Experience with the construction of an electronic repeater with an ISD circuit – modifications

In Rádiožurnál 2/1998 described OK1VEY and OK1UKE electronic repeater with ISD circuit according to Bojan Debejak S57BBD. For those, which magazine they do not own, I would recommend visiting the website of Jarda OK1VUP www.qsl.net/ok1vup, where is the scheme with Jard's modifications.

Repeater, alebo voice keyer, it is a very useful device for SSB contestmen and it is quite interesting. It didn't escape my attention either, and after getting one ISD circuit, I built the circuit. It works, but there is an annoying hum in the record, which no and not remove. Four radio amateurs responded to a request for help via the radio packet network – Jarda OK1VUP, Roman OK2CRC, Ivan OK1MMN and Luděk OK2IEN. I would also like to thank them for their valuable advice in solving this problem.
It will be best to start with their advice:

 Greetings abroad..hi If one is the same as I think , i.e.. connected to ISDxxxx and microprocessor.. atmel.. (connected according to OK1UGA), so that hum is not a problem with the ISD circuit, I just had to unplug the shielded cable with the electret microphone and there was no hum in that wound. In addition, its intensity varied depending on different angles.. the only answer is interference with the atmel or the 555 involved.. If I know, use ISD 2560 (8KHz/60 sec) it works flawlessly ... and no hum itself in the recorded signal..                                    Regards, Roman, OK2CRC Hi DR OM, I built a crowd with ISD 1020 a 1016. I solved the same problem. I spent a few days on it. It is necessary to separate analog and digital ground. In practice, I did it that way, that I used stabilization 2x 7805 for analog and digital part separately.  Everything runs without the slightest problems to full satisfaction. I do not have the manufacturer's recommended connection in front of me, but the circuit diagram of the IC with the description of the outlet shows where these two power supplies are. In the recommended connection, the two VCCs are connected, which of course is wrong. This is also corrected by the ISD manufacturer's add-on in the recommended connection.  If you still don't know what to do, I could look at the data sheet and tell you exactly which legs they are.  73 Ludek OK2IEN DR OM. I made something similar too, however, own construction, "digital" hum is caused by sampling and cannot be removed, I finally solved it with a language relay connected to the vox circuit (also own construction), which switched the output only if there was some useful signal masking the hum on it, surprisingly, it went quite well and the spin was not heard in the signal (I mean, that there was some resistance instead of an output), problem byl spis s voxem, to take over immediately and have the right time constant for the finish.. Now I'm using a sound card. I still have this VOX KEY and I use it sometimes.  73 ivan ok1mmn Hi, I also recently started building a repeater according to Ra 2/98 and with the same result - brum. I don't know the changes you write about, but I solved it by separating the land - analog and digital separately with the help of chokes. the power supply remained together. I have new web content on my qsl.net website (then under ok1vup.htm and not under index.htm, where I describe it in the technology section. Take a look at http://www.qsl.net/ok1vup/technika/opakovac/opakovac.htm, where the circuit diagram is already modified. It is true, already in solitary perception the hum is not published, but as soon as I connect it to TCVR, so it appears a little (sle⁇ weak) - they then work on it, he will probably want to separate the power supply as well. 73! Jarda OKVUP

I was the first to try the adjustment according to SPRAT No. 78 (jar 94). According to her, the ISD terminals No. 17 and 18 (microphone inputs) dynamic microphone connected. It would be galvanically separated from the ground. However, the result was bad…

The second modification modified the connection of the electret microphone according to AR-A 12/94. Also, the microphone jack is not connected directly to ground in this connection. The result is also bad…

From the information, which I got I tried two. As the first adjustment by Jard OK1VUP, which required only minor modifications to the printed circuit board. The improvement was noticeable, however, not enough. It would require longer experimentation, searching for a suitable choke, etc..

When I tried Luděk's modification with two stabilizers, I inadvertently connected the second stabilizer so, that it was powered from the first, that is, five volts. Approximately 3.8V was received at terminal No. 28 ISD – no hum disappeared. However, the record was significantly distorted. In the catalog sheet (click here to download in PDF format) there is no mention of the supply voltage range. According to an article in AR, this is it 4,5 up to 6.5V.

By further experimentation, I finally reached the state, with which I am satisfied. I left the only stabilizer, taking digital time ISD (outlet no.28) I supply it via a germanium diode GA203. It is soldered from the bottom of the printed circuit board below the circuit, where I scratched the joint. After reducing the voltage by a few tenths of a volt, the deterioration of the recording quality is minimal, but there is no sign of hum.

In addition, I have only made one adjustment in the timer circuit. I reduced resistor R1 from 47k to 22k and increased capacitor C2 from 2.2uF to 22uF. This way, the space before repeating the recording can be set from 1,2 po 6,7 seconds, which seems sufficient to me.

Overall, I rate the involvement as very successful, the printed circuit board is designed very clearly and without a single mistake. I can warmly recommend it to others after making minor adjustments.

Literature:
[1] Electronic repeater with ISD circuit, Bojan Debeljak S57BBD, translation of OK1VEY and OK1UKE, On the radio 2/1998
[2] SPRAT No.74 (jar 1994), Analogue Solid stane speech recorder, Paul Lovell G3YMP
[3] G102 digital audio memory, G103, Jindřich Tolg and Ing. Tomáš Tolg, Amateur Radio 12/1994
[4] http://www.qsl.net/ok1vup/technika/opakovac/opakovac.htm
[5] Catalog ISD14xx