Strgazr1967
Active Member
Pulled my head out of my A** and figured out the power light lol.
Mag 175-67 rebuild. Questions, Questions........
- Thread starter Strgazr1967
- Start date
That is no problem. Check your input signal signature and post it if it's not really square. If it is proper looking - you have some frequency compensation to do. A lot of that was covered in my build here:
https://audiokarma.org/forums/index.php?threads/magnavox-amp-185-question.880953/
First, make sure your v/div center switch is turned to "CAL". Otherwise, scope is not showing correct voltages. Set your probes to x1 and adjust your scope to suitable display to show 1 watt of output into 8ohm load. That is approximately 4vac peek to peek. If you have a 2v/div setting your 1watt will take 2 vertical divisions between positive and negative peeks. Recheck your square wave at 1 watt of output. Hopefully it's not that ugly.
https://audiokarma.org/forums/index.php?threads/magnavox-amp-185-question.880953/
First, make sure your v/div center switch is turned to "CAL". Otherwise, scope is not showing correct voltages. Set your probes to x1 and adjust your scope to suitable display to show 1 watt of output into 8ohm load. That is approximately 4vac peek to peek. If you have a 2v/div setting your 1watt will take 2 vertical divisions between positive and negative peeks. Recheck your square wave at 1 watt of output. Hopefully it's not that ugly.
Another thing I would do. Take your probe to calibration point on the face of the scope. It should say "cal 5vpip" or something like that. Adjust your p/div to see 2.5v up and 2.5v down. That will tell you if your scope is properly set to display 1watt of output. Higher wattage will look really ugly with squares.
I think it would be a good idea to take some measurements before you convert to the floating paraphase in order to establish a baseline for the amp. You can use the baseline as a measure of stock performance so that after you convert, you know if you are worse or better than stock performance. Hopefully the paraphase will get you somewhat better performance.
These tests are relatively easy to do with a signal generator, a scope, and two large wattage load resistors.
1. Test stability of the amp at both HF and LF.
2. Measure input sensitivity.
3. Measure frequency response and power bandwidth
4. Measure hum and noise at the speaker terminals.
5. Amount of feedback applied.
6. Output resistance--to establish the damping factor.
Also I would listen to the amp for a while (a week or so) before making any changes so you get a feel for how it sounds. Anyway these are ideas...you can go faster if you want if the amp is stable, but I think it's good practice to at least take baseline measurements before proceeding. If you have those tools listed above, you can do all of the above measurements in a few hours or less. If you don't have a signal generator, you can use your iPad if it will generate test signals from 20 Hz to 20 KHz, but you might need to pipe the output of the iPad into an outboard preamp, which then feeds the power amp, in order to obtain a drive signal large enough to drive the amp to full power.
We can walk you through the procedure for each test once you are ready.
These tests are relatively easy to do with a signal generator, a scope, and two large wattage load resistors.
1. Test stability of the amp at both HF and LF.
2. Measure input sensitivity.
3. Measure frequency response and power bandwidth
4. Measure hum and noise at the speaker terminals.
5. Amount of feedback applied.
6. Output resistance--to establish the damping factor.
Also I would listen to the amp for a while (a week or so) before making any changes so you get a feel for how it sounds. Anyway these are ideas...you can go faster if you want if the amp is stable, but I think it's good practice to at least take baseline measurements before proceeding. If you have those tools listed above, you can do all of the above measurements in a few hours or less. If you don't have a signal generator, you can use your iPad if it will generate test signals from 20 Hz to 20 KHz, but you might need to pipe the output of the iPad into an outboard preamp, which then feeds the power amp, in order to obtain a drive signal large enough to drive the amp to full power.
We can walk you through the procedure for each test once you are ready.
Output impedance measurement requires running signal with open output. In my case it was absolutely no problem but may not be the case in this build. Be ready to turn power off if amp breaks out in oscillation. When THAT starts - you can't miss it on the scope display. My amp was unstable into capacitor only loads but was perfectly happy with open output.
Frequency tuning that you may have to do is going to affect stability and frequency response. Without experience that some of this forum members have, finding reasonable compromise may take long time tinkering but it is absolutely necessary before you put amp into service, even if you think it sounds really good.
Frequency tuning that you may have to do is going to affect stability and frequency response. Without experience that some of this forum members have, finding reasonable compromise may take long time tinkering but it is absolutely necessary before you put amp into service, even if you think it sounds really good.
Strgazr1967
Active Member
Wound up working over the holiday but managed a little time last night to work on the amp. I had been thinking about the old can cap and it's condition. Even thiough I tested it I guess, in the back of my brain I didn't trust myself lol. SO I dove in and replaced the can cap with all new electrolytics. I left the original in place for aesthetics. Once I laid it out on paper so I could grasp it it wasn't hard. Came out decent I think. And yes I know the one cap was wired backwards. Thankfully caught it during the once over before powering the unit up. Phew!
I also tested the output from my Ipad using the signal gen software and to my surprise the square wave was as ugly as I saw from the amp. Thanks to @grindfix for suggesting I test it. Now I guess I need to pick up a cheap, used signal gen. But I felt better knowing it wasn't the amp that was that ugly. @kward I would like to do the testing you wrote about but will have to wait I guess till I can get a solid signal to the amp first. Any recommendations on a decent but not crazy priced signal generator? I also reused a small piece of the back of the console radio my magnavox 8601 amp came from as a cover for the bottom.
Thanks again and hope the holiday was enjoyable.
I also tested the output from my Ipad using the signal gen software and to my surprise the square wave was as ugly as I saw from the amp. Thanks to @grindfix for suggesting I test it. Now I guess I need to pick up a cheap, used signal gen. But I felt better knowing it wasn't the amp that was that ugly. @kward I would like to do the testing you wrote about but will have to wait I guess till I can get a solid signal to the amp first. Any recommendations on a decent but not crazy priced signal generator? I also reused a small piece of the back of the console radio my magnavox 8601 amp came from as a cover for the bottom.
Thanks again and hope the holiday was enjoyable.
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Strgazr1967
Active Member
It's crazy because they have clear, light tan and white in my local HD. Would love to have black. I'll have to keep an eye out for one of the BK's.
Right, a used BK 4017A is not that expensive, and is a solid generator. Not the top of the line or anything like that but it works well for what it does. It's still in production even.
poorbastards
Active Member
H
I’ve read almost too much info on this build and it’s starting to make me doubt what I should do!
My maggy 8802 was much easier! Somebody on Facebook had just restored theirs and gave me a list of exactly what he ordered. I can do the work, but my decision making skills are piss poor!
Have you changed anything recently or wish you had done something else?
Pretty good post, lots of great info. I'm compiling my parts list and hope to have it ordered
thanks for posting what you bought. I’m coming up with my own list and may use this compared to mine. So you didn’t go with .1 and .0075 for your coupling caps?
I’ve read almost too much info on this build and it’s starting to make me doubt what I should do!
My maggy 8802 was much easier! Somebody on Facebook had just restored theirs and gave me a list of exactly what he ordered. I can do the work, but my decision making skills are piss poor!
Have you changed anything recently or wish you had done something else?
GGriswold
Active Member
Sounds great- today's odd thing- I am getting bleed from one channel to the other, but not the other way around. I'll look into it.
I saw how hot that 125 ohm 10 watt resistor gets so I put a small, really small fan underneath. 140 degrees with the fan. Still going to put feet on the amp.
I saw how hot that 125 ohm 10 watt resistor gets so I put a small, really small fan underneath. 140 degrees with the fan. Still going to put feet on the amp.
