Who might be the Crown DC300 mahout in these parts? I have a project down on the bench that's gotten waaay out of hand...... 😀
So I have a 300AII that began life as a bias control amplifier in an industrial electron beam welder. After upgrading the original chassis with the updated main board (with the LF357 jfet input amps), I was setting bias and checking output. Unfortunately, when I hit about 28vac on the first channel, she zorched.... I found I had forgotten to cut the traces to the old IOC circuitry on the off-board pcb when I attached the LED wiring to the new main board.
From what I can tell, it shorted the new IOC chip, taking out the +-10v zeners supplying all the ICs, and shorting Q100, Q105, the ICs, and R116....
My problem now is a persistent -3.5vdc on the ailing channel, that starts escalating quickly. I spent most of the 90's paying bills as a bench tech at Alpha Audio Labs in St Louis, unfortunately it appears i'm a bit rusty on my theory. If anyone is familiar with this amp, any suggestions would be appreciated.....
From what I can tell, it shorted the new IOC chip, taking out the +-10v zeners supplying all the ICs, and shorting Q100, Q105, the ICs, and R116....
My problem now is a persistent -3.5vdc on the ailing channel, that starts escalating quickly. I spent most of the 90's paying bills as a bench tech at Alpha Audio Labs in St Louis, unfortunately it appears i'm a bit rusty on my theory. If anyone is familiar with this amp, any suggestions would be appreciated.....
here is a long thread I provided a little component theory to. I'm no expert on fixing them, I've never owned one. http://www.diyaudio.com/forums/solid-state/248973-crown-dc300a-repair-restore.html
What are you intending to do with this? Hydraulic solenoid valve control to a rudder actuator? Diesel motor throttle actuator? Welder bias control as per OEM?
The DC output of these things makes them highly dangerous around speakers. But they have a great case, power transformer, heat sink, fan.
On one thread, we ran into the fact that the op amp at the heart of the DC300 is obsolete. Weird pinout.
If I had one, I'd try to replace the driver boards with something more modern and incapable of producing DC output. Say an op amp differential input receiver, followed by an op amp current driver, followed by drivers then output transitors. Or a triple like a PV4C or something. A nice simple op amp driver schematic is a Marshall PA amp, it's on my other computer with the virus so I don't have the number available right now.
Obviously a board from some E-bay vendor is the cheapest option. I don't buy import stuff from ****a so I have no experience. But taking the base line from the on board missing output transistors, over to the TO3 transistor base on the heat sink, should be the M.O. I'm driving TO3 outputs about 6" away by wires from my AX6 driver board, run the amp 14 hours a day sometimes, so it is not impossible - just unfashionable. Hint-run the heat sense wires 4" away.
Any speaker costing over $10, I would run the rail voltage on this through NFETs to disconnect on DC output, ala RJ Keene, or put back to back NFETs in the speaker line to disconnect them if you dont' want to try to save the other output transistors when the first one fails. Great 50 amp fets are about $1 now. See various protection threads on this board. I don't do it with a microprocessor, everything since 8080 on an Intel $7000 development system has sat there with a blank screen for me. I copy the DC protection circuit of the Peavey PV-1.3k to instead, trip a latch and turn on the red LED and turn off the fet driver IC. Do whatever you want. it is DIY.
Best of luck with this USA classic.
What are you intending to do with this? Hydraulic solenoid valve control to a rudder actuator? Diesel motor throttle actuator? Welder bias control as per OEM?
The DC output of these things makes them highly dangerous around speakers. But they have a great case, power transformer, heat sink, fan.
On one thread, we ran into the fact that the op amp at the heart of the DC300 is obsolete. Weird pinout.
If I had one, I'd try to replace the driver boards with something more modern and incapable of producing DC output. Say an op amp differential input receiver, followed by an op amp current driver, followed by drivers then output transitors. Or a triple like a PV4C or something. A nice simple op amp driver schematic is a Marshall PA amp, it's on my other computer with the virus so I don't have the number available right now.
Obviously a board from some E-bay vendor is the cheapest option. I don't buy import stuff from ****a so I have no experience. But taking the base line from the on board missing output transistors, over to the TO3 transistor base on the heat sink, should be the M.O. I'm driving TO3 outputs about 6" away by wires from my AX6 driver board, run the amp 14 hours a day sometimes, so it is not impossible - just unfashionable. Hint-run the heat sense wires 4" away.
