PLL02A (PTBM048AOX) problem

[TM86]

Colt 480 that's been modded and demodded, possibly more than once. Came to me with no PLL action at all. Found that the VCO block had been swapped at some point, and the one that was in the radio looked like it had been attacked with pliers to pry the metal off, unsuccessfully. Swapped it with one of those aftermarket ones and VCO started oscillating.

Found that Pin 2 (Freq in) of the PLL was receiving no signal, and found that it wasnt coming out of the VCO mixer IC. I replaced the VCO mixer IC and Pin 2 now has a signal. Figured this would the root cause of the problems on this radio, since It was an original part. Well, that or someone out there has mastered the art of making their solder jobs look like factory work. Anyways, since I'm posting this you've probably guessed that wasn't the fix.

I have the proper voltage to the PLL (5.63V) and the channel selector is working. But all I get out of the PLL on the phase detector is either a high (5.29 V) or a low (8 mV). This is with two different chips (PLL02A and TC9100P, same design, different companies). I can make it jump from high to low and back again by tweaking the VCO coil, so there is some feedback in the circuit.

I do have a good 10.240MHz reference clock.

The signal coming into Pin2 on the PLL is in the 2 MHz range, and does not change in response to the channel selector being turned. It does drift around a bit.

Nothing tied to the PLL seems to shorted to either ground or +5V.

Anyone seen this or a similar problem before? Or should I break out the hammer now and save myself some time and heartburn?

 

[Dmans]

D1? A "varactor" of sorts I believe.

73
David

 

[nomadradio]

Never learned how to make sense of a faulty PLL without a 'scope. Just watching the VCO tuning voltage could reveal if the VCO tuning slug is just a couple of turns the wrong way.

Found more than one PLL that exhibited this response when the tuning slug was way off in left field.

73

 

[TM86]

D1? A "varactor" of sorts I believe.

73
David

In this chassis D1 is a switching diode to cut off the RF preamp when the lock detector goes low. It kills voltage to the base of Q1, which in turn prevents Q7 (the actual preamp) from doing anything.

Not trying to play professor here, as I'm sort of learning as I go on this one. Up till now, I've not had to muck about in the PLL loop on this chassis.

Maybe you're thinking of D3? That's a varactor for the loop downmix crystal, which seems to be fine as it's oscillating at the right frequency.

 

[TM86]

Never learned how to make sense of a faulty PLL without a 'scope. Just watching the VCO tuning voltage could reveal if the VCO tuning slug is just a couple of turns the wrong way.

Found more than one PLL that exhibited this response when the tuning slug was way off in left field.

73

Gave it a try and ran the tuning slug from the bottom to the top of it's range. All I've found is that there are two ranges where the PD voltage goes from low to high. One of those is with the slug extending out of the top of the coil, so I'm pretty sure that's not right. I set the tuning slug within the area it got the first high voltage reading out of the PD. The output of the VCO does change in response to that voltage change.

Also, for grins, I pulled and tested each of the three tantalums in the filter circuit between the PLL and the VCO. All three tested good so they went right back in.

 

[Dmans]

Maybe you're thinking of D3?

I was looking at the schematic I have for the Midland 77-882 which shows the same board number as yours.

Since the VCO is an "aftermarket", maybe the tantalums need to be a different value?

Looking at the 77-882 schematic, capacitor #'s C1, C4, C6, C8 & C22 all reference a "nominal value".

I'm out of ideas now.

73
David

 

[TM86]

I was looking at the schematic I have for the Midland 77-882 which shows the same board number as yours.

Since the VCO is an "aftermarket", maybe the tantalums need to be a different value?

Looking at the 77-882 schematic, capacitor #'s C1, C4, C6, C8 & C22 all reference a "nominal value".

I'm out of ideas now.

73
David

Different board, but only by one digit. Midland 77-882 is PTBM049AOX, this thing is PTBM048AOX. A lot of the same parts, but connected together differently.

I hadn't considered needing to change the tantalum values since the VCO block is supposed to be a drop in replacement. If that's what needs to be done, fine, but I have no clue what the values should be or how to calculate them.

 

[nomadradio]

Beginning to make me wonder if there's a compatibility issue with the replacement VCO module. Some of them are built for the PLL chip's tuning voltage to increase frequency as the voltage rises. This is done by grounding the varactor diode's anode, feeding tuning voltage to its cathode side.

Other Cybernet radios are inverted. VCO frequency falls as the tuning voltage rises. This is done by connecting the varactor diode's cathode to a fixed 5-Volt or 8-Volt source, and feeding the tuning voltage to the cathode.

Gotta look this one up next time I'm at work.

Installing a VCO that responds in reverse to the PLL chip's tuning voltage would do something like what you describe. And maybe I'm so far out on this limb that it will snap off any time now.

73

 

[guitar_199]

Interesting.....
(Just following along from the bleachers....)
I just looked in the SAMS for the Colt 480 and it doesn't say ANYTHING that I can find about the VCO. Not even a part number in the parts list. Just a box on the schematic that says "VCO". Kind of "makes it tough"!!!!

 

[TM86]

Beginning to make me wonder if there's a compatibility issue with the replacement VCO module. Some of them are built for the PLL chip's tuning voltage to increase frequency as the voltage rises. This is done by grounding the varactor diode's anode, feeding tuning voltage to its cathode side.

Other Cybernet radios are inverted. VCO frequency falls as the tuning voltage rises. This is done by connecting the varactor diode's cathode to a fixed 5-Volt or 8-Volt source, and feeding the tuning voltage to the cathode.

Gotta look this one up next time I'm at work.

Installing a VCO that responds in reverse to the PLL chip's tuning voltage would do something like what you describe. And maybe I'm so far out on this limb that it will snap off any time now.

73

If it helps any, the VCO block I'm using is the one sold by Spectrum Communications out of the UK. They advertise it specifically for this board and physically it's a pretty close fit.

The varactor anode is connected to a 22kOhm resistor and through that to the PD signal from the PLL. The varactor cathode is connected to a 6.8kOhm resistor and through that to +5V.

If you want pics of it I can do that. I bought two, because you never know when the next problem child radio will show up.

 

[TM86]

Interesting.....
(Just following along from the bleachers....)
I just looked in the SAMS for the Colt 480 and it doesn't say ANYTHING that I can find about the VCO. Not even a part number in the parts list. Just a box on the schematic that says "VCO". Kind of "makes it tough"!!!!

45 years or so down the track I can say that was kind of a jerk move on Cybernet's part.