Info needed...no SSB Tx on a Uniden Grant

Stimpy · Post by **Stimpy** » Monday 22nd Sep 2008, 0:26

Hi all,

I'm currently trying to bring a sick Uniden Grant back to life (8719 pll, PC-409AC mainboard)....

This radio works fine on AM, both Rx and Tx (although tx audio may be a tad quiet, might just be me, though!), but refuses to produce any swing on either SSB mode..

SSB receive was also dead when I got it (this turned out to be D37 and D38 gone O/C, these have been replaced with 1N4148s).

I have checked the voltages on the MB3756, shown below:

Rx Tx
7.8v 7.8v
13.53v 13.14v
7.8v 7.8v
0v 0v
13.53v 0.03v
7.8v 0.21v
0.42v 7.79v

(above readings are from rearmost pin working forwards!)

Does anyone have transistor reference voltages for this board? I suspect a switching problem, but help from a pro would be appreciated!!

Have also noticed that if I rest a finger over the rear of the PCB in the area of L23, radio will peg the needle on SSB Tx and light a dummy bulb (brightly) in the ant. socket......!

Anyone have any suggestions as to what I should try next?

Thanks in advance,
Dan

ramblingman · Post by **ramblingman** » Monday 22nd Sep 2008, 3:35

Have you checked simple stuff first..First of all i would check
the mode switch.Have seen a dirty mode switch cause this
many times.Also make sure all wires are still connected on mode
switch..Poor solder connections..

Stimpy · Post by **Stimpy** » Monday 22nd Sep 2008, 4:27

Hi Ramblingman,

I'm pretty sure the mode switch checks out fine.....think I saw around 8 volts on each position when I tested it - I'll double check that in a minute!

Would I not have receive problems if this switch was 'bad'; receive in all modes seems to be fine.......

Will go check it again anyway - thanks for your suggestions so far!

73s

Dan

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Monday 22nd Sep 2008, 5:30

It sounds like the balanced modulator has probably failed, that is where I would start.

It could also be something wrong with FT3, as both SSB RX and TX pass through it.

What I would do is start by trying to do an alignemnt and checking to see if any freqencies are missing, once they are all correct I would move on.

If I remember rightly the Grant you have there uses a 1.5KHz offset instead of a 2.5KHz offset for SSB, make sure that you have that correctly set otherwise the FT3 could be cutting off the sidebands.

Stimpy · Post by **Stimpy** » Monday 22nd Sep 2008, 5:47

Hello again

Just double checked the mode switch - all the wiring seems to check out OK, and each switch set seems to be doing what it should....the pole that deals with supplying 8volts seems to be working correctly, and doesn't seem to be getting dragged down at all (I'm seeing just under 8v on the wiper, same on each contact associated with it).

Any ideas on what I should try next??

Thanks in advance,

Dan

Stimpy · Post by **Stimpy** » Monday 22nd Sep 2008, 6:04

Hi Scott,

I checked voltages earlier on the BM, they were as follows on USB:

RX mode pins 1-7..........0.98, 3.10, 3.09, 0, 5.58, 7.05, 6.40
TX as above..................0.96, 3.04, 3.03, 0, 5.47, 6.93, 4.07

(TX readings were taken with just a deadkey, modulation didn't seem to be varying anything much, if at all)

Just yesterday I tried doing a re-alignment on the radio using your very own procedure....starting at the beginning, the 10.240 PLL osc. checked out OK, the next step where the VCO signal is peaked seemed to check out OK, but I think it was the next step (don't have procedure next to me) where you check and adjust for a 33mhz-odd signal measured off L21 - this is where I was getting hung up - I was only seeing around 23mhz!

Any thoughts? My test gear is 'basic but adequate', by the way - DVM, 2Ghz Freq. counter, 'scope good up to around 20Mhz etc...

All your help is much appreciated!

Dan