Any speaker costing over $10, I would run the rail voltage on this through NFETs to disconnect on DC output, ala RJ Keene, or put back to back NFETs in the speaker line to disconnect them if you dont' want to try to save the other output transistors when the first one fails. Great 50 amp fets are about $1 now. See various protection threads on this board. I don't do it with a microprocessor, everything since 8080 on an Intel $7000 development system has sat there with a blank screen for me. I copy the DC protection circuit of the Peavey PV-1.3k to instead, trip a latch and turn on the red LED and turn off the fet driver IC. Do whatever you want. it is DIY.
Best of luck with this USA classic.
Last edited:
Lol @ your application suggestions..... I actually picked the amp and unused replacement main board out of some industrial surplus. I had used it to drive a pair of magneplanars I had restrung until I decided to re-cap it. (one channel had a serious roll off at about 10khz) Though I have basically a tin ear, I did enjoy the sound I got out of it....
I'll definitely eyeball the thread you link for me.... thanks much.... 🙂
I'll definitely eyeball the thread you link for me.... thanks much.... 🙂
Great eyes. Congratulations.
Because they use four potentiometers to zero out the outputs, these are inherently dangerous around speakers. Pot wipers oxidize, and over the years that pretty much guarantees that some day the output will jump to a non-zero value DC voltage. That can burn a speaker coil or tear the suspension.
Stuff a DC detection speaker disconnect circuit in there, or replace the driver board with something more guaranteed by design to stay at zero. I don't personally trust anything that has transistors direct coupled to the speakers without protection; speaker wire shorts that blow transistors can come from your dog or visitors. I shorted the output of my tube amp myself by walking on the speaker wires, but tubes have output transformers that work fine forever into a short. Just my two cents.
Because they use four potentiometers to zero out the outputs, these are inherently dangerous around speakers. Pot wipers oxidize, and over the years that pretty much guarantees that some day the output will jump to a non-zero value DC voltage. That can burn a speaker coil or tear the suspension.
Stuff a DC detection speaker disconnect circuit in there, or replace the driver board with something more guaranteed by design to stay at zero. I don't personally trust anything that has transistors direct coupled to the speakers without protection; speaker wire shorts that blow transistors can come from your dog or visitors. I shorted the output of my tube amp myself by walking on the speaker wires, but tubes have output transformers that work fine forever into a short. Just my two cents.
Before they went to microprocessor control, Peavey sampled the speaker line with a 47k resistor. They filtered this with a 2.2 uf bipolar cap. If the cap went over + or - 7 v from the speaker ground, then a "SBS" broke down and allowed a current to flow out an trigger things. I did buy a replacement SBS from newark (farnell) It was the Powerex bs08d. It is like two 7v zener diodes back to back.
What Peavey did with the current was stupid in my amp because it triggered a triac "crowbar" that was supposed to short the speaker line to speaker return & blow the circuit breaker. What it actually did was short out the triac and melt the PWB lands to it off the circuit board. Without blowing the breaker.
So I'm using the current from the SBS to light up the input of a bipolar opto-isolator. I use a resistor to limit the current. Then the output of the opto-isolator connects to the set input of a flip flop, a 74HC74, to remember a fault occured. the flip flop is on a 5 v power supply, which the Peavey needs to be isolated from the channel power supply, because Peaveys have flying ground, ie the two channel speaker returns are not the same thing. You should not have that problem.
The 74hc74 Q output drives the base of a 2n2222 to pull current through a red falut led. The 2n2222 driven by the the Qbar output pulls current through a green led, and also through a couple of APV1122 FET drivers in series. Each FET driver drives one NFET, one for the + rail power supply, one for the - rail power supply. So as long as the green LED is on, the fets allow current to go to the output transistors. When the flip flop is set and the red LED is on, the current to the output transistors stops. (because the green LED is out) Which theorectically saves 8 or 9 of your output transistors, since only one failed initially.
The 74hc74 is reset by a resistor/cap combo from power up. That turns the green LED on and makes the FETs allow current to the output transistor rails. I'm using fairchild FDP52n20 Fets I think, $1.25 each. My PV-1.3k will put 22 amps on a speaker at full volume 2 ohms. Smaller current, cheaper FET will work.
Nigel7557 can do all this sense & latch and indicate with a microprocessor, but I have trouble getting microprocessor compilers to match up to my free operating system. Even when I paid the price for windows, I always had a compiler that matched some op system or main board I didn't have. So I do it with wires & IC's.
I have a diagram of this over on the other computer, the one with the virus I need to back up (without the virus installed in all the gifs this time). Will try to upload it in the morning. The diagram is over complicated because I could never find bipolar input optoisolators in stock for a couple of years, so I have 4 regular optoisolators for sense instead of 2 bipolar for the two channels. Also my original optoisolators were 20% current transfer max, and 100% current transfer is very cheap now.
