Hi

A lot depends on the circuit and the application. There are “audio” transistors out there with Ft’s below 200 MHz. The gotcah is that they most certainly will not have Hfe’s down around 20 or so. If you get into replacing germanium parts, things get even more insane. 

Bob

On Dec 22, 2023, at 1:25 PM, Jim Whartenby via Milsurplus <[email protected]> wrote:

Bob
The devices I am replacing have an fT of 5mc while just about any modern device has fTs over 200mc.

Been working on a 4kc Spectrum Generator which is one of the KWT-6/URC-32 SMO circuit boards.  I had already replaced Q1 since it had excessive emitter current.  The 2N264 I installed was one with a hFE of 40.  The normal input signal to the circuit board was enough to turn the device hard on since the measured collector voltage was less then 2 volts.  Replacing the 2N264 with one with a hFE of 23 brought the collector back to 28 volts with a decent rail to rail square wave signal.  The 4kc Spectrum Generator is now working nominally.  

I guess I could modify the 4kcSG so that voltage divider bias and an emitter resistor would tame the gain to the point where a higher hFE device will work OK.  Adding components to the circuit board will be a challenge especially since the board is conformal coated.  Not to mention that added components will have "flying" leads which may affect reliability.  The circuit boards are a tight fit into the SMO assembly so any mods need to lie low on the circuit board.

The SMO now locks at a few frequencies so I am making progress.  The inputs to the Reference IF circuit board where the frequency discriminator lives are nominal so either this circuit board has an issue or the Permeability Tuned Oscillator is so frequency unstable that a lock cannot be reliably made at most frequencies.  I have three "spare" PTOs but all have various problems.

More troubleshooting is needed.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy


On Friday, December 22, 2023 at 09:58:39 AM CST, Bob kb8tq <[email protected]> wrote:


Hi

If we’re talking about radios, HFE probably is not the number to focus on. Gain bandwidth would likely be the bigger issue. Not enough and you have no signal. To much and the stage goes into oscillation. The gotcha here is that the spec sheet numbers may not exist or if they do exist they may be misleading. Still, a better area to focus on since HFE and Ft (however you measure it) are not directly linked to each other.

Bob

> On Dec 22, 2023, at 10:17 AM, [email protected] wrote:
>
> Jim,
>
> I saw this add today in Electronic Goldmine. Have you tried 2N3903 the lower gain version of the popular 2N3904?
>
> <https://theelectronicgoldmine.com/products/g26902?mc_cid=a7184dc10f&mc_eid=9999d432a7>
>
> 73 Mike N2MS
>
>> On 12/21/2023 11:45 AM EST Jim Whartenby via Milsurplus <[email protected]> wrote:
>>
>>
>> Ray
>> The TC-1 found on eBay, see enclosure. They can be had for under $20.00 with free shipping. According to YouTube, there are clones out there so examples with the test leads and charger cord appear to be original units. YMMV
>>
>> I am working on KWT-6 SMO modules, mostly scrapped circuit boards from SMOs with mechanical problems. I guess that they were stored outside since they have frozen bearings which I don't care to replace. You can't save everything although I try my best!
>>
>> I have been successful using a TI house numbered device found at a Missouri Hamfest years ago. They are TO-18 devices that are just useable with their 1/2 inch long leads. They range in current gain from 20 to 80. So far, devices in the 20 to high 30s range seem to work well. It is hard to find modern transistors with HFEs that low. I have ordered 200 each of the 2N4400 devices (datasheet enclosed) as a backup and will try sorting them with the TC-1 for HFE and emitter current.
>> Jim
>
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