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

OK People, here is another one for you to sink your teeth into, this is a radio I fixed a few years ago now, but it is a good one to test you... it could be an easy one, it just depends on what questions you ask.

Radio Make and Model: Uniden Grant (same as President Grant)
PCB Number: PC-409AC
PLL Number: MB8719

Symptoms: SSB TX Audio severely distorted and unreadable, AM TX audio noisy but understandable, RX audio slightly distorted sometimes happens in all modes.

What is working: Everything else is working perfectly.

What has been tried: Full Re-alignment has been done.

Did it suddenly happen ?: I don't know, was like it when I got it.

Anything else that may be of help?: This is a fairly old radio, but is in OK condition considering its age.

Diagram is attached.

(note to self 15-12-05)

Are the voltage readings ok at IC6? Have you tested all the electrolytic caps in the audio circuit. Since it is an old radio some of these may be faulty. Perhap I should ask, is the PA working normally and is it noisy?

I'm just guessing as usual but I try.


Regards, Jim

IC6 voltages read as normal.
All electrolytic capacitors around IC6 audio circuit are OK.
PA audio is normal.

If you examine the output of IC 5 (TX Mixer) on a scope, does the signal look normal or is it distorted?

If you also examine the input and output of IC 6 on a scope (Audio output) does the signal look normal or is it distorted?

Do the DC voltages on IC 4 (Voltage reg) look clean on a scope?

Given the age of radio, I would automatically suspect any and all 10volt caps, and take a close look at any tantalum caps as well.

The output of IC5 looks distorted.

The output of IC6 looks normal.

Do the DC voltages on IC 4 (Voltage reg) look clean on a scope?

Yes, look normal

Are the voltages on IC5 normal?
Is the VCO output at TP2 clean?

IC5 voltages are normal
TP1 (VCO output) output looks noisy. (TP2 was the earth)

Would you test C172, 173, 174, and D63, and D64

Also have you checked for solder bridges on the track side and that all solder joints are ok concerning these?

Is there any noise on pins 1,6,and 8 of IC4?

Regards, Jim

C172, C173, C174 test OK
D63 and D64 test OK.
There are not bad solder joints on the PCB
There is no noise on pins 1, 6 or 8 of IC4

Have you tried a known good michrophone?

Jim

Microphone is OK, I always use my own test mics when working on radios, because I know how they should sound then.

Well then I think this is not as esay as you thought for us. I'm about out of guesses.

Jim

OK I will try once more. By the way your schematic is a little worn and I haven't found a better one so I have to look in the radio to find numbers.

Is there any noise on pins 5,8,or10 of IC2?
Would you test Caps. 80,81,90, and 92 for me.
Is there noise on pin 7 of IC3 TX mode?

Jim

Has the VCO Modula IC2 failed?
Has the loop mixer tran TR20 gone low gain?
His the cap which go from the input side of the choke which connect to pin 9 of the pll and ground ok? can not read the ref no it looks link 1000u at 10v
It is very hard to make out most of the part ref as the circuit diagram is so bad

James.

I would take out c-90 and c-92 and test them.
Is the signal on TP-10 normal?

Seems our test giver has disappeared perhaps James and myself angered him complaining about the schematic. LOL

JIm

eurhara:

There is noise at pins 5 and 10 of IC2, pin 8 is clean.

C80,81,90, and 92 are OK
There is no noise on pin 7 of IC3

======================
James Unit 198:

I do not know if the VCO has failed, you have to tell me what is wrong by troubleshooting the radio.

TR20 tests OK.

C84 tests OK (the one by the choke at pin 9 of PLL)

======================
timrim:

C90 and 92 test OK.

Signal at TP10 is noisy.

Check 10.240 signal at PLL, if noise is there, replace the 10.240. If still noisy, replace the PLL chip.

If you inject 3.00 volts in to TP9 with one end of R207 unhooked, is the frequency stable and free of noise at TP10 or Pin 17 of the PLL?

Is the fault with one of the caps in the low pass filter (PD) C79, C80, or C81?

timrim:
There is no noise at the crystal.

