AB International 600LX and 9620B issues

[Johnny Dior]

Hopefully this is OK to post in this section as I'm hoping Agedhorse will chime in.

 

 

I picked up an AB 600LX and an AB 9620B recently. Both have issues unfortunately, that appear to be similar. I'm a novice at best, know how to use a voltmeter and check diodes, solder caps, resistors, etc., but by no means am I an engineer nor do I have schooling is this field. It's purely a hobby at this point, so keep this mind with replies.

 

 

600LX: Turning unit on will illuminate the power LED, and the giant transformer springs to life, but not getting any sound. It almost seems like it's going straight into protection. One of the first times I powered it up the side case fan kicked on for a brief second then stopped. I can't get this to repeat. The main filter caps are getting around 70v, and a quick probe of what I think are the voltage regulators (3 legged transistor connected directly to the board but legs bent at 90 degree and tab is screwed down directly horizontally to bottom of heat sink) show voltage. I can't find any burnt resistors although R002 has a brown spot on it's outer edge. It seems to extend out from where the metal meets the ceramic.I can't tell if it's from heat or a some sort of glue/epoxy as there are similar marks on the transformer wires where they enter the unit, but there's NO damage to the wires so that seems like epoxy. The only thing out of the ordinary is the fan's wire from the thermal switch to the PCB above main caps was poorly repaired, and the speaker ground wires were cut right at the molex connector and soldered directly to the main filter cap's neg traces.

 

 

9620B: This amp is seemingly doing the same thing. At first, turning it on the power switch wasn't clicking and I thought it was broken. Tried a few more times and it finally "clicked". Fans kick on immediately and I believe the speed steps up once. That lasts about 4-5 seconds then they shut off. I've only done a quick inspection on this one and haven't checked the bottom board yet, but haven't seen anything amiss yet. It too seems to be going straight into protection, although I'm just not that familiar with these to know if this is normal start up behavior or not?

 

I tried connecting a test speaker to both amps and adjusted the volume but it's not increasing anything, although the 9620 DID crackle for a split second on the speaker but it wasn't repeatable. There is a very small amount of channel noise on both amps (talking very minute amount with speaker right at my ear), but it doesn't respond with volume, and obviously I tried connecting my XLR to RCA adapter but got nothing. I have pin 2 going to positive, with pin 1 & 3 going to ground (Hopefully that's correct? Made my own and the amp states 2 is hi and 1 is low).

 

I'm more focused on getting the 600 running first at this point but figured mentioning both to save having to create a second thread. Will be doing a restoration on both and a repaint, but want to get them at least working beforehand. I'll farm out the DC offset adjustments. AB's old site has a schematic for the 600LX, but it's not legible as they did a poor job copying it. Bob from AB mentioned they don't have any physical copies but do have the 9620's schematic (the 1400.2 is a 9620 rebadge so makes sense). So the 600 is a shot in the dark at this point, but it's not overly complex thankfully and worst case I'd just go through the entire thing with new parts. If anyone can help give me tips to at least troubleshoot the 600LX I'd appreciate it. Along with common failures of either of these when they do go.

 

Also, do these units do any sort of self-check at power up? i.e. are the fans checked for operation? Do the thermal switches ever fail? If so, would it be the same as seeing say an overheat situation i.e. go into protection? The 9620 is from 98', and the 600 is from the same time period I believe and is an overseas built unit. Thanks.

[agedhorse]

I'm afraid I have a bit of bad news for you.

 

At power on, there is a latching protection circuit that looks for excessive DC offset and if any is present shuts down the power supply via an SCR or triac (it's been a long time since I worked on one). This is what appears to be happening, which means that you have a failure of one (or both) of the amp channels. They are not easy to repair unless you do this kind of thing for a living, you need a signal generator, a scope, a variac and an understanding of the circuit.

 

Likely the same thing for the 9620.

 

You need to find a tech experienced in pro audio, and one that has experience with AB amps specifically. The 9620 is a class H amp that uses a fairly unique method of switching the 0 volt rail (IIRC) that is not used by other manufacturers. The 600LX is a relatively straight ahead dual differential topoliogy class AB amp but uses the latching protection circuit.

 

These are not DIY repairs. Not even close.

