M50 and M44 - advice

denisl · Post by **denisl** » Fri Jan 04, 2008 8:45 am

C - I just checked both amps and the caps are 10uf on both so I should be good.

Just curious.... what is the function of the large caps I'm replacing? Do they have anything to do with the sound quality? Does this do what an external 1farad cap would do (but obviously not on the same scale) ?

If so then I would see how the 6800uf wold be better.. and adding the 4th cap to the M50 would help especially if I use the M50 to run a sub...

Doc - scary pic.. I'm glad I didn't get that far. If my board looked like that I'd probably send it out or buy a new amp.

Post by **Eric D** » Fri Jan 04, 2008 9:17 am

You are correct, these caps do what an external 1 farad cap would do.

The closer you get the caps to the actual powersupply circuit, the most effective they are.

marko · Post by **marko** » Fri Jan 04, 2008 9:48 am

both eric and myself have repaired m50's with large sections burnt out near the cap area, much fun but time consuming, you have to have a passion to carry out such repairs

1moreamp · Post by **1moreamp** » Fri Jan 04, 2008 11:15 am

denisl wrote:C - I just checked both amps and the caps are 10uf on both so I should be good.

Just curious.... what is the function of the large caps I'm replacing? Do they have anything to do with the sound quality? Does this do what an external 1farad cap would do (but obviously not on the same scale) ?

If so then I would see how the 6800uf wold be better.. and adding the 4th cap to the M50 would help especially if I use the M50 to run a sub...
.

10UFD DC output termination:
My Bad, I must have added a zero in my haste on my shop work document. < My bench is not open so I was using bill tags and crib notes to verify those cap values. I will mark it for correction...Thanks

The 10 ufd and the 0.47 ufd caps are a standard output termination for power supplies. they provide little to no energy storage. their purpose is to filter out Power supply noise broad bandwidth and to add stability to the DC output fundamental. These functions are important so please replace these correctly to maintain PG spec for the circuitry.

Big Caps :
Doc is correct that these provide some stiffing to the 12 volt rail, but due to their limited size i would tend to think that they provide power supply hash filter so the switcher in the amp does not transmit hash noise back into the car 12 volt supply. I.E. they prevent power supply noise from back flowing into the car and blanking AM and FM radio so the amp complies with FFC rules and regs.
Filters are just that filters, so they do a dual purpose in the place they are located...IMO
I and Doc replace these with higher value replacements because in the time since these amps were built cap technology has changed drastically. For the same size package nowadays you can get up to 4 times as much cap value in the same size package of the caps your replacing. In this location your asking about increasing the value can only help the amp and should not cause any issues with proper operation of the amp. You just improving the noise filtering and 12 volt stiffening in front of the amp.

Changing these 12 volt caps will not change amp SQ as they are on the 12 volt side of things and nowhere near the audio signal. Changing the big blue or black caps I pointed out can change the amps SQ because these are directly located to the DC feeding the main amps and the lower rails. These do need replacement from time to time and their is a SQ related improvement based on their location and purpose inside the amp....

In the picture Doc posted the chemicals inside the cap have leaked out where the wire leads go into the cap. These chemicals are conductive and corrosive to the circuit board metals.
When they conduct they cause the chemicals to get much more corrosive to the board materials and they provide a constant short circuit across the caps wire leads underneath the cap, so the only way to stop the conducting is to remove the amp from power by disconnection from the car. until this is done the amp is shorting out the 12 volt rails and hence you get burned up boards.
By replacing these you avoid this issue temporarily until it happens again. New caps do not resolve the issue from coming back later on. My addition of RTV is to add protection to the rework and to prevent the liquid from connecting underneath the cap and shorting the 12 volt supply and thus hopefully prevent the burned board issue completely by breaking the conductive circuit caused by the liquid as it spreads between the two connection wires under the caps.
Also the RTV adds stability to the assembly and reduces the stress on the connection wires, and hopefully be reducing this stress and vibration issue the seals on the caps will live longer and seal better over time.....