=============
Rick:
There is still noise at TP10 when injecting 3V at TP9.

C79, C80 and C81 test OK.

Is the 11Mhz signal at TR-29 clean?

Have you tested L17 to see if it is working properly and is it?

Jim

timrim:
The 11MHz signal at TR29 is clean.

================
euhara:
L17 tests OK.

How about testing L16 is it the cause of the noise?

Jim

What is the voltage at Pin 10 of IC2 on channel 19?
Cold solder joint around IC2?
Is the down mix at TP 10 the right frequency?
So Pin 1 of IC2 is noisy Yes?
Is there any noise on the supply line to Pin 1 of the PLL?
Has the VCO coil L19 unstable?

euhara:
L16 tests OK.

================
Rick:
Signal voltage at pin 10 is normal, but noisy.
There are no cold solder joints on IC2, there was one on pin 8 but that has already been fixed.
The frequency at TP10 is correct, but noisy.
Pin 1 of IC2 is noisy.
There is no noise on Pin 1 of the PLL.
L19 seems to be working OK, and tests as OK.

Has R104 a cold solder joint or o/c?.
Has C91 a cold solder joint or bad?

All solder joints are now OK, on the entire PCB.

Seems we have been all over the PLL area and no luck there in finding the fault.
We have proven some things and disproved others. So would you tell us whether we have been close or need to go another directio, else give us a clue.

Regards, Jim

You are all so close...

Well then would you test L20 and see if it is keeping its setting properly?
Also Test L19 just for fun.

HOW about FET3 that looks like a good one to test. And don't forget to test L46.
Jim

His the B+ supply to the PLL and VCO free of noise and at the right voltage?

Everything requested, tests OK...

Do you want me to tell you the answer ?

I believe your answer will be that you was just joking about the problem

Regards, Jim

OK, let me put it this way...to sum up what has been covered.

It has been confirmed that there is no noise in the power supply to the PLL/VCO/TX mixer/11MHz Oscillator.

There are no bad solder joints.

There is noise on the outputs of the VCO, The PD input of the PLL, and the PD output of the PLL, and at pin 5 of the VCO when R207 is in place, but not when it is injected with a clean 3V DC at the TP9.

Then R207 must be the fault.

Jim

nope

Well I think the problem must be in IC2 one of the diodes on there must be kaput.

JIm

If c79, c80 and c81 test good, then at this time swap out IC2 and see if symptoms disappear.

WELL DONE, it was the VCO module, IC2 !

I said you were all close !!

The fault was with IC2, as the DC supply going in was clean, the PD control voltage was clean (when injected), all external components were OK, but the outputs were noisy, this meant it could have only been one thing, the VCO itself.

What makes working on the PLL and VCO circuit tricky is that it is a closed loop circuit, if something like the VCO fails in this way, its output is noisy, and this is fed into the PLL, which then tries to react to the noisy signal, so that in turn makes ithe PLL output noisy, as it rapidly adjusts itself to try and compensate for the noisy input.

The easiest way I found to isolate which part of the PLL/VCO circuit contains the fault, is to decouple the PD control voltage from the PLL and inject a new clean one into the VCO. this breaks the loop, so you then know if the PLL is making the VCO act strangely or if the VCO is making the PLL act strangely.

In my first post and only post on this thread.
I did ask if the VCO module had failed.
If you are going skirt around the truth what is the point in asking the question in the first place.
The part has failed or as not.

I noted from Rick “Test Your Knowledge” thread’s that he gave start forward answers and did not beat around the bush.

The idea of this is to troubleshoot, you asked me if it had failed, and I responded that I did not know, and that it was up to you to work out if it had or not, you asked me directly if it had failed, you did not ask for me to test it, or tell me how to test the VCO module.

So while yes your guess was correct, it was not done as a troubleshooting method, you asked me if it had failed, in a real situation, I would not have known, and would have been reliant on you telling me how test it, or ding as others did, and tell me what to check.

Anyway, this is all just for a bit of fun, how about you put something up for us all to have go at diagnosing.