Picked up a Crown XLS 202. One channel works perfect. The other channel plays but has some distortion, ? Increases with volume. Pots cleaned, relays checked, Preliminary checks, filter caps OK, voltages at all power transistor OK. I am a novice but would like to learn more. Any suggestions where to start, have schematics.
Happy New Year!
I'm attaching a schematic found at ElectroTayna.com. Does it conform to your amp?
Would you tell us about the problem and what test equipment you have available? Which channel is misbehaving?
I'm attaching a schematic found at ElectroTayna.com. Does it conform to your amp?
Would you tell us about the problem and what test equipment you have available? Which channel is misbehaving?
Yes that is the schematics. I have removed the PCB board from the case. I have build several kits (Dynaco, Hafler) but not good at trouble shooting. Iam reading alot, but would love to learn more.
Test equiptment/tools:
1. DVM
2.signal generator
3.Variac/power supply 6-30 DC volt
4.Transistor/capacitor tester (ESR meter as well)
5. Oscilloscope (just learning how to use it, Rigol 1054Z)
6. Hakko soldering and Vaccum desolderer
Thanks for any help you can give me!
Jim
Test equiptment/tools:
1. DVM
2.signal generator
3.Variac/power supply 6-30 DC volt
4.Transistor/capacitor tester (ESR meter as well)
5. Oscilloscope (just learning how to use it, Rigol 1054Z)
6. Hakko soldering and Vaccum desolderer
Thanks for any help you can give me!
Jim
Don't just swap parts, the actual problem must be found.
The current limiting circuits may be bad, so check C9 and C10, and other related parts.
If those parts are all ok then:
Measure main supply DC voltage.
Check output DC offset voltage.
Check all transistors in unpowered circuit with an ohm meter.
Also check all 0.47 ohm emitter resistors in unpowered circuit.
The current limiting circuits may be bad, so check C9 and C10, and other related parts.
If those parts are all ok then:
Measure main supply DC voltage.
Check output DC offset voltage.
Check all transistors in unpowered circuit with an ohm meter.
Also check all 0.47 ohm emitter resistors in unpowered circuit.
First off - not familiar with this amplifier, but the schematic is pretty similar to a lot of amps on the market.
My first step - check the outputs (Q13-Q20A) for any shorts with the DMM (they are big and usually easy to access the legs)? These are more likely to short, causing rail voltage on the speaker output, causing protection circuit to step-in and you say the amp still plays music, so low likely-hood.
Now You have a choice - start at the input and work forward (Q1A-Q7A) to the outputs, or work backward from the outputs checking the driver and pre-driver stages (Q9-Q12, then Q21-Q22 - if populated) comparing to the working channel? Q8A is the bias control, and wouldn't suspect that is the issue, unless the bias is zero - doubt this, since the bias is set by a resistor voltage divider.
Distortion could be coming from any stage, including out of spec resistor or faulty capacitor. Since you said you are not familiar with the O-scope, I would take the schematic and start by comparing the voltages listed on the schematic and if those check out then start taking voltages for each leg of the transistors from the working channel, then compare to the non-working channel. Likely a leaky transistor, or defective part but you should be able to pinpoint it from a voltage difference showing up.
My first step - check the outputs (Q13-Q20A) for any shorts with the DMM (they are big and usually easy to access the legs)? These are more likely to short, causing rail voltage on the speaker output, causing protection circuit to step-in and you say the amp still plays music, so low likely-hood.
Now You have a choice - start at the input and work forward (Q1A-Q7A) to the outputs, or work backward from the outputs checking the driver and pre-driver stages (Q9-Q12, then Q21-Q22 - if populated) comparing to the working channel? Q8A is the bias control, and wouldn't suspect that is the issue, unless the bias is zero - doubt this, since the bias is set by a resistor voltage divider.
Distortion could be coming from any stage, including out of spec resistor or faulty capacitor. Since you said you are not familiar with the O-scope, I would take the schematic and start by comparing the voltages listed on the schematic and if those check out then start taking voltages for each leg of the transistors from the working channel, then compare to the non-working channel. Likely a leaky transistor, or defective part but you should be able to pinpoint it from a voltage difference showing up.
