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HW-100 poor audio

Discussion in '"Boat Anchor" & Classic Equipment' started by W3WCG, Feb 24, 2019.

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  1. W3WCG

    W3WCG Ham Member QRZ Page

    I have a perfectly working HW-100 with the following exception, the audio sounds robotic and very tiring to listen to. The obvious things have been check out and found to be OK. : P.S. voltages and tube socket levels, coupling and bypass capacitors in the audio chain, speaker and tubes substituted, I've even plotted the audio response from the output of the product detector to the audio output xformer and found it to be flat from 400 to above 2500 HRZ. The 300 HRZ. level is down about 50% from the 400 level The question becomes is there something in the r.f. or i.f stages that could be causing the this problem? I have the following test equipment available : oscilloscope, precision multi function DVM, freq. counter and a TS-450 xmitter, no sweep generator. The transmitted audio sounds good (USB and LSB)according to on-air contacts especially from a unit 50 + years old. I want to sell the transmitter but can not with this problem bugging me. There is a slight difference in the pitch of the noise level when switching between USB and LSB operation. It just sounds like an audio band pass filter problem, not enough highs and not enough lows. Many thanks to all fellow hams who can point me in the right direction.
    Don W3WCG.
  2. K6BSU

    K6BSU Ham Member QRZ Page

    Your LF response is poor. Your audio chain should go down to about 200 Hz, or lower. Check the value of audio coupling capacitors between stages. You can temporarily bridge another cap across the existing ones to see if response improves.
  3. KE4OH

    KE4OH Ham Member QRZ Page

    Regarding the audio issue: Have you tried a different speaker?

    Regarding the received noise pitch between USB and LSB: That is not at all unusual for the HW-100, nor for many other brands and models. One or both crystals has probably drifted a bit. Or maybe never were right on the money. You could try using a small trimmer cap on each crystal to pull the them just exactly right.

    Or you could just ignore it. I have an HW-101 and an SB-101, which all have the same circuitry as your HW-100. My HW-101 is perfect USB/LSB. But the SB-101 exhibits the pitch difference you described. Meh.
  4. W3RU

    W3RU Ham Member QRZ Page

    Since you have a frequency counter, check the crystal frequency for the LSB and USB crystals at the center (wiper) terminal of the carrier balance pot. You must set the mode switch for the applicable sideband. It should not differ much from the 3.393.6 or 3.396.4 specification - say, 200 to 400 Hz. It will usually be low, and difficult to pad without cutting a PCB trace and adding the trimmer in series with the crystal. Easier to buy a parts board to get a set of different crystals, and take your chances.

    You must actually key the transmitter in CW mode to measure the CW crystal (3.395.6), as the receive scheme for CW uses the USB crystal.

    Also check the big ugly 20 mfd electrolytic capacitor in the 6GW8 circuit, preferably by replacing it. Since you have a scope, a 1KHz tone from an audio generator injected at the top of the audio pot, then traced through the audio amp and audio power amp stages of the 6GW8 will highlight an obvious distortion point (use low-level audio) that would be relatively easy to fix (cathode and screen resistors usually). You can also use a properly working calibrator signal to conveniently trace the entire IF and audio chain.

    A nice, pure "full range" 1KHz tone out of the speaker (versus the tinny audio) in the above test would indicate checking the stages backward through the 6BN8 detector, AVC and mixer stages for off-spec resistors. The 330 and 1000 ohm cathode resistors seem to be culprits in many of the stages from my experience.

    Jack W3RU
  5. W3WCG

    W3WCG Ham Member QRZ Page

    Thanks everyone. Some of your suggestions have already been done; it looks like the problem boils down to low freq. response as suggested by K6BNU; never thought the low end response of the HW and SB series went that low. That could be my problem because the low end frequency response of my unit is flat around the 400 hrz. level and beyond but drops off very fast and is down around the 80% level at 200 hrz.
    Thanks again W3WCG
  6. W9GB

    W9GB Ham Member QRZ Page

    Optimizing the Heathkit HW-101, SB100-102 Transceivers
    by Mark Graalman, WB8JKR (sk, 2005) the HW101.pdf

    Transmit Improvements:
    The transmitter audio quality can be improved by changing the value of coupling capacitor C11 on the modulator board from .001 μfd to .01 μfd (Figure 4). This will increase the low frequency response and give the transmit audio a little more “body”.
    If 10KΩ resistor R202 is present between the I.F. circuit board (L101 pin 4) and the bandpass circuit board (V5 pin 2), remove and replace it with a piece of insulated hook up wire (Figure 3). This will increase drive to the 1st transmit mixer. This change is already incorporated in later model HW-101s.
  7. W3DAB

    W3DAB Ham Member QRZ Page

    (1) I am interested to know a little more about what this actually means. I still have this resistor in my HW-100... is that analogous to hitting your amplifier with less then the required full 100w input?

    (2) Also, I realize in SSB that speech gain effects output in that not enough gain can mean less than 100% output power driven out of the radio, but is this mod increasing speech or output gain directly?
  8. KI4ZNV

    KI4ZNV Ham Member QRZ Page

    It sounds to me like you have BOTH high audio distortion AND poor low frequency response. Either one of these would point to a bad coupling capacitor, both of them together strongly point in that direction. I don't know how many of the coupling caps are paper but if you see any paper ones in there, replace them.

    But FIRST replace C304, which is a screen bypass capacitor and it's a big electrolytic which has almost certainly failed by now.
  9. G4EVK

    G4EVK Ham Member QRZ Page

    I am about to commence resoration of an HW-100 - d
  10. G4EVK

    G4EVK Ham Member QRZ Page

    I am about to commence restoration of an HW-100 in reasonable condition. Can anybody help me out with finding a COMPLETE assembly manual? Those listed on many site are very incomplete with pages 5-89 missing - the whole of the parts and construction details! Happy to reimburse any costs involved.

    Many thanks - hopefully another HW-100 on air soon?

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