I'm working on a Genz Benz Tranzamp g30 and it has at least two shorted out rectifying diodes.
When I got the amp on my bench it wouldn't power up. It had a blown fuse. Not seeing anything obviously burnt, I grabbed a new fuse and plugged it in. When switched on the speaker hums really loud and when I disconnected the poweramp totally out of the equation I still get quite a bit of hum...but from the transformer. If it stays on for more than 4-5 seconds, one of the rectifying diodes will start glowing and then start to smoke the PCB. They are 1n4002 diodes. I have 1n4004 on hand I am planning on replacing them with.
So my question is, should I be able to just put in the new diodes and be on my merry way? Or is it more likely something else is wrong and that's what caused the original diodes to short?
Fix it and find out. Diodes fail, all by themselves. DIodes also fail when somethiing else draws too much current through them. We won't know until we get there.
How many rectifier diodes in that circuit? Two? Four? They cost a nickel, replace them all. Any remaining "good" ones were stressed by the shorted one/s.
Check your outputs for shorts. If they show shorted, then get to work on that section too. If they SEEM OK, they probably are.
A shorted filter cap could damage a diode, but that doesn;t really happen all that often.
You usually won;t find some reason some part fails.
I checked the output before even opening the amp, it's fine. I already have a handfull of 1n4004 on hand so I will replace all four rectifying diodes and give it a go.
I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.
Quote from: MJL21193 on March 11, 2011, 09:25:52 AM
I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.
If it was mine I would consider it, but I'm doing some repair work for a local guitar shop and need to keep the cost as low as I can while still making a good repair. If it was mine I would also remove the on-board fuse and put in a chassis fuse holder.
I'll keep you guys posted. I won't have time to do the repair till Sunday.
I replaced the blown fuse and the 4 rectifying diodes. The power supply is good now, reading +27.7v/-27.3v. But when you power it up it hums like crazy (quite loud) no matter what is plugged into it. So I decided to pull the daughter board the TDA2050 is on and found that one of the pins (I think it was pin 5, V+) was scorched and the solder was mostly gone and what was there looked like ash. I cleaned it up, resoldered that pin and it still acts the same. I'm thinking the TDA2050 is cooked.
What do you guys think?
I think you can serve that TDA2050 with a side order of fries. ;)
I was wrong on the pin, it was pin 4. The output pin.
What would cause it to do that? Would it be that the diode (only one had shorted) died and cooked the TDA2050 or would it be that the TDA2050 cooked and that fried the diode?
The speaker measures 7.2 ohms and it is an 8ohm speaker, so that should be fine.
We will never know. Parts fail. There is often no correlation with some event. The 2050 is far more likely to fail first over the diode. A shorted rectifier is likely to just blow a fuse, and not partidularly likely to blow the 2050. SO 2050 first diode second is more likely, but who knows. Either way we have to replace those parts, and then the amop will be fine.
The loud hum was probably just DC on your speaker.
Quote from: Enzo on March 14, 2011, 08:19:45 PM
We will never know. Parts fail. There is often no correlation with some event. The 2050 is far more likely to fail first over the diode. A shorted rectifier is likely to just blow a fuse, and not partidularly likely to blow the 2050. SO 2050 first diode second is more likely, but who knows. Either way we have to replace those parts, and then the amop will be fine.
The loud hum was probably just DC on your speaker.
You don't think it would have fried anything else along with the 2050? I mean, I'm sure it could have, but it doesn't appear to be so. No hot marks on the top of bottom of the main board. I was just concerned it might have damaged one or more of the three JRC4558 opamps. That would be a pain to replace all of those.
Hopefully the music store will want to continue repairs and pay the few extra bucks for the chip and labor. I'm thinking you're right and a new chip would get it going again.
QuoteNo hot marks on the top of bottom of the main board.
You can't *visually* repair am amplifier, just by looking for burn marks or bulged capacitors.
