UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Amateur and Military Radio

Notices

Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

Closed Thread
 
Thread Tools
Old 15th Apr 2017, 3:06 pm   #1
G4_Pete
Hexode
 
Join Date: Feb 2014
Location: Southampton, Hampshire, UK.
Posts: 419
Default Scanti TRP1 psu problem

Hi , Todays conundrum.
I have a Scanti TRP 1 Survival radio that is set up on top band and works fine with a few watts out for our local Net but a few days ago the receive side developed squawks and birdies.

I eventually traced this to a 7.5 Mhz 100 mV p/p ripple on the 5 V psu. This is an unusual (to me) psu in that it is both linear and switch mode , it switches to linear for quiet receive and switch mode for TX with a higher current drain. However in switch mode operation it was fine the ripple only appeared in the linear mode. 5V regulation remained good so basic voltage feedback and control was working. This was a solid oscillation which started as soon as the main voltage supply was wound up even before the 5 Volt point was reached. I would have thought if this was gradual degradation there might be times when the oscillation would not start but this was not the case.

I reduced the psu to basic linear by shorting the inductor ( It is shorted by a wafer switch in this mode on the radio) and switcher input and replaced the load with simple resistors it worked fine at low currents of 10 or so mA but from that level up to my max test of 250 mA the 7.5 Mhz oscillation was present just as when operating to design loads.
I replaced the C3 470 uFelectrolytic with a newish Low ESR type and although this reduced the ripple it did not fix it. Increasing the frequency compensation C2 330 pF had little effect I even increased it up to 10 nF and all that did was make the oscillation go in short bursts. I removed and tested the transistors and they were appeared fine. The oscillation was sensitive to the base of TR1 being probed and very lightly at that. I found that putting 22pF from collector to base of TR1 instantly quenched the oscillation. I left this fix in place.

I have also re soldered the joints and checked for cracks but none found I thought the heat might encourage someting to fail completly but absolutly no change.

So the question is what might have suddenly changed and can I now trust the LM723 CN? That is trust it more than a modern rip off LM723 !! . I have now put in several 5 Volt rated transorbs and fusing on the 5 Volt rails just in case.

Any comments.
Attached Thumbnails
Click image for larger version

Name:	Scanti_PSU_5V.jpg
Views:	78
Size:	47.5 KB
ID:	141025  

Last edited by G4_Pete; 15th Apr 2017 at 3:12 pm. Reason: Inductor short clarification added.
G4_Pete is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 10:05 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.