EMRFD Message Archive 5410

Message Date From Subject
5410 2010-11-11 12:43:55 k9jri@att.net Motorola MRF-261 transistor questions
I have all three of the Small Wonder Labs PSK-xx series radios. These radios have all given mostly trouble free service until my latest build, the PSK-40.

All three radios are electrically identical other than the frequency determining components. The PSK-20 and PSK-30 have a clean PEP output of about 3 watts but the PSK-40 would output anywhere from 5- to 7 watts and would appear to be slightly unstable when the PSK-31 audio drive was started and again when the transmission ended. e.g. If the power had been adjusted to 3 watts PEP, with PSK-31 audio drive, there would be a power peak of 5 - 7 watts when the modulating signal was applied and then again when it was removed. The TX/RX circuitry is VOX operated from the PSK-31 modulating audio input signal.

After about 1 week the MRF-261 destroyed itself so I replaced it with an older MRF-261 that had been pulled from a previous project. The leads had been cut so I had to splice the leads to re-install it. This device worked perfectly (3 watts PEP with no power spikes) until a replacement device arrived from SWL. SInce the relacement did not look too good, with the spliced leads, I pulled it back out and installed the new device. The new device again produced transient power bursts or 6-7 watts and much higher sustained power output than the older device produced or than from the other two PSK-xx rigs.

I have another MRF-261 in hand, from SWL, but I am reluctant to install it without some plan to find the problem. Dave (SWL - K1SWL) was not of any trouble shooting help.... suggesting that I look at my sound card settings and probably having not received any other similar reports.

The QUESTION that I am leading up to is: Is anyone aware of a current problem with MRF-261 transitors or have counterfeits appeared on the market which have much higher gains than the original device?

Any other suggestions for me to look at? I have compared all three kits and all three seem to have been constructed correctly and then only real difference I see is that the PSK-40, with the new MRF-261, puts out far more power than the other two with similar drive levels.

73 - Mike - K9JRI
5411 2010-11-11 13:24:23 Thomas S. Knutsen Re: Motorola MRF-261 transistor questions
I'm afraid that it sounds like your MRF-261 may be counterfeits, they may
be some other device that is renamed. Could you provide an high resolution
close up photo of the transistor?
Measuring HFE, and comparing to other, known orginal devices usualy does say
somthing about the transistor without the need to send it to the producer.

Only shopping from approved vendors make most problems with counterfeit
parts dissapear, unfortunatly there is an limit of approved vendors and most
of them don't want to deal with small amounts.

73 de Thomas.

2010/11/11 k9jri@att.net <k9jri@mac.com>

>
>
> I have all three of the Small Wonder Labs PSK-xx series radios. These
> radios have all given mostly trouble free service until my latest build, the
> PSK-40.
>
> All three radios are electrically identical other than the frequency
> determining components. The PSK-20 and PSK-30 have a clean PEP output of
> about 3 watts but the PSK-40 would output anywhere from 5- to 7 watts and
> would appear to be slightly unstable when the PSK-31 audio drive was started
> and again when the transmission ended. e.g. If the power had been adjusted
> to 3 watts PEP, with PSK-31 audio drive, there would be a power peak of 5 -
> 7 watts when the modulating signal was applied and then again when it was
> removed. The TX/RX circuitry is VOX operated from the PSK-31 modulating
> audio input signal.
>
> After about 1 week the MRF-261 destroyed itself so I replaced it with an
> older MRF-261 that had been pulled from a previous project. The leads had
> been cut so I had to splice the leads to re-install it. This device worked
> perfectly (3 watts PEP with no power spikes) until a replacement device
> arrived from SWL. SInce the relacement did not look too good, with the
> spliced leads, I pulled it back out and installed the new device. The new
> device again produced transient power bursts or 6-7 watts and much higher
> sustained power output than the older device produced or than from the other
> two PSK-xx rigs.
>
> I have another MRF-261 in hand, from SWL, but I am reluctant to install it
> without some plan to find the problem. Dave (SWL - K1SWL) was not of any
> trouble shooting help.... suggesting that I look at my sound card settings
> and probably having not received any other similar reports.
>
> The QUESTION that I am leading up to is: Is anyone aware of a current
> problem with MRF-261 transitors or have counterfeits appeared on the market
> which have much higher gains than the original device?
>
> Any other suggestions for me to look at? I have compared all three kits and
> all three seem to have been constructed correctly and then only real
> difference I see is that the PSK-40, with the new MRF-261, puts out far more
> power than the other two with similar drive levels.
>
> 73 - Mike - K9JRI
>
>
>



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5413 2010-11-11 15:47:12 Dave - WB6DHW Re: Motorola MRF-261 transistor questions
Mike:
Sounds like the transistor may be oscillating, possibly at some other frequency.

Dave - WB6DHW
<http://wb6dhw.com>

5414 2010-11-11 16:14:06 k9jri@att.net Re: Motorola MRF-261 transistor questions
Dave, That is my suspicion too but it happen so quickly I cannot look for it. I do not have a spectrum analyzer available to me. When driven to 3 watts PEP the SWR into a very frequency selective load is quite low so it would appear to only be delivering significant RF within the band the load is tuned to.

My GUESS!!! is that when the vox operates after seeing sufficient audio drive the VOX circuit turns on the + voltage to the SA-612 mixer, the MAR1 amplifier, and a 2SC1970 driver. The + voltage is always applied to the MRF-261 final. In that brief interval when the VOX is turning the exciter stage(s) voltage on and off there is a brief oscillati
5415 2010-11-11 16:19:56 k9jri@att.net Re: Motorola MRF-261 transistor questions
I do not have a suitable camera but all five of the recent devices look identical under a 10X loupe. They are all the same color, shape and marked identically "MFR261 and the Motorola "M" in a circle followed by 8735. Only the older pull looks different in that the plastic is a different color, it does say MRF261 but the Motorola "M" in NOT in a circle and it is followed with the numbers 329.

I am beginning to suspect a problem in a driver stage and the old transistor is not being overdriven as it is low on gain from previous abuse (just guessing on this mind you).

My other response may shed more light
5416 2010-11-12 08:56:04 k9jri@att.net Re: Motorola MRF-261 transistor questions
Problem fixed, I believe :)

There is a double tuned circuit made up of two shielded 10.7 MHz. IF cans that feed the driver stage. The transformers have appropriate shunt caps. to tune to the 40M band. I had failed to solder the grounding tabs on the first of the two transformers. Apparently the tabs sticking through the plated through holes would intermittently make contact (when it worked as expected) and the not (when it was unstable). After soldering the two IF can grounding tabs it appears to be completely stable.

This would be consistent with the driver stage having its collector voltage turned