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Hello Joe,

my description of the relay box was a bit vague. It's what Jim mentioned:

? "A less drastic solution that I have considered is adding a low voltage circuit to the PS7 to sense the TR7 power switch and turn on mains power to that big transformer with a relay."

This is in fact what I did. I use 12 Volts that are fed into the TR7 through the contacts that were used for the mains voltage before. When the switch of the TR7 is closed, the 12 volts power a little relay which in turn switches the mains voltage feeding the PS7. Sounds a bit complicated but is very simple in fact. Nice little project for a rainy Saturday. The only disadvantage is that the 12 volts have to be there (by e. g. the usual shack power supply or a wall wart). And you have to use a 12 volts DC fan because the FA-7 would require the mains voltage.

The good things are:

- no mains in the TR7 -> safety
- no hum in the TR7 -> better audio
- no damaged TR7 switches by high inrush-current of the PS7 anymore.

My TR7 and R7 are both without mains inside. The R7 runs soooo hot when used with the internal transformer. So I use externally fed 12 volts, too.

73, Dieter DL5RDO

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Guten Tag, Dieter:

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?????????? By “relay box”, do you mean a remote power switch?

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Danke Ihnen und 73,

joe

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joseph n. graif

k4jng

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From: [email protected] <[email protected]> On Behalf Of Dieter Horst
Sent: Tuesday, November 28, 2023 7:55 AM
To: [email protected]
Subject: Re: [DRAKE-RADIO] TR7 low band TX oscillation

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Hi Patrick,

The culprit is the PS7 connector which brings mains voltage in your radio including hum you won’t get rid of.?

So it is not the PSU itself that brings the hum in your radio.?

I use a little relay box to switch on my PS7.?

So leave your electrolytics, just get the mains out of your radio and you‘re done.?

73 de Dieter DL5RDO

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Von meinem iPhone gesendet

Am 28.11.2023 um 07:00 schrieb Patrick TK5EP <pegloff@...>:

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Hi Jim,

After a lot of tests, PSU swaps, etc... I came to the conclusion that this TR7 is VERY VERY sensitive to any noise, AC spurs, hum that are present on the 13,5V supply voltage.

Even is powered with a external PSU but leaving the PS7 control connector connected, there is a hum on the signals.

All signals (RF, audio) are affected with these noises, sometimes it is very noticeable on the receiver audio.

The scope on the loudspeaker shows that on the 50 Hz main is wobbling the AF signals.

Even with a "clean" PS7 output, it is noticeable.?

The first thing I'm thinking about is the VCO board. I will order all caps and change them.

Has anyone already treated this problem ?

Thanks and 73,
Patrick

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