Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: NannerPuddin on November 09, 2009, 07:46:23 pm
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I know this is just for tubes, but I have a Carevin Bass amp from 1992 that just bit the dust. I think it is the power rail, as it keeps blowing fuses, but the PT checks out ok. Anyone able to direct me to some resources maybe for getting some help in fixing this?
Thanks,
Doug
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Most likely a blown power transistor which can take out all kinds of other thangs along the way. Gotta schematic? Do you know how to check a transistor? You can sorta check em like a diode with an ohm meter. I say sorta cuz you're really only testing for a shorted or open device. It could still be bad and check out good on an ohm meter.
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There are about a dozen bass amp schematics at the Carvin Museum site. Maybe yours is close enough to one of those to start diagnosing.
The Carvin Museum tech pages (http://www.carvinmuseum.com/techdocs.html)
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There is no such thing as a broken SS amp. It is a tube amp carcass. :grin:
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There is no such thing as a broken SS amp. It is a tube amp carcass. :grin:
Amen, j! I gutted an early 70's Peavey Pacer and built a 5E3 in it. I've been told it's the best sounding Pacer ever! :toothy10:
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Nonsense. 1 it's a bass amp & 2 Phillip Lithman used a Peavey. I might one day create the greatest boutique amp ever made (not likely), but that won't change the fact that I am a horrible guitarist. I suck in ways most people haven't even thought of.
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Ok, so it has been a while since I posted, but I finally got a schematic from Carvin and have had time to look it over. I am including a couple of pics. First, the main MOSFET power supply all comes out in one unit. It has the rectification and filtering, as well as the transistors. I do not really know how to check a transistor. I have noticed, that they are very cheap to buy. I am tempted to replace the four transistors and give it a try.
The transistors are 2 MTH1520's and a couple of TIP31c.
Does anyone have any advice?
Thanks!!!!
Doug
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> I am tempted to replace the four transistors and give it a try.
It's your life. Do what you think is best.
But I see a lot of small transistors, and I bet you the price of a TIP31 that one of them is shorted, and will burn-up your big boys again.
> I do not really know how to check a transistor.
In THIS case: there are 6 ways to connect an ohm meter to a 3-leg part, both polarities. If any of those ways reads DEAD-short, the transistor is ruined.
Unfortunately, if a transistor stays shorted and the power supply has some guts, the short burns-open. Detecting this needs hard thought.
As you say, the transistor is cheap, but the labor and mess to replace (and re-replace) it isn't.
Look: pull the wires off the big transistors. Power-up and see if the fuse holds and some of the rectifier leads come up to +30V and -30V DC. If so, the rectifiers and caps are good. That's not great news: these are the cheap parts. BUT if it still blows fuses, replace the rectifiers (bigger is better) and go ahead and replace the big caps too.
At some point we have to say: teaching you to diagnose discrete transistor power amp failures is more time and effort than an old 50W bass-amp can possibly be worth. If this was 1977 all over again, it might be a good life-skill. (Was for me.) But most of this old stuff either never fails or has already failed and been dumpstered. Fixing solid-state amps is not lucrative and has no future. Gut it and put a 5F6A in there.
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PRR said"At some point we have to say: teaching you to diagnose discrete transistor power amp failures is more time and effort than an old 50W bass-amp can possibly be worth. If this was 1977 all over again, it might be a good life-skill. (Was for me.) But most of this old stuff either never fails or has already failed and been dumpstered. Fixing solid-state amps is not lucrative and has no future. Gut it and put a 5F6A in there."
Yeah there is definitely no money in diagnosing some older solid state amps.I do it at work and am no where near as proficient as PRR yet.I just did an SVT pro-3 and got lucky.I changed all the outputs and a couple of little three legged transistors and a few resistors and it's working fine.I brought it up slowly on the variac and checked for heat at the outputs.I find lots of problems by just feeding small voltages into the amp.
It's a bid headache and I only recommend doing it if you have to and you need to know how to check those transistors.But don't assume they are good with an ohmeter check.Sometimes they won't hold up when powered up.
You might change a few parts and get lucky,but those little driver transistors are like phase inverters and they often blow whan the outputs go south.sometimes it takes a few resistors with it too and the odd time a voltage regulator.
A 5F6A seems like child's play in comparison.
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> a Carevin Bass amp from 1992
Ya know, it really does help to say WHAT you have (not just "a Carevin"), so your friends do not go barking up wrong trees.
This is a FET100.
> a couple of TIP31c.
No, a TIP31C and a TIP32C! And these will not blow fuses (R20-R21 would smoke first).
If it holds a fuse with the big MOSFETs disconnected (and NO load), I would ask what the output voltage is. But I am not sure what the right answer is on this MOSFET circuit. Most BJT amps, no load, big devices out, will still try to force themselves to zero-DC output and make some attempt to pass signal. But I'd have to stare at this quasi-comp driver to guess if it even tries to get happy without its big-butt devices. (McEuen has always been too clever for my brain.)
