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Old 08-11-2020, 08:07 AM   #1
amelbye
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Default C-Audio RA500 troubleshooting

Hi, I'm trying to repair a C-Audio RA500 amplifier (one dead channel). In fact, I have two identical units with the same problem, so I'd really like to figure this one out. I could just throw parts at it, but I'd prefer to understand the design, and that's why I'm asking for help. I'm hoping someone here is familiar with the design - I'm not a pro and have no relevant education, I just enjoy solving problems.

I can't find a schematic for this unit, and apparently it was manufactured by some other company and is not similar to other C-audio designs.

It's a single rack unit stereo amplifier. Output devices are K135 and J50, and the majority of op amps are TL072CP

I'm puzzled by the voltages I find inside this unit.

The outputs from the primary windings connect to the main power section symmetrically at +-64V, all seems well in that area.

However the secondaries confuse me: using the transformer center tap as ground reference - the output from the secondary rectifier is 24V+ and 0V. The rectifier diodes test fine. My first thought is to start looking for a shorted component, however looking at the traces - the negative output of this rectifier goes right back to the center tap on the transformer. To me this is strange. Knowing that the TL072's need positive and negative supply I check their voltages:
one of them gets +24V and 0V, most of them get 0V and -15V.

Right now I lean towards thinking that the +24 is only used by the protect and limiter circuits, and that the TL072's (except the one recieving +24) is powered by the primary power rail, tough some drop down resistors or regulators that I have not located yet. Possibly the positive failed and that's why I have -15V and 0V.

OR is the input stage supposed to be asymmetrical? Perhaps the center tap is not the correct zero point for this part of the amplifier? If so, the voltages may actually be +7.5 and -7.5 and everything is fine in that section too...

There's also a set of 1n4004s very close to the inputs, that seem to be doing absolutely nothing. No AC or DC touching anything. Weird.

Last edited by amelbye; 08-11-2020 at 08:11 AM..
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Old 08-11-2020, 09:16 AM   #2
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Default Re: C-Audio RA500 troubleshooting

might be in protect .. are there lights on the front to say ?
am guessing its class D ?
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Old 08-11-2020, 09:47 AM   #3
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Default Re: C-Audio RA500 troubleshooting

It does actually come out of protect, but seems pretty sensitive, it some times enters protect just as I touch things with the probe of my multimeter.

I'm guessing the amp is class AB, it's an old design.

I just noticed a high amount of AC on the positive main rail - could be the multimeter misinterpreting but I'm guessing I have a lot of ripple on there - my output caps seem healthy but I'll try swapping in a fresh one and see. Could be the ripple has blown something down the line.
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Old 08-11-2020, 10:01 AM   #4
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Default Re: C-Audio RA500 troubleshooting

is it a shared power supply ?
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Old 08-11-2020, 10:05 AM   #5
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Default Re: C-Audio RA500 troubleshooting

The power supply is shared between the channels, each channel has its own protect circuit - possibly operated by the same chip
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Old 08-11-2020, 10:14 AM   #6
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Default Re: C-Audio RA500 troubleshooting

Hello!

You wrote that you could not find a schematic
if that helps, here is a service manual.
https://elektrotanya.com/alesis_ra500.pdf/download.html
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Old 08-11-2020, 12:41 PM   #7
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Default Re: C-Audio RA500 troubleshooting

Please attach good clear pictures of the inside of the unit so we can see how all the boards are connected together, then the pictures of the boards.
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Old 08-11-2020, 01:39 PM   #8
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Default Re: C-Audio RA500 troubleshooting

Quote:
Originally Posted by local View Post
Hello!

You wrote that you could not find a schematic
if that helps, here is a service manual.
https://elektrotanya.com/alesis_ra500.pdf/download.html
that is for alesis
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Old 08-11-2020, 02:09 PM   #9
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Default Re: C-Audio RA500 troubleshooting

I'm afraid the Alesis RA-500 and the C-Audio RA-500 are not the same unit.

