uA703 replcement module - Page 1
Forum section: electronics / projects

author: HMartins
author_rank: Newbie
timestamp: November 17, 2020, 12:32:20 pm
content: Hi.I have a Pioneer SX-990 to repair. It seems to have a faulty IF amplifier.The diagram shows two IF amplifier versions, one with 2SC460 and the other with TA7060P but in fact LM703 is installed.These are Jurassic ICs. Looks like uA703 and NTE703 are direct replacements for LM703 but none of them basically not available (I live in Portugal). Looks like some NTE703 are available in USA but EU does not make things easy. The are some from China but I suppose they are probably quite used and reused …OK. I am trying to make a small module to simulate this 703.I draw the circuit and simulate it (PSpice) and it seems to work. Nevertheless I can not understand Q3 and Q4. I suppose Q4 behaves like a constant current somehow controlled by Q3 but these are just my suspicions. Q3 B to C shunt makes me nervous and there is no resistor on Q4. I selected 1N4148 just because I have many of them.Now, the transistors are MMBT3904, SMD versions of 2N3904. Not sure if this type is suitable. Seems to work in simulation, God knows how.I will draw the PCB so that I can have several versions, TO-78-6, 8 pin DIL, 6 pin DIL, 6 pin SIL, 5 pin SIL, … looks like there are all sort of versions for the same matter.After all this trouble, to avoid RF feedback I don’t know if I should provide some screening for the modules. If so, I suspect I must do some extra hand work to get a proper screening…Could I have some opinion about this?Thank youH. Martins

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 17, 2020, 01:12:48 pm
content: Welcome to the forum.You ask us for our opinion on what you do.......Why look for problems where there are none?Not everything that comes from China is necessarily fake or faulty.There are LM703 DIL8 from China at 6 euros to sell on ebay, including shipping costs.Buy one and try it .....Attention, it is even possible that you are mistaken and that it is not the µA703 which is defective but the 10.7Mhz transformer which precedes it or which follows it.I already had this case in a pioneer receiver

author: David Hess
author_rank: Super Contributor
timestamp: November 19, 2020, 11:34:19 pm
content: There is an assumption in this design that Q3 is matched to Q4 and Q2 is matched to Q5.

author: TimFox
author_rank: Super Contributor
timestamp: November 20, 2020, 12:10:37 am
content: I believe that the transistors in the 703 IC were based on the 2N918 RF transistor, which has ft> 600 MHz. The 2N3904 has spec ft> 300 MHz.

author: floobydust
author_rank: Super Contributor
timestamp: November 20, 2020, 12:13:03 am
content: More details about the LM703 in National Semiconductor app. note AN-6 March 1968 "Tuned Circuit Design using Monolithic RF/IF Amplifiers". The LM171 is also discussed.App note list found here :http://www.projects.scorchingbay.nz/dokuwiki/electronic/datasheet/national/archiveorg-an

author: David Hess
author_rank: Super Contributor
timestamp: November 20, 2020, 12:39:26 am
content: Quote from: TimFox on November 20, 2020, 12:10:37 amThey probably are that fast but there is a good chance that discrete 2N3904s will be fast enough in this application.