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In Reply to: RE: Need MCS 3245 power supply repair help/advice posted by PJN on July 08, 2008 at 09:31:09
Is this a Tuner, amp, preamp, or receiver? Is that charred residue or just the adhesive that has dried up and changed colors from the heat? I can't tell that good from the picture. Sometimes the adhesive looks like the caps have failed but it just has discoloration due to heat. One way to find out is get a DMM and check to see if the caps are short or open. Did any fuses blow? Do any lights come on? If this is a receiver then you then you need to find out which fuses blew either the power supply fuses or the positive/negative rail fuses, which could mean that some of the output transistor maybe wasted.
Please let us know and we will try to help you out.
The MCS 3245 is a receiver sold by J.C. Penny in the 70's. It has several circuit breakers instead of fuses and the breaker on the power trips whenever the power is turned on. The marks at the bases of the caps don't look like adhesive to me, more like something scorched. I did test the resistors and caps with my multimeter, the resistors all seem to be reading right on their stated specs, however most of the caps seemed to have failed. I'm getting very low resistance ( < 0.5 ohm) on most of then including the big smoothing caps. Only a few of the small ones seem ok they had very high resistance (off of the scale). I don't have the capability to test diodes or transistors. Every other board in the reciever looked perfect.
You need to desolder the caps (write down the orientation) and check them out of circuit to be sure which cap it is and which rail failed. Since this reciver is old and you already had a problem with it I would just replace all of the caps on that power supply PCB only after you have found out which cap has failed so you know what other areas to look if there are other problems. Get the values with increased voltage ratings because some of the designs have the voltage ratings too close to the operating voltage, this will give some safe operating values. Very good and cheap capacitors to use are the Panasonic FC and TS series caps, you will notice a big difference in sound quality once you do this.
Does you DMM have a diode checker? Please make sure you check the bridge rectifier to insure there is nothing wrong with it either, remember you should have a .6-.7 voltage drop across each diode.
Thanks for the help. I agree with you on the caps, I was figuring on just replacing all of the caps, they're cheap and after shipping it makes sense to buy everything you think you'll need once. I don't have a diode tester on my DMM, can I just test the voltage drop using the DMM ? Diode and the bridge rectifier are also fairly cheap. It seems to me that the most fool proof thig to do is just replace everything on the board. There are very small diodes and larger diodes can I just replace them all with the larger type for simplicity ? One last question, do you know what the thing is in the picture that I have circles and do you think that I should replace that as well ?
I see 4 caps that have boiled out, several "dry, overheated solder pads around some resistors, a chip off a high-temp power resistor, and a few other issues.. The devices you are wondering about are "transistors"... they are either NPN or PNP (pointless to go into a electronics lecture here..). They may be fine or not- you really need a "diode test" function on your DVM (I assume it's a digital tester), and the knowledge of how to interpret the readings. They are semi-conductor devices like diodes for testing purposes... In this portion of the circuit they will be common transistors, and very inexpensive (3904's or 3906's or something like that..)... you can replace everything if you wish, but something drew too much current from the power supply in the first place, and it may just smoke again... from your pic you have about $20 of parts to replace on this board (very approximately)... good luck...
Thanks for all the input. I've decided not to bother spending my time or my brother's money chasing repairs, there are to many unknowns and no guarantee of a good outcome. Besides as I recall I never was very impressed with the MCS, my old Kenwood and Sony recievers at the time sounded much nicer. I'll build a chip amp out of the parts instead.
Bad caps do not trip breakers or blow fuses except in the rare cases when they are shorted. That junk you see on the board is not cap crap, it is glue used during manufacturing. Sometimes when it turns color it becomes conductive but almost never enough to blow a breaker or fuse.
You don't want to give up yet ? Get a DVM and check the outputs. Doesn't matter what they read unless it is 0.000. That is a short when you use the -|> |- scale. You find that, remove them and the drivers. The other channel should work.
People always think caps, it ain't. Caps in the PS can cause hum and noise. Caps in the amp can cause loss of gain and response. I have only seen shorted big caps that break a breaker or blow a fuse in 40 years twice and one of those units was subjected to overvoltage, got 240 instead of 120.
I was the audio guy at three places I remember, and when I talked, they listened. We were always on a budget and that means not replacing $50 worth of parts when what it needs is fifty cents.
If it failed during a thunderstorm it could be the main rectifiers, get the DVM and I will tell you how to find out.
May I add? To spend ANY amount of money on this unit is of questionble merit, but I also understand FULLY the importance of "sentiment"- if it is THAT important, then spend as much as you can afford.... I suspect that you will smoke that same ps filter board instantly though... so get a variac and ramp up the 120 Vac slowly while monitoring the o/p voltage (speaker terminals) and stop the instant you see ANY increase in DC voltage- I'm betting the o/p transistors (one or both channels) or the driver transistors are shorted... so don't go too far down the trail unless it's that important....
I suppose those little diodes might be zener diodes, and I don't know what values they need to be. does anyone know where I might find a circuit diagram for a MCS 3245 ?
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