Cobra 148 GTL DX MK1 - Mic Audio Mod

[ch25]

I would not go higher than 1uF, not much benefit.
Btw, what microphone are you using?

[Otter]

For a try no need to replace, just add new, parallel to existing ones on the foil side.

Are you saying that these new 470nF caps need to be added in parallel on the solder side , or do they replace the original part ?

You can add them in parallel on the solder side to try them out. They are so much larger values than the original capacitors that the ones that are in there will have little effect on the total capacitance.

[Phuture]

Astatic Teardrop.

[ch25]

Junk mic.
Want play with audio get a good one.
Behringer XM8500 for example.

If you want cheaper than that, get Panasonic WM61A. Will sound excellent.

wm61a.jpg

[Phuture]

I actually bought a Panasonic WM61A a while back and have been on the look out for a preamp circuit with separate bass and treble controls that I could use inside a fist mic, but haven't found one yet. The Behringer looks impressive, but I guess, as you say, would need to be used with an audio mixer, etc.

I have yet to carry out full AM and SSB tests on the 148 but with the replacement of the 3 caps so far, without injecting directly into the AM modulator (collector of TR50), there should already be an improvement in Tx audio on AM, right? I know I have to see if the SSB cap C129 needs changing as well to improve the audio there.

[ch25]

Now your microphone is a bottleneck.
Use this wm61a. Mod is simple. Put cap and resistor inside radio. 8V is present inside in many places.

xm8500 works good plugged directly, no need for extra gear.

[Phuture]

I see what you're saying. I'll try it out. Thanks Lemmy.

[Phuture]

Meant to ask, how critical is the tolerance figure of the caps? Up to what percentage is okay?

[ch25]

Don't bother the tolerance, you won't hear it.

[Phuture]

Okay, thanks Lemmy.

[Phuture]

Okay, just to conclude this thread, I went for WIMA metallised polyester film (MKS2) 1uF caps in the end. 148 Tx sounds really good now and got good audio reports. Thanks Lemmy.

[ch25]

Good work!

[cb4ever104]

Okay, just to conclude this thread, I went for WIMA metallised polyester film (MKS2) 1uF caps in the end. 148 Tx sounds really good now and got good audio reports. Thanks Lemmy.

Hi P!

So that people don't have to read through this whole thread . Could you just say which caps you eventually changed , and their values .

Thx

Sammy

[Phuture]

Hi Sammy,

Sure, no problem. There is actually no change from the original cap values Lemmy suggested replacing, so C135, C128 and C217 (MKT) for FM.

I changed mine to WIMA 1uF MKS2 caps. I don't think there is any difference between caps marked MKS and MKT as "S" and "T" are used to indicate polyester by different manufacturers, according to what I read. I tried CBB (Metallised Polypropylene Film) caps which are supposed to be even better for audio than metallised polyester and they worked well, but once I discovered MKS were the same as MKT I found more sellers on eBay selling the values I wanted to try (560nF, 680nF, 1uF) in metallised polyester. Honestly, I could not tell (nor local stations listening) any audible difference between the MKS and CBB cap types. I presume this is because we are not dealing with high-fidelity audio in the 148 GTL DX, but since the MKS caps come in smaller packages (less than 1cm wide) I went for MKS caps as they fit neatly into the original spaces in which the caps are located. The CBB caps I sourced at 100V were thick and much wider and although you can get 63V and 100V CBB caps that are around 1cm wide I could not see any benefit in using them.

I was not too bothered by the AM Tx audio of my 148 as AM seems to have a bias towards a slightly bassier Tx frequency response anyway (in my experience) and I don't use AM much anyway.

As Lemmy suggested, the value of C129 can also be increased to widen the Tx frequency response on SSB, but no one fairly local to me seems to have a problem with the original 148 SSB Tx audio quality. I will check with a few DX stations in the future, however, to see if my SSB Tx audio at distance is decent enough to keep as it is.

That's it. Hopefully it's useful to someone. Thanks again to Lemmy for the original help and advice.

[cb4ever104]

Thanks P !