Maybe 5% of bad parts "show"it; for the other 95% you'll have to actually measure.
QuoteI was just concerned it might have damaged one or more of the three JRC4558 opamps. That would be a pain to replace all of those.
Don't get why a failed PSU diode or a power amp chip might damage a preamp chip.
Quote from: J M Fahey on March 15, 2011, 03:41:33 PM
QuoteNo hot marks on the top of bottom of the main board.
You can't *visually* repair am amplifier, just by looking for burn marks or bulged capacitors.
Maybe 5% of bad parts "show"it; for the other 95% you'll have to actually measure.
QuoteI was just concerned it might have damaged one or more of the three JRC4558 opamps. That would be a pain to replace all of those.
Don't get why a failed PSU diode or a power amp chip might damage a preamp chip.
I was concerned about it throwing AC through the chips - nothing to be worried about?
A shorted diode would let AC current pass freely, yes, but that will be shorted to ground by the filter capacitors, blowing the main fuse (that's what happened on your amplifier).
Besides that, preamp ICs are fed through a couple resistors and Zeners, which would further block any undue voltage that tried to reach them.
That's why I said that it's not impossible but quite unlikely a failure.
If anything, the power amp chip or transistors *might* be in danger because they are fed through a more direct path.
Personally I connect every suspect amp through a series limiting lamp, until I am sure it works properly.
Quote from: J M Fahey on March 16, 2011, 10:16:19 AM
A shorted diode would let AC current pass freely, yes, but that will be shorted to ground by the filter capacitors, blowing the main fuse (that's what happened on your amplifier).
Besides that, preamp ICs are fed through a couple resistors and Zeners, which would further block any undue voltage that tried to reach them.
That's why I said that it's not impossible but quite unlikely a failure.
If anything, the power amp chip or transistors *might* be in danger because they are fed through a more direct path.
Personally I connect every suspect amp through a series limiting lamp, until I am sure it works properly.
I should start doing that as well (the lamp thing). Do you simply have a light bulb in series with the amp? If so, what wattage would suffice? I think I might make a special bulb holder with outlet on it to plug into when testing amps.
Quote from: MJL21193 on March 11, 2011, 09:25:52 AM
I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.
A bridge rectifier is four diodes in a pre-configured package, so it isn't inherently safer. If you replace lower current diodes with a higher rated bridge rectifier, you might keep the diode/bridge from failing but you'll just be inviting some other problem, since something is still drawing too much current through the rectification circuit. I just finished repairing a Valvestate 8200 amp that was blowing the PC mount fuses and some series resistors in both the + and - rails of the low voltage power supply that feeds all sub-sections of the amplifier (preamp, reverb, chorus, etc.). I had to isolate the problem first by disconnecting sections of the amp from the power supply until I could find where the current overdraw was coming from, and then further isolating it to a component. In my case it turned out to be an opamp in the reverb circuit that was pulling too much from both rails. I suggest you try a similar approach in your Genz Benz amp.
I fixed the amp. Put in a new TDA2050 and we're good to go. One of the traces had lifted from all the heat when the old one blew! :grr
But it seems fine, I played it for an hour with no issues at all. Also, they really gooped on the thermal, I think part of the issue may have been too much thermal limiting heat transfer.
I removed the light bulb limiter info into a new thread since it is so useful. It's a sticky too. :tu:
http://www.ssguitar.com/index.php?topic=2093.0
Quote from: guitarkitbuilder on March 16, 2011, 09:23:09 PM
Quote from: MJL21193 on March 11, 2011, 09:25:52 AM
I learned my lesson with diodes (had 1N5406's short and nearly smoked the transformer) - I only use bridge rectifiers now, for anything that draws serious current.
Might be worthwhile to replace those with a 15-25A bridge.
