Hi, first post - I'm a perpetual electronics noob, so pardon some of my ignorant explanations...
I just picked up a Crown D60 today for $40, untested, at an antique shop. It's in pretty lousy shape so I knew I was taking a risk, but it was ok. Got it home, first test I did was fire it up and check for DC voltage - not getting anything right. I am getting AC voltage of 120v when I probe around and follow where the power cable comes in and connects to the little stripboard behind the transformer. When I test DC, I get something like 19.5mv on my meter. Also, my scope doesn't register anything when I probe the circuit board.
I pulled out the two gigantic 10000uf caps and tested them with a Peak meter. Both register at 13.50mf and about 14.50mf... I don't know if that reading is somehow abbreviating the thousands on the can, or if the reading is literally 13uf (mf) and they're goners... I'd expect that they are shot since the unit is 45 year old but, I don't know assuming.
Does anyone have explicit experience with these amps and know if I'm on the right track testing? I did check both fuses (outside and inside) and both are ok. I have not tested the amp with any speakers or signal yet. It just doesn't appear to be getting power correctly.
I apologize, I like to tinker with stuff and I have equipment, but I don't really know what I'm doing, save a few things... I can usually diagnose and part-swap though.
I just picked up a Crown D60 today for $40, untested, at an antique shop. It's in pretty lousy shape so I knew I was taking a risk, but it was ok. Got it home, first test I did was fire it up and check for DC voltage - not getting anything right. I am getting AC voltage of 120v when I probe around and follow where the power cable comes in and connects to the little stripboard behind the transformer. When I test DC, I get something like 19.5mv on my meter. Also, my scope doesn't register anything when I probe the circuit board.
I pulled out the two gigantic 10000uf caps and tested them with a Peak meter. Both register at 13.50mf and about 14.50mf... I don't know if that reading is somehow abbreviating the thousands on the can, or if the reading is literally 13uf (mf) and they're goners... I'd expect that they are shot since the unit is 45 year old but, I don't know assuming.
Does anyone have explicit experience with these amps and know if I'm on the right track testing? I did check both fuses (outside and inside) and both are ok. I have not tested the amp with any speakers or signal yet. It just doesn't appear to be getting power correctly.
I apologize, I like to tinker with stuff and I have equipment, but I don't really know what I'm doing, save a few things... I can usually diagnose and part-swap though.
So I rechecked the fuse on the outside, and it had blown after whatever I was probing with the meter. As a test, I plugged in a sound source and a junky speaker just to make sure. I was probing around at the on-off switch connection and i sparked, heard about a split-second of my sound source and then blew the internal fuse. 2.5amp fuse which I naturally don't have on hand.
If you don't have it yet, service manual at http://akdatabase.org/AKview/albums/userpics/10007/Crown D60 Service Manual.pdf
> 10000uf caps ... 13.50mf and about 14.50mf... I don't know if that reading is somehow abbreviating the thousands on the can, or if the reading is literally 13uf (mf)
A modern meter, when it says "m", *probably* means "milliFarad". 10,000 microFarad is 10 milliFarad. I say the caps are in the right ballpark. Hardly matters if it is 10 13 or 14. The "mildly high" value may come down after voltage is applied and the oxide re-forms. (Or it may explode....)
Anyway, even no caps would show "some" DC voltage. Lower than expected and buzzing bad, but shows DC voltage.
A modern meter, when it says "m", *probably* means "milliFarad". 10,000 microFarad is 10 milliFarad. I say the caps are in the right ballpark. Hardly matters if it is 10 13 or 14. The "mildly high" value may come down after voltage is applied and the oxide re-forms. (Or it may explode....)
Anyway, even no caps would show "some" DC voltage. Lower than expected and buzzing bad, but shows DC voltage.
The striped wires are the secondary. They go to the bridge rectifier. But first you need a 100 watt light bulb in series with the AC power line.
The outside fuse is 2 amps, the one hidden inside is 2.5 amps. Intent is to prevent stupid folks from putting in too large a fuse in the fuse holder they know about.
I think the fuses should be of the time delay type.
