BIAS Transformer T203, and Associated Circuitry: Difference between revisions

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*<b>BIAS Testing</b>

Test-time had arrived. I carefully metered the entire Bias circuit, and didn't like the way the "Bias Adjust" potentiometer contacts were nearly touching the front-panel. A slight bend, and two layers of 600V electrical tape eased my concern. All connections seemed sound, and there were no detectable shorts.

I made up an AC power-cord with alligator-clips, and hooked onto the (not-yet-wired-in) Bias Transformer primary, with the 12V secondary still un-connected and capped-off. This would test the Bias supply only.

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[[File:Bias_test_90v_cw.jpg|500px|thumb|right]]

CW (fixed) Bias test

Mode switch in the CW position, internal meter set to "Bias Voltage", my Fluke meter connected to the V101-socket grid-connection and the common + of C209 and C210. YAY! -90V showing on both meters. And especially ... <b>NO SMOKE</b>

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[[File:Bias_test_60v_adjustable_ssb.jpg|500px|thumb|left]]
Continuing, I placed the mode-switch in the SSB postion, and found I could easily adjust the SSB negative bias from about -22V to the CW level of 90V. Having read that I would get the right plate-idle-current with around -55V to -60V bias, I set it to -60V for now. Again, it was nice to see agreement between the Collins internal meter and my Fluke.

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This first power-application test was really gratifying, after the long idle and rebuilding periods. And because I believe this was the troublesome circuit that caused my smoke-out, I'm actually elated to see this first section working!


[[Repair|Back to Repair]]
[[Repair|Back to Repair]]

Latest revision as of 20:39, 11 November 2020

30s-1 replacement bias transfomer.jpg

The replacement Peter Dahl transfomer! It took 2-1/2 months from initiating the order, until delivery. I made the actual purchase through Digi-Key, who provided the customer-facing fulfilment for Hammond. The transfomer also includes a 12.6VAC (at 800ma continuous commercial service) winding, which is used for three distinct purposes within the 30S-1:

  • the AC voltage from this winding powers the incandescent dial and meter bulbs
  • rectified and filtered DC runs through the HV-interlock circuit
  • rectified and filtered DC is used to operate the Antenna Relays (when an external short-to-ground occurs - often an Amp T/R relay inside the exciter)

  • 30s1 t203 laced installed.jpg

    T203 installed, laced and soldered into it's new home!

    When I look at the previous T203, especially the type of insulation and the under-chassis un-laced wiring, it makes me wonder if my burned T203 was already a replacement...? Hmmm... I will thoroughly check each and every circuit associated with this transformer, because it seems possible that this BIAS transformer, in this amp, has been an on-going issue. I intend to firmly quash all the problems now!


    Biasdiodes mounted fuseholder.jpg

    The original selenium rectifiers were long-gone, having been replaced with a terminal-strip and newer silicon diodes. Rather than replace or simply fix what was already there, I wanted to prevent another bent-and-shorted kind of tragedy (if it can happen once...).

    I borrowed Collins' own idea from their nearby 12V rectifiers: I bought a triple-fuse mounting block (designed for AGC tubular glass fuses), and mounted my replacement 1N4005 diodes to this fuse-holder. The diode-leads were lightly clinched, then soldered to the fuse-clips, while the circuit connections were made to the holder's solder-tabs.

    These diodes aren't going anywhere! And the printed component-labelling may help me (or someone else) in the future.


    C209 c210.jpg

    C209 and C210 upgraded, mechanically secured, and labelled A pair of new capacitors - same 50uF, but uprated from 150VDC to 250VDC, working temperature range -20C to +85C. The capacitors are labelled, as is the T203 Bias-Transformer centre-tap connection (I may be the next person helped by this clear labelling!). An additional layer of clear heat-shrink was added to ensure HV sturdiness, and the nylon mounting clamps are spaced a further 8mm off the chassis.


    Whoops terminal strip.jpg

    I broke this brittle terminal-strip.

    While working underneath the Relay Shelf, I broke a terminal-strip. Unable to find an exact replacement, I opted for two smaller terminal-strips, turned 90-degrees to the original. And, this allowed me to neatly separate the two functions on the original: the Step-Start circuit, and the BIAS connections.


    C203 step start.jpg

    Replacement C203 step-start capacitor, securely mounted.

    I replaced C203, making sure it's well-secured, and well-insulated. There should never be a problem of fat-fingers causing future short-circuits! The new C203 is rated at 200VDC, which improves on the original 150VDC rating, especially comforting considering how the incoming AC line-voltage has increased over the decades: designed with 115VAC/230VAC in mind, my current voltage is ~6% higher at 122/244 (just measured it).


    Underside relayshelf tidy.jpg

    Underside of Relay Shelf, now tidied.

    Whew! All the parts I indicated on the schematic with yellow arrows have now been replaced - YAY! Now, on to some of the upgrades...


    • BIAS Testing

    Test-time had arrived. I carefully metered the entire Bias circuit, and didn't like the way the "Bias Adjust" potentiometer contacts were nearly touching the front-panel. A slight bend, and two layers of 600V electrical tape eased my concern. All connections seemed sound, and there were no detectable shorts.

    I made up an AC power-cord with alligator-clips, and hooked onto the (not-yet-wired-in) Bias Transformer primary, with the 12V secondary still un-connected and capped-off. This would test the Bias supply only.

    Bias test 90v cw.jpg

    CW (fixed) Bias test

    Mode switch in the CW position, internal meter set to "Bias Voltage", my Fluke meter connected to the V101-socket grid-connection and the common + of C209 and C210. YAY! -90V showing on both meters. And especially ... NO SMOKE


    Bias test 60v adjustable ssb.jpg

    Continuing, I placed the mode-switch in the SSB postion, and found I could easily adjust the SSB negative bias from about -22V to the CW level of 90V. Having read that I would get the right plate-idle-current with around -55V to -60V bias, I set it to -60V for now. Again, it was nice to see agreement between the Collins internal meter and my Fluke.


    This first power-application test was really gratifying, after the long idle and rebuilding periods. And because I believe this was the troublesome circuit that caused my smoke-out, I'm actually elated to see this first section working!

    Back to Repair

    On to Screen Supply Overhaul