Fender Passport DP 250

[bwinkel67]

So I read an older thread off of this to help me fix the Passport:

Fender PA problem. Passpoprt 250

I fixed it about 18 months ago after a fellow DJ gave me his old broken system as he was leaving (we ran a dance together). Not an expert in electronics but I have a multi-meter and found the bad transistor A1962 and bought one for $2, replaced it and it worked for a few hours until the other one blew (C5242) and replaced it as well. The system ran for 18 months with about a dozen DJ's using it weekly for 3 hours for about 100 people dancing. Then the original owner who blew it many years ago came for a visit and DJ'ed and blew it again (yes, ironic and not sure what he did...he had a lapto with card and somehow his card is the culprit).

Now I haven't been able to fix it. This time I had to replace the above two as well as two smaller ones (MPSA06 & MPSA093) as well as a 150 ohm resistor...and a capacitor at c202 needed to be resoldered for some reason.

In any case, now it works for an hour and blows one of the big transistors. I've replaced like 3 of them now. It's always the left channel. I've been over the board and compared everything from left channel to right channel and it matches. I have the schematic and things seem to match. No open circuits. I'm at a loss. I read something about biasing the board. I didn't do that last time and have no idea what that is.

Note that the thing will play for hours unless you give it a bigger input signal and then drive it...that's when it blows one of the transistors.

Any help would be appreciated. User Enzo has had some great insight in the past, hoping he's still on this forum. Perhaps the problem is biasing the board, dunno. Didn't have to do it last time but maybe I got lucky. Or could it be somethign else wrong, like the power supply?

Cheers,

TM

[Enzo]

I don't have the drawings handy, I'll have to look them up.


Yes, when changing output transistors, one needs to reset the bias on adjustable amps. Even on non-adjustable ones, we need to check that current draw is not excessive, and can alter the circuit if need be.

[bwinkel67]

So I'm assuming that is what continues to blow my transistors. I must have gotten lucky the first time I replaced them. Do I need to worry about the left and right offsets as well?

There was a brief description of how to adjust the bias in one of the older posts but I wasn't clear what exactly was being checked. I'll re-read that. It sounded like you had the amp on and adjusted the bias until some signal dropped. It'll have to be trial and error again...

One more weird question...I'm not an expert in electronics but I thought resistors are bi-directional. There are a couple of 120K ones that when checked in the other direction read 10K. It's consistent on both channels. What's going on?

Thank you!

TM

[Enzo]

If you disconnect one end and lift it from the board so the resistor is now alone, it will measure OK. WHat was happening is that some semiconductor junction somewhere is more or less in parallel, anad your meter turns that junction on only one direction.

OR... there is some residual voltage in a cap somewhere confusing the meter. A classic example of that is a resistor that reads WAY high one way, and the other way it reads "negative resistance." Just a meter on ohms facing a small voltage.

Probably one of those.

Offset adjusts the output to sit right at zero volts DC. You want that dead on.



I'll be more specific later. One simple bias method is to monitor the current the system draws from the wall. Put an ammeter in series with the mains to your amp. Watch the current as you adjust the bias a little bit each way. If one way causes the mains current draw to reduce, keep going that way. Once it stops reducing, go no farther, it is a cold as it will get. And further reduction from that point will increase crossover distortion. Any adjustment that increases mains current is the wrong direction.

[bwinkel67]

For bias adjust using the ameter approach I assume that no speakers attached and level and volumes turned all the way down? The last time I blew I had no input coming in just had volume on 4 or 5 with no speakers and had it on for 5 minutes and the one transistor I just replaced got sizzling hot and blew again. Before that I assumed that there actually needed to be a signal coming in to amplify for the thing to blow but that wasn't the case. I also read that sometimes you get a batch of bad transistors....I got these on eBay in packs of 4 and the first set worked for 18 monts. Still, when the new ones blow you can see the metal backing gets warped...the originals don't do that.

TM

[bwinkel67]

Board Bias'ed

So I ended up getting the final pieces and biased the amp using a current meter. It ended up not needing any adjustments and of course, though it hasn't blown yet, I fear it will.

So, quick question, two years ago when I initially fixed it I replaced all 4 transistors at once. This time I replaced them one at a time. I just got an entire set of original Toshiba trnsistors and am wondering if I replace the mix I have with these that it may solve the problem. Any feedback?

