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Crate CR-212.....HUMMMMM

Started by matthias, July 09, 2006, 12:01:31 AM

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matthias

well, first off ...............
Hello. I've been looking for a place like this, although this seems more on the building side but I'm sure that some of you are great at trouble shooting  failures. If this is absolutely the wrong place for this post, just say. and/or point me to another SS forum, that would be great.
So, here goes.
I have this Crate CR-212 and it Hums, bad.
The hum is still there and unaffected with all pots at zero, same volume as when all knobs dimed.
I've checked the ground
I've replaced the input jack for the 0db and shorted the proper traces for the -6db
I've replaced the filter caps,
and I've replaced the rectifier diodes.
the hum is in the realm of a Baritone low C.
Where to next?

RDV

#1
First, welcome, and we'll help if we can.

I've got some questions for you.

Did you own this amp before it hummed? Or do you just get it, and it was humming when you got it? If there is a headphone jack or line out, does it hum from those outputs. If it does not hum from those outputs then we can narrow it down to the power amp.

You say you replaced caps, what portion of the amp were these located in? Power Supply, Preamp, or power amp sections?

This is quite an old amp, you could have a bad transformer or power transistor quite easily.

RDV

matthias

Hello RDV,
When I got the amp it hummed, also both input jacks were trashed so the guy was plugging straight into the footswitch jack which allowed him to get straight to the poweramp section. I don't know if this is right, but I have no schematic for it so? 
I ran a cord from the line out into a valve jr. and the hum came along for the ride.
I noticed that the speaker jacks have Iso washers (of course) while the line in/outs do not.....should they?
The caps that I replaced are located on the power amp board but are the PS caps, the rect. diodes are right there as well.
As far as age, yeah it old and its seen some heavy abuse, but it does sound nice..........well, minus the hum of course.
the Transistors are cheap enough to just go ahead and replace. I believe they are 3055's as of right now.
don't know if they are original.
the tranny, well I havn't checked it out at all runs cool and I havn't noticed any excessive vibration from it.

RDV

The hum is probably not coming from the poweramp as you get it from the line out as well.

If you access the power via the footswitch jack is the hum there then? If not we can narrow the problem to the preamp & or preamp power supply. Without a schem it's hard for me to know what's going on there. If the preamp gets it's power from the main supply, it either does it with regulators, or with zener diodes. I doubt it has a seperate transformer or anything.

Try isolating the jacks in front cause that could be a ground loop source and a relatively cheap solution if so. In the Crates I've owned they were plastic jacks which would be isolated so that may be it.

If that doesn't do it then we'll need to examine how the preamp is getting it's power and see if it's getting too much or too little or if you getting ripple in there causing oscillation at a low frequency.

RDV

Crystallas

check the pots.
What power conditioner are you using? Ive had a few amps that were sensitive to being plugged straight into the wall that would hum.

matthias

Hey RDV,
That makes some sense about the poweramp not being it, except that the hum is there when you plug into the footswitch or into the line in.
The Input jack is a regular metal one, but I have ISO washers on it.
also, like you Imagined, there is no seperate tranny for the preamp, I'll have to look into which way it is powered
Crystallas, Hello.
Check the pots? I'm guessing for DC and/or cold joints?
I'm not using a power conditioner at the present time but I have tried that with one used for computer set-ups and that one made no difference.

I'll check these ideas out tommorrow..or ahh.....later today.  Thanks alot guys.

joecool85

I would consider larger filter caps on your PS section.  Some amps just come with crappy small ones, I know you already replaced them, but if it was me, I would try replacing them with ones double in value or so.  How many amps is it?  I'm assuming the 212 means 2 x 12 speakers not the wattage.  If it is upwards of 100w RMS, I would consider two 10,000uF caps on the PS section along with new diodes.  Also, it might be a poor ground somewhere in the amp, double check that all connections are good, no cold solders etc.  Also see if there are any wires for sound (input etc) going near the PS, that will make it hum like a bumble bee.  Some amps just have poor design out of the factory, not sure if this is one of them.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

matthias

Hello Joe,
It claims 60 Watts RMS.
the PS caps are 4700uF.
As far as going up on capacitance, could I just parallel each with the same value? That should bring it up to 9400uF at the same 35V rating.
I guess if I do this I would also need to beef up the rectifier diodes? Right now I have some 4004's in there.

joecool85

The diodes would be fine.  On that same note, 4700uF caps should be fine.  If you are worried about it at all, you can parallel caps of the same value with them like you asked.  It won't hurt, and it *might* help.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

matthias

#9
Alright so,
I just disconnected the preamp from the poweramp board, so it is not part or the circuit anymore.
The hum is still there. So, at least I know that its not the pots nor the jacks or anything in the pre-amp. I guess I'm down to the poweramp or the transformer,  I'll check the ground for the Poweramp again. Should the preamp out and the poweramp in jacks be isolated as well?

RDV

Quote from: matthias on July 09, 2006, 03:56:23 PM
Should the preamp out and the poweramp in jacks be isolated as well?
Were they originally isolated? If so yes, if not you could try it and see.

RDV

matthias

well, just to see, I Isolated the pre out and power in jacks and heard nothing different.
I checked out the board for cold/broken joints, reflowed anything that was somewhat questionable.
I beeped out all grounds and even disconnected the chassis ground, grinded off the paint and sanded the terminal lugs clean and reassembled 'em just to make sure we were grounded.
HUMMMMMMMMM...................
maybe I'll go all out here and use one of these Mr. Yuk things..........:-\
I guess I could change out the transistors...........I have two new ones but I dont have new Insulation thingies

joecool85

Insulation thingies?  I would say try changing your diodes.  If its not that, there is a good chance its a poorly made/old/worn out transformer.  Have you tried checking the voltage on the transformer with a good digital multimeter?  If it fluxes, even 0.01volts, thats a bad sign.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

RDV

#13
Disconnect the speakers and turn on the amp and measure the DC offset by measuring the + & - speaker terminals and post the voltage reading. If it is high, then you likely have a bad transistor or a short somewhere.

I used to have a Crate G-60 which is pretty much the same amp. It used to hum like crazy and I could hit it on the top of the amp & it would stop! Eventually it started switching channels all by itself and I took it apart. Probably could've fixed it now.

RDV

matthias

#14
By the insulation thingies I mean the mylar type washers that keep the collector from contacting the heat sink.

O.K.
D.C. offset=1.184V
not sure what to do with this.

Transformer First time thru:   20.86V- 0 - 20.86V (did not notice any fluctuation)
Transformer 2nd time thru :  20.96V - 2.2 mV - 20.96V (on this measurement I did notice some fluxuation)
so From what you say joe, this doesn't look so good?

Oh Yeah, hitting the top of the amp, a.k.a. the old "fonzie" move, that was tried way back.