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  1. Jon1982

    SX-1980 Intermittent Fault

    Just to give closure, I replaced the Q13 / Q15 pair and the problem hasn't recurred - been fine for a couple of months now so I think I've nailed it. If anyone else HAS to change TC0 on the FM Front end, a 3-10pF vario cap gave me the correct adjustment. IIRC the problems all started when I...
  2. Jon1982

    SX-1980 Intermittent Fault

    Got an issue whereby tuner will suddenly drop out - a station tuned to say 106mhz then re-appears at 104.5mhz. Just tapping TC0 with a plastic trim tool remedies it. I <think> at some point I absent mindedly adjusted TC0 when I was trying to follow the APC setup procedure... clumsy of me and...
  3. Jon1982

    SX-1980 Intermittent Fault

    Thank you Mark, I believe you may well be correct. I'll get some new trimmers on order. While I'm at it, does anyone know what value the 'TCO' trimmer should be on the FM front end? I definitely have an issue there
  4. Jon1982

    SX-1980 Intermittent Fault

    Thanks for the info regarding the auto reset. Might be getting warmer, taken a voltage reading at D6 which I believe to be a bias diode that sits on the heatsinks on flying leads, about -1.7V which would appear normal, when the 'thud' occurs this shoots all over the place up to 30 odd volts if...
  5. Jon1982

    SX-1980 Intermittent Fault

    Bit more background from a couple of months back which I forgot to add The amp was at the point where it would reliably cut out with the 'thud' after a few minutes (as it is again now) I thought I would check around the rectifier / filter caps, I found a couple of connections I wasn't happy...
  6. Jon1982

    SX-1980 Intermittent Fault

    Just checked and it looks like I replaced that cap when I did the recap a couple of years ago (for another electro) still worth swapping out for the film cap as you suggested?
  7. Jon1982

    SX-1980 Intermittent Fault

    Confirm 'thud' occurs with no input signal DC offset drifted up to about 35mV before thud Will change that cap, thanks
  8. Jon1982

    SX-1980 Intermittent Fault

    Re-soldered pretty much everything but to no avail. The 'thud' seems to be cyclical, about once per second, and seems to be accompanied with a loss of source signal, until there is a big enough 'thud' to cut the speaker protection in. Thought that may give a clue as to where I should look next...
  9. Jon1982

    SX-1980 Intermittent Fault

    Thanks very much for the suggestions guys The prob doesn't seem to be the molex connectors, given them a good shine up and problem still apparent. I don't remember changing any tx's on the power amp boards. I take it there's some which are failure prone? I rechecked the bias & DC offset as...
  10. Jon1982

    SX-1980 Intermittent Fault

    Wonder if one of the gurus could help me track down an intermittent fault on my 1980 which is somewhere in the output stages I have checked it is not to do with the pre-amp stage by using a separate power amp via the pre-outputs Unit has been re-capped, power supply mod done and has worked...
  11. Jon1982

    SX1980 Heatsink Grilles

    Zebulon, I'm glad I allowed you to indulge yourself :) I must say Idunno, that unit looks in fantastic condition. I couple of my grill slats have been pushed in, although I suppose I could take them off and straighten them. Although, if they're worth that much, I might take them off and sell...
  12. Jon1982

    SX1980 Heatsink Grilles

    Just me being curious, this one.. My 1980 has what I would describe as grilles which cover the heatsinks. Looking at pictures of other 1980's I'm struggling to find another one that has these. Its clearly factory, but I can't see them in the service manual. My unit is a European 220 / 240V...
  13. Jon1982

    SX1980 Filter Cap Replacement

    Thankyou for taking the time to reply zebulon, your endorsement of the chemicon caps is useful. I've ordered some 27000 uf / 100V ones which look to be the same diameter as original but a little shorter. Ordered from mouser although look to be on back-order. I read about your recent bereavement...
  14. Jon1982

    SX1980 Filter Cap Replacement

    Could also go for 160V / 27000 uF.
  15. Jon1982

    SX1980 Filter Cap Replacement

    Hi folks, Going to open the SX-1980 back up and change the filter caps, as my meter suggests they're starting to degrade, and they're the only electrolytics I haven't changed. I'd ideally like to keep the same can diameter (3"). I have found some Vishay caps 100V / 22000 uf as per original, I...
  16. Jon1982

    SX1980 Voltage Regulator / current source question

    I seem to have cured it! Q203 remotely mounted via flying leads and a bad solder joint onto the PCB. I'd never looked at it closely enough it seems. Also those following this saga will note I had similar problem with the -80V and I found a suspect solder joint there, too. So now with that taken...
  17. Jon1982

    SX1980 Voltage Regulator / current source question

    Might have found the problem here. There seems to be a bad connection to Q203 collector. Could this have the affect of making little Q201 do all the work?
  18. Jon1982

    SX1980 Voltage Regulator / current source question

    New voltage measurements: Input (supply side R202) 28.5V Q201 Base 14.9V Q202 Base 6.7V Q202 Emitter 6V Q203 Base 14.2V Q203 Collector 28.1V Q203 Emitter 13.4V
  19. Jon1982

    SX1980 Voltage Regulator / current source question

    Found a mistake - I had accidentally put a 24V zener in. I had both ammo reels on the table and must've grabbed the wrong one. The result I now get the correct voltage 13.5V at pin 13, but Q201 gets even hotter and starts to smoke after a few seconds, so I'm none the wiser as to finding the...
  20. Jon1982

    SX1980 Voltage Regulator / current source question

    Just taken a reading of 16V across the 6.2V zener, so I think I can safely say that has failed. Will substitute and see what happens
  21. Jon1982

    SX1980 Voltage Regulator / current source question

    Mark, I am sure no offence intended, and none taken, as of course you don't know me from Adam. As one of my comedy T-shirts will attest, I am not a complete idiot, some parts are missing. :banana: As requested: Q201,202,204 - All now KSC945. Which seems to be the same as 2SC945A, except base &...
  22. Jon1982

    SX1980 Voltage Regulator / current source question

    I have removed all real loads, and placed a 100 ohm resistor between pins 13 (13.5V) & 10 (Gnd). It tries to regulate but the voltage keeps wandering up and then sharply dropping down in a peculiar manner. Q201 gets very hot. Q203 does not get at all hot. I assume that R202 should be 3.3 ohms...
  23. Jon1982

    SX1980 Voltage Regulator / current source question

    The plot thickens... I've today gone over the 13.5v, 8v, 5v regulator with a fine toothed comb, checked all transistors, resistors etc, checked for shorts, nothing apparent. I checked R201 and its the original 22k. I removed Q203 and checked the voltage at collector... 35V! Same at Q202. Any...
  24. Jon1982

    SX1980 Voltage Regulator / current source question

    Current sources built, tested (both outputting 2.2mA) and installed and working. I notice the + / - 80V's seem to ramp up now over a few seconds rather than immediately, assume this is a characteristic of the circuit. Transistors used... KSA1013YTA (PNP) KSC2383YTA (NPN) Something still wrong...
  25. Jon1982

    SX1980 Voltage Regulator / current source question

    I took up your suggestion. And saw that I have made a daft, albeit slightly forgiveable, mistake :) If I attach the circuit diagram with a little addition to make things a bit more obvious to fellow dumbasses, you can probably guess what I'd done wrong!
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