[TenTec] Century 21 problem - key thump in phones?

[swiftm at btopenworld.com]

Hi all 

I have a lovely old Century 21 (analogue model) in "as new" condition, with one small but annoying problem - I'm hoping another C21 owner may be able to advise me... 

The problem is this: 

I can hear an audible thump at the point where I un-key the transmitter - ie. at the point where the radio switches from transmit back to receive. This is only evident when listening through phones; I'm unable to tell clearly if the same problem exists through the speaker. It seems not to, or is certainly much less apparent (perhaps this is due to the frequency response of the speaker, though?). 

The problem is particularly noticeable with the sidetone set at a reasonably low volume level. As I increase the sidetone level, the thump is less and less noticeable - however, I find the sidetone uncomfortable to listen to at higher levels, so keeping it up loud is not really an option (nor good for the ears!). 

Interestingly, the thump is only really noticeable when I have the audio filter set at either the 500 or 1kHz positions. At the 2.5kHz position, it sounds fine (that's not to say there isn't any thump, it's just I can't hear it). 

Could the problem may be related to the un-muting in the audio power amp stage (on page 3-13 of the manual)? I note that D1, C1 (1uF), R1 (10K) and R2 (6.8K) are intended to delay the switch-on time of the audio input signal to IC1 (LM380 amp), via Q1 (an MPSU01). Are the component values listed for this delay circuit optimal? How would I go about increasing the delay time - I assume it's a case of increasing the value of C1? If so, what value would you recommend? Otherwise, would you suggest changing the value of R1 or R2? 

Alternatively, I wonder if the thump is being introduced in the Audio Pre-amp stage (page 3-11). With either the 1kHz or 500Hz filter position selected the audio signal passes through IC1, an LM3900 quad op-amp. In the 2.5kHz filter position, when no thump is obvious, IC1 is bypassed. Have you any suggestions as to what might cause the thump in this area? Perhaps the amplification of the recovered audio by IC2 (an LM387) is part of the problem, as this feeds into the IC1? 

The head-phones I'm using are good quality low impedance (32 ohms) domestic units, typical of those used for general "hi-fi" listening. Looking at page 2-3 of the manual it recommends using a simple "attenuating network" with low impedance phones so that, when transmitting, "residual noise and audio feedback will be reduced". Could this be the cause of my problem? I think it's very unlikely, but thought I should mention it, given that I can't obviously detect the thump when running through the speaker instead of my phones. 

Finally, if none of the above seems plausible and I'm just going to have to live with this quirk, I have been given the circuit details for a very simple headphone "thump filter". This consists of a 10uF capacitor in series with a 330uH choke at the phones output. I understand this should reduce the thump, but may compromise receive audio for non-CW signals (not a *big* problem for me). I'd welcome your thoughts on this "last resort" option... 

Many thanks in advance for any help or advice! 

73 

Mike 
M0CQG