The outside fuse is 2 amps, the one hidden inside is 2.5 amps. Intent is to prevent stupid folks from putting in too large a fuse in the fuse holder they know about.
I think the fuses should be of the time delay type.
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Well, I found a box of fuses and happened to have a 2.5A, so I dropped it in and powered her on. Still nothing. So I remembered how I had that glitch at the power switch yesterday, and decided to alligator clip an on/off switch in place of the original. Sure enough, it powered on and the LED lit, but moments later started smoking and blew the internal fuse.
The transformer got super hot and I think the smoke was coming from the bridge rectifier. So the switch is definitely bad but not the source of the problem.
Need to do a little reading. I’ve never worked on a power amp before and don’t feel like blowing myself up just yet.
The transformer got super hot and I think the smoke was coming from the bridge rectifier. So the switch is definitely bad but not the source of the problem.
Need to do a little reading. I’ve never worked on a power amp before and don’t feel like blowing myself up just yet.
If your a novice, be aware of the 1 hand at a time rule. Any voltage over 25 across your heart can stop it. Use an alligator clip lead for the negative meter probe. Wear no jewelry on hands or neck, 1 vdc at 20 amps from stored energy in capacitors can burn your flesh to charcoal. Touch no metal bare handed over 1 v. Wear safety glasses, solder can splash and transistors can explode, especially with no AC limiter in. Don't work alone and don't work distracted by kids or media.
An early test is what DC voltage you are getting on the rail caps. WIth 100 W light bulb limiter in the AC line this should be at least 80% of the nominal voltage on the schematic. Or 70% of rating printed on the side of the cap.
What ESR did the peak meter show? e-caps that read 25% high are usually dried out. ESR should be .03 or lower per the chart in the peak manual. High ESR will limit maximum wattage out of the amp. Also old rail caps tend to leak, which makes a mess. Leaky rail caps could cause the rectifier to smoke. Buy new caps only from authorized distributors like newark, digikey, mouser, arrow, alliedelec, RS in UK and various german ones. e-caps deteriorate even on the shelf. I do buy * bargain caps from newark but expired shelf life means you have to reform them before use. I use the ohms scale of the dvm for that. e-bay and alibaba are known for counterfeiters.
If those are screw terminal caps, build a holey board to mount snap in caps. Much cheaper. Use glue if clamps don't fit. I prefer ones with longer service lives from Panasonic, nichicon, rubicon, some vishay. You should be able to find 3000 hour or more service life caps. Newark & digikey have the service life in the selector table, others make you download the manufacturer's datasheet.
An early test is what DC voltage you are getting on the rail caps. WIth 100 W light bulb limiter in the AC line this should be at least 80% of the nominal voltage on the schematic. Or 70% of rating printed on the side of the cap.
What ESR did the peak meter show? e-caps that read 25% high are usually dried out. ESR should be .03 or lower per the chart in the peak manual. High ESR will limit maximum wattage out of the amp. Also old rail caps tend to leak, which makes a mess. Leaky rail caps could cause the rectifier to smoke. Buy new caps only from authorized distributors like newark, digikey, mouser, arrow, alliedelec, RS in UK and various german ones. e-caps deteriorate even on the shelf. I do buy * bargain caps from newark but expired shelf life means you have to reform them before use. I use the ohms scale of the dvm for that. e-bay and alibaba are known for counterfeiters.
If those are screw terminal caps, build a holey board to mount snap in caps. Much cheaper. Use glue if clamps don't fit. I prefer ones with longer service lives from Panasonic, nichicon, rubicon, some vishay. You should be able to find 3000 hour or more service life caps. Newark & digikey have the service life in the selector table, others make you download the manufacturer's datasheet.
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Thank you for the help. I left off with the amp passing some kind of signal but not audio.
First, with additional advice, I removed the +/- from the bridge rectifier and powered it on. No smoke, no popped fuse. I also ordered a replacement just to be safe. Then I removed each of the power transistors and tested them using the Peak Atlas meter. Every one of them showed 'good' in terms of no shorts, but one had a very low Hfe (12) while the others were somewhere in the range of 65-90. But after soldering everything back together and throwing a new fuse in, I got power, no smoke, no popped fuse and LOUD hum and that weird voltage-like sweep sound coming from the speakers. So something is passing but I don't know what.