The cap electrolyte is conductive, this is how the cap works inside, and passing 12 volts through this leaked liquid increases corrosion and conductivity by < electro-chemical reactions like that inside of your car battery>. RTV'ing the base of the caps along its channel will block these chemicals from making a circuit, and hopefully prevent board burn up.

I came about this idea by trying to figure out how this happens to these amps, and by studying the end results and root causes. If RTV'ing is the caps is too much trouble you can bypass this step. I do it just to give that extra attention and effort my clients deserve. So far it works rather well, but only time will tell......C

denisl · Post by **denisl** » Fri Jan 04, 2008 12:23 pm

I will definately use RTV. My recap job will be performed exactly as you (and Doc) meticuliously outlined. Your experience and willingness to share it is very generous and I'm extremely grateful.

I only wish I was able to find the 6800uf caps at a reputable seller...
If you find a part number anytime soon let me know and I'll be happy to shell out another $20 for the higher capacity caps. Otherwise, 5600uf it is.

1moreamp · Post by **1moreamp** » Fri Jan 04, 2008 12:28 pm

and Thanks to Doc for reposting all his info for everyone also....C

Post by **Wakeup** » Fri Jan 04, 2008 1:21 pm

Ya C and Doc are great guys! Much props to the both! I wish i had the steady hand and patience to do the stuff they do! great work!

thingy · Post by **thingy** » Fri Jan 04, 2008 1:42 pm

Sorry for hijacking - but C - could you explain what RTV is (like spell it out for me) - being Danish my English has some holes and the tech stuff is part of where the holes are

andy600rr · Post by **andy600rr** » Fri Jan 04, 2008 3:58 pm

I have a question for C and/or Doc........

Have you ever replaced the large resistors for like a 5 or 10 watt ceramic unit that will dissipate more heat????

OR

Is it not practical to do this due the difference in tolerance values between metal film vs ceramic?????

Im glad I'm not the only one who's opened up an M44 to see a brownish area of the cct board around these resistors...

1moreamp · Post by **1moreamp** » Fri Jan 04, 2008 6:40 pm

thingy wrote:Sorry for hijacking - but C - could you explain what RTV is (like spell it out for me) - being Danish my English has some holes and the tech stuff is part of where the holes are

Sorry for being offline so long...

RTV = Pure Silicone Sealant manufactured by GE and others. I prefer Clear RTV silicon Sealant.

Hope this helps, sorry for the vagueness in my description. RTV is a common sealant type adhesive here in the US.....C

1moreamp · Post by **1moreamp** » Fri Jan 04, 2008 6:50 pm

andy600rr wrote:I have a question for C and/or Doc........

Have you ever replaced the large resistors for like a 5 or 10 watt ceramic unit that will dissipate more heat????

OR

Is it not practical to do this due the difference in tolerance values between metal film vs ceramic?????

Im glad I'm not the only one who's opened up an M44 to see a brownish area of the cct board around these resistors...

Installing higher wattage devices will only ensure a larger body to sustain the heat that these resistor are going to dissipate. The Heat is a given level because of the physical laws surrounding the voltage drop that they must sustain for the circuitry to work correctly.
By increasing the physical size of the device the heat will be spread over a larger area, but this really will do very little except maybe slow down the oxidation rate on the wire leads that connect the device to the board.

I have seen 5 watt sand blocks installed in these amps by PG so they even went larger on them in later models. So yeah go ahead and use larger wattage rated devices as it will not alter circuit operation......C

1moreamp · Post by **1moreamp** » Fri Jan 04, 2008 7:00 pm

denisl wrote:I will definately use RTV. My recap job will be performed exactly as you (and Doc) meticuliously outlined. Your experience and willingness to share it is very generous and I'm extremely grateful.

I only wish I was able to find the 6800uf caps at a reputable seller...
If you find a part number anytime soon let me know and I'll be happy to shell out another $20 for the higher capacity caps. Otherwise, 5600uf it is.

