Loss of some channels on TX.

[ChubbyChops]

....... we digress

[QRZ-934]

I did note that pin 11 of the upd7508 is the lock detect signal , so the first job is to meter that pin on all channels on receive and transmit to check that the pin remains high and the VCO is locked on all channels.
I'll check back in here later if my headache doesn't get any worse

Hope your headaches better, stand by for another.

Ok, page 15 of the manual shows the upd7508 and pin 11's states should be: H=Unlock, L-Lock which is backwards to what you suggested.
Not an issue as ive not taken the top cover off yet to get at the bugger.

I did however remove the bottom cover to access the component side of the board and after locating the various test points on the diagram and the manual i did the test for vco voltage suggested in the paperwork, Test point 1 appears to be the correct location for that test and connects to pin 8 of the vco which is accessible from the component side of the board.
According to the manual it should be between 3.5 to 4.5 volts when the radios on Channel 1 ( which it was) however voltage was too high at 5.11volts.
After monitoring the voltage I noticed it creeping upward the longer it was left on until it finally stabilised at 6.20volts.
Me being me suspected it was drifting cos of temperature and after blowing cool air on the front board for a few moments it started to creep back down a few tenths, dont know if thats just a side issue or a sign of something else that needs investigation tho.
I must state right now Ive not adjusted anything in this radio, it had this issue when it was purchased, the only other work I did to it was to recap the set with all new panasonic/nichikon capacitors, a few of the old ones were out of spec by a mile.
Now Ill go take off the other cover......

[ChubbyChops]

Not uncommon for radios to "warm up".
I always leave on for 30 mins to stabilise before adjusting anything.

[QRZ-934]

Not uncommon for radios to "warm up".
I always leave on for 30 mins to stabilise before adjusting anything.

Yes indeed, I dont think some people understand that even in this day and age that kind of thing happens.

Also checked the outputs of all the voltage regulators, all seem on the money, looks like theres an AN 6541 feeds the vco block on pin 8 via R6 560ohm resistor.
It also feeds pins 4 and 6 so to my mind unless the regs on its way out theres and issue connecting to that somewhere.
Will try and do the lock/unlock checks on the cpu chip in a while see what that shows me.

[Alucard]

Ok, page 15 of the manual shows the upd7508 and pin 11's states should be: H=Unlock, L-Lock which is backwards to what you suggested.

Got a headache again tonight ffs. 2 in 2 days and I never get headaches
You're quite right, my mistake I'm so used to the lock detect being high for lock I just presumed that was the case.

[QRZ-934]

Ok, page 15 of the manual shows the upd7508 and pin 11's states should be: H=Unlock, L-Lock which is backwards to what you suggested.

Got a headache again tonight ffs. 2 in 2 days and I never get headaches
You're quite right, my mistake I'm so used to the lock detect being high for lock I just presumed that was the case.

No worries sir, Hope your head feels a tad better soon.

[QRZ-934]

With a logic probe on pin 11 of the cpu, i tested each channel on rx and tx for a state of change.
Nothing discovered there apart from a slight change in tone from the probe when keying the radio onto transmit.
Otherwise appears to be locked on every channel.
Vco voltage after about an hour rises to 7.75volts and appears to want to keep on going.
I checked the first few items on the alignment section , battery backup volts as should be on 5.0volts.
Avr voltage also at 5.0 volts.
Voltage on pin 18 of pll should be 5.6, I had 5.07.
Still being nagged by that excessively high vco voltage so I went back to the diagram and found D17 (MA1051-m) is a zener. Gets 9volts from the an6541 thrust at its input but other side which appears to show 4.0volts should be present.
Havent actually tested that yet as its buried down the side of the chassis. Its just like working on peugeots and renaults.....
So Im wondering if the diodes doing a number on me.
Dont know yet.

Regarding that frequency discrepancy in the channel assignments table.
Does anyone know why the RX frequency would =TX plus 58.1125 mhz???
It makes no sense to me at all.
Ch40 on lo band for example states tx= 933.9875 and rx =992.100mhz.....?
Im stumped at that.
Whats it mean?

[DX-Digger]

Something of interest in the spec sheet however thats got me scratching my head.
in the channel assignments section of the manual its specifies that say for channel 1, the tx frequency is 934.0125, the rx frequency is specced as being 992.125mhz....???
At the bottom of the page it states: " RX frequency =TX frequency +58.1125mhz".

I dont understand that at all..... anyone else have a clue?

Any help appreciated.

Basically there is Xtal oscillator in the radio set to 58.1125 MHz
It mixes with the incoming 934.0125 MHz signal and produces either the sum or the difference.
In other words you can either up mix or down mix the signal.
So receiver is either 992.125MHz up mixing
or 875.900 MHz down mixing.
In this particular radio it has an IF (intermediate Frequency) of 992.125MHz thus it is up mixing.