I do need to replace the caps, I know. I haven't ordered them yet because they were extremely expensive for the screw-terminal type and I don't think I'll have the patience to build an extra mounting board to use snap-ins. But I will order them, I just want to know if these are characteristics of the input caps or something like the output resistors?
First, with additional advice, I removed the +/- from the bridge rectifier and powered it on. No smoke, no popped fuse. I also ordered a replacement just to be safe. Then I removed each of the power transistors and tested them using the Peak Atlas meter. Every one of them showed 'good' in terms of no shorts, but one had a very low Hfe (12) while the others were somewhere in the range of 65-90. But after soldering everything back together and throwing a new fuse in, I got power, no smoke, no popped fuse and LOUD hum and that weird voltage-like sweep sound coming from the speakers. So something is passing but I don't know what.
I do need to replace the caps, I know. I haven't ordered them yet because they were extremely expensive for the screw-terminal type and I don't think I'll have the patience to build an extra mounting board to use snap-ins. But I will order them, I just want to know if these are characteristics of the input caps or something like the output resistors?
If your amp has suffered a field wiring short, blowing output transistors, then output resistors can blow. Other than that, not a usual problem.
Output transistors the gain doesn't matter much in most designs. Most designs use output transistors and the drivers as emitter followers, and gain doesn't matter much in that service. Where there are multiple output transistors in parallel, the Vce match is critical to keep the load current shared evenly between the transistors.
Come on, a mount board for snap in caps can be made of a surplus plastic sheet sawed of a toy, or even out of plywood. You need a drill to drill the holes, and a long screw tube & nut to mount it securely to the case. I use #6 x 1 1/2" screws and elastic stop nuts from mcmaster.com. Also some glue like 3M windshield adhesive to glue the new caps down so thay don't move around. You could buy a new drill motor for the cost of a screw terminal cap. A plug in 3/8" dewalt was $69 at Rural King this spring. Use safety glasses with power tools. Don't waste money on Harbor Freight "tools" they look like tools but are really toys for adults.
If you want to test your expensive caps before replacing, leakage at significant voltage is the paramater that will make a rectifier smoke. Put a 1 ohm resistor after the bridge rectifier and before the cap, measure voltage. After the amp charges up and the input is shorted, you want no more than 100 mv on that 1 ohm resistor, ie 100 ma.
Another thing that could cause the rectifier to smoke is oscillation. Pretty rare on crowns, takes a scope or a long involved test with an analog AC voltmeter and various value blocking caps on the negative lead to spot ultrasonics. Digital AC voltmeters only read 50/60 hz signals except the $180 fluke RMS meter which tops out at 7000 hz and would read zero falsely on ultrasonic oscillation.
Output transistors the gain doesn't matter much in most designs. Most designs use output transistors and the drivers as emitter followers, and gain doesn't matter much in that service. Where there are multiple output transistors in parallel, the Vce match is critical to keep the load current shared evenly between the transistors.
Come on, a mount board for snap in caps can be made of a surplus plastic sheet sawed of a toy, or even out of plywood. You need a drill to drill the holes, and a long screw tube & nut to mount it securely to the case. I use #6 x 1 1/2" screws and elastic stop nuts from mcmaster.com. Also some glue like 3M windshield adhesive to glue the new caps down so thay don't move around. You could buy a new drill motor for the cost of a screw terminal cap. A plug in 3/8" dewalt was $69 at Rural King this spring. Use safety glasses with power tools. Don't waste money on Harbor Freight "tools" they look like tools but are really toys for adults.
If you want to test your expensive caps before replacing, leakage at significant voltage is the paramater that will make a rectifier smoke. Put a 1 ohm resistor after the bridge rectifier and before the cap, measure voltage. After the amp charges up and the input is shorted, you want no more than 100 mv on that 1 ohm resistor, ie 100 ma.