Some people add faults for overtemperature heatsink, and even overcurrent through current transformers or hall effect sensors, to trip the fault latch before any output transistors blow. The set line is a wire AND function where any number of optoisolators could be connected. Any low in makes a low out and makes the flop set. There is so much damage upstream from a shorted OT when one blows. My peavey CS800s has the current transformers & the microprocessor fault disconnect, and hard relays that an experienced repairman in Michigan told me hardly every fail. So I trust that one as is. The PV-1.3k, costing $400 less than the CS800s, that is the one that melts itself instead of protecting the speaker.
Read some of the protection threads. Most do not cover multi-transistor PA amps though, just 1 or 2 output transistor pairs. That's why I don't recommend the diyaudio store protection board, its relay may be somewhat wimpy for a 300W amp.
What Peavey did with the current was stupid in my amp because it triggered a triac "crowbar" that was supposed to short the speaker line to speaker return & blow the circuit breaker. What it actually did was short out the triac and melt the PWB lands to it off the circuit board. Without blowing the breaker.
So I'm using the current from the SBS to light up the input of a bipolar opto-isolator. I use a resistor to limit the current. Then the output of the opto-isolator connects to the set input of a flip flop, a 74HC74, to remember a fault occured. the flip flop is on a 5 v power supply, which the Peavey needs to be isolated from the channel power supply, because Peaveys have flying ground, ie the two channel speaker returns are not the same thing. You should not have that problem.
The 74hc74 Q output drives the base of a 2n2222 to pull current through a red falut led. The 2n2222 driven by the the Qbar output pulls current through a green led, and also through a couple of APV1122 FET drivers in series. Each FET driver drives one NFET, one for the + rail power supply, one for the - rail power supply. So as long as the green LED is on, the fets allow current to go to the output transistors. When the flip flop is set and the red LED is on, the current to the output transistors stops. (because the green LED is out) Which theorectically saves 8 or 9 of your output transistors, since only one failed initially.
The 74hc74 is reset by a resistor/cap combo from power up. That turns the green LED on and makes the FETs allow current to the output transistor rails. I'm using fairchild FDP52n20 Fets I think, $1.25 each. My PV-1.3k will put 22 amps on a speaker at full volume 2 ohms. Smaller current, cheaper FET will work.
Nigel7557 can do all this sense & latch and indicate with a microprocessor, but I have trouble getting microprocessor compilers to match up to my free operating system. Even when I paid the price for windows, I always had a compiler that matched some op system or main board I didn't have. So I do it with wires & IC's.
I have a diagram of this over on the other computer, the one with the virus I need to back up (without the virus installed in all the gifs this time). Will try to upload it in the morning. The diagram is over complicated because I could never find bipolar input optoisolators in stock for a couple of years, so I have 4 regular optoisolators for sense instead of 2 bipolar for the two channels. Also my original optoisolators were 20% current transfer max, and 100% current transfer is very cheap now.
Some people add faults for overtemperature heatsink, and even overcurrent through current transformers or hall effect sensors, to trip the fault latch before any output transistors blow. The set line is a wire AND function where any number of optoisolators could be connected. Any low in makes a low out and makes the flop set. There is so much damage upstream from a shorted OT when one blows. My peavey CS800s has the current transformers & the microprocessor fault disconnect, and hard relays that an experienced repairman in Michigan told me hardly every fail. So I trust that one as is. The PV-1.3k, costing $400 less than the CS800s, that is the one that melts itself instead of protecting the speaker.
Read some of the protection threads. Most do not cover multi-transistor PA amps though, just 1 or 2 output transistor pairs. That's why I don't recommend the diyaudio store protection board, its relay may be somewhat wimpy for a 300W amp.
Last edited:
If you can't find a 20 amp dpdt ice cube relay for less than $6 on the surplus market you're not looking hard enough. Big enough for most amps, and better than what they put in a QSC.
If you want beefier they can be had. Found some giant 4pdt's at skycraft of $15. I even used those for the reversing switch on the lift winch in my shop. If that's not big enough to interrupt a fault to a loudspeaker nothing is. And they were $15.
Relays can produce distortion, but you have to weigh that against the ease of use and the less potential for collateral damage like smoked pcb traces compared to a crowbar.
If you want beefier they can be had. Found some giant 4pdt's at skycraft of $15. I even used those for the reversing switch on the lift winch in my shop. If that's not big enough to interrupt a fault to a loudspeaker nothing is. And they were $15.