A bridge rectifier is four diodes in a pre-configured package, so it isn't inherently safer. If you replace lower current diodes with a higher rated bridge rectifier, you might keep the diode/bridge from failing but you'll just be inviting some other problem, since something is still drawing too much current through the rectification circuit.
Replacing the diodes with a bridge wasn't meant to be 'safer', it's not supposed to act like a fuse. A bridge will withstand a lot more instantaneous current than 4 diodes combined. It keeps on doing what it's meant to do - rectify AC. If there is trouble further down the line, having the rectify blow doesn't help.
Quote from: MJL21193 on March 22, 2011, 07:18:49 AM
Replacing the diodes with a bridge wasn't meant to be 'safer', it's not supposed to act like a fuse. A bridge will withstand a lot more instantaneous current than 4 diodes combined. It keeps on doing what it's meant to do - rectify AC. If there is trouble further down the line, having the rectify blow doesn't help.
I guess I fail to see the difference between using 4 x 1n4004 diodes or 1 x bridge rectifier. Electronically they are the same, right?
The spec to look at is Peak Forward Surge Current. 1N4004 has 30A, a 15A bridge will handle ~240A. Sounds excessive but if the power supply has a large capacitor bank to charge (or a fault somewhere), it can come in handy.
I don't fix things, I build them and for anything drawing more than a couple hundred mA, I'll use a bridge.
The question then arises, are all your other parts also 10x overspec, or just the bridge?
Quote from: Enzo on March 22, 2011, 09:54:38 PM
The question then arises, are all your other parts also 10x overspec, or just the bridge?
Would a 1A bridge be 10x overspec for a 200mA supply? Equal to or less than the cost of 4-1N4004. 4 leads to solder, not 8. Higher peak current.
Like I said in my first post, I learned the hard way by nearly losing an expensive transformer through ignorance and trying to save a few cents. To me, it's easy - use a bridge and worry about other things.
Quote from: MJL21193 on March 23, 2011, 12:15:44 AM
Quote from: Enzo on March 22, 2011, 09:54:38 PM
The question then arises, are all your other parts also 10x overspec, or just the bridge?
Would a 1A bridge be 10x overspec for a 200mA supply? Equal to or less than the cost of 4-1N4004. 4 leads to solder, not 8. Higher peak current.
Like I said in my first post, I learned the hard way by nearly losing an expensive transformer through ignorance and trying to save a few cents. To me, it's easy - use a bridge and worry about other things.
I guess if you are building an amp from scratch it would make sense. But I would think that converting an existing amp to use a bridge would be a pain in the butt.
One thing that puts a load on the diodes in a rectifier is the filter capacitors, during power up. The capacitors are initially empty, and a surge of current flows through the rectifier to fill them up. Sometimes you may hear a 60 Hz "twang" when you turn on some power supplies. That's the rush of AC being drawn to fill up the filter caps.
The bigger the filter caps, the harder the full-wave bridge rectifier has to work to load them up on power up. (Some people think that the filter caps are like the amplifier's testicles: the bigger the better).
A leaky capacitor will also add to the workload of the rectifier diodes. Of course any kind of short circuit condition anywhere in the device.
Quote from: Kaz Kylheku on April 20, 2011, 03:26:35 PM
One thing that puts a load on the diodes in a rectifier is the filter capacitors, during power up. The capacitors are initially empty, and a surge of current flows through the rectifier to fill them up. Sometimes you may hear a 60 Hz "twang" when you turn on some power supplies. That's the rush of AC being drawn to fill up the filter caps.
The bigger the filter caps, the harder the full-wave bridge rectifier has to work to load them up on power up. (Some people think that the filter caps are like the amplifier's testicles: the bigger the better).
A leaky capacitor will also add to the workload of the rectifier diodes. Of course any kind of short circuit condition anywhere in the device.
Good point about the PSU caps. I had wondered about the size of them and how big is necessary. The LM3886 amps at chipamp.com use PSUs that have 2 x 10,000uF caps! They're huge! Of course that same PSU board is used for the stereo and dual mono kits. For a mono application it's way overkill but for a stereo application it may well be a good idea.