I'll post pictures tomorrow - I have one of the units cracked open at my desk at work
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Old 08-12-2020, 04:59 AM   #10
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Default Re: C-Audio RA500 troubleshooting

Hello

It really is different.
unfortunately it is only similar.It may be used in parts.This here C-audio RA 1000. https://audio-circuit.dk/ssmc/
Or even similar here.https://www.hifiengine.com/manual_library/c-audio.shtml
Or here https://audio-circuit.dk/ssmc/

Last edited by local; 08-12-2020 at 05:01 AM..
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Old 08-13-2020, 04:28 AM   #11
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Default Re: C-Audio RA500 troubleshooting

Quote:
Originally Posted by local View Post
Hello

It really is different.
unfortunately it is only similar.It may be used in parts.This here C-audio RA 1000. https://audio-circuit.dk/ssmc/
Or even similar here.https://www.hifiengine.com/manual_library/c-audio.shtml
Or here https://audio-circuit.dk/ssmc/
Thanks, local, I'll check them out!

I put some more time into this yesterday. Of the two amplifiers one has CH1 completely dead, the other has CH2 completely dead - so the thing upsetting this amplifier is probably concentrated to only one of the channels.

On one of the units the power transformer died yesterday, for no apparent reason. Fuse did not break. I guess I'll part that one out and use the chassis for a class D build. I'll still try to revive the second unit.

There are capacitors here that show signs of overheating - however they test fine on my ESR meter, so I've left them in place. I'll recap if I manage to get the amp working. A couple resistors have overheated and started to drift in value - I replaced them with uprated components. (R204 & R205 3.9k uprated to 3W resistors)

R204 and R205 must be part of the +-15V power supply. They have +-55V on one leg and +-15V on the other (when both rails are working)

I might have missed one, but I believe I have checked every diode.

This morning I tried removing all the TL072s from their sockets - and the sockets now have +15V and -15V as expected! So this design is not that weird afterall - the TL072s are supposed to run symmetrically. I measured every single one and none of them were shorted. I started reinserting them and the strange thing is the voltage on the +15V rail drops gradually as I add more TL072s. I was expecting to find a single shorted chip, but this is not the case.

Perhaps a bad electrolytic afterall? I've ordered an LCR meter, as I figured that my ESR meter may be giving me false positives...Seeing if capitance is within 20% could maybe be useful. Perhaps next step is to test all electrolytes out of the circuit?

I'll try to post some pictures shortly
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Old 08-13-2020, 04:55 AM   #12
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Default Re: C-Audio RA500 troubleshooting

A thought that just came to mind: Is it possible for a capaitor to test good for ESR and still leak?

I haven't come across such behaviour before, but it seems that this might be what's going on here...
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Old 08-13-2020, 09:26 AM   #13
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Default Re: C-Audio RA500 troubleshooting

Quote:
Originally Posted by amelbye View Post
Thanks, local, I'll check them out!

I put some more time into this yesterday. Of the two amplifiers one has CH1 completely dead, the other has CH2 completely dead - so the thing upsetting this amplifier is probably concentrated to only one of the channels.

On one of the units the power transformer died yesterday, for no apparent reason. Fuse did not break. I guess I'll part that one out and use the chassis for a class D build. I'll still try to revive the second unit.

There are capacitors here that show signs of overheating - however they test fine on my ESR meter, so I've left them in place. I'll recap if I manage to get the amp working. A couple resistors have overheated and started to drift in value - I replaced them with uprated components. (R204 & R205 3.9k uprated to 3W resistors)

R204 and R205 must be part of the +-15V power supply. They have +-55V on one leg and +-15V on the other (when both rails are working)

I might have missed one, but I believe I have checked every diode.

This morning I tried removing all the TL072s from their sockets - and the sockets now have +15V and -15V as expected! So this design is not that weird afterall - the TL072s are supposed to run symmetrically. I measured every single one and none of them were shorted. I started reinserting them and the strange thing is the voltage on the +15V rail drops gradually as I add more TL072s. I was expecting to find a single shorted chip, but this is not the case.

Perhaps a bad electrolytic afterall? I've ordered an LCR meter, as I figured that my ESR meter may be giving me false positives...Seeing if capitance is within 20% could maybe be useful. Perhaps next step is to test all electrolytes out of the circuit?

I'll try to post some pictures shortly
Helló Amelbye

If you disassemble the transformer, there is a fuse inside the winding on the primary side.
I’ve had a mistake before, but it’s measurable.

As you write, if the value of the resistance changes from heating it can cause a voltage drop, and if there is little capacity, which is on the + -15V terminals of the operational amplifier supply voltage.

