I could not agree more. For example, I am a car guy as my other hobby. I did however work in the auto industry for years as a mechanical engineer and I can totally see where you are coming from. No amount of equipment will help you in anyway if you do not understand the underlying theory. So your point is well taken. I just want to try and accumulate the proper equipment or at least shop for deals while I am ready, studying and watching videos. And I want to stress again a comment I made somewhere; sorry I do not remember where. I am not expecting to be able to become a tech or even close to know what I am doing, even in the time frame of a year. I am just a middle-aged guy trying to start out and hopefully over the next 5-10 years I can become somewhat competent and maybe by retirement be able to propagate this hobby and have a small business, maybe even one day pass on what I learned to someone else 🙂
I have some uA739 chips also for my own use. Bought a bunch of real ones ages ago. It's a really good chip. They are used in the Marantz 3600 and 3800 preamps, which is why I bought spares.
Sorry to hear you were ripped off indianajo. I am insured, you can never stop someone intent on breaking in to your place.
Not to be insensitive at all, but if you need certain equipment to do a job, you need that equipment. Period. There isn't any getting around it. I would be unable to work without my equipment and would be out of business until it was replaced.
Also yes. Having the gear without understanding how things work or how to use it does not enable you to do a job. It all goes together.
Sorry to hear you were ripped off indianajo. I am insured, you can never stop someone intent on breaking in to your place.
Not to be insensitive at all, but if you need certain equipment to do a job, you need that equipment. Period. There isn't any getting around it. I would be unable to work without my equipment and would be out of business until it was replaced.
Also yes. Having the gear without understanding how things work or how to use it does not enable you to do a job. It all goes together.
I am not in business. Before I had speakers with 25 db down harmonic distortion, I listened to amps with a $40 headphone with low distortion. If a replaced parts amp sounded like 1% HD, I might have put the 100 capacitors in the scope to make it work. PITB job, the boards were glued in the case. Never happened. Compensation problems causing oscillation on certain US products were so infamous I have avoided buying those models (Hafler, SWTC, phase linear) , or followed the cookbook oscillation fixes (dynaco ST120). I do not work on imported products. The one oscillating product I repaired, an op amp mixer, showed undue heat on certain parts. Osclllation confirmed with the VOM & $.50 capacitor. Frankly a $30 non-contact thermometer is a much more reliable product than a 40 year old oscilloscope. Doesn't take as much bench space either. I do not have a room dedicated to test equipment. I repair in a room full of 2 organs LPs & CDs. The oscilloscope sits out of the way where I have never needed to use it.
I have repaired amps repaired by the impatient, (3 initials on heat sink) that did not pursue why output the was whanging with DC to rail voltage more than once. Their solution was a DO NOT USE CHANNEL A label. Very professional. Finding a good looking solder joint that pops loose under certain temperature conditions or mechanical stress , is much more more common than improper compensation. I have salvaged a TV board (for the heatsink) repaired by a pro (company sticker) where a years old used capacitor was put parallel to a failed capacitor. Talk about sleaze. I have one failure, a 75 w mixer amp that goes silent when inside the case, but works fine outside. As I paid $40 for it and it is monaural, tracking that fractured land or bad solder joint has not been a priority. I'm much more concerned about 40 year old roof shingles (1/8 left to do), about the broken hammer shaft on the Steinway, or the Mercedes that got 1000 miles after motor overhaul by a sleazebag before the oil pressure dropped and the compression went too low to start. 10 days after the burglar was released from jail, the two cars I own that do start had the windshields smashed out with a hammer. Unavailable part. Will be replaced by flat glass when I manage to order aluminum channel. BTW the roof was repaired in 1956 & 1980 by sleazebags who cut the new shingles flush against the dormer window, leaving the 1936 shingles to repel water down the valley. Lots of ways to cheat the customer. I paid $850 for a V8 motor overhaul, caught the guy putting in cast iron rings when I specified stainless, then got 600 miles before a gallon of antifreeze started leaking into the oil. I do much better with motor overhauls I do myself.
Note, I never said you do these sorts of things. Many pros do.
