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  1. dcgillespie

    New 800B

    To make sure it’s not just the output tubes not biased correctly, remove all four output tubes and try the test again. If the bulb still stays bright, then something in the HV B+ section is at issue, but if it goes dim then the bias circuit to the output tubes needs checking or adjusting — but...
  2. dcgillespie

    New 800B

    So now at least you know where the problem lies. Something in the Low Voltage DC Heater/Bias Supply is shorted, connected wrong, or has failed. Concentrate on that area to find the issue. Don’t reconnect the red transformer wire until the fault is found so you won’t hurt your output tubes. Dave
  3. dcgillespie

    New 800B

    I’m in N Ga, about an hour north of Atlanta. But first, disconnect either one of the red power transformer wires and tape it off. Then see if the light stays dim. If not, then also disconnect one of the power transformer wires to the bridge rectifier, tape it off, and then repeat the test and...
  4. dcgillespie

    Untapped Potential: Heath's W-5M

    The 24 volt transformer can work perfectly as well: just use a voltage doubler rectifier and you’ll have all the bias voltage you need. I did this in one of my own units and it works just fine. Dave
  5. dcgillespie

    Untapped Potential: Heath's W-5M

    I would agree because you’re also running out of voltage drop for good filtering. Usually a raw supply of about double the actual need works well after the filtering and adjustment networks are in place. Dave
  6. dcgillespie

    Another RCA RS 171 Mod Question

    Technically correct Gadget. But it needs to appear across the winding, because it’s across the winding in the other channel. Dave
  7. dcgillespie

    MC240 right channel problem

    The usual problem is a dirty service selector switch. Clean it real good with some Deoxit and the problem just might be resolved. Dave
  8. dcgillespie

    Another RCA RS 171 Mod Question

    There were a number of ways that were devised to either try to produce stereo out of a single amplifier or, enhance the performance of ordinary single ended stereo designs beyond their otherwise normal limitations. The example Gadget provided is one such approach, but not that which your RCA...
  9. dcgillespie

    Fisher SA1000 => KT88s?

    The real problem in using KT88 tubes is that it will cut the NFB applied though the partial cathode coupling feature basically in half -- besides requiring more drive and bias voltage -- meaning that the unit will never meet specs if you make that substitution. If made however, the global NFB...
  10. dcgillespie

    Mc30 hum goes away when gain,hum or rca is touched

    That is incorrect information. When a cap that designed for AC bypass duty is installed, if it should ever fail, it is designed to fail open — so there is no safety issue. Dave
  11. dcgillespie

    Actual watts....who can shed light on this subject?

    I’ve never specifically analyzed Mac power meters, but unless they monitor both voltage and current being delivered into the load, then any power meter indication will be an approximation. Most power meters are designed to indicate based on a fixed impedance, so that only voltage output needs...
  12. dcgillespie

    Crime of the Mid Century

    What an inspiration for all of us! The usual words of praise just seem so inadequate. You have certainly done Avery proud. Congratulations! Dave
  13. dcgillespie

    Mc30 hum goes away when gain,hum or rca is touched

    I’ve sent you another PM to see if you are receiving my responses. Dave
  14. dcgillespie

    Mc30 hum goes away when gain,hum or rca is touched

    My response was in your PM. Dave
  15. dcgillespie

    McIntosh 50W-2 Amplifier

    The tubes operate under the same conditions that ol’ Leo Fender ultimately ran them, so 50 watts is very possible indeed. What a true piece of history you have — the original implementation of the Unity Coupled concept! It deserves to be brought back to all the excellence that history has shown...
  16. dcgillespie

    Mc30 hum goes away when gain,hum or rca is touched

    Did you try the grounding experiment I mentioned? Dave
  17. dcgillespie

    Conrad Johnson MV75A ... revisiting a pretty old thread for some help !

    Measure the voltage at pin 8 of each output tube. If the op-amp comparator circuits are working correctly, they should all be the same. Also, check that all the 20 Ohm cathode resistors are within the their rated tolerance of 20 Ohms as well. Use the cathode voltage and resistance values for...
  18. dcgillespie

    C76 Barrier Insulator

    Hayseed has them as well. Dave
  19. dcgillespie

    Fisher 500B FM Stereo drops out after 20 Minutes

    Unfortunately, this sounds like a classic case of one (or more) of the IF transformers having developed mica disease. After a period of time from a cold start, the transformers warm. If an internal connection to one of the resonant caps in the base of the transformer becomes compromised by...
  20. dcgillespie

    Phase shift question

    I've been enjoying this thread and the parallel thread on power and distortion immensely. Long ago, I thought all of the various vintage designs were an absolute thing regarding the amount of feedback applied, how they were stabilized, and the resulting performance achieved -- that there was...
  21. dcgillespie

    Magnavox Hot grid??

    It also appears that you have a SS replacement rectifier plugged in, in place of the stock 5U4GB, which will raise the B+ considerably, and aggravate the issue as well. Dave
  22. dcgillespie

    Fisher X-101-C 12ax7s no power

    The X-101C employs a voltage doubler power supply rectifier arrangement. If the main B+ is low, then invariably it is not doubling as it should. This can only be caused by one of more of three things: 1. One of the diodes has gone open (quite unlikely since diodes invariably fail shorted, which...
  23. dcgillespie

    Auxiliary 2 input fisher x-202-b

    You can't picture a scenario today -- but back in the early to mid 60s, making and playing tape recordings was the next big thing. Typical recorders had only 2 heads -- erase, and record/playback. But the better decks employed three separate heads -- erase, record, and playback. Fisher was...
  24. dcgillespie

    Rx For the Magnavox 8800 Series

    The other issue is that the output stage could not be pulling any significant current at all with virtually no screen voltage on the output tubes (check the cathode voltage, which will be non-existent if that's the case). If you currently have 310 vdc supplied to the output stage but with...
  25. dcgillespie

    FM Volume about 1/2 of AUX/Phono/Tape Mon. 500-B

    Glad you got it resolved! Dave
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