Phoenix gold MS – 275 Blown channels?

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Chesley
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Phoenix gold MS – 275 Blown channels?

Post by Chesley »

Good evening fellas I’m hoping somebody can help me out today after learning that it is pretty much impossible to get the electrical schematic for this a.m. I’m wondering if anybody here on this form can help? On my 275 I had a blown right channel. Static he sound in very low volume out of that right side. My father‘s good friend is an electrical engineer so we opened up the M check the fuses everything looked OK except that on that right channel the two red LED lights were not on. The two red LEDs on the left channel were on in that channel worked fine. He pulls out his Ossola scope (not sure if that’s how you spell it probably not. Big machine that he used probes to poke around the motherboard). Also we were using a hand-drawn schematic the only one I could find on the Internet while trying to test these components. While he was comparing the good side to the bedside trying to track down the problem he touched something and two red LEDs on the good channel went off and did not come back on. Also I noticed originally on power up the red overload light would come on first then go out and then the green power light would come on. Now that both channels LEDs were not coming on on power up it went straight to green power light. It was around this time that he said this was not the right schematic. So this is the dilemma I am in hopefully somebody on here has the knowledge to at least walk me through what the test to see if it’s bad. He was thinking it went into protection mode however we cannot figure out that circuit. Any help would be greatly appreciated. Thank you all for your time.
Chesley Shapiro
ttocs
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Re: Phoenix gold MS – 275 Blown channels?

Post by ttocs »

sillyscope is the short hand slang, oscilloscope if you care is the correct term. welcome to the phorum your learning already. I am ok for swapping caps on amps but funny enough as a trained electrical tech I do not get down to circuit board repairs often enough to be any good here. One thing to know is that we always recommend replacing the large caps on the ms amps. PG didn't know it at the time but the caps they use after 15-20 yrs will start to leak a fluid that if it gets to the right part of the circuit can cause the cap to explode. Yes I do mean explode as often it takes a good portion of the circuit board with it. Once that is done its a great little amp.
what else can I say I am a grumpy asshole most of the time.
Chesley
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Re: Phoenix gold MS – 275 Blown channels?

Post by Chesley »

Awesome thanks for the advice I will change out the caps. Also a little update on some information I’m looking for bias voltages on driver and power transistors. Thanks again for the help
Chesley Shapiro
Chesley
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Re: Phoenix gold MS – 275 Blown channels?

Post by Chesley »

Can anybody tell me where I can find these resistors for sale.
22, 33, 68 ohm, 1/4 watt, fusible resistors?
ttocs
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Re: Phoenix gold MS – 275 Blown channels?

Post by ttocs »

not sure what a fusable resistor is but digikey or mouser is where a lot of us shop. You can get the caps there as well.
what else can I say I am a grumpy asshole most of the time.
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Jacampb2
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Re: Phoenix gold MS – 275 Blown channels?

Post by Jacampb2 »

I don't do a lot of work on ms amps, but I do work on a lot of PG. As far as I could find, PG didn't use fusible resistors, but rather flame proof. If you do find any official PG schematics, the flame proof resistors always have a (f) under them in the prints.

You can order them from mouser. Iirc Koa spear has a complete flame proof line I normally order from. I'm going out to the shop in a minute, I'll verify brand and series.

For bias current, it's normally adjusted to between 1 and 2 milivolts dropped across the emitter resistors. If the red leds are of, you will have no bias current. The leds are part of the constant current source that provides the bias.

If the amp is in protect, it won't light green. Any ms/mps schematic is going to get you in the ballpark. The amps are all virtualy the same with difference in power supply rails and bumper of parallel devices. One schematic is good enough for any of them if you know what you are doing.

Good luck,
Jason
M: M100, M44 for a custom amp project
Zx: Zx500, Zx450, Black Zx350
ZxTi: 4 Zx600Ti's, 1 Zx400Ti
Ti: 5 800.1's & 900.7 for a custom amp project. 1 1200.1, 1 1000.2
Tantrum: 2 1200.1's, 1 600.4, 1 500.2
XS: XS6600
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Jacampb2
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Re: Phoenix gold MS – 275 Blown channels?

Post by Jacampb2 »

I just looked, they aren't Koa, but Yageo FRM series are what I have the most stock of for flame proof. I also use some vishay-dale flame proof resistors, but they are a lot more expensive so I only buy them when I can't find the value needed from Yageo. Mouser stocks both brands.
M: M100, M44 for a custom amp project
Zx: Zx500, Zx450, Black Zx350
ZxTi: 4 Zx600Ti's, 1 Zx400Ti
Ti: 5 800.1's & 900.7 for a custom amp project. 1 1200.1, 1 1000.2
Tantrum: 2 1200.1's, 1 600.4, 1 500.2
XS: XS6600
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Eric D
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Re: Phoenix gold MS – 275 Blown channels?

Post by Eric D »

From the limited research I have done, fusible and flameproof resistors are technically different but in most circles people use their names interchangeably.