**edit**
Does anyone have a good equation for how large the filter caps should be?
The basic idea is that *all* power supplies have some ripple (unless regulated) which depends, among other things, on capacitance and the amount of current they have to supply.
So you decide "I can live with 4Vpp ripple on my 100W into 4 ohms power amp" and the formula suggests a capacitance value.
If you want half ripple, you double capacitance, just like that.
S the general idea is: double capacitance does not hurt a bit, on the contrary.
On a *tube* rectifier PSU, input capacitors matter and you can't go overboard because the initial current pulse that charges them may (will) stress the tube rectifier, but on SS diodes, that's not an issue.
Economic (or space) considerations will override any technical worries.
Quote from: J M Fahey on April 21, 2011, 10:43:40 AM
The basic idea is that *all* power supplies have some ripple (unless regulated) which depends, among other things, on capacitance and the amount of current they have to supply.
So you decide "I can live with 4Vpp ripple on my 100W into 4 ohms power amp" and the formula suggests a capacitance value.
If you want half ripple, you double capacitance, just like that.
S the general idea is: double capacitance does not hurt a bit, on the contrary.
On a *tube* rectifier PSU, input capacitors matter and you can't go overboard because the initial current pulse that charges them may (will) stress the tube rectifier, but on SS diodes, that's not an issue.
Economic (or space) considerations will override any technical worries.
So it's not really a concern that 10,000uF caps for instance will burn out the diodes on charge up/turn on?
Not likely.
There is also another unseen current limiting factor there which is transformer copper wire resistance, so in the real world, you will *never* have a zillion Amperes charging current at all.
To put some real world numbers into it, say you have a 22+22VAC transformer with a 30V peak value (a typical chipamp transformer), and internal resistance 1 ohm (actual secondary winding resistance + reflected primary resistance): no matter what size capacitor you will never have more than 30A peak current.
What capacitor size will influence will be for *how long* that charging pulse will last.
We are talking "one time" (turn on) , measured in milliseconds.
A typical diode used there (as part of a bridge) will be a 1N5402 (3 amperes continuous current).
Check its datasheet , http://www.fairchildsemi.com/ds/1N/1N5408.pdf (http://www.fairchildsemi.com/ds/1N/1N5408.pdf), it's full of very interesting data, that's what they are written for, and you'll see:
QuoteIfsm (non repetitive Peak forward surge current) 8.3 mS (milli Seconds) single half wave (the duration of the initial 60Hz charging halfwave) = 200A
200 Amperes, it's not a typo.
It means that a typical chipamp power supply, 22+22V transformer rectified with 4 x 1N5402 diodes charging 2 x 5000 or 10000 uF , which I earlier estimated as causing an around 30A peak for a few milliseconds, will never burn the diodes at turn on.
If the chipamp is stereo, it will use a larger transformer, higher capacitance, and a larger bridge rectifier with correspondingly higher single pulse rating, so in practice no sensibly designed PSU will burn "on its own" at turn-on.
There is always a (wide) safety marging built-in.
Then why do power diodes fail now and then?
My guess is that a shorted power amp causes those 30A to flow for a much longer time (although *for us* it still looks quite fast)
, until the fuse actually blows, and *that* stresses them to death.
A fuse may take from 1/4 up to 1 full Second to blow, meaning 250 to 1000 milli Seconds, way over the guaranteed time.
Even so, most times the specified Fuse protects them (not always)
In power supply design, it is important to calculate the surge current when the filter cap is larger than 1000 uF. A common design is to insert a series surge resistor to limit the surge current rather than using more expensive and larger diodes.
If your TD3050 had a bad solder joint, that could have caused arcing and produced the surge current that destroyed the diode. The shorted diode, in turn, shorted the secondary of the transformer, which blew the fuse.