[Johnny Dior]

Thanks for the quick reply AH and nice to meet you. Is there a way I can check to see if the DC offset is high enough to trigger this event? Would I just probe the speaker terminals? Also, if I put all new parts in i.e. new Toshiba 2SB554, 2SD424, caps, etc., are you saying that won't work? I got these cheap enough that putting $100 worth of parts into one wouldn't bother me. The 600 looks straight-forward with part replacement. Or is there something I'm missing? These amps came from the same person that was in a band. Any idea what typically causes something like this? Maybe a surge? Or a faulty pre-amp? Is it possible the DC offset pots (not sure if that's the correct name for them) have failed, and are causing the unit to output incorrect values? I've read where someone else doing a repair ran into a faulty one.

[agedhorse]

Can't check the DC offset whenever the latch triggers the shutdown. There are procedures for disabling the latch protection for troubleshooting purposes (you MUST have a variac and a current limiter) but it's way beyond any DIY approach.

 

Replacing the output transistors is more likely to result in more bad output transistors than anything else. There are many other problems that can occur during a failure, I recently trashed a 600LX because it wasn't worth repairing (and I have all the parts, equipment, docs and experience) because there was enough damage that it was more effort than it was worth.

 

This is especially true of the class H products, there are some specialized techniques for diagnosing these. They are not easy to repair unless you are very experienced and have the proper equipment. Most of the 2 tier procucts switched the supply rails, the 3 tier products switched the lowest tier on the opposite side or something odd like that. I haven't looked at a 3 tier product in over 10 years.

 

It's unlikely to be DC offset pots, and I do not recall the 600's having offfset trims. (at least the 600A docs show no DC offset trims).

 

There were a lot of versions over the years of all of these amps, so there are variations that I am not familiar with. Most of my experience was with the products during the last 10 years of production.

[Johnny Dior]

I appreciate your candor AH. Is the 600LX a decent enough sounding unit when working? I bought these solely because the reputation of being workhorses that last awhile was appealing, and wanted to see how these sounded. Given no one ever has anything negative to say about these was also a driving factor in taking a gamble with purchasing them.

 

 

I contacted AB about the 9620. Figure it's best to just go straight to them for that one.

 

 

Do you happen to have a clear schematic of the 600? Bob from AB stated they had none, and their old web site's .pdf schematic of the 600 LX was the only thing available. Unfortunately, it's not legible which he noted. I could possibly have it fixed locally if a schematic was available.

 

 

P.S.

I wish you still had your 600LX as I would have gladly bought it for parts as I need a channel pot and both knobs.

[agedhorse]

I don't have the vesion that you have, and mine is not that great either. There were USA versions and taiwan versions of the amp, they were very different. Then there were the 600, 600A, 600LX, Prescident 600 etc. The differences were really significant.

[RoadRanger]

Why bother with those when you can get a nice RMX850 for $150 ? Still made and serviceable though probably cheaper to replace than repair.

[Johnny Dior]

The 600LX is rated at:

• 270w per channel @ 8 ohms
  
• 425w per channel @ 4 ohms
  
• 750w @ bridged mono @ 8 ohms
  

The RMX850 is rated at:

• 200w per channel @ 8 ohms
  
• 300w per channel @ 4 ohms
  
• 530w @ bridged mono @ 8 ohms
  

Granted the 850 is 2 ohm stable, but I don't need 2 ohm loads for full-range stereo operation. Wattage-wise, a RMX1450 is closer to a 600LX, although the RMX line apparently only has around a 300 damping factor, vs. around 250 with the 600LX. The 600LX has a better signal to noise ratio of 104 vs. the RMX line of 100. Suppose if I really wanted, the 600LX with better cooling and more capacitance, along with some tweaks could do 2 ohm.

