Hum in Crate guitar amp

[Monsteranus]

I have a Crate CR-160 guitar amp that has a low hum with all level controls all the way down. The hum does get louder as I in crease the volume levels somewhat.

It has two 4700 mfd 35 volt filter caps in place but before I go removing those I was wondering what other conditions may cause the hum other than the caps?

Thanks!

 

[Richard Ellis]

Is the suspect hum around 120 Hertz??? If so, yes by all means change out those 4700uF caps.....if the power supply board is of an "open" type & there is room, bump the voltage up to 50V, this will increase the lifetime of these caps.....keep the value the same at 4700 tho'.
These large caps dry out pretty consistently, changing the values ..your unit was made around 1983 & who knows how much it's been used...assuming your not the original owner of course.

_____________________________________________________Rick..........

 

[eyoung]

Rick is right, replace the electrolytics in the p/s. modern caps are going to be smaller for a given value so going to a higher voltage typically will result in the same physical dimension. Newark, Mouser, or Digikey are good sources for parts.
or ck the swap meet forum

Regards, Elwood

 

[Monsteranus]

Im not a good judge on pitch so I cant say for sure what hertz it counld be.

If the O.D. of a 50v cap is larger would be a problem to install the way this unit is assembled. Whats the advantage of increasing the volt value of the cap?

 

[Monsteranus]

I found some caps of the same value but rated for 50 volt in a scrap unit I hold on to and temporarily installed in the amp. The hum was not present at initial power up but after a few minutes it began to hum. This is with all level controls at full off.

I then put the original caps back in to have the same result as mentioned above.

 

[Richard Ellis]

OK.It seems the caps are OK at this point. You need to start trouble-shooting this unit. Has the unit allways had this hum? If not when and under what circumstances did the hum "start".
First off you want to make a very comprehensive visual inspection.......the very first step in troubleshooting. Make some room where you can eyeball the unit, have some very good bright lighting, find yourself a magnifying glass (preferred) ...your looking at all the solder joints thruout the circuit board.....your looking for poor connections & hairline cracks on the joints.
A typical joint looks like a "Hershey Kiss", silver in color of course, A typical crack will be a hairline crack all the way around said joint...circular. Further you are looking for burn't components, and evidence of overheated components (Discoloration) discoloration of the board too. Look extremely close....We are trying to solve your problems......by taking the first steps..........Granted we all have our "favorite" failures........and sometimes it works out just fine skipping right to it, but there is no replacement for a full "investigation" here.
Recent designs in engineering of amps & such are very poor in design & we all here can upgrade & improve the performance, reliability & function of you unit. I've seen the schematic of this particular amp & I'd bet real money it can be improved.

_____________________________________________________Rick....

 

[llwhtt]

Check the 15 VDC zenered supplies, the parts run hot in that area. Check all components mounted on the PCB that extent thru the front panel, look for cracked/bad solder joints. Since the hum gets worse after time sounds like a heat issue, zener supplies.

Craig

 

[geraldfryjr]

IMO, I would call the factory and tell them to fix it! jer

 

[llwhtt]

This thing is 30 years old, I don't think the factory will have anything to do with it.

Craig

 

[geraldfryjr]

Okay , my bad. jer

 

[geraldfryjr]

Here is what I found. jer
sorry file too big....?
P.M. me if you need the schmatic. jer

 

[Monsteranus]

Rick, trying to give a good visual inspection as well as wiggle testing is usually the first thing I do. The main power board is mounted upside down in this unit so thinking about it again I will give a good second looking over and wiggle test since I have the power board out at the moment.

Thanks for the suggestions, I will apply them.

 

[jaycee]

http://www.crateamps.com/pdf/manuals/CR160_OM.pdf seems they do have the schematic online!!

edit: If this amp has never been serviced, then I would say the first thing to do would be to replace ALL of the electrolytic capacitors. These are ancient now, have a limited lifespan, and die with heat.

Try feeding a signal straight into the "LINE IN" jack. This should help tell you if the problem is in the preamp or the power amp stage.

 

[jaycee]

Just noticed you said the hum gets louder as you turn up the volume - so the problem is almost certainly in the preamp stage.

 

[Monsteranus]

Thanks Jay!

I noticed the main output transistors were RCA brand, is that typical for Crate parts?

 

[jaycee]

For the age of this amp it's expected. 2N3055 is likely a good substitute, or MJ15003 if you want something that will take a bit more abuse.

 

[cozmcc]

Any success on this? I have 5 CR160's and a CR212 chassis. 1 of the 160's was sounding like a ring modulator and I put in new filter caps and now I have the mother off all hums (same problem with the CR212, I've been replacing part by part in that one to no avail)

I would really love to find out how to fix this hum - and yes it's 120hz

Thanks!