Tell us a bit about your Variac and power supply. Are they separate devices? Power supply has adjustable voltage? Dual supplies maybe?Adjustable current limit?
They may present an appealing way to power the amp while troubleshooting.
They may present an appealing way to power the amp while troubleshooting.
Leaky transistors can happen at the back end where shorted speakers or wires have damaged them, but are unlikely at the front end. This product is ancient. I would plug up signal generator through y cable to both channels, put 1.0 vac signal in, with scope look at output of C7a good channel versus bad channel. The input cap. Don't even have to power up for this test. If different, with AC unplugged, remove & replace connectors CN1 or CN2, only the one on the bad side. Oxide can happen to connectors. Remove & replace scrapes oxide off. There is also a switch on the input board, erase contacts or spray out with contact cleaner (watch out may be flammable) or whatever you can do to get oxide off. Still bad, replace C7a on the bad channel.
Still bad, further follies required. If there is a big transformer and 2 big filter caps, you can safely work on it using one hand and a alligator clip to P.S. ground on the negative DVM probe. Never use 2 hands after the power has been on. Measure mains caps below 2v before touching any metal with hands. Discharge main caps with alligator clip leads and 1000 ohm 10 watt resistor as required. If by chance there is a switcher supply, little transformers & coils, do not touch that area. No metal on hands wrists or neck. Wear safety glasses, solder splashes and parts can explode.
I work on transistor amps & organs back to 1962 but after I find out what is wrong and correct it, I replace a lot of rubber sealed caps. Keeps me from going back in for the next one all the time.
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Just fixed an xls602. Same thing, more power. The input board has two tl072, just swap them out for good measure.
They tend to go bad if molested, the input section of a pro (?) power amp is not a good place for them as they have jfet inputs, not very good voltage clamps and tend to latch up if mistreated or jolted with static electricity.
I would mod the +-18v section to work with ne5532s or so. Lower input impedance, diff clamp diodes onboard, hard to kill, hard to make misbehave, impossible to beat for $/performance. I put DIP8 sockets for what-if on mine but still running with TL072s.
Then there was another problem with the dropper resistors feeding +-18V to the input board. Two new resistors and done. They are located right by the flat cables going to the preamp on the main board. May be covered in glue/gunk/caulk.
My particular amp was mistreated, running on bad gensets for god knows how long (bulging psu caps from gross overvoltage), the cover is missing so the cooling is bad as it needs the top cover for airflow over the heatsinks, lots of gunk and dust. But it is still mostly an old Crown, even if made in china. The output transistors are old school TO-3 metal cans, simple class AB topology and the power circuitry is very simple and easy to fix.
Recomendations:
Chuck both TL072s in the preamp board, put sockets, put in new TL072s from TI. BTW, easy way to tell if the opamp is bad or about to go bad: check for offset voltages at the outputs and inputs, if present (more than 0.0n volts) = dead or dying.
Check/Mod the +-18V section, put higher wattage zenners and lower the value of the dropping resistors accordingly. Maybe up the wattage a little.
They tend to go bad if molested, the input section of a pro (?) power amp is not a good place for them as they have jfet inputs, not very good voltage clamps and tend to latch up if mistreated or jolted with static electricity.
I would mod the +-18v section to work with ne5532s or so. Lower input impedance, diff clamp diodes onboard, hard to kill, hard to make misbehave, impossible to beat for $/performance. I put DIP8 sockets for what-if on mine but still running with TL072s.
Then there was another problem with the dropper resistors feeding +-18V to the input board. Two new resistors and done. They are located right by the flat cables going to the preamp on the main board. May be covered in glue/gunk/caulk.
My particular amp was mistreated, running on bad gensets for god knows how long (bulging psu caps from gross overvoltage), the cover is missing so the cooling is bad as it needs the top cover for airflow over the heatsinks, lots of gunk and dust. But it is still mostly an old Crown, even if made in china. The output transistors are old school TO-3 metal cans, simple class AB topology and the power circuitry is very simple and easy to fix.
