[GreenKeys] Flesher TU-170 Loop Key Transistor

Sat Oct 31 12:13:58 EDT 2020

Thanks to everyone for the comments. I've only started looking at this. I
did not notice C40, which is not on the PCB, and did not notice the
internal loop supply!

Since C40 would be directly across the selector magnet when the keyboard
is closed (idle), I suspect C40 is more for suppressing keying noise from
Q6 getting into the receiver. The selector magnet and C40 will form a
parallel resonant circuit with the only loss being the selector magnet
resistance. I SUSPECT there will be quite a bit of ringing when Q6 turns
off. I'll have a look at it on a scope when I get the TU-170. I think C40
would limit the peak voltage to twice the loop voltage. The voltage table
shows the collector of Q6 at 79 V, so twice that is considerably below the
300 V rating of Q6. It will be interesting to see what the ringing looks
like.

The manual does not say what the loop supply voltage is, but the voltage
table shows 79 V on the collector of Q6. From my measurements of the model
15 show 2.3 V across the selector magnets when there is 60 mA loop current
(selector magnet resistance of 38 ohms). Of course, the resistance will
vary depending on whether the selector magnets are pulling (series) or
holding (parallel). But, in any case, it appears the loop supply is about
79 V + 2.3 V or about 81 V.

Also, I see that Q6 is configured as a current source instead of a
saturating switch, so no external current limit resistor is needed. The
dissipation in Q6 is about 4.1 watts.

Again, thanks for all the info! I look forward to receiving the TU-170. I
have to do some interface circuitry to interface it to my SEA245 radio (
https://w6iwi.org/SEA245/ ).

Thanks!

Harold
https://w6iwi.org

> Looks like C40 & R58 + R59 constitute an RC snubber across Q6 (MJE340)
> collector & emitter.
>
> WayneWB4OGM
> -----Original Message-----
> From: Harold Hallikainen <harold at hallikainen.com>
> To: w9ddd at tapr.org <w9ddd at tapr.org>
> Cc: hwhall at compuserve.com; harold at mai.hallikainen.org
> <harold at mai.hallikainen.org>; greenkeys at mailman.qth.net
> <greenkeys at mailman.qth.net>
> Sent: Fri, Oct 30, 2020 10:22 pm
> Subject: Re: [GreenKeys] Flesher TU-170 Loop Key Transistor
>
> Did not realize there was an internal loop supply! Missed that as I was
> reviewing the manual. I am temped to put a clamping zener across the
> printer connection to the TU-170.
>
> Thanks!
>
> Harold
>
>> I see you mention the 152590 loop supply, you realize that the TU-170
>> has
>> an internal loop supply?
>>
>> I'd better shut up, half asleep and making mistakes.
>>
>> John, W9DDD
>>
>>
>>> On Oct 30, 2020, at 11:02 PM, hwhall at compuserve.com wrote:
>>>
>>> I've built keyers using that transistor and always had an RC snubber
>>> across them.
>>>
>>> Wayne
>>> WB4OGM
>>>
>>> -----Original Message-----
>>> From: Harold Hallikainen <harold at mai.hallikainen.org>
>>> To: Green Keys <greenkeys at mailman.qth.net>
>>> Sent: Fri, Oct 30, 2020 8:20 pm
>>> Subject: [GreenKeys] Flesher TU-170 Loop Key Transistor
>>>
>>>
>>> I'm looking at the manual for the TU-170 and see that it keys the loop
>>> with an MJE340 which has a maximum Vce of 300 V. 152950 loop supply has
>>> a
>>> loaded output voltage of 125 V. Can the MJE340 take the inductive spike
>>> when it opens the loop, or should a clamp be added?
>>>
>>> Thanks!
>>>
>>> Harold
>>>
>>>
>>>
>>> --
>>> FCC Rules Updated Daily at http://www.hallikainen.com
>>> Not sent from an iPhone.
>>>
>>>
>>>
>>> --
>>> http://w6iwi.org
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>>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
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>>
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>
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>

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