PRC-320 Clansman Warble in SSB

[nissguy]

Hello All,

I've been playing with a PRC320 that I received a week ago and until two days ago it was functional with no complaint.

Now it seems that it has a warble and chirp in SSB only. AM is fine.

You can see/listen to it here: https://youtu.be/btVkeSLfm10

Would this be due to the tantalum caps in the synth I've read so much about?

Many thanks & 73!

[Radio Wrangler]

Sounds likely. Frequency fluctuations from the synthesised LO will be heard on SSB or CW, but probably be inaudible on AM.

David

[nissguy]

Thanks for the reply! I'll check it out further!

[Jon_G4MDC]

Before attacking the circuitry - what is your power supply?

[G6Tanuki]

The tantalum caps do sound like a good call, yes.

If it's not been used for some time you can also get frequency-instability due to the wiper-contacts in the bandswitch/turret. Sometimes rotating the band-select knob [Clockwise only please, to avoid potential damage to the contact-fingers] a few turns to clean-up the contacts.

if you do need to replace the tants, do the high-voltage ones in module-5 [PSU] first. They're only rated at 130V and the 'synth' rail runs at 120V. I replace them with nearest-value ordinary 105C 160V electrolytics from a reputable manufacturer [Panasonic, Rubycon, Nippon Chemi-Con].

Yes you *can* still buy the original metal-cased tantalums but they cost probably more than you paid for your PRC320 in the first place!

[nissguy]

Hi All,

You can see the radio in operation here: https://youtu.be/btVkeSLfm10

Since then I've changed the high voltage caps in Module 5 to electrolytic which did not resolve the issue, and so then I did the same for the two caps in Mod 9f.

It still did not rectify the issue. I've been investigating further and trying to understand how the synthesizer works and take a look with the DSO.

In short I found that some voltages were higher than they should be in the manual, and the frequency from the divider changes based on switch position. I understood that the Ramp Mono frequency was supposed to stay at 250hz, and the pulse width was supposed to change based on the divider output.

My summary notes here: ********************8QL8O39

9F-3 Ramp Mono - ********************WG19aEf

9F-2 Ramp Output - ********************kwsBVTf

9F-Tr2 Emitter Output - ********************bhU37k8


I'm hoping someone can point me in the right direction from here.



Thanks!

VA3UBC - Niss

[nissguy]

Edit to the above post:

I'm a bit of a dummy. I did all this without an antenna, counterpoise, or handset.

I'll add more photos later, however with the radio "loaded", the RAMP OUTPUT frequency is within the 1hz of 250hz spec as advertised, and I have a good +110V on 9F, yet the problem remains.

Now I'm wondering if the problem is no longer in the synth module, and back at the VFO? Perhaps with the varicaps?

[Sparky67]

Hi Niss,

I just watched your YouTube video. The symptom sounds the same as when the battery volts are failing on a PRC320 Tx. I had a 320 which I was told swung completely outside the Rx bandwidth as the battery volts dropped...it was perfect on a good battery.

Martin
G4NCE

[G6Tanuki]

Yes, if you're running it from an external PSU, and you don't have a good RF-ground for the radio, RF following the path-of-least-impedance and going-to-ground via the PSU can cause all sorts of horrible-wobblies in its regulator.

[I run my PRC320 from a "MASCOT 9320" switched-mode 24V 3A PSU - these were specced for first-generation analog cellphone base-stations and have really-good, industrial-strength RF-immunity/decoupling.]

[nissguy]

Thanks guys, but I have +25.8V on the battery.

[nissguy]

Hi Again!

So I did some more digging with regards to 9c Reference divider & comparator (********************dEWuv52) and it looks like my Ramp Mono and Sample Mono are mostly within spec (********************a/6UUjOYM)

However, the phase angle output is supposed to be a square wave (and I imagine at logic voltage) however it is sawtooth and in millivolts. ********************5C8sYlv

You can see the attached video of the Oscope and where I was connected: https://www.youtube.com/watch?v=nQeIdpdPSls

I can't for the life of me see where the Phase Angle Output goes in the radio, and what it gets used for. (********************3J9gXzj)


Could it be that maybe ML4 (Frequency Comparator) is bad, if it's putting out a bad Phase angle output, and if it's bad then perhaps it's feeding erraniuos information tot he counter, which would then send a bad reference voltage to the VFO? If its possible, any idea on how to verify?

Thanks again!

[Sparky67]

Hi Niss,

The symptoms do appear to be associated with high current peaks being drawn, so I would suggest similar dynamic tests would be worth investigating. I am presuming the current drawn on AM doesn't vary anyway near as much as on SSB, and any shift in frequency would probably stay within the Rx bandwidth and not be so noticeable.

Unfortunately I don't own a PRC320 at the moment, otherwise I would have been happy to compare notes with you. I had 5 here at one stage in various states of faultiness!

Martin
G4NCE

[nissguy]

Thanks for the info Sparky!

Does anyone else have any possible guidance? I'd really like to get this back on the air. This is my only HF rig, and I could certainly use the connection to other people on the air at this time!

Cheers!