Diode drop to match transistor VBE drop question

> I don't know that the diodes need to be high current

I don't know either. But a hi-current diode will give a lower bias and transistor current. A really-small diode on high-current transistors opens the chance of HIGH idle current and quick failure.

Then I noticed the BJTs were TO-66 (remember that?) 25W 1A 40V 3MHz parts. Bigger base than the 100mA TO92 jellybeans used in smaller gear. Use a big diode.

And 1N4007 should be dirt-cheap in any repair shop. So it should be a no-cost safe first try. If it actually biases too cold, we can work from there.

I have no clue how the diodes could go bad yet the transistors stay OK. I don't see a way for excess current to flow, or for reverse voltage to happen. So there's still a mystery.

On third thought: I'd be real tempted to lose the entire buffer and strap two 5532 chips as a parallel unity-gain buffer. Use individual 10 ohm output series resistors for current-sharing. This will drive the apparent 150 ohms. All the bias and protection is already in there. Yes, this "adds 5532 sound" to older vintage gear. But is there any complete (mike to speaker) reproduction chain which does NOT already have multiple 5532s in the path? You still have whatever goodness is in that A1000 chip: it will still do the input sensing and all the voltage gain.

Actually, there might not be any 5532s in this studio at all, except for maybe the headphone amp.

It would not be difficult to configure these diodes that they're thermally coupled with the BJTs either. I think I'll look into the first two suggestions before I go too far beyond what's already on these PCBs.

Also, I don't know that these specific diodes have gone bad, just that many other ones in this console are or have...

Let me answer your question, with a question.

Have you repaired any of these modules simply by replacing those diodes?

I don't see how the diodes could die since M1 probably can't source enough juice to kill them.  Secondly, I'd bet that one or both of those transistors has gone bad as well and taken out those diodes.

All 5 of the line amps I have are functional and seem to be working (at least audibly) properly.

I'm recapping the whole console and am also checking diodes as I go through. There are 3 1N4148s on each console channel on +24V supply traces, and the rate of failed or failing instances here are probably about 50%.

With the discovery of how many diodes are failing on the supply rails, I'm thinking it is probably worth replacing the diodes in the circuit in question. My ultimate goal with this thread is to make sure that I'm not doing to install a diode that has different specs than the originals, and damage the amps.

ok, understand now.  Since the early part of the discussion ended up with a schematic of base bias diodes, I thought we were talking about those dying specifically...

I'd leave those alone if you aren't finding any problems with them.

even old diodes were decent enough.  It sounds like something external killed those other supply rail diodes.

Is there a part # on them?

No part number on most of them. One amp module has 1N4148s in it.

bobschwenkler said:

Also, what are factors that contribute to diodes going bad? Are transistors susceptible to the same manner of failure (going short)?

Click to expand...

Common failure mode is short circuit from melting. This can be from too much current in forward direction, or reverse voltage breakdown, and again heating from current x full reverse voltage..

JR

Hm, interesting. Maybe just age as well? It doesn't seem like this many of these diodes are being subjected to that extreme of conditions. Many that have failed are powering one A-1000 op amp and that's it.

4148s are low current switching parts, not power rectifiers..

The small signal diodes  1n4148, 1n914, etc, can transiently handle around an amp, but only for very short transients, since their small die will quickly overheat.

JR

Svart said:

4148s are low current switching parts, not power rectifiers..

Click to expand...

Though I think as they are on these cards, I don't see that they could be drawing more than 50mA at the very high end, which is pretty well below their average forward current of 200mA or so I think?

They're just passing DC at the inputs to the channel strips, no rectification.

still, DC or AC or whatever, it's currents in time.  They might be able to stand an amp for 1us but might not be able to handle 500mA for 1s.  Who knows really, that's why they have ratings.

How much decoupling is on the cards in question?

Sometimes you might be getting start up or shutdown transients from charging the caps on the rails.  I've seen inrush currents blow diodes on boards even though operating currents were well within spec.

You could scope the area out and see if you have voltage transients, or do a math function on a dual channel scope and use a low value resistor to find the current transients if you really wanted to go that far.

Maybe it could be as simple as putting a 5 ohm resistor in series with the 24v to the boards and switch those 4148 to something like a 4007.  If it's DC there is no use in using a 4148 over anything else really.

Decoupling is either 47 or 100 uF behind each diode on these channels.

So am I hearing that diodes being run well within their limits should have a better chance at life than the specimens I've got on hand (~50% failed or failing at 40 yrs old)?

well anything being run within it's spec will run forever(practically). 

The diodes are going bad for some reason.  They don't just wear down due to age.  Something is killing them.  The culprits are as John mentioned before, overvoltage in forward biasing, overvoltaging reverse biasing, overcurrents, etc.  This could be from a lightning strike, static, bad ripple on the DC rails, who knows but whatever it is has to have been one of those killer conditions.

Sometimes parts can be hit with something but only partially fail.  They will assuredly fail with time though.  Maybe this console got hit with static from a lightning storm and the diodes are finally failing when you get a line spike when the power company kicks on the backup generators..

So either you spend a lot of time and energy tracking this down or you can try higher voltage and current diodes and hope that nothing else breaks.. 

Ok, I see. I'd thought that they maybe just have a life span, sort of like electrolytics. Thanks. I'll look a bit further into it at some point.

Those are the bias diodes. Replace them with 1N4001, etc.
If you can find them, a single MZ2361 will replace both diodes.
Had the same problem with Electrodyne line amps  about 25 years ago.