The best C-meter is to replace the capacitor.
The problem there is that the C-meters operate at 9V, although capacitor ESR reveals a lot.

Last edited by local; 08-13-2020 at 09:32 AM..
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Old 08-13-2020, 12:04 PM   #14
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Default Re: C-Audio RA500 troubleshooting

So the failed transformer shows open windings?
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Old 08-13-2020, 03:48 PM   #15
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Default Re: C-Audio RA500 troubleshooting

BTW, this site has some free C-AUDIO schematics.
https://www.schematicsunlimited.com/c/c-audio
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Old 08-19-2020, 02:49 AM   #16
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Default Re: C-Audio RA500 troubleshooting

The transformer shows continuity on the windings, but I have no voltage coming out of it - I'll get back to that unit in time - for now I'll focus on the unit that still is partially working.

I'll post some "unneccessary" info here in case someone with a similar problem comes across this thread in the future:
I pulled all the electrolytes. C207 and C208 were in fact faulty. They mesured fine for ESR in circuit, but there must be something in parallell that throws of the reading. I also replaced all the 14 small 22uF capacitors, as one of them was bad.

All the blue rubycon caps were fine, but I uprated C212 to a 105C part, as it looks like it takes a lot of heat.

Fired the thing back up and it's still the same problem. I started poking around with my multimeter. TR201 seems to have to do with voltage regulation on the +15 rail. It is shorted base to emitter. The part is a C2910 (2sc2910) which is obsolete. 2SA1208/2SC2910 is a pair of PNP/NPN transistors. 2SC3503/KSC3503 are (according to another thread) current production replacements. They can also handle more heat/power which should prevent problems in the future. If you replace one, I guess that at least in audio circuits you'd replace both.

I probed around some more and found another faulty C2910 at the driver circuit for one of the channels. Probably explains the dead channel. For now I'll take parts from the broken unit. If the transformer turns out to be OK, I'll get modern replacements for the other unit.

As I'll be replacing transistors I guess I should also check bias and such. There are 3 pots per channel. I'm guessing one is center point, one is bias, but what would the third one be?
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Old 08-19-2020, 06:33 AM   #17
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Default Re: C-Audio RA500 troubleshooting

Solved it - by taking 2 working C2910 transistors from the dead unit, voltages stabilized and the dead channel started working again.

There is one more resistor that should be replaced. R202 has been running hot. It seems to be a 2k2 resistor, both units measure the same but I can't tell from the color bands, they are too faded. I'll upgrade it to a 3W part, and add heat sinks to all TO92 parts.

Then I'll have to figure out what the 3 pots pr channel do. Any clue what the 3rd one would be for?

I'll leave this unit on the side for now and see if I can figure out the tranformer issue on the other unit.
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Old 08-19-2020, 04:13 PM   #18
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Default Re: C-Audio RA500 troubleshooting

Pictures of the boards and inside the unit will help so we can see what caps and resistors you are talking about are.
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Old 08-20-2020, 10:42 AM   #19
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Default Re: C-Audio RA500 troubleshooting

Sorry for the delay, here are some pictures. I did one overview of the whole amp, one of where most the stuff is going on (left side of the heatsink is only rectifier and smoothing caps). Then I did three where I point at problem areas: one pointing at the two capacitors that measured fine in circuit but turned out to be faulty, one pointing at the overheated area causing the +15 rail to drop and one pointing at the component in the driver circuit that caused one channel to go silent.
Attached Images
File Type: jpg 20200820_183202.jpg (663.3 KB, 8 views)
File Type: jpg 20200820_183209.jpg (784.7 KB, 6 views)
File Type: jpg 20200820_183225.jpg (677.5 KB, 4 views)
File Type: jpg 20200820_183308.jpg (760.6 KB, 4 views)
File Type: jpg 20200820_183237.jpg (774.4 KB, 3 views)
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Old 08-20-2020, 10:45 AM   #20
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Default Re: C-Audio RA500 troubleshooting

The second unit: I must have been doing something wrong - the transformer is working again!

I like to build a shopping list large enough to get free shipping before ordering parts - I'm not quite there yet. I need to get started on the next project and will probably have enough to place an order next week. When both units are up and running I'll look into what the trimpots do. Any advice on what could be the purpose of the third pot on each channel would be great!

Last edited by amelbye; 08-20-2020 at 10:49 AM..
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