Recomendations:
Chuck both TL072s in the preamp board, put sockets, put in new TL072s from TI. BTW, easy way to tell if the opamp is bad or about to go bad: check for offset voltages at the outputs and inputs, if present (more than 0.0n volts) = dead or dying.
Check/Mod the +-18V section, put higher wattage zenners and lower the value of the dropping resistors accordingly. Maybe up the wattage a little.
I can put in socket for the TI072s but modding to work with ne5532s maybe beyond my capablities at this times. Thabnks for the input!! Once I get fix the board apears to allow components to go all the way to a 802.
The only way to go all the way to an 802 is to up the power supply voltage to what it is on the 802. But it can’t hurt to install all the MJ15024/5’s that you physically can. They probably left out a heat sink or two, though. The board was somewhat modular (I had an 802, but got rid of it - a friend needed it, I didnt need another one that small).
I have a couple of amps with TL072 (dual) or the 4 amp version TL074. No problems. Be sure to check with the DVM the output pins (1,7) are the same DC voltage as the input pins (2,3,5,6) with no signal. Before replacing. Be sure when probing not to short the power supplies to the output pins. This will burn one up. One is the pin with the dot on the upper left from the label side. Count counterclockwise. See the datasheet.
NE5532 uses more current than TL072 and would require bigger wattage zener reglators on the power supplies with smaller value resistors. Quieter amps for same curretn like NJM4580 are available in the same pinout. Modern Peavey uses a lot of these.
The cost of blown xls 802s etc is so low, I don't see the benefit of modifying the output stages for higher power. Xls-202 is fine for home use. I listen mostly at 1/8 watt, with peaks of maybe 50 w/ch on the cannon shot in 1812 overture. My speakers are 98 db 1w1m.
NE5532 uses more current than TL072 and would require bigger wattage zener reglators on the power supplies with smaller value resistors. Quieter amps for same curretn like NJM4580 are available in the same pinout. Modern Peavey uses a lot of these.
The cost of blown xls 802s etc is so low, I don't see the benefit of modifying the output stages for higher power. Xls-202 is fine for home use. I listen mostly at 1/8 watt, with peaks of maybe 50 w/ch on the cannon shot in 1812 overture. My speakers are 98 db 1w1m.
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The TL072 is actually pretty good about input overload. Limit the input CURRENT to 4 mA and you will never have issue, even if driven beyond the rails. Ive tried some of the more advanced versions as power amp input stages, only to have them blow and latch the speaker to the rail with the slightest bit of clipping. Limit that input current on the 072 and that won’t happen. Even sending the speaker to the rail and putting 80 volts on the feedback input won’t do anything, provided the feedback resistor is large enough to limit the resulting input current (20k or more on 80 V rail). This amp doesn’t have them in the feedback loop, but a lot of other Crowns do. I keep the 072s rather than putting in something “better” because of their fault tolerance.
Mine likely blew the TL072s and the drop resistors on the supply because, somehow, mains voltage got to the inputs. As I said, I'm using TL072s again. I've seen JFETs (maybe it was TLE2071s, not the original OPs), going south on a Crown DC300-II after (again!!!) getting some mains on the inputs. I don't know what the heck, but Spain and proper grounding don't go together much. This was in a recording studio of all places, but it looks like the UPS was miswired or something. It took me a couple of weeks to find out the failure mode after it randomly fried a pair of NS-10s.
On power-up, at random, it would latch, but just slightly, sending around 10V to one speaker or the other.
I've seen a lot (all of them) of partly fried TL074 on an Altair MF-24, a Spanish take on a class H pro amp (of the Iron Pig variety)... but overengineered where it doesn't matter while the really important stuff is underengineered, like the LF cutoff point on the pre, which makes it unusable for sub duty. Same story, mains got into the inputs and cascade fried everything. The outputs were fine... I may end up modding it for better LF and use it on a friend's sound system.
The DC300 got modded to use vintage NE5534s as the original OPs are unobtanium on top of being noisy on a good day, unusable on a bad one. No problems ever since and it is dead quiet too.
On power-up, at random, it would latch, but just slightly, sending around 10V to one speaker or the other.