You really need to get to the bottom of why the amp failed first.
Just replacing outputs could end up with new outputs blowing as well.
Check drivers too, Check Vbe multiplier and its trimmer.
I would test all components on the pcb, look for shorted caps as well as diodes transistors etc.
Just replacing outputs could end up with new outputs blowing as well.
Check drivers too, Check Vbe multiplier and its trimmer.
I would test all components on the pcb, look for shorted caps as well as diodes transistors etc.
Thank you @nigelwright7557 I agree, I plan to take all outputs off, same with the drivers, test those and then like you said, maybe work back and check/inspect other components, thank you! To me this is part of the learning process even if parts of the work seem redundant or trivial, it is still good for learning!
Depends upon the amount of pain derived from the first mistake.
In the amp with "DO NOT USE CHANNEL A" label, 131 components replaced, most <$.10 . It was used with failed output transistors so often, the power supply regulator diodes were blown, the input & DDE (speaker high freq protection) op amps were blown, and even some 0.1 uf 50 v capactors bypassing +-15 rails near the op amp open, not shorted. Root cause was a good looking ittermittant solder joint on feedback pin of the input op amp. Bad solder joints can be found by listening to the output with a trash speaker protected from DC by back to back 3300 capacitors, for pops. You push on all the pins with a chopstick or meter probe, then when it pops, look to see if some place downstream of your probe has done something bad like whang to a stupid DC voltage. A little faster but more expensive is circuit cool spray. Components can have an INTERNAL bad weld that won't respond to pushing on the solder joint with a stick, but will pop in response to cooling or alternately a hair dryer.
Don't forget when you fire it up again run it thru a incandescent bulb. You may miss one thing and end up blowing everything you just replaced.
I would procede as follows. 1 with dvm check double diode test on outputs, drivers, pre-driver, various diodes, back to Q201 after op amp. Replace any shorts or opens. 2. Leaving output transistors out, power up through 60 w bulb. Check power supply rails symmetrical if lower than +-48, +-15 to op amp nominal. Negative of DVM to speaker ground. replace parts as required to get there. 3. check inputs & output of op amp 2,3,6 all at same voltage. If not you may need one or two. 4. look for plausible voltages on collectors of transistors; between rails, symmetrical, all b-e junctions reading about 0.6 or -0.6 volts. If light bulb is burning, one or more transistors may be shorted c to e or diode not blocking DC, etc. Other resistors or capacitors may be open or shorted, since rail voltage ran back from output transistors into areas where parts were not rated +-48 v. Double diode test does not prove semiconductors etc good at higher voltages. Running on 60 w bulb stresses those parts near rated voltages. Parts declared good with 2 v diode test may short or open when presented with near full voltages. Note Q203 204 are VI limiters, they and associated parts may have been damaged by rail voltage raging out base line of output transistors.
5. When series light bulb is out and D234 D235 symmetrical voltages with ground, (may be many parts replaced to get here) put 1 vac in from transistor radio or signal generator. Look for ~1 vac at op amp output, little higher vac at Q201 , higher vac at Q208 Q212 predrivers. Use 2vac scale of analog vom for front, 20 v scale for predrivers, 100 vac scale near predrivers. or use scope. Seies .047 cap series positive probe of analog VOM, negative probe to speaker ground. Scope probe has its own series cap. E to C of Q211 should be about 2.4 vdc. 6. All tidy, put in drivers Q401 Q402. With no signal speaker out should be at 0 vdc with 1 vac input, music or tone should come out speaker jack on trash protected speaker. 7. put in output transistors, Do dc check on speaker out, check current on r410 r403 about 10 to 20 ma (100-200 mv across 0.1 ohms). Adjust R225 R226 until it is. 8 that nice & stable around 0 volts, listen to music or tone from speaker jack, all still with light bulb series AC line. 9 check function of leds from the 339 comparitor. 10 eveything stable and symmetrical, No undue ac voltage on speaker through 390 pf cap (no ultrasonic oscillation) cooked for several hours into trash speaker (protected by 3300 back to back caps). 