The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

hi folks,
does anyone know what components need changed on the blue display 158edx?
there is a mod for the old 158 but the components have been changed on the new echo board and the components seem a lot smaller.
the board no is EPTOSSB501.
i would like to enable full fm modulation or as near as ...

I think the capacitors that are being referred to would be C120 and C75 on the EPT3600 chassis.

I would try changing C120 to 270pf and C75 to 56pf (use NP0 caps), then performing a re-alignment of the 14 MHz oscillator in accordance with the service manual.

To be honest I don't think that ...

Ok then Mick, we have a mod, 1n8 it is then!

"The Superstar 158 audio mod" (c)Brian Morris and Mick T.

Have you set up the bias for the driver and the final in accordance with the service manual? Check R263 connected the base of the final, sometimes this burns out and goes open circuit.

Brian.

Hi Brian
Tried a number of different values of capacitor and the 1800pf was the best in this case.
Thanks for your help.

Ok, the reason I chose 4n7 was that in theory it should give a cutoff frequency of just over 3kHz. Was there a vast difference between using 4n7 and 1n8?

Would you say that ...

Or, get a TIP42 from Bardwells, cheap as chips!

http://tinyurl.com/6j4gc7

Brian.

Maplin don't stock much in the way of components these days, but a TIP2955 should do the job, but not quite the same case, so you might have to do some frigging around to get it in.

Regards,

Brian.

Hmm, seems a bit strange that, if anything I would have expected to see 8 Volts on AM and LSB since you say that AM and LSB are working properly.

You should see 8 Volts on the appropriate tag for the selected mode, if you don't then either the switch is knackered, or something has gone short ...

Ok Shaun, rather than guesssing, I think we'll have to start fault finding and do some voltage measurements; let's start with the mode switch, what voltage do you get on the USB tag when in USB mode, and what voltage do you get on the LSB tag when in LSB mode?

Brian

Check L28 and D38 too.

If you have a continuity tester, put both probes accross the diode, then swap them round, it should conduct one way but not the other way.

These days most multimeters have a diode test setting on them, in which case the meter should show 0.6 to 0.7 volts one way round, and ...

They are probably just standard silicon diodes, in which case a 1n4148 will be fine as a replacement. Definitely worth cheching that L25 isn't open circuit as well.

Brian.

Did you test the diodes when you removed them, or replace them with new diodes?

Brian.

DIRTY HARRY wrote:Hi Scott would the Balanced modulator be IC3 which is a AN612 ?

Apologies to Scott for jumping in and answering, but yes that's correct IC3, the AN612 is the BM. I doubt very much that this is the problem, otherwise it wouldn't be working on LSB.

Regards,

Brian.

D33 definitely worth checking, also D37.

Regards,

Brian.

Right, I think I can see what's causing this problem.

The first three components in the audio path on the echo board form a crude band-pass filter; the value of the resistor (R1) has increased from previous versions of the Ranger echo board, and this has had the effect of making the low-pass cutoff ...

Hi Shaun,

All understood. If I get time, I'll have a look over the diagram myself and see if I can come up with any ideas.

Regards,

Brian.

There can't be that much wrong with it if it's only USB that's not working, should be easy enough to find the fault with the circuit diagram (see link below):

http://www.cbtricks.com/radios/uniden/grant/index.htm

Maybe start at the mode switch and, and then follow the diagram from there.

Regards ...

Hi,

If you can hear the roger bleep then this means pretty much all of the audio chain is working. Check the mic, the mic socket pcb (check caps and chokes), check the mic gain pot and it's associated wires, and check that R224 is still 10k. There's not really anything else it can be.

Regards ...

Does anyone have a circuit diagram of the echo board? It seems a shame to bypass the echo board completely, it would be better if we can work out where the audio frequency response is being messed up on the echo board, and then come up with a fix.

Brian.

This is an easy mod; you need to change 3 components, Remove D33. Change varicap diode d31 to SVC251 or BB809. Change R231 from 6K8 to 3K3. Remove R168 and connect the top end of the Clarifier control (Looking at my diagram) to the always 8-volt line, you can pick this up at the Collector of TR39 ...