problems with?
in the DC-30OA. The detector disconnects the regulated
supply voltages to the output stages during these turn-on
and turn-off periods, thus not allowing the DC offsets at the
output to occur.
This may be what I am having problems with?
I would check the voltages of the working channel against the voltages of the non-working channel. You've got the idle voltages on the schematic, you are way ahead of my last project.
Although, my latest amp I fixed with with neither channel working. Vbe was not 0.6 to 0.7, or Vce wasn't somewhere between the power supply rail and 1.5, (Amps don't use transistors for switching except for the turn on timer circuit), Or a diode wasn't dropping 0.6-.7 or back biased, or the op amp wasn't centered on speaker ground with no signal, or something like that. Resistors werent the right value, same or lower than the size printed on them. Or capacitors didn't have DC across them, If you've got DC on the output with a load on it (use a resistor, not a speaker) something is turning the output transistors on that side on. What is doing it? If IC1 pin 1 is at 1.55 v like the print says, why is Q102 conducting? (if your offset voltage is plus). The bad channel is A, right?
If zero input voltage (short the input jacks) causes DC on the output with the speaker/load resistor connected, perhaps something is bad in the feedback circuit (the dotted line in the schematic). Feedback is how the IC and stuff around it even knows there is a speaker connected.
As far as shipping costs on all the little parts you find blown up, I found old junk a useful resource. A 1n4148 is the same in an amp or a dead PC power supply, a bigger diode might be found in a dead computer display, resistors and capacitors might come out of anything. Of the 117 components I found blown up in my last amp, I only bought new about forty of them. Mostly I bought new output transistors, drivers, electrolytic capacitors. I had 15 volt zener diodes, I needed a 16 for this amp, I just put a regular diode in series with the 15 v zener to make 15.7 v. I needed a 0.015 uf capacitor in the VI limiter, I paralleled .01 and a 4700 pf.
Although, my latest amp I fixed with with neither channel working. Vbe was not 0.6 to 0.7, or Vce wasn't somewhere between the power supply rail and 1.5, (Amps don't use transistors for switching except for the turn on timer circuit), Or a diode wasn't dropping 0.6-.7 or back biased, or the op amp wasn't centered on speaker ground with no signal, or something like that. Resistors werent the right value, same or lower than the size printed on them. Or capacitors didn't have DC across them, If you've got DC on the output with a load on it (use a resistor, not a speaker) something is turning the output transistors on that side on. What is doing it? If IC1 pin 1 is at 1.55 v like the print says, why is Q102 conducting? (if your offset voltage is plus). The bad channel is A, right?
If zero input voltage (short the input jacks) causes DC on the output with the speaker/load resistor connected, perhaps something is bad in the feedback circuit (the dotted line in the schematic). Feedback is how the IC and stuff around it even knows there is a speaker connected.
As far as shipping costs on all the little parts you find blown up, I found old junk a useful resource. A 1n4148 is the same in an amp or a dead PC power supply, a bigger diode might be found in a dead computer display, resistors and capacitors might come out of anything. Of the 117 components I found blown up in my last amp, I only bought new about forty of them. Mostly I bought new output transistors, drivers, electrolytic capacitors. I had 15 volt zener diodes, I needed a 16 for this amp, I just put a regular diode in series with the 15 v zener to make 15.7 v. I needed a 0.015 uf capacitor in the VI limiter, I paralleled .01 and a 4700 pf.
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without load....i switched the amp off and on a couple times with DVM on output. I did see a flash of sometype of voltage then oV. It was not everytime. Tonight with load on it I will confirm plus or neg DC offset. I did confirm that this occurs on both channels. As stated I still have not got the bias setting correct on channel 1, but based on djk's comment the bias setting has no impact on DC offset.
I set the 200ohm trim resistor to 154ohm, 4 more ohms than the current hand select resistor for channel 1 bias setting. I jury rigged the 200ohm trim pot on to the socket for the hand select resistor, used to set bias setting. I was short on time so I turned amp on quickly and thought I saw a reading in the low 300mV range and thought that looked good. In hindsight I should have let the amp warm up. When I got back to the amp I measured closer to where the bias was originally, 410mV. Sadly I purchased 25 pieces of 154ohm resitor.
I need to solder a section of round lead on to the trim pots rectangualar lead so it fits in to the resistor socket better. I will then adjust trim resistor insitu to ensure the bias setting is moving...hopefully be able to get it in to the 310-340mV range with what range I can get out of the 200ohm trimmer.
I agree it is time to step back and get measurements everywhere I can on both channels. These can then be baseline measurements, published for comments.
Also I have the IOC pcb along with the IOC led board and the gain pots free floating from the amp. All was removed from the aluminum front face plate. I am sure this doesnt create any problems?
I have not measured any of the idle volatges between the IC and the predrivers. Will do this as well. thanks indianajo
Yesterday I looked at the negative driver on the output board long enough to learn that it has +0.6/0.7V from the negative rail volatge sitting on the emitter. Different than how the positive driver is operated. I dont remember the voltages well but the base has to have +0.6/0.7V potential from the emitter voltage which is at neg. rail voltage. I sort of panicked when I measured large neg volatge on both the base and emitter until I looked more careful and saw the 0.6/0.7 difference in the two readings.
I set the 200ohm trim resistor to 154ohm, 4 more ohms than the current hand select resistor for channel 1 bias setting. I jury rigged the 200ohm trim pot on to the socket for the hand select resistor, used to set bias setting. I was short on time so I turned amp on quickly and thought I saw a reading in the low 300mV range and thought that looked good. In hindsight I should have let the amp warm up. When I got back to the amp I measured closer to where the bias was originally, 410mV. Sadly I purchased 25 pieces of 154ohm resitor.
I need to solder a section of round lead on to the trim pots rectangualar lead so it fits in to the resistor socket better. I will then adjust trim resistor insitu to ensure the bias setting is moving...hopefully be able to get it in to the 310-340mV range with what range I can get out of the 200ohm trimmer.
I agree it is time to step back and get measurements everywhere I can on both channels. These can then be baseline measurements, published for comments.
Also I have the IOC pcb along with the IOC led board and the gain pots free floating from the amp. All was removed from the aluminum front face plate. I am sure this doesnt create any problems?
I have not measured any of the idle volatges between the IC and the predrivers. Will do this as well. thanks indianajo
Yesterday I looked at the negative driver on the output board long enough to learn that it has +0.6/0.7V from the negative rail volatge sitting on the emitter. Different than how the positive driver is operated. I dont remember the voltages well but the base has to have +0.6/0.7V potential from the emitter voltage which is at neg. rail voltage. I sort of panicked when I measured large neg volatge on both the base and emitter until I looked more careful and saw the 0.6/0.7 difference in the two readings.
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measured output A of dual op amp chip UA739, with neg. lead on channel A neg. speaker terminal, pos. probe to pin 1, -.53V. Should be +1.35V.
measured output B of dual op amp chip, with neg. lead on channel A neg. speaker terminal, pos. probe to pin 13, -.51V. Should be +1.35V.
Measure V+ at chip pin 14, measured +9.24V. Should be +10V
Measure V- at chip pin 7, measured -9.81V. Should be -10V
Did I take these measurements correctly? Seems the outputs are off quite a bit?
measured output B of dual op amp chip, with neg. lead on channel A neg. speaker terminal, pos. probe to pin 13, -.51V. Should be +1.35V.
Measure V+ at chip pin 14, measured +9.24V. Should be +10V
Measure V- at chip pin 7, measured -9.81V. Should be -10V
Did I take these measurements correctly? Seems the outputs are off quite a bit?
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