I hope that makes sense.

[QRZ-934]

Something of interest in the spec sheet however thats got me scratching my head.
in the channel assignments section of the manual its specifies that say for channel 1, the tx frequency is 934.0125, the rx frequency is specced as being 992.125mhz....???
At the bottom of the page it states: " RX frequency =TX frequency +58.1125mhz".

I dont understand that at all..... anyone else have a clue?

Any help appreciated.

Basically there is Xtal oscillator in the radio set to 58.1125 MHz
It mixes with the incoming 934.0125 MHz signal and produces either the sum or the difference.
In other words you can either up mix or down mix the signal.
So receiver is either 992.125MHz up mixing
or 875.900 MHz down mixing.
In this particular radio it has an IF (intermediate Frequency) of 992.125MHz thus it is up mixing.

I hope that makes sense.

Yeah that helps somewhat, im just not particularly au fait on all the in depth stuff.
Basically I know enough to get myself into trouble.
Located D17 on the board after much messing, voltages are fine at the two points so rules that out.
Also located r6 on the board ( its unmarked and an smt device-joy) volts into it are 8,8 ,on the other side theyre at 7 so tallies directly with TP1 volts but still over the top for whats specified at between 3.5-4.5.
Has to be a reason for it.

Ho hum, more messings must follow.

[Alucard]

Have you got a frequency counter? Might help to know where it's going to when it's not on frequency.

( I know Chubby's gonna check this thread.. I can't reply in your wanted thread for some reason but I've probably got a LC7132 somewhere if you still need one.)

[ChubbyChops]

( I know Chubby's gonna check this thread.. I can't reply in your wanted thread for some reason but I've probably got a LC7132 somewhere if you still need one.)

Sorted now thanks mate

[QRZ-934]

Have you got a frequency counter? Might help to know where it's going to when it's not on frequency.

( I know Chubby's gonna check this thread.. I can't reply in your wanted thread for some reason but I've probably got a LC7132 somewhere if you still need one.)

Alas not at the moment. It was something i was looking at for a while but cash is a tad tight at the moment, however the one ive got my eye on is in the Rf explorer series.
I know at some point Ill need one but Im wee bit tighter than a ducks butt and loathe to spend it if I dont have to.
Looks like Ill have to though to monitor it properly.

[QRZ-934]

Ok. Im an ass. I didnt need to prostrate myself and beg for your forgiveness but here I am.
I am an ass.

Turns out I couldnt fault the radio on TX after all.
Couldnt fault it at all, cos Ive been testing the wrong one........(gets me coat).

So its actually a receive fault on the other one. Same symptoms however so at least I got something right.
After testing the pin 11 PLL I got channels 1 to 11 locked. All others from 12 up start pulsing hi so are unlocked.
Probes making a tone that rises and falls, the higher up the band you go the faster it pulses.
Sounds like a capacitor charging and discharging to me.
Bear in mind I recapped these both and this one did have the fault prior, maybe I missed one.
Unless anyone else has a suggestion.....no Im not doing that to myself!

[Alucard]

Sounds like the VCO is just out of lock. Try giving it a small tweak preferably with a non metallic tool if you have one.
Also a frequency counter needn't break the bank, I bought a few of these..

Freek.jpg

From Ebay cos they were cheap (8 quid each) not expecting much and they were amazing, very accurate, well made have programmable IF offsets etc.
The only possible downside is they need to be quite close to the transmission to get a good reading but really that's no different to my main counter that cost 25 times as much

I didn't buy mine from the seller whose watermark is on the pic it's just the first image I found. You can still get them for 8 quid from Ebay.

[QRZ-934]

Sounds like the VCO is just out of lock. Try giving it a small tweak preferably with a non metallic tool if you have one.
Also a frequency counter needn't break the bank, I bought a few of these..

Freek.jpg

From Ebay cos they were cheap (8 quid each) not expecting much and they were amazing, very accurate, well made have programmable IF offsets etc.
The only possible downside is they need to be quite close to the transmission to get a good reading but really that's no different to my main counter that cost 25 times as much

I didn't buy mine from the seller whose watermark is on the pic it's just the first image I found. You can still get them for 8 quid from Ebay.

Thanks for that, il have a snudge around and see what I cant get.

Dont want to tweak anything at this point until I know I can return the trimmers to their exact starting point, I did just touch the top of the trimmer with a metallic tool to see if the interaction of me attached to the end would make any state changes, it didnt.
But I have found something else thats giving clues out.
At cold switch on, the channels are locked up to around channel 25 and as the radio warms up the lock is lost and the lock then passes down to the next channel which then starts to lose it and so on the longer the radio s on for.
Something overheating or changing its resistance perhaps.....
Still cant see why that TP1 voltage should be so high when it clearly states on the diagram to be 3.5-4.5 volts and as far as I can determine theres no way to alter it.