Zapco Model 151 Mods (or How do I bring stereo back) - diyAudio
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Old 14th February 2010, 09:02 PM   #1
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Default Zapco Model 151 Mods (or How do I bring stereo back)

I just picked up a model 151 with the power supply. I hope to run some front stage on it, but I have a small problem. It is a mono model. I am decent with a soldering iron, but I don't have the knowledge to reverse engineer and restore stereo output. From what I have read on here, all model 151s were 4ohm stereo, non-bridgable. If you wanted a mono amp, you send it back to get modded, or heat up your soldering iron. I have it setup with a power supply running my AR-71s, and it sounds killer.(Side note, kick my quadraflex 650-fet to the curb, doesn't do these speakers justice.) If i hook up a stereo input, the low end goes away, but it does appear to be stereo. It has a sticker on the case labeling it as monaural. (It is kinda funny, since my online presence has been monoaural for long time). I live down the street from the zapco offices, but I kinda want to get my hands dirty on this one. I also got an AEX in the deal, but that is missing the power supply, and may go on ebay to help fund a PX, or another 151 to bi-amp. Also, anyone have an ESM they want to get rid of?
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Old 14th February 2010, 11:07 PM   #2
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My favorite amp!!!
You will need to resolder a cap and cut a solder bridge (or put one in) ... It all depends on which board of 151 you have. I think I have 4 different boards for them. Can you post a pic? Also check to see if the cap in the picture is cut. If it is cut, it is a mono amp. If it is not, then it is a stereo amp. Here is a pic of a stereo and a mono board. The cap is missing on the mono one rather than cut. Post a pic of your board and I can get you pics of it in stereo mode.

Click the image to open in full size.

Click the image to open in full size.
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Old 14th February 2010, 11:44 PM   #3
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It is missing the cap.
Click the image to open in full size.
The solder bridge is located on the four empty pads? My picture is a bit washed out there, but all four pads are separate.
I believe you are the gentleman that has enough 151s to make a SPL beast. Is there any good place to read up on the early Zapco stuff? I live in the heartland of zap, but most info I get is "You aint **** unless you got a zap on yo' lap." (In fact, my shop is working on a truck with a SEQ installed.)
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Old 15th February 2010, 02:27 AM   #4
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I went through 6 of the mono amps that I have. Most are close to the same board. (all I have is stereo with that exact board) You have a middle of the 151 run production board. The later ones seem to have the jumper. Here is what you need to do to run it back stereo. 1. change the cap, or reattach it... 2. Make sure there is no jumper between the cap + and -. That is it. Some of the mono ones like yours (close to your board) that I have do not have the jumper at all, and they still work in mono. So, if there is no jumper across the legs of the cap (should NOT read 0 ohms) , then you just reattach cap. I think I have 4 with no jumper and they play fine in mono. Let me know if you have anymore questions on the 151's or the SEQ/PEQ/PX/150's. I have a lot of them for reference if you need pics...

Last edited by tomtomjr; 15th February 2010 at 02:31 AM.
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Old 15th February 2010, 03:06 AM   #5
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Would this be the a good replacement? Parts-Express.com:330pF 500V Ceramic Disc Capacitor | voltage vacuum test PAC PA mics mic ic ceramic caps cap ac It looks like yours has a 1kv volt rating. Mine just has a 331 on it, which would make it 330pF. Doesn't have anything else on it.
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Old 15th February 2010, 07:50 PM   #6
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I will dig into it later today. I will open some back up and see what I can figure out. There are several different caps used in the different boards. Will get one of the stereo ones opened up (your board) and see what I can come up with for a value...
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Old 22nd February 2010, 07:33 PM   #7
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cool little amps
How much wattage do these things make ?
What are the full specs ?
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Old 22nd February 2010, 07:59 PM   #8
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They came in 2 basic flavors over the years, 75W/Ch and 100W/Ch -4 ohms minimum load. Introduced in 1978 as System 150/150L/150H, they evolved over the years, already using 4-layer PCB's by 1984 or so.

Power supply was a separate piece, +/-30V for the 75W system and +/-40V for the 100W system. The amplifier module was the same in either case.

An optional 18,800uF "energy storage module" went between the PS and amp.

The System 151 and 200 versions were also available as 151A/200A, which featured hand selected/matched components.

They were expensive, between $600 and $900 around 1986 depending on the package.

And Zapco's forte was custom work. If you had the cash, they's build it the way you wanted it.
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Old 22nd February 2010, 08:51 PM   #9
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Quote:
Originally Posted by tsmith1315 View Post
They came in 2 basic flavors over the years, 75W/Ch and 100W/Ch -4 ohms minimum load. Introduced in 1978 as System 150/150L/150H, they evolved over the years, already using 4-layer PCB's by 1984 or so.

Power supply was a separate piece, +/-30V for the 75W system and +/-40V for the 100W system. The amplifier module was the same in either case.

An optional 18,800uF "energy storage module" went between the PS and amp.

The System 151 and 200 versions were also available as 151A/200A, which featured hand selected/matched components.

They were expensive, between $600 and $900 around 1986 depending on the package.

And Zapco's forte was custom work. If you had the cash, they's build it the way you wanted it.
So, if I find a model 200 PS, I can get a bit more oompf out of my 151? I haven't heard it yet in an automotive application, but i have a feeling I am gonna want a bit more power.
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Old 22nd February 2010, 09:23 PM   #10
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Quote:
Originally Posted by monoaural View Post
So, if I find a model 200 PS, I can get a bit more oompf out of my 151? I haven't heard it yet in an automotive application, but i have a feeling I am gonna want a bit more power.
Yep, the 100W and 200W units used the same amp module. Somewhere around here I have the schematic from Zapco.

But try it before you get concerned about power output, it is no slouch by anyone's 75W/ch standards.

One tip -these amps like work well with input signals in the 1-2V range, which was nearly non-existent back then. Zapco signal processors and line drivers provided a hefty output, and gain control was done through the outboard processor.

If you do end up opting for a 40V supply, I'd be happy to take that 30V model off your hands...



Robcrx: I don't have spec's nearby. Quoted THD was low enough to place it squarely alongside the best of any group. Also, IMD was actually specified, and a battery of test results/plots and comparisons against other respected car amps were promoted as well. If you were buying an amp in the mid-80's and basing your decision mostly on spec's, you probably would have been choosing between Zapco and Orion.
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Last edited by tsmith1315; 22nd February 2010 at 09:27 PM.
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