GME Electrophone final replacement

[falcott]

Hi all - this is my first post, so be gentle with me

I have a GME Electrophone black model TX830N (26MHz for New Zealand legalities) with a blown final transistor (2SC2078) due to a broken antenna.

I have access to a 2SC1969 transistor that I would like to fit in place of the C2078 that I don't have. I understand that this is a drop-in, but what else do I need to tweak?

I have a circuit diagram for the TX830N, but I'm not sure how to read it exactly.

I also have the use of a Hewlett Packard RF communications test set, though there is no one who can tell me now to drive the thing. I can switch it on and find the screen that tell the Tx frequency and power easily enough... Is it necessary to attach a load when using the test set, and if so, how?

I'm sort of a newbie when it comes to fiddling inside radios, though I've built hi-fi equipment before. Can someone give me a leg-up? Thanks!

[The Defpom]

The GME CB's are very basic, you can just drop in the new final and be done with it, unless you need to align the TX coils (unlikely unless someone has fiddled with them), if you do need to adjust the TX you can get away with simply putting the radio on CH19 and peaking T6, T7, T8, T9.

RX alignment would require an RF signal generator (probably has one built into the test set you have access to), again, peak on Ch19, T1, T2, T2a, T3, T4, you do this with a little signal level as possible from the generator, then set the generator to -67dB output no modulation and adjust RV1 to not quite light the +30 LED.

One modification I do is add another series diode to he diode located next to C92, this reduces the power output a little, and helps to give the radio more powerful modulation, the AMC is in the middle towards the front under the wires.

I am not sure if the 1969 will work, I have never tried substituting one, I have always replaced like with like.

[The Defpom]

Also, there is one more thing, it is likely that the audio transformer is fried as well, they often burn out when a final is blown on GME's, replace the final and see if the radio has audio, you may be lucky.

[falcott]

Thanks Scott, your advice is greatly appreciated!

I assume you mean Rx audio. Yes, it is likely that this has gone too.

Do you know where I can source an audio transformer and 2SC2078 transistor?

Cheers!

(PS: I guess you can swap this thread to a more appropriate forum now )

[The Defpom]

I get parts from the US (try rfparts.com), but as far as the audio transformer goes, I have been lucky, as I managed to obtain some junked radios with good transformers in them, however, I have seen that Jaycar and Dick Smiths do have some, I have not tried them out yet though, it may take some experimentation to find a suitable match, if you do end up going down this route please post back here about the results if any of them worked in the GME.

The transformer is used in TX as well as RX, to produce the AM modulation.

[falcott]

Hi Scott,
I managed to pick up a modulation transformer from Spectron in Auckland. They are GME agents and I have an account with them. I didn't try Standard Communications who supply GME in NZ.

The TX830N certainly looks like a basic radio, as you observed. Only about 3/4 of the board is populated - looks like it was cobbled up from a different radio.

Cheers!

[The Defpom]

I am impressed, you managed to get OEM parts, you obviously have some knowledge when it comes to repair work and electronics, I will have to note down those sources as places to look when I need GME specific parts.

How much did the transformer set you back ?

[falcott]

The transformer was only $8.24 incl GST, and it was the only one in stock. They have only a very limited stock of parts for the TX830.

I have only a little knowledge of electronics. My grandad, God rest his soul, was a bit of a wizz with valve/tube radios, and I didn't learn as much as I should have from him. Over 50 years ago he had a ham radio filling a bedroom, and when I was a kid we used to pirate broadcast to each other on ex-army ZC-1 transceivers on occasion, though we lived next door to each other! In his last few years he enjoyed CB DXing. I suppose what I'm saying is that I have more inspiration than knowledge!

[kiwizook]

Hi,

I am currently trying to fix a tx830n, which has also been used without an aerial, as the connection had broken off.
i am also suspecting a blown final, but can't seem to locate the right transistor on the board, as I don't have the schematic for it, any pointers for this?
also I see you can get the transistors of RS components easily enough

[The Defpom]

It is a 2SC2078, I do not know if RS stocks them.

[kiwizook]

RS stocks them, as I ordered a few yesterday. I mean to say I cannot find the physical position of the transistor on the board, so you got any idea where exactly it is located on the circuit board of the tx830n?

[kiwizook]

Turns out RS show stock on their website, but don't actually have them as they have been discontinued. just ordered some of ebay, so will see what they are like when they arrive next year

[kiwizook]

does anyone have the specs for the audio transformer used in the tx830N?