Hey Guys, My bass player has an Ampeg SVT 4 Pro that kept popping his mains breaker. First thing I noticed was the On/Off Switch was toast and R218 was toasted on the board.
I used the other side of the Power switch so that problem is solved.
I checked all the transistors around that resistor and Q202 was shorted, the others seemed fine so I just replaced Q202 and R218 and now R251 and R258 went up in flames.
Should I start pulling output devices and drivers in that channel? Anybody have issues with the bias/driver supply with these things?
Thanks,
-bird
I used the other side of the Power switch so that problem is solved.
I checked all the transistors around that resistor and Q202 was shorted, the others seemed fine so I just replaced Q202 and R218 and now R251 and R258 went up in flames.
Should I start pulling output devices and drivers in that channel? Anybody have issues with the bias/driver supply with these things?
Thanks,
-bird
Hi Enzo, I'll be damned. It looks like the emitter resistors are open. Usually those large coffin resistors puke their guts out when they fail, they looked fine so I didn't even think to check them. I replaced the R218 and Q202 and fired it up and poof. I guess I will have to pull the output devices and drain the gate capacitance voltage off and see if there is a short but it looks like the output is toast in that channel
You don't have to pull an output transistor to see if Source is shorted to drain. The reason those fractional ohm resistors went open is very likely because the transistor they serve was shorted.
When you do this stuff, think of the current. Resistors do not burn out on their own, there is always excess current through them. If R251 and 258 burnt, that means current came from somewhere. It makes sense that it came through Q214, Q216. And the reason it went through those small resistors is because the path through the large power resistors was open.
Now look where R251,258 go. They join others on that line over to Q208. I wouldn't be surprised if it were toast, as it provides the other end of that failure current path.
When you do this stuff, think of the current. Resistors do not burn out on their own, there is always excess current through them. If R251 and 258 burnt, that means current came from somewhere. It makes sense that it came through Q214, Q216. And the reason it went through those small resistors is because the path through the large power resistors was open.
Now look where R251,258 go. They join others on that line over to Q208. I wouldn't be surprised if it were toast, as it provides the other end of that failure current path.
I always seem to not be able to check them in circuit well. Something seems to charge up the gate and which turns the device on and looks like a short. I will try in circuit first as it would be easier.
I too was kinda looking at Q208 so I will check that as well.
I guess I am just curious what went wrong? Like, what was the chain of events to lead to the output stage devices going? I don't want to replace them only to have it happen again. I remember looking at one before and it was working fine but the output stage devices were not sharing current well so they must not have been matched. I wonder if the same thing happened here, a few of the devices were pulling more than they should be and finally failed. I see matched sets for sale on ebay but they are expensive. It's going to cost $150 in output devices
Well I have time to take another look at the amp so I will report back.
I too was kinda looking at Q208 so I will check that as well.
I guess I am just curious what went wrong? Like, what was the chain of events to lead to the output stage devices going? I don't want to replace them only to have it happen again. I remember looking at one before and it was working fine but the output stage devices were not sharing current well so they must not have been matched. I wonder if the same thing happened here, a few of the devices were pulling more than they should be and finally failed. I see matched sets for sale on ebay but they are expensive. It's going to cost $150 in output devices
Well I have time to take another look at the amp so I will report back.
I got hung up this afternoon but just had time to look at it again.
Yup one channel is blown. I pulled a couple FET's and their emitter resistors to make sure and they are toast. It's weird the coffin resistors physically look fine, the last time I had ceramic coffin emitter resistors fail they were clearly destroyed visually.
I will get matched sets from epray and hope they are fine.
I have seen other Ampeg SVT's where the output stage devices didn't match at all which is what I am accounting the failure too. Has anyone else seen SVT's with poorly matched FET's from the factory? Is this common or did my buddy just get a dud. I mean it worked for him for years but I know he isn't the only bass player to have his SVT blow the outputs from normal use. I know for a fact he doesn't abuse this amp. Another friend of mine in Michigan brought his to a tech and he replaced both channels which ran him $600 for the repair. When he picked it up he said his tech made it sound like this was a common problem with these amps and he hates doing them because it's not easy to flip the board out for repairs which I kind of agree with, they don't make it easy. So anyone else out there in the service field see this a lot with these?
Yup one channel is blown. I pulled a couple FET's and their emitter resistors to make sure and they are toast. It's weird the coffin resistors physically look fine, the last time I had ceramic coffin emitter resistors fail they were clearly destroyed visually.
I will get matched sets from epray and hope they are fine.
I have seen other Ampeg SVT's where the output stage devices didn't match at all which is what I am accounting the failure too. Has anyone else seen SVT's with poorly matched FET's from the factory? Is this common or did my buddy just get a dud. I mean it worked for him for years but I know he isn't the only bass player to have his SVT blow the outputs from normal use. I know for a fact he doesn't abuse this amp. Another friend of mine in Michigan brought his to a tech and he replaced both channels which ran him $600 for the repair. When he picked it up he said his tech made it sound like this was a common problem with these amps and he hates doing them because it's not easy to flip the board out for repairs which I kind of agree with, they don't make it easy. So anyone else out there in the service field see this a lot with these?
