Hello.
I have an old Heathkit 5-watt monaural FM receiver, the AR-27. I need to replace the diode that isolates the two output transistors (schematic attached).
The original one was a small "can" type; both leads come out of the same side. It was small and mounted to the same heat sink as the output transistors.
But I can't find diodes of that type. Can I just use any old regular silicon diode? What amperage would I need it to have? etc. etc.
Any help will be appreciated!
Thanks,
Bob H.
Whatever diode (or VBE multiplier circuit) you use, don't fail to heed your own words: that thing is mounted to the heatsink for a reason. The temp of that diode has to match the transistors to keep the bias current correct. If not, it will screw up the tracking and mess up a bunch of things, some of which might be disasterous.The original one was a small "can" type; both leads come out of the same side. It was small and mounted to the same heat sink as the output transistors.
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The translation is, I don't think the diode's VBE is super critical because this is definitely not a precision circuit and the total of resistive degeneration in both chains would make it somewhat forgiving of VBE mismatch. I'd still put the diode on the heatsink.
I don't think the diode is super critical because the circuit uses a single diode VBE and a series 4.7 Ohm resistor "matched" against two transistor VBEs and a couple of small 0.5 Ohm resistors in their emitters. If you read the schematic, the voltages at the nodes are there. I believe the diode in question and the series 4.7 resistor have 1.2V across them total. That means the resistor has about 0.5V across it. The output transisitors also have 0.5 Ohm emitter resistors to provide degeneration. That's why I think the diode vBE is not that critical, but you could increase the 0.5 Ohm resistors or adjust the 4.7 ohm resistor to dial the diode in:Like I said above, using the exact correct diode is absolutely critical, overwise thermal runaway is almost guaranteed. I repaired a LOT of these types of poor designs back in their day, if you didn't use the correct diode you just trashed it again.
I fully agree on mounting the diode on the heatsink though.
I suspect I know the failure mode you describe: if the diode was such that the VBE was a lot bigger than nominal, it would increase the output idling current (heating the transistors) and could cause thermal runaway, especially if pushed hard. I believe if the external resistors are adjusted to force the output current to the correct value, that won't happen. Just my opinion.
If you're redesigning the amplifier to that extent, why not do it properly and just stick a Vbe multiplier in?.
Like tunedwolf says, those old amplifiers were a real pain, you had to stock the exact correct diode for every single amplifier you repaired.
I am not sure I understand. A VBE multiplier would have a little different characteristics than a discrete diode. If the point is to match the original, that would seem to be more risky to me. It's been a while, but I seem to recall VBE multipliers work best when they are set up to create at least two or three VBEs worth of voltage across the VCE of the transistor, so it will be in it's linear region. In this case, you only want one or a little over one VBE, I think that puts the transistor pretty close to being in saturation which changes the current draw of the circuit. Anyway, that was an interview question once so it's what I remember. If you push the VBE multipliers down towards sat, they don't act like VBEs down there.
As I said, i can't follow your reasoning where you said I should not redesign it by changing the 4.7 resistor, since putting in a 2VBE circuit will require that as well. The initail design uses a single diode and a 4.7 Ohm resistor to force the voltage across the two output trans VBE's. I certainly agree a BETTER design would be to have two matching diodes (or diode connected transistors) in the left chain to set the bias, especially since that is the design I use when I build a discrete output stage.You actually need a little over 2 x Vbe, in order to turn both transistors slightly on. It's a perfectly normal and common method, and a transistor tracks the transistors far better than a diode does.
You can certainly vary the resistor to set the quiescent current, but it still doesn't track correctly, the diodes used are VERY specific and specialised.
That's a surprising value. I thought the output idling current would be higher? Based on schematic, I think the current flowing down the diode chain is supposed to be about 100mA. I would have guessed the output current would be in the same ballpark. Is there a reason it's so low?It really depends on the OP's objective. If it's authenticity, then he needs a diode identical to the original. Maybe there are markings that will help us find one.
If he wants almost authentic, but is willing to tolerate poor reliability, then a heatsink-mounted diode with a proper adjustment to R128 might meet his objective. Set R128 so that the output stage current is 5-10ma (<10mV from the top of R129 to the bottom of R130) with no audio.
That's a surprising value. I thought the output idling current would be higher? Based on schematic, I think the current flowing down the diode chain is supposed to be about 100mA. I would have guessed the output current would be in the same ballpark. Is there a reason it's so low?
OP hasn't stated whether he wants to improve the amplifier or preserve it.easiest technique is to fit a Vbe multiplier
100mA is FAR too high for such a small amplifier - easiest technique is to fit a Vbe multiplier (like I've said all along). Set the bias to zero with the preset, then input a VERY low level sinewave and monitor the output with a scope. Adjust the preset until the crossover distortion just disappears, then check the actual current (it will be a LOT less than 100mA). Leave the amp to warm up, and keep checking it.
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