IMPORTANT
I need to get you to up the wattage on those 91 ohm resistors to 5 watt. My Documents say 2 watts but in your picture PG used 5 watt versions of these in metal film.
I am asking you to change the wattage of these based on your pics, my crib note document say otherwise but it may be a typo error in them. In your pic the value is 5 watt for those 91 ohm resistors. Please change this ASAP so you get the correct part.

As for the 6800 vesus 5600 caps, go with what fits as you are already increasing the value substantially and again this in not something that will affect SQ and the basic upgrade you achieve with 5600 is just fine in this area.... Sorry about the wattage typo...C

denisl · Post by **denisl** » Fri Jan 04, 2008 9:00 pm

Thanks C - no prob I'll get those 5 watters..

thingy · Post by **thingy** » Fri Jan 04, 2008 11:33 pm

1moreamp wrote:
thingy wrote:Sorry for hijacking - but C - could you explain what RTV is (like spell it out for me) - being Danish my English has some holes and the tech stuff is part of where the holes are

Sorry for being offline so long...

RTV = Pure Silicone Sealant manufactured by GE and others. I prefer Clear RTV silicon Sealant.

Hope this helps, sorry for the vagueness in my description. RTV is a common sealant type adhesive here in the US.....C

Thanks C - I'll try to search for it - most silicone sealant here in DK is a very powerfull glue and i dont want that on the board so i suspect it is not that

andy600rr · Post by **andy600rr** » Sat Jan 05, 2008 2:10 am

1moreamp wrote: Installing higher wattage devices will only ensure a larger body to sustain the heat that these resistor are going to dissipate. The Heat is a given level because of the physical laws surrounding the voltage drop that they must sustain for the circuitry to work correctly.
By increasing the physical size of the device the heat will be spread over a larger area, but this really will do very little except maybe slow down the oxidation rate on the wire leads that connect the device to the board.

I have seen 5 watt sand blocks installed in these amps by PG so they even went larger on them in later models. So yeah go ahead and use larger wattage rated devices as it will not alter circuit operation......C

Thanks.... as always.

denisl · Post by **denisl** » Sat Jan 05, 2008 12:06 pm

Can you believe digikey doesn't carry any 5W 91ohm resistors!!!

BTW - my first order arrived!

1moreamp · Post by **1moreamp** » Sat Jan 05, 2008 12:15 pm

Search using the search link I listed in this post it will find them for you....C

denisl · Post by **denisl** » Sat Jan 05, 2008 12:21 pm

C - I did. I just went through every distributer on findchips.com and none of the results matched. I then went to each of the sites and searched for a 5W 91ohm metal film resistor and nothing.

If you found it can you shoot me the link?

Also - for my edification...
A 9.10K resistor is 9100 ohms right? Move decimal over 3 positions for K ?

1moreamp · Post by **1moreamp** » Sat Jan 05, 2008 12:44 pm

denisl wrote:C - I did. I just went through every distributer on findchips.com and none of the results matched. I then went to each of the sites and searched for a 5W 91ohm metal film resistor and nothing.

If you found it can you shoot me the link?

Also - for my edification...
A 9.10K resistor is 9100 ohms right? Move decimal over 3 positions for K ?

Yeah 9.10K is 9100 ohm

You might be stuck reworking your original 91 ohm resistors... If so just remove the original ones and clean all the solder off the end wires, and then scotch brite them till the oxide is removed and they are shiny, then you can resolder them and the solder will adhere properly. As long as the solder has been reworked your in the good with these....C

denisl · Post by **denisl** » Sat Jan 05, 2008 12:47 pm

I was hoping to take advantage of a new resistors longer leads and raise them up a bit... can I use some of the lead wire from the new caps and solder extensions on the old resistors to raise them up?

1moreamp · Post by **1moreamp** » Sat Jan 05, 2008 1:01 pm

denisl wrote:I was hoping to take advantage of a new resistors longer leads and raise them up a bit... can I use some of the lead wire from the new caps and solder extensions on the old resistors to raise them up?