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First, thanks for all of the ideas on this. Second, I apologize that I did not include more info in my original post. At that time, I did not really know exactly what I had. I took some doing, but I was able to get a schematic of the FET100 from the tech guy at Carvin.
I agree with you guys whole heartedly that it does not make a lot of sense to try to make a habit of fixing these things. Fortunately, I am hopeful that it will involve a small amount of money for parts, and a somewhat larger investment of time, since Little Money and Loads Of Time is exactly what I have these days. In other words, it should beat feeding the pigeons at the park.
Also, it's just a little of a challenge, I like the amp, and I would like to be able to play it again.
PRR and Psyco, your last few posts have helped me quite a bit. That gives me alot to work on for the time being. I really appreciate your help since this obviously is NOT what you guys got into amp building for.
Thanks again!!!!
FWIW I have included the FET100 schematic that Carvin sent me.
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> I have included the FET100 schematic that Carvin sent me.
It was already on Doug's website:
http://www.el34world.com/charts/Schematics/CARVIN_FET_100.pdf
> The transistors are 2 MTH1520's
I do not know what an MTH1520 is. Like the TIP31 +AND+ TIP32, you gotta get the right part. If MTH is Carvin's house-number, you may have to find out what commercial part is suitable.
First off: EXAMINE for burnt resistors. They are clues.
Also solder a 270 ohm across the ends of P1, the idle current adjust pot. If/WHEN that pot goes bad, FET current goes to infinity, which is bad.
On each big FET, disconnect the Drain wire (you may be able to guess from the connections, or look-up the pinout for the -exact- device you have installed).
Disconnect one end of R13.
Disconnect any speaker load.
In this form, the amp "should" come up to full power-rail voltages (perhaps +50V and -50V DC), and the speaker-jack hot terminal "should" sit at "zero" V (less than 1VDC).
Fix R13.
Now float your voltmeter across R10. You are measuring the voltage which goes to the Q6 FET Gate. It should be small, maybe nearly zero. Un-shunt P1 and adjust it. The voltage across that Gate should rise smoothly to almost 5V, with NO jumps to higher voltage. If it twitches past 5V, the pot is shot and must be replaced. In the end you want to leave the pot set for about 1.0V-1.5V at each Gate. This is "too cool" for best small-signal sound, but that's safest.
If you have ANY doubt that P1 can set a RELIABLE 2V-4V bias, without fail, then replace it with a fresh high-reliability pot. (I would even consider tacking-in a fixed resistor: the exact bias is not critical, but lost-bias is VERY bad.)
Put 100 ohm 10W resistors to connect the MOSFET Drains to where they should go. The idea is to pass "some" current so we can check some more, but not pass BIG current and smoke anything precious.
If a 100 smokes right away, that FET is shorted.
In this form, the amp should bring its output to zero volts DC (<0.1V).
Wire a 100 ohm 10 Watt resistor in series with a speaker, and connect it ("108 ohm speaker"). In this form, the amp should "play", soft, but clean. Soft because most of your power is lost in the several 100 ohm resistors: kills gain and clobbers maximum output. But if something is still wrong (and it often is), you are out a $1 tack-on resistor instead of a $10 screw+goo transistor.
When it appears to work as good as it can with all the choking, take out the added resistors and cross your fingers. (An intermediate step is to try 10 ohm 10W resistors: it will almost play right or it will smoke very fast but cheap.)
A final step is to measure the idle current and trim P1 (or select fixed resistors) to get the suggested 200mA through the FETs (or the whole amp). But IMHO, this may not be necessary in a bass stage amp. It cleans-up the sub-Watt sound, but a bass stage amp never plays sub-Watt, and any slight distortion will be blamed-on/credited-to the guitarist. As long as it does not idle HOT, and doesn't make decays sound like a rock-crusher, it is biased fine.
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looking at blurry pics in previous post... confirm MOSFET part number please - i read MTH15N20
research shows that MHT15N20 is/was a motorola part and of course, it's now defunct. some further research revealed that yorkville sound used this part in at least one of their products and provide a cross to an IOR device - IRFP250 - it may work in your circuit, it may not... your mileage may vary...
trust in the man (PRR), it IS likely that a small signal xistor(s) caused your calamity.
info link.
http://www.yorkville.com/downloads/other/PartsCat.pdf
please note that you have to follow the cross - they reference an in-house number (6928) and indicate the crossed part# when you search for the in-house number. it's a PDF so, search the pages - section 12 xistors - MFETs.
the IRFP250 seems to have been either more popular (likely), or at very least a newer part. it's now defunct as well, however is seems as though jameco still has some stock... there are links to datasheets provided in the jameco pricing link below... two bucks each - not bad, u fix amp for less moolah than a 6-pack of decent ale, IF the MOSFETs didn't survive electron carnage.
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=670080