11. you may remove light bulb box from AC line. Check again for oscillation, undue heat on any part, and if not, play music on protected speaker for day or two. 12. all stable, nothing too hot, Change 50 year old electrolytic caps 1 or 2 at a time to make sure you are not back in here in days, weeks or months as they fail. Do function test with light bulb box in AC line each 1 or 2 caps to make sure dc voltages stay symmetrical. music flows through as designed, no ultrasound 13 All tidy symmetrical DC voltages, playing music on trash speaker, no ultrasound, Play on a protected speaker for several days, if don't want to listen to it use 150 w 8 ohm resistors. All good 14. do full power check on load resistor or cheap speakers rated at 100 w for an hour. Check no dc, no ultrasound, no parts too hot, put covers on. 15 play on cheap speaker for days or weeks to prove everything is stable. I listen to piano music, tinkly bells, bass drums at this point to prove HD & IMD below 0.1%. If not further probing with sound probe or scope to see where distortion is originating. 16. If that proves okay, may proceed to your speaker with the $280 drivers. Again check distortion with hard to reproduce tracks. 1% HD amps do not sound like a Steinway, particularly on top & lowest octave. Tinkly bells sound like sssh with HD. I use Beethoven Appassionata for low octaves, Peter Nero When I Fall in Love for solo top octave piano, Martin Denny Hawaii for tinkly bells.
5. When series light bulb is out and D234 D235 symmetrical voltages with ground, (may be many parts replaced to get here) put 1 vac in from transistor radio or signal generator. Look for ~1 vac at op amp output, little higher vac at Q201 , higher vac at Q208 Q212 predrivers. Use 2vac scale of analog vom for front, 20 v scale for predrivers, 100 vac scale near predrivers. or use scope. Seies .047 cap series positive probe of analog VOM, negative probe to speaker ground. Scope probe has its own series cap. E to C of Q211 should be about 2.4 vdc. 6. All tidy, put in drivers Q401 Q402. With no signal speaker out should be at 0 vdc with 1 vac input, music or tone should come out speaker jack on trash protected speaker. 7. put in output transistors, Do dc check on speaker out, check current on r410 r403 about 10 to 20 ma (100-200 mv across 0.1 ohms). Adjust R225 R226 until it is. 8 that nice & stable around 0 volts, listen to music or tone from speaker jack, all still with light bulb series AC line. 9 check function of leds from the 339 comparitor. 10 eveything stable and symmetrical, No undue ac voltage on speaker through 390 pf cap (no ultrasonic oscillation) cooked for several hours into trash speaker (protected by 3300 back to back caps). 11. you may remove light bulb box from AC line. Check again for oscillation, undue heat on any part, and if not, play music on protected speaker for day or two. 12. all stable, nothing too hot, Change 50 year old electrolytic caps 1 or 2 at a time to make sure you are not back in here in days, weeks or months as they fail. Do function test with light bulb box in AC line each 1 or 2 caps to make sure dc voltages stay symmetrical. music flows through as designed, no ultrasound 13 All tidy symmetrical DC voltages, playing music on trash speaker, no ultrasound, Play on a protected speaker for several days, if don't want to listen to it use 150 w 8 ohm resistors. All good 14. do full power check on load resistor or cheap speakers rated at 100 w for an hour. Check no dc, no ultrasound, no parts too hot, put covers on. 15 play on cheap speaker for days or weeks to prove everything is stable. I listen to piano music, tinkly bells, bass drums at this point to prove HD & IMD below 0.1%. If not further probing with sound probe or scope to see where distortion is originating. 16. If that proves okay, may proceed to your speaker with the $280 drivers. Again check distortion with hard to reproduce tracks. 1% HD amps do not sound like a Steinway, particularly on top & lowest octave. Tinkly bells sound like sssh with HD. I use Beethoven Appassionata for low octaves, Peter Nero When I Fall in Love for solo top octave piano, Martin Denny Hawaii for tinkly bells.
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@indianajo
I took the liberty of reformatting your post into min-paragraphs--- my eyes were crossing... Hope you don't mind. 🤔
indianajo said:
I would procede as follows.