I'm an amateur, I've done a couple of SS amps and the emitter resistors looked fine after they blew. Since this is an instrument amp with an internal speaker probably, after replacing the output transistors I would check that the speaker doesn't have a shorted turn. Turn it up to vol 10 and check the current across the emitter resistors using ac voltage scale. Check against the dc soa rating of the output transistors.
Like on MJ15024/25 no more than about 3 amps. You'll get a more accurate reading on an analog VOM, DVM, most of mine produce random numbers on music that is not 50-60 hz. Of course immediately after replacing output transistors you have to test every semi before that since rail voltage went rampaging out the base line, overstressing everything marginal. Resistors blow, op amps blow, even filter caps on the op amps in PV1.3k, they were rated 50 v.
My craftsman dvm checks transistor eb and cb junctions fine with the 2000 ohms scale. Good transistors read 450-780 ohms forwards, ---- backwards. My newer Parts-express meter doesn't do that on ohms, the open circuit voltage is too low, but it has a diode scale that works with similar readings. I found TO3 transistors in the 450-500 ohm range were overstressed and would blow up on rail voltage before I built a room heater AC line load. (PV1.3k amp would not power up on a light bulb). I personally wouldn't waste any money on e-bay transistors.
There are a couple of guys that post here that sell there, but parts from ***** home of fake handbags?I got a fake bicycle motor last year from *****, $280 to a US vendor not even e-bay, and I got 60 miles out of it before it blew up. OTOH my $2000 new bicycle made in ***** from a respected US vendor was superb. Depends on who is supervising QA in *****. Newark multicomp stuff from ***** has always been fine, also the new On semi parts are made in ***** through newark mouser and digikey. I buy transistors from newark, mouser, digikey, alliedelec. People on the west coast like arrow. Your e-bay bargain transistors, check Iceo before installing. With a 12 v to 24 v power supply , what current do you get through 200 ma scale with a 10 k resistor in series, from the collector to the emitter with base open. Should be microamps. And watch the vceo Ic and P ratings at distributors, newark will sell On semi rejects with special parts numbers with dodgy ratings right there in the selector table. I bought some NJW21194 transistors with 250 mw rating, and they would blow up on 85 v. $1.93 each, worth about $.01.
Like on MJ15024/25 no more than about 3 amps. You'll get a more accurate reading on an analog VOM, DVM, most of mine produce random numbers on music that is not 50-60 hz. Of course immediately after replacing output transistors you have to test every semi before that since rail voltage went rampaging out the base line, overstressing everything marginal. Resistors blow, op amps blow, even filter caps on the op amps in PV1.3k, they were rated 50 v.
My craftsman dvm checks transistor eb and cb junctions fine with the 2000 ohms scale. Good transistors read 450-780 ohms forwards, ---- backwards. My newer Parts-express meter doesn't do that on ohms, the open circuit voltage is too low, but it has a diode scale that works with similar readings. I found TO3 transistors in the 450-500 ohm range were overstressed and would blow up on rail voltage before I built a room heater AC line load. (PV1.3k amp would not power up on a light bulb). I personally wouldn't waste any money on e-bay transistors.
There are a couple of guys that post here that sell there, but parts from ***** home of fake handbags?I got a fake bicycle motor last year from *****, $280 to a US vendor not even e-bay, and I got 60 miles out of it before it blew up. OTOH my $2000 new bicycle made in ***** from a respected US vendor was superb. Depends on who is supervising QA in *****. Newark multicomp stuff from ***** has always been fine, also the new On semi parts are made in ***** through newark mouser and digikey. I buy transistors from newark, mouser, digikey, alliedelec. People on the west coast like arrow. Your e-bay bargain transistors, check Iceo before installing. With a 12 v to 24 v power supply , what current do you get through 200 ma scale with a 10 k resistor in series, from the collector to the emitter with base open. Should be microamps. And watch the vceo Ic and P ratings at distributors, newark will sell On semi rejects with special parts numbers with dodgy ratings right there in the selector table. I bought some NJW21194 transistors with 250 mw rating, and they would blow up on 85 v. $1.93 each, worth about $.01.
Last edited:
IRFP240PBF Vishay Siliconix | Discrete Semiconductor Products | DigiKey
IRFP9240PBF Vishay Siliconix | Discrete Semiconductor Products | DigiKey
These are made by Vishay.
Are you suggesting just buying 5 of each and just pop them in? Or are you suggesting I get a bunch of them and spend the time to match up the closest ones?
The ones on ebay I was looking at were here: 6+6 MATCHED SET IRFP240 & IRFP9240 POWER MOSFET 200V 20A N&P CHANNELS TRANSISTOR | eBay
Honestly I don't think Ampeg even bothered matching output devices which is why they blow up frequently.
