Bugera BVP5500 fault

[valvusmusicus]

Hi There
Yes thought of that-tested with a scope at the input of the power amp section and the speaker output-No DC ofset or oscillations-must be the idling current
Will keep all posted-hopefully this will help some other poor sod who bought one of these
They seem to have at least a 1 in 10 failure rate out of the box as Ive read in other posts and forums!!
CHeers again

Paul

[Steve Conner]

Did you just buy it? Even with a 1 in 10 failure rate, getting a replacement under warranty has a 90% chance of fixing the problem.

[valvusmusicus]

Naa-my fault I guess. Bought as a cheap backup amp about 20 months ago-tested in the shop-jsut made sure it lit up. Was going to put it thru its paces at a later date and then use as a back up
Guess what, Main Ampeg SVT has gone tits up-so I thought Id get this out
.................and the rest is history!!

[valvusmusicus]

Ive put it back together now. There is one high amp fuse after the transformer AC feed to the output amp bd (no dc fuse on each rail after rectification). So I stuck an amp meter in place of the fuse-starts off at about 0.9 amp from cold-then as the heatsink heats up, it goes down toa stedy 0.37amps when hot. This with the fan connected
Is there a similar topology of power amp with these devices - I shall have a look around

Regrads

Paul

[Enzo]

Hi valvus, welcome to the forum.

I am a Benringer repair center, and I can;t imagine where you get the idea that 1 in 10 of these are bad from the box.

I don;t mean this unkindly, but reading your posts it seems you are keenly focused on the amp needing to be redesigned, when it probably just needs repair.

MOSFETs, new transformers, extra fans, and so on. The problem is not that it is getting hot, the problem is causing it to get hot. Adding fans here is like taking pain killers when you have a stone in your shoe.

If the amp is getting hot just at idle, then too much current is flowing. Changing the power suply voltage is not an answer. Trimmer VR1 on the power amp should be an idle current trim - your bias control. It works with D15,16 to make the amp bias. NTC component R7 is parallel to the trimmer. If either the trimmer or the NTC is open, then idle current will be excessive. Chances are there is no open diode, but if one has slipped high in junction drop, your amp can be mis-biased.

[MarkusBass]

Adding fans here is like taking pain killers when you have a stone in your shoe.

Enzo always know how to express complex ideas in a simple way . If the amp gets hot quickly after it is turned on, it just needs a repair. You mentioned the the output transistors were shorted. In such a case it is not enough to replace the transistors. You have to check bias circuit, drivers and possibly several other components. The amp is at least incorrectly biased. I hope you are almost ready with the power amp's photos (you say that there is no bias trimmer, Enzo says it is there) . I also suggest to measure voltages on ballast resistors - this is the place where you check quiescent current.

Mark

[valvusmusicus]

Hi There
Thanks for taking the time to analyse the situation and reply
I am struggling due to the fact that no schematics are available for me to safely adjust anything. If I new some measurement points-or had a schematic-I wouls have a go
I can make certain assumptions, but dont want to twiddle anything that I dont know about
Thanks for the detail you have given me, I will start from there
What should the standing current be for the output stage please? Is it ok to measure it accross the single AC fuse I mentioned?
Ive put it away now, so could you tell me if VR1 is clearly marked and is one of the 3 visible by the fuse please.
I am sorry to suggest that 1 out of 10 fail, but Ive read some bad posts about the amp. I cant remeber where I saw the 1 out o 10 failure comment-but Ill have a look and tell you-perhaps you might have a word with the person who posted this-and find out why
I do not think the amp should be re-dsigned-perhaps a larger heatsink?
On reflections I guess my comments were a little premature, and I should have done some more research!!
I guess it is impossible for you to send me a schematic of the PA section, as this may affect your relationship with Bugera
But thanks for the info anyway

Regards

Paul

[valvusmusicus]

Sorry-sent last reply before reading MarkusBass's last
I will check the ballast resistors
Thansk all for being so helpfull
I think I have a lot to learn about power amps!!!!!!!!

Paul

[valvusmusicus]

Here are the photos-you can clearly see the 3 trimmers in one of the photo's
I guess the big 3/5watt resistors are the ballast resistors?

[valvusmusicus]

Sorry I meant the 3 watt resistors

[MarkusBass]

It looks like a typical Behringer power amp used e.g. in Behringer BX3000T. So this is 300W @4 Ohms (minimum). There is no need to enlarge the heatsink, nor to add a fan, nor to change the transformer. You just need to fix it or bias correctly. Can you measure the voltages on 0.22Ohm resistors? A symbol of every component is printed on the printed board. The symbols for the trimmers may be behind them - the picture is not focused - it's hard to see anything on the third photo.

Mark

[valvusmusicus]

Many tahks-will do

[Enzo]

Yes, if this circuit is not the same, it is DARN close to the BX3000 power amp.

And it is quite possible all it needs is an adjustment.

Behringer tends to be good about printing the part numbers and values on the circuit boards. I mentioned above th bias control is VR1, and that is 100 ohms with the NTC also in parallel. SO look for VR1 and/or 100 next to one of the controls. If the pot doesn;t say 100 ohm it, it might say 101. the other two pots are current limits, and are 2.2k VR2, VR3. Could be marked 2k2 or 222.

I am not sure which fuse we are talking about. I'd just monitor mains current. Turn VR1 towrds cooler, and see if the mains draw reduces too. If so, keep going. Once the thing stops reducing the mains draw, there is little point in turning down further. However, that said, I usually just set it maximally cold, and turn it up from there until I get where I want to be.

I don;t know which direction is colder, watch the ammeter and see. I will say that whichever extreme of VR1 has the least resistance is the coldest setting.

I have no idea what current through the output devices ought to be. I consider that method rather arbitrary and I never use it. A solid state amp like this adjusts to crossover distortion quickly and easily. Adjust so the notch just disappears, then back off just a hair. Or cheap and dirty, adjust bias upward until mains draw starts to climb, then back off. Same thing.


A word about failure rates. First, unless one works at Behringer or whatever brand is in question, one wouldn;lt likely have access to real failure percentage numbers. One would be guessing. Second, problems are ALWAYS over-represented on the internet. Someone gets a problem, BOOM online they go to gripe about it. SOmeone gets a new amp in the mail that works like they extected, they don;t waste time telling strangers. This place is like the service counter at best Buy. If you habg out there, all you ever see if people having problems. No one ever stands in line to tell the repair guys that their new TV worked right out of the box.

Well, I might, but then old hippies are weird.

[Steve Conner]

Well, I can see the left-hand of the three pots is a "101", so that'll be it.

Did you replace the output transistors? You never mentioned that. As was pointed out, you have to rebias, same as when replacing power tubes in your SVT or whatever. There is also the possibility that you got fake devices that don't meet spec, if you bought them off Fleabay.

How did we end up redesigning this amp anyway? The thread kind of got off on the wrong foot. It would have been better if the original post had just listed the setup, the symptoms observed, and any work previously done. Did your personal amp actually splutter, fart, melt down etc. rather than repeating a bunch of Internet rumours, which are worth the paper they aren't printed on.

And yes, the heatsink looks more effective than the one in the SVT-3Pro.

[MarkusBass]

Did you replace the output transistors? You never mentioned that.

Actually, I mentioned this and not Paul. It's because Paul mentioned in his first post about: "meltdown, or start spluttering and farting" .

Mark