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Everything posted by ampaddict

  1. Hey Jake, I have a near mint SA-9800 here in Perth. Happy to set it up, do a demo video and upload to YouTube ... I can post the link here once done
  2. Would you consider posting to WA 6280? Happy to pay what you ask and postage on top, up front. thanks, Brad
  3. Traced it back to one of the decoupling caps I added to the underside of the board. I've simply removed them both and hey presto, all is good. Are they really necessary? Thanks everyone for your input. I have another 555A that I'm going to do a real number on to include a nickel plated chassis and upgrades similar to this: Leestereo's Restoration/Upgrade of a Sansui AU-555A | Audiokarma Home Audio Stereo Discussion Forums Cheers, Brad
  4. Legend, thanks Mike. Will let you know how I get on ...
  5. Thanks Mike. I have a Rigol DS1052E digital oscilliscope but not that experienced in using it. I also have a digital signal generator. Would you advise to send, for instance a 1KHz sine wave into the Left Aux RCA then measure the signal from the speaker terminals back through the signal path? Will begin with an audio probe as this seems less complicated 🙂
  6. I agree Pete. I also have another PCB untouched original that is identical - photo. I'm really stumped as to where the LH distortion is stemming from. RH is outstanding. Listening through headphones makes no difference either. I know it's something in the main amp but I seemed to have tried everything.
  7. That bridge is part of the standard PCB tracks as per SM. rails are 48v each measure at F002/003.
  8. double and triple checked caps and replaced all transistors on both sides, that's what's very strange. Even replaced the ceramics, just in case. Edited my original post with a lot more information ...
  9. Hey folks, I held off for a few weeks to try and nut this one out myself but have hit a wall. I've recapped the entire amp and replaced all small transistors. RH sounds absolutely amazing but LH is very distorted. 1. Checked that it was not a speaker issue by swapping LH/RH 2. Checked that it was not a preamp issue by swapping Aux LH/RH inputs - LH amp out still distorted 3. Swapped out tone boards from a spare AU-555A - no change 4. Swapped out Preamp boards from a spare unit - no change. To me, this points to only an issue with the main amp. Swaps: TR801/802 - XA495G ->> KSA992F TR803/804/807/808 - 8002-1 ->> KSC2690A TR805/806 - 2SC281 ->> KSC1815 (attached to heatsink) TR809/810 - 9002-1 ->> KSA1220A TR815 - 2SC627 ->> ZTX694B PS caps upgraded to Vishay BC C001 2200uF ->> 4700uF C002/004 470uF ->> 1000uF C003 1000uF ->> 2200uF Main amp C819/820 1500uF ->> 1800uF All other electros swapped like for like values with Nichicon FG/Panasonic FM and WIMA MKS2. All resistors checked Left to Right and all in spec and matching. Output transistors swapped for MJ21194G but not difference (old ones reinstalled) Centre voltage set to 28.5V Bias voltage set to 12mV DC offset at around 20mV at the speaker terminals Sorry, my previous post sent too early 🙂 hfe_sansui_au-555a_schematic.pdf
  10. to confirm I stuffed up one of the KSA992 pairs, I had the bases connected, instead of the emitters. Thanks again!
  11. pickle me Grandma! I swapped out IC1 and 2 from the working unit and all is fine. You were right, it must have been my poor pairing of the KSA992's. I'll go back and redo the KSA992 pairs carefully, as the 2SA798's in the ICs are noise prone. Thanks for all you help Mike, you're a legend!
  12. I swapped out the 2SC1951 with same from a working unit as I thought they looked sus. I have ZTX694B on order from E14. There's something else causing all these discretes to appear shorted I think. Pulled the Zener ZD601 (EQA01-13R) and diode tested OL one direction and 0.67V the other - that seems ok.
  13. Double checked and same. Using a DBT so may explain the lower voltages? TR10 looks a little more normal e= -.063V b= -1.2V c= -29.35V TR03 e= -29.33V b= -29.35V c= 24.85V TR05 e= 24.90V b= 24.96V c= 25.05V
  14. thanks Mike. R35 checked spot on 6.8. I don't have a replacement to hand but I do have a 6.2ohm .5W carbon. TR09 diode tested fine when removed and here are the gnd Vdc readings: E = 23.98V - 24.15V fluctuating C = 23.95V - 24.55V fluctuating B = -29.35V stable I'm recapping as I go, so perhaps the fluctuation is the caps settling in?