IRFP9240PBF Vishay Siliconix | Discrete Semiconductor Products | DigiKey
These are made by Vishay.
Are you suggesting just buying 5 of each and just pop them in? Or are you suggesting I get a bunch of them and spend the time to match up the closest ones?
The ones on ebay I was looking at were here: 6+6 MATCHED SET IRFP240 & IRFP9240 POWER MOSFET 200V 20A N&P CHANNELS TRANSISTOR | eBay
Honestly I don't think Ampeg even bothered matching output devices which is why they blow up frequently.
Right under my nose. I remembered the Pass amp using the same output devices so I checked the diyaudio store.
F-4/BA-3 Transistor Kit – diyAudio Store
I will just order them from here Easy Peasy
F-4/BA-3 Transistor Kit – diyAudio Store
I will just order them from here
Easy Peasy
diyaudio is a good choice IMHO. Matching is less critical if emitter resistors are .5 ohms instead of .22 ohms. I haven't downloaded your schematic to discuss details. Peavey puts in .5 ohm emitter resistors so they can match from the factory your old amp output transistors from decades ago. Higher emitter resistors will cut power a smidge, negligible in terms of decibels but highly important when advertising the amp's power rating. I match junction transistors with a 12 v power supply(wall transf) , a dvm, a 100 ohm resistor (base) and a 10 ohm resistor (collector). I don't know how it is done on fets, but I would think a resistor divider for the gate, to make 5 v Vgs, and a 10 ohm resistor PS to the drain should give you a voltage Vds to match up. When matching you have to buy extras to put them back in the parts bin.
Time matters when matching transistors due to self heating and different gain at different temperatures. So count off to take the Vds reading at the same time on all transistors. Like one second if you can move your eyes from the clip lead to the meter face that fast, or 1.5 sec if you need that time. Freight bills are $8 min for me from newark, 7 parts-express, 12 mouser, so I try to get stuff in all one box. Getting your emitter resistors plus the driver transistor+resistor plus heat sink grease plus mica washers (multicomp kit) could save you enough freight bills to pay for a couple of output transistors from distributor. I bought 2 extra OT, got away with it, had exactly two outlier voltages.
Time matters when matching transistors due to self heating and different gain at different temperatures. So count off to take the Vds reading at the same time on all transistors. Like one second if you can move your eyes from the clip lead to the meter face that fast, or 1.5 sec if you need that time. Freight bills are $8 min for me from newark, 7 parts-express, 12 mouser, so I try to get stuff in all one box. Getting your emitter resistors plus the driver transistor+resistor plus heat sink grease plus mica washers (multicomp kit) could save you enough freight bills to pay for a couple of output transistors from distributor. I bought 2 extra OT, got away with it, had exactly two outlier voltages.
Last edited:
The Ampeg does use .47 emitter resistors so they probably didn't have to closely match from the factory.
I still feel better getting matched devices, I think it will help. Maybe it won't. I don't do a lot of transistor gear but I have seen even with .47 emitter resistors the quiescent current draw was not even close. The channel that does work has a large difference in current draw, some are pulling less than 10mA and a couple are at 30mA.
I literally just got off the phone with my bass player that owns the amp and he doesn't want to throw it out and so he would like to fix it. We discussed options and he agreed on just doing both channels anyway with the matched devices from the diyaudio store.
I still feel better getting matched devices, I think it will help. Maybe it won't. I don't do a lot of transistor gear but I have seen even with .47 emitter resistors the quiescent current draw was not even close. The channel that does work has a large difference in current draw, some are pulling less than 10mA and a couple are at 30mA.
I literally just got off the phone with my bass player that owns the amp and he doesn't want to throw it out and so he would like to fix it. We discussed options and he agreed on just doing both channels anyway with the matched devices from the diyaudio store.
Svt 4 pro
I just got done working on a Ampeg Svt 5 pro. I wasnt able to get it fixed in time before customer came and got it. When i first started working where i work , one of the techs that quit , worked on this amp . I had no clue what he did , so i adopted the amp to work on. Then the store where i worked wanted to move to a smaller location . So that set us back even further . When i finally got a chance to work on the Svt , i got it working all the way up to the relays. ,but for some reason it still wouldnt pass signal to the outputs. Thats where it was when customer decided to come and get it. They are difficult to work on , givin that there is one big board along with 8 smaller boards.
I just got done working on a Ampeg Svt 5 pro. I wasnt able to get it fixed in time before customer came and got it. When i first started working where i work , one of the techs that quit , worked on this amp . I had no clue what he did , so i adopted the amp to work on. Then the store where i worked wanted to move to a smaller location . So that set us back even further . When i finally got a chance to work on the Svt , i got it working all the way up to the relays. ,but for some reason it still wouldnt pass signal to the outputs. Thats where it was when customer decided to come and get it. They are difficult to work on , givin that there is one big board along with 8 smaller boards.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.