 

 

The 9620B is rated at:

• 850w per channel @ 8 ohms
  
• 1400w per channel @ 4 ohms
  
• 2000w per channel @ 2ohms
  
• 3000w bridged @ 8 ohms
  
• 4000w bridged @ 4 ohms
  

The RMX 2450 is rated at:

• 500w per channel @ 8 ohms
  
• 750w per channel @ 4 ohms
  
• 1200w per channel @ 2ohms
  
• 1500w bridged @ 8 ohms
  
• 2400w bridged @ 4 ohms
  

The 9620B puts out roughly double what a RMX 2450 puts out at every load. It also has an insanely high 1000 damping factor vs. 300 damping on the 2450, plus it still has a slightly better signal to noise ratio of 101 vs. the RMX line of 100. This will be a straight-up sub amp, capable of powering 4 to 6 subs with ease right at their rated values. Been running channel 1 and it sounds great. Very clean bass. Did I mention I already own both

 

 

After some troubleshooting, it appears the 9620B has a dead channel 2, so the amp board is getting shipped off to AB to get fixed. I don't mind putting money into them to give them another lease on life.

 

 

[agedhorse]

Damping factor difference is effectively zero, it does not matter.

 

Can not convert the 600 to 2 ohm operation, there's more to it than capacitance and cooling. Seems like that's the standard answer on internet

forums, but there's a whole lot more that goes into it. Not just tweaks, that's why the 900 is SO much different in every significant way.

 

The S/N ratio difference is insignificant.

[Johnny Dior]

Damping factor might not matter much on a really tight paper woofer with little to no travel, but it does matter on actual sub woofers that are capable of an inch or more of travel. I've done tests with higher-end (600w rms) car audio amps bridged, running to identical subs (i.e. two different amps bridged with each powering an identical 12" sub @ 4 ohm with both running at same time). Only using a freq. cut of 100 hz from the head unit (amps set at full range) and nothing else, with gain set so they both appeared near equal while being fed a mono signal, the amp with 100 damping had the sub's movement wildly sloppy (excess and exaggerated over even minute bass notes) travel/over travel vs. 500 damping and controlled the other sub much more precisely with minimal movement. It was visually apparent watching them which was which. I experimented and even tried coloring the higher damping amp with a bass boost around 45 hz and even upped the gain a bit to see if it'd induce more uncontrolled movement, and it still controlled the sub's movement better. I set the amps so they "sounded" nearly the same and left them alone. A year later, the sub running on lower damping amp had excessive wear. With no power going to them, physically pushing the sub by hand saw it extremely loose vs. the other one which was still taut (these were $500 a piece subs mind you running right at their rated values). I can also tell an audible difference with the 9620B powering two 15" woofers (1 channel @ 16 ohm) vs. any of my recapped vintage gear at low volumes where it holds sine wave-type bass notes very nicely. If the bass sounded worse, I'd definitely say so.

 

I read in the literature for the 600LX that it supposedly will go below a 2 ohm reactive load before incorporating the safety. I assumed that with a mid/tweeter only set up on the amp at moderate to low levels with extra cooling, it would tolerate 2 ohm. If not, I'll take your word for it as I won't try it. IME some older amps can often do a step below rated like 6 ohm instead of 8, so long as they are over-built to some degree and don't have overly strict protection circuitry AND have fans keeping board temps down.

 

 

S/N doesn't matter? I was under the impression a higher S/N ratio implies less background noise? Or are you saying inter-modulation distortion should be more of a factor?

 

 

 

[agedhorse]

The difference between 250 and 300 and even 1000 doesn't matter one bit in practice. Down between 50 and 100 there might be some difference.

 

The sub that was loose was damaged from overpowering, the suspension components were degraded. It was not due to the damping factor. You have no idea if one of the amps might have had a high pass filter built into it's internal signal path that was responsible for what you think you saw. We also do not know what the REAL ratings of either the subs or the amps in question are. The care audio industry makes up their own numbers that often have little bearing to reality. Trust me, ifthe sub is rated at 600 watts, it's not 600 watts RMS like the (real) pro audio industry rates their products at.

 

2 ohms reactive for protection circuit envelope is a far cry from the overall SOA of the output stage. It does not mean that the amp can drive a 2 ohm nominal resistive load, which would lead to much lower reactive components.

 

Without knowing the exact S/N measurement metrics and weighting (which we do not know because they are not stated), the difference of a few dB is not significant. How it works in practice is what really defines real world signal to noise performance. IMD is evem less of a factor.

 

You are going into territory where your statements and assumptions are gentting more and more questionable. I don't know what your background is, but the information you are stating looks internet derived. I have designed many commercial amplifier products, I would never make the assumptions that you are making because I know that they are not true.