Transmission1

Hi,

This Tristar 747 I'm trying to fix seems to have more than its fair share of faults. Further to my previous post and adding the roughly a 10Kc shift (thanks to this ibcomms and this forum ), my initial belief (before any work was done) was that the distorted transmitted FM/AM audio was due to poor alignment. However, since carrying out a couple of mods and a full realignment the distorted TX audio persists. I've seen a couple of audio mods (as per below) but I am wondering which, if either of these, will cure the problem.

Interestingly, when feeding the radio a 500Hz to 5Khz sine wave, there is no distortion apparent on the oscilloscope trace of the transmitted output. When keying up and speaking into the mic, however, the audio distorts badly and clips as if over mod-ing - this happens wherever the deviation preset (RV1) is set.

So my question is will either of these audio mods fix the problem?

From http://m0ysu.com/CBrepairsGENERAL1-2017.html

PTBM121D4X
Modulation is seldom brilliant on FM. Firstly ensure that the TX frequency is absolutely right. Use a frequency counter with at least 7 digits. Lift out R29 (100k) near the VCO block and cut the track between TP1 and the VCO block. Solder a 4k7 resistor across the break in the track. Connect a 1M resistor from the VCO block side of the 4k7 to C32 (where it is joined to R29). The audio AGC action is fierce so, to reduce it, cut the track between D38 and the other components (C125 C173 R150) or lift one leg out of the PCB. Connect a 1K preset in series with D38 and adjust for the most pleasant modulation. The Altai DM315E echo mic works well. The AGC action can also be reduced by soldering an 18k resistor in series with C175 (3u3) next to RV14.

From Cubwolf.com

https://cubwolf.wordpress.com/2015/02/2 ... rovements/

Thanks!

Is the distortion present on all modes or just FM? If it is on all modes, have you tried another microphone?

If the audio "distorts badly and clips as if over mod-ing" that indicates a fault, not a need for a performance improving modification. Get the radio working correctly as a stock unit first.

Disconnect the base of the driver (2SC2166) and listen to it on a second radio right alongside. Is it still distorted? This will tell you if it's an RF issue.
PS: this test will only work on USB, LSB, and FM, not AM.

Ground the mic pin that's normally grounded on Rx only and open on Tx. This will put your Tx audio through the speaker of the radio. If it's distorted there, the audio output circuit (a TA7205AP IC from memory or one of its associated components) is in trouble.

Hope this helps...

Hi phuture i would do what sue / lbcomms said and I would suggest you carefuly go thru the radio starting from mic audio input wire Check all ceramic and electrolics and regulators transistors/etc and check all solder joints. i agree with her 100% get the radio working right before doing any audio mods to your radio . Gd luck n Cheers n 73's

Thanks ibcomms and speeddemon.

Ibcomms, Yes, I thought it more likely to be a fault since it isn't affected by the deviation level and I just couldn't see the audio being considered acceptable as standard even with the PTBM121D4X's reputation for having a somewhat below average mod quality. I guess I was thinking a mod might cure the fault in the process since trying to trace the fault using a sine wave signal, as you would do with an audio amp, seemed pointless. Yes, I've tried a couple of dynamic mics and power mics, but there's no improvement. The distortion is obvious on received FM, but it's less clear as a fault on SSB modes as it's so noisy. In fact, SSB seems to have a lot of random screechy noise when audio is sent through the mic socket (perhaps this is a different fault?). Sorry, AM is actually very good.

I disconnected the base of the driver (C2166) and found the distortion was received exactly as before on the other radio in the affected modes. I grounded the mic socket as you described and the distortion is very clear in both FM and SSB modes. I checked AM as well and it's nice and clear. I guess the obvious question is if it's the audio IC, why would it not affect AM? Presumably the fault is in one of the audio section components not used by AM?

Thanks for your great troubleshooting advice - its clearly on the right track!

That tells us that it's not an RF feedback issue. Because the audio is good on AM, that also clears the microphone and the audio power amp IC as the cause.

This leaves you with the switching between AM, SSB, and FM. Quick and dirty test here is to make sure the FM trimpot only affects transmit audio on FM. Ditto for the SSB Tx audio gain and the AM modulation limiter, they should only affect SSB and AM audio levels respectively.

There are a bunch of diodes and a few capacitors that perform the mode switching. Could be any one of them. Rare but regular. But do the above quick test first to narrow down the search...

Hi ibcomms,

I'm not sure which preset you mean by "FM trimpot" as I can't see it marked on the schematic that's generally available for the PTBM121D4X (see attached). The presets seem to be labelled slightly differently depending on which diagram you find on the web, so the attached image might not necessarily be definitive.

Thanks

part 2

Thanks speeddemon but unfortunately I'd already checked out that service manual several months ago and found many of the references, e.g. component numbers, test points, voltages, etc, don't match my radio and I presume the UK version of the Tristar 747 et al. Additionally, the PCB layout is not for the PTBM121D4X.

Anyway, possibly being a bit over cautious here and going to assume the obvious in that it's the FM deviation preset, RV1, in which case the answer is yes, the deviation preset adjusts the FM audio output on FM only, ibcomms.

Send it to Nick

I could but I'd prefer to try to repair it myself and learn from the experience. Hopefully others interested in increasing their electronics knowledge in radio will also find this post useful.

Phuture, I'll do a detailed reply in a few hours when I get home, at work right now.

Phuture wrote: ↑23 Nov 2017, 17:05 Thanks speeddemon but unfortunately I'd already checked out that service manual several months ago and found many of the references, e.g. component numbers, test points, voltages, etc, don't match my radio and I presume the UK version of the Tristar 747 et al. Additionally, the PCB layout is not for the PTBM121D4X.

Anyway, possibly being a bit over cautious here and going to assume the obvious in that it's the FM deviation preset, RV1, in which case the answer is yes, the deviation preset adjusts the FM audio output on FM only, ibcomms.

The Lou Franklin manuals are written for one of the Cybernet boards (they're also copyright).

All the boards are similar enough that it's not difficult to translate component numbers, VRs, coils etc between them but you do need to be able to read the schematics and have a bit of an understanding of how the radios work.

Get the Ham Jumbo 2 service manual or the Concorde II manual, they're much closer to what you have.

http://www.cbradio.nl/ham/Service_Manua ... er_ENG.pdf

It's a process of elimination from here. There are lots of things that can cause "distortion" but the devil is in the detail, for example distortion caused by a failed transistor or capacitor sounds different to the distortion caused by a bad filter, and distortion caused by a bad bias component sounds different again.

First up, do you have any means of recording audio at wired line level such as the blue jack sockets on a computer(i.e. not by using a microphone or by using a smartphone app)? If not, no big deal, just a different method.

Will reply with more detail once you answer us on that one.