  15. Hey folks, I'd be grateful for a little guidance in pinpointing a voltage problem with my AU-4900 please. Having read dozens of forums here on SNA and on AK, I've tried all of the known weak points (I think). It seems my issue is not uncommon so I hope I'm just missing something simple. I'm especially thankful for the knowledge shared by @skippy124 - I thoroughly enjoy your work John! I'm handy with a soldering iron and have all the common test equipment and my trusty DBT. Problem: Amp won't come out of protection mode due to high voltages on Left output devices. Initially, I was able to adjust the VRs to get the bias set correctly but then protection kicked in. As the voltages to the Right amp section seem good and the PS section is sending the correct voltages out, my first thoughts was that this is a protection circuit issue. I've basically replaced all components in the protection circuit, to no avail. I've also replaced all of the transistors on the tone board and 90% on the amp board and all diodes on the amp board bar the Zener (on order). Replacing components has not fixed the issue, nor made is worse. I've attached a circuit diagram of the protection circuit with some voltages (via a DBT). Here are the components I've replaced: Amp Board D01, 02 -> 1N4148 D605, 607-615 -> 1N4007G TR601, 602 (SDC1364) -> KSC1945CGTA TR15, 16 (2SC711) -> KSC945CGTA C602 -> 1uF 63V WIMA MKS2 C604 -> 470uF 25V Panasonic FM C605 -> 100uF 50V Panasonic FM TR601, 602, 602 (2SC1364) -> KSC945CGTA TR03, 04 (2SC1951) unchanged and tried swapping from a working 4900 TR05, 06 (2SC711) unchanged and tried swapping from a working 4900 TR07, 08 (2SD438) -> KSC2690AYS TR09, 10 (2SB560) -> KSA1220AYS TR11, 12 (2SD188A) unchanged TR13, 14 (2SB545A) unchanged IC1, 2 (dual 2SA798) -> paired KSA992FTA with matching Hfe Fusable resistors R37,38,39,40 I've also swapped out the 4 germanium diodes from a working 4900. Tone Board All (2SA726) -> KSA992FTA All (2SC1313) -> KSC1845FTA Voltages on output devices: TR14 E= -29.5mV, B= -629mV, C= -29.6V TR13 E= 24.75V, B= 24.75V, C= -29.62V TR12 E= 20mV, B= 600mV, C= 29.68V TR11 E= 24.78V, B= 24.95V, C= 29.68V I've even tried removing the output devices but that has no effect on the Left voltages. Any other measurements that would be helpful for diagnosis, please shout. Thanks in advance. Brad
  16. reposted in correct category - apologies Brad
  17. Thanks folks and sorry for the delay. Great call on the US parts suppliers Mike. I use RS and E14 but Mouser seem to have a greater range of the vintage replacement parts. Will update once the rebuild begins ...
  18. Item: Pioneer PR-101 or PR-100 10.5" silver reels and PP-220 adaptors Price Range: Negotiable Item Condition: New or Used Extra Info: 2 matching pairs of silver reels (1 + 2) plus adaptors ****FOUND**** had to sell a kidney! To suit Pioneer RT-909 etc. Must be matching pair or can buy separately to get 1 + 2 and must be silver. No warping and no blemishes on presentation side. Not asking for much am I 🙂 Now, where's that hen's tooth ...
  19. That's better. Multi-turn trimpots reinstalled. Bias good. It sounds amazing and I'm assuming will only get better once the caps bed in ...
  20. ... hangs his head in shame but happy all that work wasn't for nothing! So used to measuring across a pair of resistors, I didn't think to change my approach. It sounds bright and detailed with the WIMA and Nichicon FG series and all new trans. Thanks everyone!
  21. Thanks Pete. I was measuring across the pairs of resistors and getting 26V but when I measure across one of the resistors, I can adjust the bias to spec. Seems I was simply measuring it incorrectly!
  22. am I correct to measure the bias across the pairs of emitter resistors (R837/R835) (R836/R838)? What's the target voltage please?
  23. Problem is same on both channels. Amp works but is slightly distorted on both channels, very noticeable when applying base. Resistance in circuit as follows: R837 0.5ohms R835 0.5ohms but seemed to fluctuate up to 1ohm before settling on 0.5 - could be just a dirty connection R836 0.5ohms R838 0.5ohms To confirm, do the VR's adjust DC offset or BIAS voltage and where exactly to measure? Just the speaker posts as MBZ pointed out? If DC offset, am I aiming for a reading as close to zero as possible? On other Sansui's like AU-2900, I've measured bias voltage across the pair of emitter resistors to get a low, mV reading and used the VRs to bring it to spec. I assume the 2900 is not a cap-coupled amp like the 101? Thanks for your patience and help
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