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Speech amp on HW-100

Discussion in '"Boat Anchor" & Classic Equipment' started by W1BVV, Jul 6, 2020.

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  1. W1BVV

    W1BVV Ham Member QRZ Page

    I'm refurbing a new to me HW-100. After cleaning and alignment the receiver is working like a champ, but using a EV -619 that came with the transceiver, the modulation is unsuitably low even with the mic level maxed. To date I have:

    Verified all test voltages per the manual
    Checked all the resistor values
    Replaced 2 20 mfd electrolytics ( speech amp and audio amp)
    Checked mic cableing and plug
    Checked mic output against other dynamic mics I have
    Traced signal from mic input though speech amp and into modulator

    The gain provided by V1 seems to be very low. I swapped the other 3 6EA8's into the specch amp with no change. I then ran another dynamic hi impedance mic into the set with no change. Curious, I ran the mic through an outboard mic preamp and into the Hw-100 at a gain I would normally use for input to a line level device, and I get expected modulation and good on air audio reports.

    I have NOS 6EA8's on order, but it doesn't seem to be a tube problem considering the swap.

    Any chance the HW-100 needs more mic output than I dynamic produces. I assumed the EV-619 was a matching era mic.

    Dave
    W1BVV
     
  2. W3RU

    W3RU Ham Member QRZ Page

    Dave,

    Simplest thing to do at this point is to inject a 1 KHz audio tone into the mic input, then trace out the signal through V1A/B and through V2 to the balanced modulator using a scope. Try to identify exactly where the signal is not amplified sufficiently in a given stage, then suspect a defective coupling or bypass capacitor. To verify that the mic is working with a high enough level, see if voice peaks approach the p-p levels observed from the 1 KHz sine wave on the scope. You won't see any level increase from V2 since it is a cathode follower, but verify you do not have any loss in signal level. Bad cathode resistors are common culprits in this stage.

    Verifying that all voltages are at spec is an excellent first step. The 4.7K B+ resistor (R4 I think) for V1's plates and screens is often high and usually stressed due to a bad 20 mfd e-cap which you stated was replaced, but then you would have experienced low plate voltages which you stated were OK.

    Another desirable change would be to install 1N5711 Schottky diodes in the Balanced Modulator - a tremendous improvement over the original '914's with greatly improved carrier nulling, especially when replacing the pot with a 10-turn cermet 200 ohm component ($.20, if that). I also change the .001 mfd coupling capacitor to .01 mfd to improve bass response in today's "hi-fi" environment.

    Regards and good luck,

    Jack W3RU
     
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  3. W1BVV

    W1BVV Ham Member QRZ Page

    Jack,

    Thanks for the suggestions. I'll hook up my signal generator and do a trace through the V1. Looking at mic signals at the top of the mic level pot (between V1a and V1b (the follower) the gain seems to be around 3x. My tube circuit analysis is a little rusty, but I would expect it to be much higher. I'll re-check the cathode resistors and the coupling and bypass caps.

    Dave W1BVV
     
  4. W3RU

    W3RU Ham Member QRZ Page

    Dave,

    Good start! I would be expecting a gain of at least 10 through each amplification stage, but not the cathode follower.

    Also, check the 22K input resistor just in case. And, a check of the VOX relay operation with proper sensitivity control would demonstrate that the first audio stage is working OK.

    I am assuming you have a manual with the readable schematic for reference.

    Jack W3RU
     
  5. WA5VGO

    WA5VGO Ham Member QRZ Page

    HW-100’s are known to have marginal mic gain. This is especially true on 40 meters. Try the following:
    The balanced modulator output transformer, T-1 usually has two peaks. Be certain it is set to the upper one. Also, change the 6EA8 speech amplifier tube to either a 6CQ8 or 6LM8. Both of these are drop in replacements.
     
  6. W1BVV

    W1BVV Ham Member QRZ Page

    Had a few minutes at lunch to poke around a little more. With the original V1, gain at 1kHz from mic connector to top of Mic level pot (wiper goes to follower grid) is x10 (.6v RMS). Signal at the grid of the follower is 0.3V but pin 8, the cathode of the follower, is 0V.

    When I substituted a NOS V1, gain V1A gain went up to 18x but follower cathode is still 0V.

    With the new tube, there appears to be some modulation, I didn't have my PEP meter hooked up so not sure how much. Haven't checked T1 tuning yet or the caps. VOX does NOT work with either tube, but does work when I use my external preamp and mic. More work tonight.

    Thanks for all the suggestions - seem to be making progress.

    Dave W1BVV
     
  7. W3RU

    W3RU Ham Member QRZ Page

    ERROR CORRECTION:

    Dave,

    I had the Collins 32S-1 audio circuits on my mind while writing the earlier post. Collins uses two audio stages before the cathode follower, but Heath uses only one. Accordingly, just substitute V1B for references to V2 which I made in error.