Relays can produce distortion, but you have to weigh that against the ease of use and the less potential for collateral damage like smoked pcb traces compared to a crowbar.
Think I'll get any distortion through these babies? 😀
https://www.surplussales.com/Relays/mercury.html
https://www.surplussales.com/Relays/mercury.html
No. But suspect mercury wetted contacts won't break a 20 A DC arc. My 1300 W amp will probably make a 1000 A short if the 10000 uf caps dump.
Cool, a surplus house closer than LA or Syracuse: Nebraska I'll be looking them over.
Did you see the post 208 P21 in this thread: http://www.diyaudio.com/forums/solid-state/197468-diy-short-circuit-protection-21.html
They built a discrete flip flop out of a an octave of discrete parts instead of a 74HC74, but otherwise similar idea. They used a hard contact relay, I use two nfets. They used a darlington relay coil driver, I used a FET driver IC.
Andrew lebone on that thread is always trying to sell his board.
Cool, a surplus house closer than LA or Syracuse: Nebraska I'll be looking them over.
Did you see the post 208 P21 in this thread: http://www.diyaudio.com/forums/solid-state/197468-diy-short-circuit-protection-21.html
They built a discrete flip flop out of a an octave of discrete parts instead of a 74HC74, but otherwise similar idea. They used a hard contact relay, I use two nfets. They used a darlington relay coil driver, I used a FET driver IC.
Andrew lebone on that thread is always trying to sell his board.
Good point. I have a couple odd mercs around here. I'll have to do some experimenting....
....I lived in Phoenix in the '80s, they had a massive surplus house just south of town... I think it was called Apache Electronics...
....I lived in Phoenix in the '80s, they had a massive surplus house just south of town... I think it was called Apache Electronics...
Sell the transistors and driver board on EBay. I recouped the entire cost of a DC300A doing that.
Then start from scratch.
If it smells like nicotine, Rock 'n Rye and cannabis, you can soak it in a dilute solution of borax.
Then start from scratch.
If it smells like nicotine, Rock 'n Rye and cannabis, you can soak it in a dilute solution of borax.
I do.
My PV-1.3k blew TO3 transistor tops to the ceiling before I discovered series resistance in the AC line. I used a room heater element, something never mentioned before on this forum.
With 124 replacement parts, 90 of them from salvage, the first full power test will be another adventure. One I'm not embarking on until I actually finish the protection circuit described previously. $100 in new output transistors - a bit of a risk. Also that 1300 W making the resistors glow on the coffee table.
Playing keyboard & singing before a live audience needing that power - another adrenilyn charge. So far my biggest audience solo was about 50. So they didn't pay? If they don't leave before I do I win.
Oh, BTW, in rehearsal for that Christmas cantata gig with the 50, the ST120 amp caught fire - big 1' diameter blue fireball as the base stopper resistors lit up. The junior choir was very impressed.
My PV-1.3k blew TO3 transistor tops to the ceiling before I discovered series resistance in the AC line. I used a room heater element, something never mentioned before on this forum.
With 124 replacement parts, 90 of them from salvage, the first full power test will be another adventure. One I'm not embarking on until I actually finish the protection circuit described previously. $100 in new output transistors - a bit of a risk. Also that 1300 W making the resistors glow on the coffee table.
Playing keyboard & singing before a live audience needing that power - another adrenilyn charge. So far my biggest audience solo was about 50. So they didn't pay? If they don't leave before I do I win.
Oh, BTW, in rehearsal for that Christmas cantata gig with the 50, the ST120 amp caught fire - big 1' diameter blue fireball as the base stopper resistors lit up. The junior choir was very impressed.
Last edited:
I picked up a DC300 and IC150A from a med student at NYU for $100, both with their nice condition wood-veneer cases. The IC150A sports a Jung regulator and an NE5534 opamp. Sounds very nice to me! The DC300 did undergo a transistor-cum-driverboard-ectomy as mentioned above and now has some lateral mosfets (K1058 and J162) and an LM4702 driver board. The transformer, carcase and connectors are all that remain.
Lol.... Fireball? Try a 60KV oil bath power supply shooting craps when a novice rebuilt the electron gun wrong 😀
We've learned to leave the bolts out of the tank..... rather clean up oil than shrapnel..... 😉
We've learned to leave the bolts out of the tank..... rather clean up oil than shrapnel..... 😉
Too late to edit -- i tried 5 or 6 opamps including opa627, lme49710 etc. but the '5534 really sounds nice in this situation.
Unfortunately, I spent too much time too close to the speakers in my youth..... hence I'm easy to please....
- Status
- Not open for further replies.