You would still need to clean the old leads so they will take solder.
AND you must hand wrap the wires together to form a stiff connection.

By this I mean you must wrap the extension wire around the resistor wire in a tight circular fashion so it actually supports the weight of the resistor even without solder, And I would not consider it well done with less than 3 or more complete turns on each connection.

The connection will see high heat so it must be excellent even before you apply solder to it, and it must take solder across the entire wrap area just to hold up....

denisl · Post by **denisl** » Sat Jan 05, 2008 1:06 pm

I wouldn't do it anyother way

Thanks again C.. So I have all my parts.

One last (yea right!) question..
My new 330omh resistors are considerably smaller then the ones currently in the amp... Is this okay or are they wrong?

1moreamp · Post by **1moreamp** » Sat Jan 05, 2008 2:15 pm

denisl wrote:I wouldn't do it anyother way

Thanks again C.. So I have all my parts.

One last (yea right!) question..
My new 330omh resistors are considerably smaller then the ones currently in the amp... Is this okay or are they wrong?

They are probably the wrong wattage, try and reuse your old ones with the extensions we just talked about.

On a side note these resistors your working on RARELY fail, so reworking them is just fine. The additional height will give better ventilation BUT because its sealed up inside the amps case the ventilation is limited at best.

A better idea is to find a better replacement and if possible mount that replacement to the case as a sink for the excessive heat build up. Ahh my thoughts on this have lead me to research TO-220 case type resistors by Vishay
These are quite expensive but worth it in many instances. Unless your up to spend several dollars a piece then reworking the resistor should solve any current issues there may be in your amps.....C

Ahh one other thing, You should try and find some Silver solder to rework these with, Silver solder has a higher temp rating ~ about 100 degree's higher so you iron must be adjustable to get the temp needed to even melt and flow this solder. It will last much longer under heat stress...C

denisl · Post by **denisl** » Wed Jan 09, 2008 10:30 pm

Hey C -
I happen to be in San Jose for work and walked into a FRY's electronics... WOW! We don't have them on the east cost. I was like a kid in a candy store.. Anyway.. I happen to find 2W & 1W 330 ohm resistors except they're +/-2% not 5%. They look just like the ones in my amps except a last band of red not gold. I'm assuming that's okay.

I couldn't find any 91 ohm. I'm sure the answer is no but can I use something close to 91ohms? like 100? Just checking..

1moreamp · Post by **1moreamp** » Wed Jan 09, 2008 10:53 pm

denisl wrote:Hey C -
I happen to be in San Jose for work and walked into a FRY's electronics... WOW! We don't have them on the east cost. I was like a kid in a candy store.. Anyway.. I happen to find 2W & 1W 330 ohm resistors except they're +/-2% not 5%. They look just like the ones in my amps except a last band of red not gold. I'm assuming that's okay.

I couldn't find any 91 ohm. I'm sure the answer is no but can I use something close to 91ohms? like 100? Just checking..

You were here at my home and did not PM me ?????:shock:

Glad you like Fry's they turned the electronics business into a 7-11 operation out here

The fry's kids dad owned a small grocery chain, they went into electronics go figure eh? They are really rich because of those stores, and I mean Lambo/Ferrari rich.

Anyway you should have let me know I have tons of parts just sitting and I know better places for you to get 5 watters out here. A place called Halted Specialties has the Best precision resistor assortment and reasonable to...

As for the 91 ohms, its a engineered number, it provides a current limit for the zeners in the 15 volt rails and the current for the input op-amps at 15 volts.
Now you might get away with a slightly lower value but a higher value might prevent the supply from driving the frontend chips properly. To work around this I would need to do some math for that circuit to make sure we don't float something too high, or too low.

I would also look for two 45 ohm 5 watt resistors, then just connect them and VIOLA 90 ohms 10 watts, and 90 ohms is within a 5% margin the original device is rated tolerance wise, and way long leads this way.

So there are options as you can see. And 2% tolerance is just fine, actually tighter then original so no biggy..

Anyway LMK whats next, and when your coming back out....C