I've seen a lot (all of them) of partly fried TL074 on an Altair MF-24, a Spanish take on a class H pro amp (of the Iron Pig variety)... but overengineered where it doesn't matter while the really important stuff is underengineered, like the LF cutoff point on the pre, which makes it unusable for sub duty. Same story, mains got into the inputs and cascade fried everything. The outputs were fine... I may end up modding it for better LF and use it on a friend's sound system.
The DC300 got modded to use vintage NE5534s as the original OPs are unobtanium on top of being noisy on a good day, unusable on a bad one. No problems ever since and it is dead quiet too.
If you’re going to plug input jacks into the wall you deserve to have to fix your amp. Usually the 1/4 watt resistors will fry, along with any semiconductor it hits next. One reason to always use a separate package for the balanced input - not the other half of the op amp being used to control the overall feedback. At least the collateral damage stops. But you do want an op amp that won’t die if the power amp section heads for the rails (ie, open VAS/current source). The versions using a single ended VAS (DC300, the PL400/700-2, Peavey CS, GK) like to latch momentarily and correct themselves after a few ms if the op amp’s offset is the wrong polarity. Perhaps that Altair amp uses that topology. On a dim bulb they latch permanently if you fire them up on a DBT with the speaker connected. You have to fire them up without the speaker, THEN connect it when servicing. With the symmetrical (BGW-type) VAS they don’t do that regardless of offset polarity. Another good case for using speaker relays, regardless of other problems they may cause.
Yup... It wasn't me... Sticking a jack into an outlet.
This is 50Hz land, we have 230v in the outlet. This makes for some fun if grounded and ungrounded wiring clashes. The neutral and ground tend to be bonded at the mains distribution box (key word, tends!!!), some outlets are grounded, some aren't. Using grounded extension cords in ungrounded outlets is common, this means that any switching power supply and a lot of linear power supplies have a pair of Y2 caps between the two legs and ground. This lifts the ground to roughly half the mains voltage. 115v on the ground pin... Then you connect sometihing properly grounded and... Bzzzzzt... A couple of angry mains filter caps just discharged their anger into your audio cabling. The worst offender tends to be Jack to XLR cables, as you touch the ground with the tip of the jack first.
Then, sometimes, you have different phases (400v!) feeding different outlets on the same podium/bandstand/room... Bzzzzt/puff/bang... That's most likely what happened to the Altaïr...
I'm still trying to wrap my head around a mini mixer I lent out on a gig, most mike inputs got fried... Like... How???
About the DC300, we are working on it, it will get a protection board running off the stock transformer, no big modifications needed... I need to finish the board layout...
This is 50Hz land, we have 230v in the outlet. This makes for some fun if grounded and ungrounded wiring clashes. The neutral and ground tend to be bonded at the mains distribution box (key word, tends!!!), some outlets are grounded, some aren't. Using grounded extension cords in ungrounded outlets is common, this means that any switching power supply and a lot of linear power supplies have a pair of Y2 caps between the two legs and ground. This lifts the ground to roughly half the mains voltage. 115v on the ground pin... Then you connect sometihing properly grounded and... Bzzzzzt... A couple of angry mains filter caps just discharged their anger into your audio cabling. The worst offender tends to be Jack to XLR cables, as you touch the ground with the tip of the jack first.
Then, sometimes, you have different phases (400v!) feeding different outlets on the same podium/bandstand/room... Bzzzzt/puff/bang... That's most likely what happened to the Altaïr...
I'm still trying to wrap my head around a mini mixer I lent out on a gig, most mike inputs got fried... Like... How???
About the DC300, we are working on it, it will get a protection board running off the stock transformer, no big modifications needed... I need to finish the board layout...
the laws of the ground are often impenetrable...
I had a dual mono PA amp like that, as soon as I grounded it, it instantly burned the ground track between the two volume potentiometers, taking the source with it in the process.
I had thought about a galvanic leak between the two power transformers but I never managed to prove it.
it worked perfectly with one source per side.
I had a dual mono PA amp like that, as soon as I grounded it, it instantly burned the ground track between the two volume potentiometers, taking the source with it in the process.
I had thought about a galvanic leak between the two power transformers but I never managed to prove it.
it worked perfectly with one source per side.