In the M and MS series amps there are about 6 of these in critical locations in each gain stage. I have been replacing them with 0.33W fusible resistors from Digikey for years. Hopefully this is fine and I can continue to do so.

I am pretty sure that many years back when I first started repairing amps, I could not figure out what one of these was when I found it to be burned "open". At that time PG was still in their original facility and had their customer service in house. I sent them a message asking what part this was and their manufacturer and part numbers pointed me towards 0.33W fusible resistors.

Either way, these are often to blame for the exact symptoms you are describing in your MS275.
Got "schooled" by member shawn k on May 10th, 2011...
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Jacampb2
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Re: Phoenix gold MS – 275 Blown channels?

Post by Jacampb2 »

As far as I know, all fusible resistors are flame proof, but the opposite is not true. I too had a real problem child pg amp years ago that dropped me down this rabbit hole. I never questioned PG about it, but when they are shown on the schematics, they never show any "fuse" rating as is common practice. Maybe it was just a quirk of the PG designers and they always used the same fuse rating, but even so they were normally very good at calling stuff like this out.

I don't think you can really go wrong either way unless maybe you got a fusible rated much too low. You have far fewer options to choose from when looking for fusible though, so for me I'll stick with flame proof.

Later,
Jason
M: M100, M44 for a custom amp project
Zx: Zx500, Zx450, Black Zx350
ZxTi: 4 Zx600Ti's, 1 Zx400Ti
Ti: 5 800.1's & 900.7 for a custom amp project. 1 1200.1, 1 1000.2
Tantrum: 2 1200.1's, 1 600.4, 1 500.2
XS: XS6600
Chesley
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Re: Phoenix gold MS – 275 Blown channels?

Post by Chesley »

Awesome guys. I will let you know how it goes I really appreciate your help guys thank you
Chesley Shapiro
Chesley
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Re: Phoenix gold MS – 275 Blown channels?

Post by Chesley »

Awesome guys. I will let you know how it goes I really appreciate your help guys thank you. And the other two, fatter sized electrolitic's should be replaced? and if so should I replace with higher voltage limit
Chesley Shapiro
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Jacampb2
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Re: Phoenix gold MS – 275 Blown channels?

Post by Jacampb2 »

The wet caps do not need to have the voltage upgraded, it doesn't hurt to increase the capacitance and use higher temp rated devices as long as they will fit in the factory footprint. I believe there is a thread in the "how to" section of the forum here. It shows best practice for replacing the big bulk capacitance. The board is easily damaged, so please do yourself a favor and review the available material. I believe there are also instruction on David's registry page.

Power supply input bulk capacitance absolutely without a doubt need swapped of its got its original caps. They leak around the electrodes and mostly are an invisible problem until your amp catches fire.

Good luck,
Jason
M: M100, M44 for a custom amp project
Zx: Zx500, Zx450, Black Zx350
ZxTi: 4 Zx600Ti's, 1 Zx400Ti
Ti: 5 800.1's & 900.7 for a custom amp project. 1 1200.1, 1 1000.2
Tantrum: 2 1200.1's, 1 600.4, 1 500.2
XS: XS6600
Chesley
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Re: Phoenix gold MS – 275 Blown channels?

Post by Chesley »

Thank you Jason your help is much appreciated
Chesley
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Re: Phoenix gold MS – 275 Blown channels?

Post by Chesley »

update. We have switched out the capacitors and a couple of resistors that were burned up. Now on power up the green light comes on and The four red LEDs light up on the motherboard. However as soon as we Push any kind of volume through it it goes into protective mode. Or the red overload LED comes on. And the amp shut off. Any ideas?
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Jacampb2
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Re: Phoenix gold MS – 275 Blown channels?

Post by Jacampb2 »

Are you 100% sure that your test speaker is good? Even more important, what kind of power supply are you using? If it doesn't have enough current an amp can behave like this as well. If everything is good, leave it unconnected and power the amp up, let it idle for a while to warm up, then measure the voltage across each emitter resistor. None should be higher than a few milivolts. If one is much higher than the rest, suspect the output transistors that have their emitters connected to that resistor. It is really rare, but the outputs can become "leaky" and not fail outright. If our if circuit the output tests OK, it could be something further back in the driver or our driver stage causing them to conduct. I'd probably start with the channel that had the open flame proof resistors.
M: M100, M44 for a custom amp project
Zx: Zx500, Zx450, Black Zx350
ZxTi: 4 Zx600Ti's, 1 Zx400Ti
Ti: 5 800.1's & 900.7 for a custom amp project. 1 1200.1, 1 1000.2
Tantrum: 2 1200.1's, 1 600.4, 1 500.2
XS: XS6600
Chesley
Posts: 8
Joined: Mon Feb 05, 2018 11:52 am

Re: Phoenix gold MS – 275 Blown channels?

Post by Chesley »

Problem solved!!! Thank you guys for all your help and it happened to be a little resister we missed plus the bias was off. Once again thank you very much for your help y’all have a wonderful
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