Another thing that could cause the rectifier to smoke is oscillation. Pretty rare on crowns, takes a scope or a long involved test with an analog AC voltmeter and various value blocking caps on the negative lead to spot ultrasonics. Digital AC voltmeters only read 50/60 hz signals except the $180 fluke RMS meter which tops out at 7000 hz and would read zero falsely on ultrasonic oscillation.
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A 100W light bulb is a poor way to limit current, because you have little control over amp current and high current discharge of PS caps into an amp fault.
Disconnect amp form PS after the caps and power amp with a HEWLETT PACKARD 6227B DUAL DC POWER SUPPLY. Set the voltage to 25 and current limit to 50 to 100 mA or just above the bias current level. The power supply does not use large output caps. Several HP 6227B recently sold on ebay for $20 to $50.
This allows troubleshooting the amp without destructive current levels.
Disconnect amp form PS after the caps and power amp with a HEWLETT PACKARD 6227B DUAL DC POWER SUPPLY. Set the voltage to 25 and current limit to 50 to 100 mA or just above the bias current level. The power supply does not use large output caps. Several HP 6227B recently sold on ebay for $20 to $50.
This allows troubleshooting the amp without destructive current levels.
Ok... so, this is a disappointment...
I'm ready to give up on this amp. I think it's been left outside in the rain. I wasn't getting anything on the scope so I popped the chip out and that's how it came out; all rusted and broken. Same thing happened to 2 of the caps I swapped out. Parts-wise, I'm only in it the filter caps... everything else to me is throwaway stock.
Power supply is good, reads 34v where it should be 30 so that's ok... I get 30v+ around the mainboard, including on the chip so I figured something was shorted. I was avoiding pulling the IC until I did the caps and when I saw the caps fall apart, I knew I was in trouble.
Last call... chuck it or try the IC replacement?
I'm ready to give up on this amp. I think it's been left outside in the rain. I wasn't getting anything on the scope so I popped the chip out and that's how it came out; all rusted and broken. Same thing happened to 2 of the caps I swapped out. Parts-wise, I'm only in it the filter caps... everything else to me is throwaway stock.
Power supply is good, reads 34v where it should be 30 so that's ok... I get 30v+ around the mainboard, including on the chip so I figured something was shorted. I was avoiding pulling the IC until I did the caps and when I saw the caps fall apart, I knew I was in trouble.
Last call... chuck it or try the IC replacement?
Ok, I gave up after snapping off an allen key in one of the rusted knobs. I then plugged in some headphones and now the amp is just blowing fuses on power up. Screw this - I already pulled the circuit board and I'm tossing the rest of this thing in the trash. I'll buy a good one at some point.
You could buy amp boards on ebay and use the chassis, heat sink and power supply to make a great project.
Low cost universal amplifier chassis
Take a look at these:
LJM MX50 kit amp
NAP-140 Clone Amp Kit on Ebay
Low cost universal amplifier chassis
Take a look at these:
LJM MX50 kit amp
NAP-140 Clone Amp Kit on Ebay
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Amplifier 101:
1. check all power transistors for shorted or open state before you apply any power.
2. Do not connect a speaker until it looks like it is working (with a volt meter and scope).
3. check the bridge rectifier for shorts, although the connected transformer secondary will be close to a short.
4. put an incandescent light bulb etc in series to limit initial power up current. When the amp is working, it will not draw enough power (undriven) to light the lamp.
nuff for now.
Steve
PS. I suggest you replace the guts with a module(s) from amazon, reuse the box and power supply.
1. check all power transistors for shorted or open state before you apply any power.
2. Do not connect a speaker until it looks like it is working (with a volt meter and scope).
3. check the bridge rectifier for shorts, although the connected transformer secondary will be close to a short.
4. put an incandescent light bulb etc in series to limit initial power up current. When the amp is working, it will not draw enough power (undriven) to light the lamp.
nuff for now.
Steve
PS. I suggest you replace the guts with a module(s) from amazon, reuse the box and power supply.
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Yeah, the expensive part of an amp project are the heat sinks, box, transformer. +-34 working there should be lots of boards you could build or buy and put in there.
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