    Another thought if there is low amplification (you stated 3x earlier) in V1A: the C2 10 mfd/16 volt e-cap in the cathode circuit should be replaced after all these years. This, alone, may solve your problem - I've found a few bad ones here. Also, the same cap c212 in the V17 Vox sense circuit on the bandpass board could be bad - it is hard to unsolder the ground lead, however, because the chassis does an excellent job of heat sinking that land, so loosen the 2 screws on either side of this ground land to cut the heat sinking effect. I happen to use a 63 volt cap as a replacement which is the same physical size as the 50 year old 16 vdc caps, because I have a bunch on hand.

    A quick check for these two circuits would be to set the mic gain to minimum and the vox sensitivity to minimum (wiper on their respective ground ends of each pot), then check your scope for the V1A gain again as a baseline - it just may have increased to 10X. Then, vary each of the two pots independently to see if one of them is dragging down the audio signal to 3x, which would be a major troubleshooting clue. If not, I would place my bet on C2 needing replacement if all the other voltages for V1A are OK to get the V1A cathode back to proper operation. I have also picked up shorts on the little VOX panel, one of which could well be at the Vox sensitivity pot, due to mechanical abuse while servicing the O-rings.

    Continued good luck

    Jack W3RU

    PS After posting, I see you got in there before me, and have most likely answered half the questions I just raised. I think the pot tests will tell you a lot, since both pots are supplied from the same point off the V1A plate circuit, and would certainly interact with each other if one circuit is working improperly.

    Verify that the V1B CF cathode will pass the audio through that .2 mfd cap at about the same level without amplification if you scope it on the balanced modulator side of the cap.

    If you managed to get a good 18X signal after substituting an NOS 6EA8 in V1 after all those previous subs using V5, V12, V15 etc, and to no avail, you may want to consider replacing all those tubes as well! Here is where a tube tester would be beneficial. I once picked up an SB-102 at a hamfest that had EVERY tube replaced with a bad one! Not normally a problem, except the SOB even replaced the 6GW8 with a bad one - now, THAT could have been expensive, but I did have 2 spares; the rest are cheep tubes, and the $$ 6HS6's replaced with 6AU6's work just as well.
     
    Last edited: Jul 6, 2020
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  8. W3RU

    W3RU Ham Member QRZ Page

    Dave,

    After reading again, you now have a clue - the preamp is driving the circuit so hard, that enough signal is leaking through the VOX pot to activate, where this should never have a problem under the original design condition.

    The audio circuit input likes a high impedance mic and input, and every hi-Z mic I've seen supplies plenty of audio - especially the D-104 without the preamp. So, perhaps you have either a low impedance mic output (check the internal wiring or switches for hi/low selection), a bad mic element, or a circuit defect. If your audio generator can be placed on a low level by itself, feed this (alone, through a .01 cap) directly to the mic input in VOX mode to see if you can trip the VOX and note the audio level with the pot set at mid-point. Set the remaining two pots at mid-point (delay, anti). Then, trace through the rest of the audio chain, which should be supplying adequate 1 KHz sine wave audio signal level for perhaps 75-100 watts power output assuming you can get that power level on TUNE or CW. If you can trip the vox on a low audio level and without the external preamp, your V1A stage is probably OK, so concentrate on V1B and then through the balanced mixer, T1 and V2 isolation amp with the carrier added, through the filter and up to the V3 grid with the scope synced to 3.395 MHz. If you can get to this point, then connect the mic to see what you get in peaks.

    Jack W3RU
     
  9. W1BVV

    W1BVV Ham Member QRZ Page

    Jack,

    I really appreciate your help. I have tried to help a lot of people on an audio forum troubleshoot reel to reel tape decks, something I have some experience with. Most however ask questions the ignore the advice and continue to flail around. I'm trying not to be that guy!

    Let me do a reset and try to be specific as to what and how I've measured. I am at a disadvantage that I don't have a real scope. I'm using a usb scope module that is only good up to a few 100 kHz. With the "new" tube - not sure the old one was a problem- 62 mV in the mic terminal yields 1.1 V at V1 pin 6, 0.6 V at pin 9 (grid of follower) and 0 V at pin 8 (cathode) all rms readings. Its the 0 V at the cathode of of V1b that confuses me. I seems it should be equal to the grid, thats the point of a follower?

    The EV 619 microphone is specd with a sensitivity of -57dB.

    By running a dynamic microphone through a preamp (not sure of the gain or output - I can figure it out) and into the mic input jack, it was easy to find a preamp gain setting where everything worked (PTT, VOX, modulated power out and good audio reports and good bandscope display on my Icom 7300 (antenna disconnected). So Mic directly in - No modulation, No VOX , Mic with preamp into mic input - Good mod, VOX works. In both cases there is no indication of ALC on the S meter during transmit.

    The new 6EA8 tube increased V1a gain by 70% (10x to 17x) but no signal seems to get through the follower.

    Dave W1BVV
     
  10. W1BVV

    W1BVV Ham Member QRZ Page

    Disregard everything I've said about VOX. After tracking down the specs on the 619, turns out the mic element is shorted until the PTT is engaged.
     

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