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Old 5th September 2012, 04:41 AM   #11
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I finally had a chance to check the DC voltage on the speaker terminals. I get a quick reading of around 2.5VDC on both channels before it goes into protection. I don't see anything like that if I perform the same test on my Opti Drive 50.
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Old 5th September 2012, 05:30 AM   #12
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Are you sure its 2.5 volts DC? or could it be 2.5 Milli-volts DC? what range is your meter on ?

In most amps the DC offset out will trigger the amp to shut down at much lower levels somewhere between 50 and 300 milli-volts DC in many designs.
At 2.5 volts DC out your amp will fry voice coils in many circumstances and that level might increase with audio drive levels. So I am thinking your opto's are just fine and that there are some issues with both channels producing that level of DC output.
A new amp will likely being the 0 to 5 milli-volt range, a used amp in the 5 to 15 milli-volt range. Many manufactures recommend 15 milli-volts or less DC offset on the speaker terminals on rebuilds and less on new unused gear.
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Old 5th September 2012, 05:45 AM   #13
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Well, I thought I had it sitting at 20v range for DC. If I put it in 2000mv range and it would go out of range. I have a crappy Centech multimeter, so maybe it was reading wrong. I will see if I can borrow a good one and try again. I connected this thing to some old speakers that I don't care about and they played just fine. I was assuming that alot of DC voltage would cause the speakers to pop loudly and eventually fry. Maybe not?

So if I really have that much DC hitting the speaker terminals, where do I go now?
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Old 5th September 2012, 06:33 PM   #14
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High DC offsets are usually caused by damaged transistors that are leaking and drifting with thermal heat build up. I start with the input pairs myself but it could be any of the transistors in the main amp stage. Each main amp has a DC blocking input capacitor. The purpose of that cap is to block DC from the preamp buffer input of the amp< the IC's near the RCAs>. So it is doubtful that the DC problem could be anywhere else but in the main amp sections.

Amps that are driven into clipping hard all the time seem to have damage of the input transistor pairs by my experience, and when the damage is not there its usually in the next stage of the amp. outputs and drivers can be the source of the issue but they tend to fail outright and short most of the time and are easily found by using a meter on its low ohms range. Using a ohm meter to find leaky transistors is a needle in a haystack situation IMHO. In this case your going to have to disable the protection opto somehow so the amp will stay on long enough for you to trouble shoot the main amp stage. lifting the input to the opto on the audio side might disable the protection so the amp will stay online for you to tech it out.

If time is a factor and you are limited in training on this sort of work you might just want to just replace the input pairs of each channel with new devices and then recheck your amps output DC. They are usually jelly bean transistors costing 3 cents each in bulk, so depending on the input configuration your looking at 12 to 24 cents to replace all of those transistors with new ones and not worry about the possible leakage issues. retail prices of those transistors will likely be much higher but I have only bought in bulk so I only know wholesale bulk pricing.

Have you looked into Perry's training web-site ? just look under any of his posts including in this thread and you will see links to click on. Perry's site is the best info on the web, period ! I suggest you look into his training info to see where this amp repair is likely headed. He has tons of info there and he might also have some info on this series of Lanzar amp. For now though please recheck the DC output of the amp first...
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Old 5th September 2012, 08:52 PM   #15
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Since I am a complete novice to this I don't mind throwing some new parts on it and see what happens. I have already replaced the caps on the power supply section and the mosfets as well.

Are you talking about replacing the tiny cylindrical transistors on the board that live in the center secion? I will get some full res pics up later tonight of my amp.

I'm going to borrow my friend's fluke tonight and check for DC voltage again. I think my meter was acting up on me. I was reading about having DC bleed into the speaker terminals and fro what I've read I should be getting a loud pop and of course will most likely damage my speakers. This one comes on and plays just fine for that short period.

I've checked out Perry's website and it is helpful, however still a little over my head. I have to say I am having fun trying to learn as I go. Thank you so much for all the fantastic advice. Although it would have probably been more wise to pay a professional to repair this amp, I'm still glad that I'm going through the process of trying to learn this. I've thrown away some really nice amps in the past that may have just required a few bucks worth ofparts and some knowledge to get back up and running.
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Old 6th September 2012, 06:08 AM   #16
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Alright. Looks like my meter had a low battery and was giving off funky voltage readings. It appears like I get around 250-300mv for a moment and then it drops down to 15-20mv before it goes into protection. After it goes into protection DC volts come back up to around 250mv and slowly drop off. I noticed that when it goes out of protection I hear a faint click sound on the board. I can't tell where it is coming from other than possibly somewhere in the power supply section or maybe towards the middle. I'm thinking that maybe I should get this over to someone else to figure it out. I reposted the pics below. They come out a bit better.

Last edited by Buickmike; 6th September 2012 at 06:11 AM.
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Old 6th September 2012, 06:09 AM   #17
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Old 6th September 2012, 06:10 AM   #18
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Old 6th September 2012, 07:43 AM   #19
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One of those opto's should be connected to a small signal transistor located on each channels output. That transistor should be the offset trigger for the opto to inhibit the amps power supply.
The other opto is likely used to detect balanced rail supply, and if it is not satisfied that both rails are present and balanced it also inhibits the power supply.

The readings your getting sound OK as all amps have some sort of turn on output that settles to normal conditions. Could you measure the plus and minus power supply voltages while doing this turn on test again? if the rails are not correct and balanced this could also cause the power supply to inhibit.

The input pairs are located in the center of your picture just behind the op-amps chips. but check the rail supply before you do anymore component removals, and let me know what voltages you get with your negative probe lead connected to the RCA shield or the center tap ground of the power supply toroid transformer, both should be the same reference level for the black lead of your meter.
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Old 6th September 2012, 06:49 PM   #20
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Quote:
Originally Posted by 1moreamp View Post
The input pairs are located in the center of your picture just behind the op-amps chips. but check the rail supply before you do anymore component removals, and let me know what voltages you get with your negative probe lead connected to the RCA shield or the center tap ground of the power supply toroid transformer, both should be the same reference level for the black lead of your meter.
Are you talking about the long chip to the left of one opto and below the other? Am I trying to find the input voltages to a couple of the pins on that to determine rail voltage? Also, I have 2 coils and I'm not sure which one is actually the transformer. Sorry, still very new to this and am struggling with some of the terms. Still trying to grasp exactly what rail voltage and input pairs are. I keep re-reading Perry's basic amp repair site and keep getting lost.
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