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  1. dorke

    Obsolete Hewlett Packard Transistor

    Another thing, Q2 is part of the 5V regulated digital voltage of the unit. It is the driver of the series-pass Tr. Q1 in the regulator. It also serves as part of the current limit and Over-voltage limit circuit . If you are lucky, the failure in the P.S wasn't such that the 5V rose to a level...
  2. dorke

    Obsolete Hewlett Packard Transistor

    Well, I think you may have the Q2 pins mixed: Emitter is indeed tied to Q1 base. Base is connected to J2 pin5 Collector is connecetd to GND via chassis(TO- 220 metallic case with no isolator?) Does this make any sense?
  3. dorke

    Beginner questions..

    You should check prices ,I find Ebay to be the cheapest,Aliexpress may be cheaper at times. I like Ebay for the use of PayPal. example: http://www.ebay.com/itm/DALLAS-DS18B20-18B20-TO-92-Thermometer-Temperature-Sensor-/262136735487?hash=item3d089126ff:g:DAkAAOSwk1JWe11x...
  4. dorke

    Beginner questions..

    The cheapest place is most likely Ebay -cheapest parts from China . There is a catch in that ...some times you don't get the real McCoy :( Dave, I thought this is a "world based" forum,isn't it?;)
  5. dorke

    Help me to understand this circuit

    The MP2307 is a Buck step down converter. Like @hevans1944 (Hop) said above: The DC voltage regulated output is set by R3 and R4 to be about 5.22V. The diode D1 is a Schottky diode(although the symbol in the schematic is that of a Zener) . Depending on current and temperature, the max voltage...
  6. dorke

    Obsolete Hewlett Packard Transistor

    Well, Please post a clear schematic of that logic card. I would imagine the only reason to keep on using that "museum instrument" is it is a part of an old automatic measurement system with HP-IB software, with no budget to replace/update ;)
  7. dorke

    38Hz flash

    With the Arduino no extra component is needed. You simply use a single output pin ,and turn it on and off to drive a LED. each period at about [1/2] *[1/38] = 13.158 mSEc. BTW an Arduino is a gross overkill here. The simplest would be a 38Hz square wave osc with enable controlled by the...
  8. dorke

    Looking for eyelets that work for .1" spacing

    @Schectonater, What do you mean by "nasty environments"? Conformal- Coating(epoxys,silicones etc.) of PCBs is the way to go.
  9. dorke

    Flyback Transformer

    The name is HV-Block (splitter). It basically splits(distributes) the 30-40kV anode voltage from the fly-back to the 3 CRTs.
  10. dorke

    Flyback Transformer

    A CRT projection TV uses 3 CRT one for each of the 3 colors. That is why you have a fly-back transformer with 3 High Voltage terminals .
  11. dorke

    Technics SA-828 - Static with audio, then stopped working

    That spark could mean your speaker is blown. Check it's resistance. But to carry on with the amp we need the measurements of #251 and #253. All tests should be done with no speakers connected and volume at minimum.
  12. dorke

    Technics SA-828 - Static with audio, then stopped working

    Confusing. So now the enabling/disabling of the speakers has no effect on protection or not?
  13. dorke

    Technics SA-828 - Static with audio, then stopped working

    Do you have speakers connected to the unit? What is the voltage at center pin of Z602 when you enable/disable the speakers from the panel and the amp goes in/out of protection?
  14. dorke

    Technics SA-828 - Static with audio, then stopped working

    Well, Are you saying that if you disable the speakers from the front panel,the amp isn't in protection mode? Could you please complete the tests of #251. Please test voltages( relative to GND )on both sides of R634,R644 and R664,R672.
  15. dorke

    Technics SA-828 - Static with audio, then stopped working

    D608 Anode = -0.393 (from #248) and Q622(C)= -63.2(from #250),are in miss-agreement. Being the same point,they should be the same voltage. Which voltage is it? Please re-measure voltages relative to chassis GND on Q622. Please re-measure voltages relative to chassis GND on both sides of...
  16. dorke

    Please explain circuit

    Hop,Agree with you . Like we said before it's a Bad design...and has such, it is good methodology and practice to avoid such. Nevertheless , it can still work,it all depends on the switch type/behavior. The problem of switch bouncing can be "properly solved" by introducing a dedicated HW...
  17. dorke

    Technics SA-828 - Static with audio, then stopped working

    1. Resistance between pins 8,9 of IC1002 is too low, should be about 150K. 2. Q628 and D616 may be bad 3. Please check DC voltages relative to chassis GND on both sides of: R664,R672. 4. Please check DC voltages relative to chassis GND on Q622. 5. After that please check with...
  18. dorke

    Technics SA-828 - Static with audio, then stopped working

    Z601 voltage is fine. Z602 voltage at fault , and the reason the amp is in protection. That means the right channel is faulty. With volume at minimum. Please measure DC voltages relative to chassis GND on: anode of D608 and Cathode of D610 . anode and cathode of D616 . anode and cathode of...
  19. dorke

    Technics SA-828 - Static with audio, then stopped working

    The reading of -39.4v @D1001 is strange can you verify it ? The conditions show your AMP is in protection mode(pin6=37.8V.) Is the protection LED on the front panel(top left side) "on"? Please measure the DC voltage on the center pins of Z601 and Z602. Pin 8 voltage is not fine, please...
  20. dorke

    Need help with pcb repair

    You can use thin wires if you can, that would probably be the easiest way to go. The IC can be glued( after you verify the repair works) if needed as well
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