1. with dvm check double diode test on outputs, drivers, pre-driver, various diodes, back to Q201 after op amp. Replace any shorts or opens.
2. Leaving output transistors out, power up through 60 w bulb. Check power supply rails symmetrical if lower than +-48, +-15 to op amp nominal. Negative of DVM to speaker ground. replace parts as required to get there.
3. check inputs & output of op amp 2,3,6 all at same voltage. If not you may need one or two.
4. look for plausible voltages on collectors of transistors; between rails, symmetrical, all b-e junctions reading about 0.6 or -0.6 volts. If light bulb is burning, one or more transistors may be shorted c to e or diode not blocking DC, etc. Other resistors or capacitors may be open or shorted, since rail voltage ran back from output transistors into areas where parts were not rated +-48 v. Double diode test does not prove semiconductors etc good at higher voltages. Running on 60 w bulb stresses those parts near rated voltages. Parts declared good with 2 v diode test may short or open when presented with near full voltages. Note Q203 204 are VI limiters, they and associated parts may have been damaged by rail voltage raging out base line of output transistors.
5. When series light bulb is out and D234 D235 symmetrical voltages with ground, (may be many parts replaced to get here) put 1 vac in from transistor radio or signal generator. Look for ~1 vac at op amp output, little higher vac at Q201 , higher vac at Q208 Q212 predrivers. Use 2vac scale of analog vom for front, 20 v scale for predrivers, 100 vac scale near predrivers. or use scope. E to C of Q211 should be about 2.4 vdc.
6. All tidy, put in drivers Q401 Q402. With no signal speaker out should be at 0 vdc with 1 vac input, music or tone should come out speaker jack on trash protected speaker.
7. put in output transistors, Do dc check on speaker out, check current on r410 r403 about 10 to 20 ma (100-200 mv across 0.1 ohms). Adjust R225 R226 until it is.
8. that nice & stable around 0 volts, listen to music or tone from speaker jack, all still with light bulb series AC line.
9. check function of leds from the 339 comparitor.
10. eveything stable and symmetrical, No undue ac voltage on speaker through 390 pf cap (no ultrasonic oscillation) cooked for several hours into trash speaker (protected by 3300 back to back caps).
11. you may remove light bulb box from AC line. Check again for oscillation, undue heat on any part, and if not, play music on protected speaker for day or two.
12. all stable, nothing too hot, Change 50 year old electrolytic caps 1 or 2 at a time to make sure you are not back in here in days, weeks or months as they fail. Do function test with light bulb box in AC line each 1 or 2 caps to make sure dc voltages stay symmetrical. music flows through as designed, no ultrasound
13. All tidy symmetrical DC voltages, playing music on trash speaker, no ultrasound, put covers back on.
Last edited: 25 minutes ago
I took the liberty of reformatting your post into min-paragraphs--- my eyes were crossing... Hope you don't mind. 🤔
indianajo said:
I would procede as follows.
1. with dvm check double diode test on outputs, drivers, pre-driver, various diodes, back to Q201 after op amp. Replace any shorts or opens.
2. Leaving output transistors out, power up through 60 w bulb. Check power supply rails symmetrical if lower than +-48, +-15 to op amp nominal. Negative of DVM to speaker ground. replace parts as required to get there.
3. check inputs & output of op amp 2,3,6 all at same voltage. If not you may need one or two.
4. look for plausible voltages on collectors of transistors; between rails, symmetrical, all b-e junctions reading about 0.6 or -0.6 volts. If light bulb is burning, one or more transistors may be shorted c to e or diode not blocking DC, etc. Other resistors or capacitors may be open or shorted, since rail voltage ran back from output transistors into areas where parts were not rated +-48 v. Double diode test does not prove semiconductors etc good at higher voltages. Running on 60 w bulb stresses those parts near rated voltages. Parts declared good with 2 v diode test may short or open when presented with near full voltages. Note Q203 204 are VI limiters, they and associated parts may have been damaged by rail voltage raging out base line of output transistors.
