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?
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
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.
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
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.
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.
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.
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.
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.
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?
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
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
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.
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
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.
That D.C offset is pretty high, but not so high as to stop the amp from working or anything.
I'm wondering now if this is just the way this amp sounds. I can only suggest larger PS caps like 10,000µF or even higher but that still may not do it or even be worth the cost on such an old amp.
Sometimes designers shoot themselves in the foot by crowding things together and causing proximity effect hums and buzzes. Of course I'm guessing here cause we don't have a schematic or layout to go by here.
Hum usually means DC where there isn't supposed to be any. A Zobel network on the output might stop some oscillations. It will require a 2.7 ohm 2 watt resistor and a .1µF cap.
Like this. (http://fatboy.ssguitar.com/schems/Zobel.GIF)
HTH
RDV
Actually, DC makes very little noise compared to AC where it shouldn't be.
How does a Zobel network reduce DC offset?
Heres the problem, your trying to re-invent the wheel to get this amp going.
I say scratch that, start fresh, and use the housing to fit in a new project. You may aswell, for as much as you have trouble shooted this one, its going to just cost you a lot for a little payoff.
Well, since it seems like you already have tried pretty much everything except total rebuild then maybe it's time for something far-fetched: Have you tried the amp in another room or apartment? Sometimes too long mains wiring, very long extension coords etc. causes a very irritating hum. Also, any motors or fluorescent lights nearby?
Anyway, that DC offset indeed seems too high: I'd suspect there's something breaking down and I'm sure Crate did not design an amp that would heat the voice coils with such a high DC figure. One possible source for unbalanced output is in the power amp input stage, which is very often a differential amp. I'm pretty sure this amp has a discrete one, (although a schematic would verify it). Any inbalance there will increase the DC offset - at least a bit. Maybe some components, solder joints or something have gone/are going bad in there. Besides a bad grounding, increased hum is as well a symptom of a stressed output stage. (The stage draws more current than it normally should). Are some of the transistors or resistors getting hotter than they normally should? Perhaps a busted CCS circuit somewhere in the PA is messing up the operation, causing inbalance and probably an excessive current draw - which then leads to supply sag, which causes an increase in ripple.
Can you test the supply voltages without the load of the power amp circuit to see whether you have sag even when idling? If this is the case then adding more capacitance would help in reducing the hum but would not cure the real problem. If the voltages still fluctuate notably (without the load of PA circuit) you most likely have either problems in the mains distribution or a faulty transformer.
It seems the preamp is good, so don't totally start from scratch. Just do a new PS and power amp section, those are the easiest to build anyway!
I'll try to measure the transformer with out a load and see whats up there.
Alright, I think I'm comming to my senses.
I think It's the pre-amp that I like anyway.
Matthias:
Did you ever resolve the hum? I'm having the same issue.
Reflowed the whole board, replaced the the 4700/35 caps.
Attached is schematic from cr-160 power section which is the same board.
I love these amps and am dying to get it running right.
Thanks,
Scott