I’m seeking help troubleshooting my dead Phoenix Gold MS250 amplifier, an early 1990’s vintage. It served me well for many years, then one day I’m driving along listening at low volume and it just stopped. No pop, no click, no smoke.. just as if it was turned off. And hasn’t given me any output since.


Now I get a green power light, but absolutely nothing on the outputs. No click or pop when it turns on or off, not even a blip on the scope. Feeding it with a 1kHz 1Vpp sine wave input (gain in the center) also yields no output (see scope screen below). The manual claims 2A idle current @14v, I measure only .36A idle current @14v input. After several minutes powered up, ambient temp 74F, the SG2525A PWM chip is slightly warm at about 80F, but the 4 bluish resistors just above it, with a cap between them (R9, R10, R11, R12) are HOT at 155F. The caps are all cool and I didn’t notice any other hot spots on the board.


Does anyone have suggestions on where to go from here? I thought I would dig up the data sheet for the SG2525A chip and see if it’s giving any PWM output, but I’m certainly open to any and all suggestions.


The best I could do for a schematic was an unofficial MS275 schematic. I’m told the MS250 and MS275 were nearly identical, but I don't see some of the key IC's on the schematic so I'm not sure how helpful it is... I’ve added a link to the manual and some pictures below. If you click for full screen images, you'll get the high resolution versions.

Owners Manual:
https://drive.google.com/file/d/0B7i1pd8bSu5oQlFId1VsLWhYNTQ/view?usp=sharing

Schematic (unofficial):
https://drive.google.com/file/d/0B7i1pd8bSu5ocU1SWG9HckRkV1U/view?usp=sharing

Input and output:


Hot resistors marked:


Various pics:

 

Check the supply voltages first +/_ 35, 45 volts lines, any fuses blown, if the pwm chip isnt working, there wont be any ht rails, you need the correct circuit diagram.

 

Oh jeez, is 1990 now consirdered vintage?

 

Thanks for the tips! The fuses are all good, I did check those. I'll try to figure out where the supply lines are (amplifier newbie) and check those. I'll keep trying to find the schematic...

Oh jeez, is 1990 now consirdered vintage?

LOL, I guess so! I bought this thing for my first car way back in high school, 1991 I think... I don't even remember how many burgers I had to flip to afford it.

 

Check all your output power transistors. I have had a couple where these went bad. If so, replace them all. Common on this unit.

 

For both channels to go down at the same time suggests the power supply. I am waiting to see if you read any supply voltage.

 

For both channels to go down at the same time suggests the power supply. I am waiting to see if you read any supply voltage.

Agreed. I really should pay more attention when I read. Start with power supply.

 

The best I could do for a schematic was an unofficial MS275 schematic.

The pictures (coil, sg2525, mosfet output devices) seem to suggest a class d amplifier while the schematic a linear amplifier. I think you should try to figure out whether it is digital first.

If it is digital, I would try to find out if the oscillator is working;

If linear, I would start with idle working points.

But figuring out what it is will be key.

 

I don't think it is a class D. One, 1990 is too early for class D and two, there aren't the required inductors in the amplifier side. (Look at third picture.)

 

Thanks guys. With 14V supplied to the amp, I can't find anything inside higher than source voltage. The oscillator is giving good output @70kHz, the 4 resistors are getting a square wave at 35kHz, so I think that part of the power supply is OK. But the MOSFETs all have source voltage (+13V) on the gate and drain, and 0v on the source. The rectifiers all show only milivolts DC on both pins, and some AC voltage, but it's at 60Hz and I think it's from my bench power supply.

One thing that seemed odd; the 4 resistors that get really hot (R9-R12), they all get identical input, the output from R9 has significantly more amplitude than the other 3. See the attached images.

I attempted to compile all the readings into a picture so you can see what I'm measuring, I also attached the images individually.

Data sheets:
IRFZ44 MOSFETs - Gate and drain (left 2 pins) 13v, source 0v
MUR810 rectifiers - milivolts on pins


All readings compiled into 1 image


Individual:
Oscillator output, R9-R12 input:


R9-R12 output side


MOSFETs and rectifiers

 

The pictures (coil, sg2525, mosfet output devices) seem to suggest a class d amplifier while the schematic a linear amplifier. I think you should try to figure out whether it is digital first.

If it is digital, I would try to find out if the oscillator is working;

If linear, I would start with idle working points.

But figuring out what it is will be key.

It's a high end class A/B

 

The mosfets look to be part of a SMPS, not the amplification side.

The TO246 devices (four of them, two for each channel) appear to be the amplifier itself.

I would start with the power supply: making sure that it is there.

It's a high end class A/B

It is a pretty mundane design. I would give it credit for being symmetrical (dubious audio value, however). The use of an inverting amplifier for audio is quite rare, however.

Given the output devices used, output power is probably 20-30w RMS/ch. Heatsink / rail voltage would suggest lower wattage.

 

1990 is too early for class D

The concept of class d amplifier goes way back, to 1940s. and certainly 1960/70s with semiconductor devices.

 

You need to look at the outputs from the SG3525 as per the datasheet, it may be in figure 9/10 mode looking at the pulse coil on the right side

(its marked up on the pcb CT, AC, HV on the screw terminals,)

if so it uses two fets per output, look at the gates/ drains of the fets Q7 to Q10 for square waves, if no signal then trace back to the sg chip.

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00000958.pdf

 

@Dodgydave - Thanks, I think I understand what you're saying. On the SG2525, pin-4 is giving good output, but I neglected to check pins 11 and 14, I will check those tonight. There is a nice square wave across the resistors (see attached image), but from memory there was no square wave on Q7-Q10, I will double check this tonight too.

SG2525 Pin-4 output, resistor input:




@dannyf - I think I am looking at the power supply side? I added a picture below of what I think is the output side. It's rated (RMS) 50w/ch @4ohm, 90w/ch @2ohm, or 180w @4ohm mono. The specs should be pretty close, I never measured the output but several blown 30A fuses (back when I was young and didn't care about my hearing) says that it does draw more than 30A @14V under full load as stated in the manual. Here's the manual with the specs and better explanation:

https://drive.google.com/file/d/0B7i1pd8bSu5oQlFId1VsLWhYNTQ/view

Output side, the 10Ga black and red wires are output leads:

 

In addition to what Dodgydave suggested, note if there is 12 volts on the drain.

 

In addition to what Dodgydave suggested, note if there is 12 volts on the drain.

If I've got the pins right (IFRZ44 mosfet), then there was 13v on the gate and drain (left 2 pins), and 0v on the source (right pin), for all 4 transistors.

 

I think DD suggested the best path. Find where the pulses stop between the SG2525 and the gates.

 

I think I am looking at the power supply side?

I think so. It is a car power amp, right? It's taking the 12vdc and convert it to +/- 35vdc for the power rails to the (analog) power amp.

So all the smps stuff is related to the dc converter, not the amp itself. Thus, I would first try to make sure that the rail voltage is right.

 

Output side, the 10Ga black and red wires are output leads:

That's the (analog) power amp side.

Those 10GA wires ***only*** look impressive. For the kind of power it can deliver, it is not really needed.