TM

[J M Fahey]

The symptom "biases well, does not overheat, works perfectly at, say 50% of full power but blows in an hour or less when pushed" stinks of fake transistors, the modern day plague, and even more in Toshibas.
You paid $2 for them, what did you expect?
Buy *only* from very reputable suppliers or pay a little more and buy straight the OEM ones (such as buying from Fender or Peavey) because *they* buy straight from the original Factory and screen their prooducts, for obvious reasons.
A smaller electronics shop only cares about the "sales price minus cost" equations, which means "sales profit".
Just to freeze your blood read:
Counterfeit Transistors
Counterfeit Transistors
counterfeits
### Counterfeit transistors - diyAudio
Danger: Fake and Counterfeit Transistors on Ebay " Exposing the truth " - YouTube
Ugh !!!

[bwinkel67]

Thank you, that is very useful information and likely what has been happening to me. I finally threw out the old ones otherwise I'd have cracked one open. I can get them locally through a dealer and got one which was in originally packaging and most likely real. The four I just got in came from China...not so set anymore on using them. It's sad because I got 4 two years ago from China and they seemed to have been genuine. These new ones seem to have identical printing on them as the original TOSHIBA ones but the ink is different...the back metal is flat but I would think counterfitters likely wise up to that as well...grrrr.

TM

[bwinkel67]

So six months after fixing my Fender I had a weird blip. Been working fine and we've been using a pre-amp to reduce stress on Fender--i.e. we plug a Behringer 502 mixer into the Fender and all our DJ's plug into the pre-amp. This keeps us from driving the Fender as the volume now stays around 3 (as opposed to 6).

So a few weeks ago, one of the few times I was away unfortunately, system goes into safe mode and is assumed to have blown. I just got it and flipped the power on and it works. Back in December the same thing happened but then it went into safe mode and when power was cycled it worked but only for a song--note though that in December we weren't using the Behringer and volume was at 6.

In both instances it was the same DJ (not one of our regulars) using the Native Instruments Traktor Audio 2 (Amazon.com: Native Instruments Traktor Audio 2: Musical Instruments) on his laptop. Going through a pre-amp mixer, I would think that should protect the amp, esp since it's not being driven so much. So two questions:

1) What's going on? Why does it go into safe mode for a few minutes and then work again?
2) Why would this specific external USB audio device so uniquely interact with the Fender, pre-amp or not?

P.S. I will drive the Fender for a while to see if it is still robust (last time I tested it for 90 minutes at volume 6 without problem). Have run it for 30 minutes at volume 3 just to see that it could get through a few songs without blowing and currently it's working...

[J M Fahey]

Just guessing: speaker impedance is nominal, and close to spec at mid frequencies, but varies a lot at very high and low frequencies.
Not only the ohms value, but it often behaves inductive or capacitive, which overheats the power transistors.
So your amp may work happily with a "regular" music program, but maybe some "DJ Special Sound Enhancer" patch produces a TON of "wall shaking" bass; I have seen some which even synthesize a sub octave which was not even in the original music, which is a great speaker killer and power amp overloader.
Only explanation I can imagine for amps working for months buy dying fast with certain DJs or when going straight from the notebook to power amp.
I think that building a sharp filter whick kills everything below 80Hz (or 60Hz, but not lower) will make amp and speakers live much longer.
I have made experiments with synthesizers and bursts of 100Hz sound sound incredibly chest thumping, go figure, while, say, 60Hz ones were audibly deeper ... but much weaker, believe it or not.

[bwinkel67]

Thanks for the quick response! As for speaker impedance, isn't that a function of the Fender system? I don't have the spec's handy for it currently. If not, how do I measure?

The sound cards spec's, as stated on Amazon, says that:

o Cirrus Logic 24bit/96 kHz AD/DA converters for pristine audio quality
o Two high-gain stereo outputs for punchy output levels

(P.S. I love how it claims to be the most portable when, uhm, you can buy USB sticks that do this...well, maybe not at that level of quality)

The music we play is older recordings from 20's and 30's (swing music). I think this DJ plays more of the older tunes that lack bass in the recordings. We did have him connect through our Behringer but I suppose that doesn't necessarily filter the signal as you suggested. Can you purchase variable Hz filters so you can set max and min levels. Seems like a useful device.

[J M Fahey]

"Old" Fender and "new" Fender is not the same.
Brand alone does not mean much nowadays.
That said, Passports are fully designed, developed and built by an Asian (Taiwan?) Company; Fender just distributes them.

[mozwell]

I didnt see this in any post (or in hte original thread), so here is the service manual for future reference....