5. When series light bulb is out and D234 D235 symmetrical voltages with ground, (may be many parts replaced to get here) put 1 vac in from transistor radio or signal generator. Look for ~1 vac at op amp output, little higher vac at Q201 , higher vac at Q208 Q212 predrivers. Use 2vac scale of analog vom for front, 20 v scale for predrivers, 100 vac scale near predrivers. or use scope. E to C of Q211 should be about 2.4 vdc.
6. All tidy, put in drivers Q401 Q402. With no signal speaker out should be at 0 vdc with 1 vac input, music or tone should come out speaker jack on trash protected speaker.
7. put in output transistors, Do dc check on speaker out, check current on r410 r403 about 10 to 20 ma (100-200 mv across 0.1 ohms). Adjust R225 R226 until it is.
8. that nice & stable around 0 volts, listen to music or tone from speaker jack, all still with light bulb series AC line.
9. check function of leds from the 339 comparitor.
10. eveything stable and symmetrical, No undue ac voltage on speaker through 390 pf cap (no ultrasonic oscillation) cooked for several hours into trash speaker (protected by 3300 back to back caps).
11. you may remove light bulb box from AC line. Check again for oscillation, undue heat on any part, and if not, play music on protected speaker for day or two.
12. all stable, nothing too hot, Change 50 year old electrolytic caps 1 or 2 at a time to make sure you are not back in here in days, weeks or months as they fail. Do function test with light bulb box in AC line each 1 or 2 caps to make sure dc voltages stay symmetrical. music flows through as designed, no ultrasound
13. All tidy symmetrical DC voltages, playing music on trash speaker, no ultrasound, put covers back on.
Last edited: 25 minutes ago
Item 7 100 to 200 mv across .1 ohm emitter resistor is 1 a. Look for 1 to 2 mv to make idle bias current 10-20 ma. Sorry.
Frankly, 0.1 ohm emitter resistors are a bit risky. Thermal runaway may occur. I would change to 0.33 ohm or 0.47 ohm per Peavey practice, to harden the amp for hard use. You may have to increase resistor wattage since they will be dropping 3 to 5 times the power of .1 ohm. May have to stack vertically to fit in 5 watt resistors. With bigger resistors, voltage increases on emitter resistors to get 10-20 ma idle current. I=V/R
Distortion check may not work if your ears have no response above 7000 hz. I have 14000 hz ears. In that case scope check on square wave input is the distortion check. If you do have good ears and garbage speakers, listen for distortion on headphones. Good headphones can be had for <$40, and I've had some $6 phones that were fine.
Frankly, 0.1 ohm emitter resistors are a bit risky. Thermal runaway may occur. I would change to 0.33 ohm or 0.47 ohm per Peavey practice, to harden the amp for hard use. You may have to increase resistor wattage since they will be dropping 3 to 5 times the power of .1 ohm. May have to stack vertically to fit in 5 watt resistors. With bigger resistors, voltage increases on emitter resistors to get 10-20 ma idle current. I=V/R
Distortion check may not work if your ears have no response above 7000 hz. I have 14000 hz ears. In that case scope check on square wave input is the distortion check. If you do have good ears and garbage speakers, listen for distortion on headphones. Good headphones can be had for <$40, and I've had some $6 phones that were fine.
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Hi @indianajo I have a question about this step, you are saying to power through a 60 w bulb, is there any way to do this using a variac or other means? Or do I have to build a light bulb limiter ?
Thank you
The variac I have owned does not isolate one from the AC line. Autotransformer, 60 w. Better make sure the neutrals in your house are not the hot. Was surprised by a TV I was working on, big blades in that house were hot.
The light bulb provides a useful clue. Flash then goes out, no great short in there. Bright lamp, start checking voltages to see which component(s) are shorted.
With tungsten bulb sold only at Savation Army resale etc. One may have to put one cup coffee boilers or the like in series with the AC line.
The light bulb provides a useful clue. Flash then goes out, no great short in there. Bright lamp, start checking voltages to see which component(s) are shorted.
With tungsten bulb sold only at Savation Army resale etc. One may have to put one cup coffee boilers or the like in series with the AC line.