Pioneer SX-990 FM radio issue - signal strength too low - Page 1
Forum section: electronics / repair

author: nuno
author_rank: Frequent Contributor
timestamp: November 17, 2020, 03:34:31 pm
content: Hello all,Trying to fix this vintage (~1972 I think) radio and amp and in need of some help, never dealt with radio stage stuff before, but have the concepts. Any pointers are appreciated.Radio seems to have low sensitivity, the 3 or 4 strongest FM stations have a high volume (what I would classify as "normal"), others are audible but much lower volume ("good" audio quality, though).Tuning meter looks ok, although it doesn't deflect full-scale (some 30% of scale on the strongest stations).Signal strength meter barely moves, even on the strongest stations. Never goes into stereo.The MUT signal from the IF stage enables/disables the MPX ("stereo decoder"); I cut it and forced the MPX to be enable, and we have stereo.So the issue is on the signal strength (MET1) and MUT out from the IF board. The meters are mechanical needle meters. I took the measurements in the figure in attach. From the measurements it looks like there's a problem (broken or out of tune) with T3 transformer / filter... what do you think? The diodes D2 - D4 seem ok on in-circuit measurement, and the signal is rectified. On measuring T3's pin 4 I know the scope probe lowers the amplitude because the sound volume goes a little down (well, scope probe capacitance is same order of magnitude as 3pF cap...). The MUT pin 5 output is at ~160mVdc level on the conditions of the attached image.The IF-sine I refer to in the image is the 10.7MHz IF sine.Thanks!

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 17, 2020, 04:40:51 pm
content: The problem of lack of sensitivity as you can see often comes from a defective FM front end.The fet 2SK15 is in fact more exposed than other semiconductors to electric discharges picked up by the antenna.Are you sure that what is faulty is the FM IF unit and not the FM front unit?

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 17, 2020, 04:53:37 pm
content: If you are measuring strong station with 5V pp IF on output of Q4 to the detector,does that not mean the RF front end and IF train is good?Another 2 dB and it will be clipping.Typically there was no AGC on these old FM broadcast receivers and I don't see it on your circuit.

author: nuno
author_rank: Frequent Contributor
timestamp: November 17, 2020, 05:43:02 pm
content: I suspect the transformer (T3) because the signal amplitude after it is much lower than the amplitude before. But I have no sensibility for this kind of circuit, I don't know what I should be reading there. The MUT and MET1 outputs have loads. the signal there is definitely too low because the meter barely moves and the signal on the MUT output is unable to enable the MPX circuit.The front end is attached. I'm using 2 pieces of wire as antenna, but I live 20 - 30Km from the transmitter, my experience with radios is that I just need a piece of wire to receive the same stations as any other radio.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 17, 2020, 05:57:14 pm
content: If you suspect T3, replace it with another transformer removed from another SX 990.These kinds of spare parts are no longer available and you will need another "for parts" SX 990 to cannabilize it.What often happens in vintage repairs is to make one device working from two broken ones.

author: GerritMax
author_rank: Contributor
timestamp: November 17, 2020, 08:38:05 pm
content: I would say start looking at thishttps://www.youtube.com/c/xraytonyb/videos.He does a lot of tuners and receivers among other things.

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 17, 2020, 09:41:49 pm
content: Older receivers often benefit from an alignment. Easy to do at least a passable alignment on a broadcast station using a scope.Simply max out the IF envelope using a weak signal.Better quality alignment does require specific test equipment which we don't all have.Personally I use a Sound Technology 1000A for aligning analog FM tuners.I would not assume a component failure without trying alignment first. A 50 year old receiver will definitely have drifted.

author: nuno
author_rank: Frequent Contributor
timestamp: November 18, 2020, 05:36:04 am
content: Quote from: akimpowerscr on November 17, 2020, 05:57:14 pmWell, if that's the case, it won't happen. I'll have to leave it as is.

author: nuno
author_rank: Frequent Contributor
timestamp: November 18, 2020, 05:41:09 am
content: Quote from: mzacharias on November 17, 2020, 09:41:49 pmSo I just scope the IF Vpp after the "offending" transformer and tune its screw for maximum Vpp, right? I'm a bit reluntant on doing this because I'm afraid to leave it worse than it is. This radio is not mine and I wouldn't like to return it in a worse condition than I got it and "gain responsability".

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 18, 2020, 09:24:11 am
content: The biggest problem is trying to do a reliable diagnosis without having a sweep generator.It would be enough to check the sensitivity and the response curve of the FM IF unit as indicated in the service manual to know if this unit is faulty or not.The localization of the fault in the FM IF unit can be done by checking the influence of the transformer cores on the response curve.Note that when an IF transformer is defective, it is generally the internal capacitors of these transformers which have lost their insulation (leakage current).

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 18, 2020, 06:35:02 pm
content: Quote from: nuno on November 18, 2020, 05:41:09 amBest to find a low impedance node (like the emitter resistor of a transistor) to which you attach your x10 probe, if there's enough envelope there to look at.Or even at the output of a crystal filter, if any. Adjust for maximum IF amplitude with a small signal strength. It won't be perfect but close enough to tell if there's room for improvement.With the service manual you should be able to tell which transformers to adjust for maximum at the lower end of the scale, and which trimmer caps to adjust at the higher end (while looking at your signal strength meter).Be aware that maximum signal strength with everything maxxed out may result in slightly higher distortion, but should not be high enough to be a problem.

author: nuno
author_rank: Frequent Contributor
timestamp: November 19, 2020, 01:59:31 pm
content: Ok, thank you all. I decided it's best to leave the radio section for someone else with more experience than I.Cheers!

author: jh15
author_rank: Frequent Contributor
timestamp: November 19, 2020, 02:58:21 pm
content: I replaced lots of those ua703's ESPECIALLY the black plastic versions.First was encountered in a car radio, 1971 or so. Spent eons trying to find why the stereo wasn't decoding although the signal was full quieting. Later, I would replace all the 703's easy and cheap. The black ones failed even when new, I replaced many under factory warranty.(also have a sound technology 1000A bought new).

author: nuno
author_rank: Frequent Contributor
timestamp: November 19, 2020, 03:35:59 pm
content: These are the metal can version. Actually there are versions of the SX-990 with another similar chip, TA7060 (plastic).