Old military radio: Collins PRC-66 teardown! - Page 1
Forum section: electronics / general

author: TheMG
author_rank: Frequent Contributor
timestamp: May 07, 2014, 03:57:00 am
content: While watching Dave's most recent mailbag episode and the cordwood puzzle made me think of a teardown I did a while ago of the Collins PRC-66 I have. It is a UHF "manpack" transceiver which was primarily used for portable ground-to-air communications. It operates in the frequency range of 225-399.95MHz in 50kHz channels. Modulation is AM only, and output power is 2 watts. As far as I know, it was manufactured some time around 1967, so this was cutting-edge technology at the time, far ahead of any consumer electronics of that era!When I acquired it, it was in a non-working condition. I was quickly able to determine that several "wet" tantalum capacitors in the voltage regulator module had gone short, taking out a tiny fuse in the process. I replaced these with modern tantalum capacitors and replaced the fuse which got the radio working nearly perfectly!Without further ado... the teardown in pictures with captions!Outside view of the radio. Classic military green paint. Chassis is presumably some sort of magnesium allow. Rubber seal along the edges of the bottom cover keeps water and moisture sealed out. 28V NiCD battery pack (not shown) clamps onto the back end of the transceiver.Obligatory nameplate shot.Front panel with connectors and controls. BNC connector on the left is for the antenna, blue connector in the middle in the for the earphone and mic or handset. The connector labelled "wideband" as far as I know could have been used to connect a modem and transmit/receive data, though I have no confirmation of this. Controls are fairly simple and typical of a lot of transceivers. Squelch control labelled "SQ", volume control, power switch which also selects the operating mode, and the frequency controls. Note that this is a frequency-synthesized transceiver, and therefore the frequency is selected in precise 50kHz steps.Bottom cover removed, showing the modular construction. Modules can simply be pulled out using the conveniently provided pull handles. This allowed quick and easy field repair using spare modules.Modules remove to bare chassis. Note the function and location of each module is clearly indicated.Close-up of the tuning mechanism. Each of the frequency selection knobs operate PCB-type wafer-like switches to set the correct bits for the frequency synthesizer to get the desired frequency. Also there is an intricate gear mechanism including a differential, this is used to turn the tuning capacitors in the receiver front-end filter as well as the transmitter section output filter by the appropriate amount for the selected frequency.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 07, 2014, 04:18:12 am
content: Innards of the regulator module. You can see some of the offending faulty "wet" tantalum capacitors, they are the silver tubes with red on one end. If you look closely, you can see severe corrosion on one of the leads. This is due to the acidic electrolyte eating away at the capacitor after so many years. Note this regulator is actually an SMPS type.Side view showing the "cordwood" construction. This is why the mailbag episode with the cordwood puzzle made me think of this teardown. Excellent example of the cordwood construction used to save space. It sure is tight in there!Another view, with the side PCB folded down.This is the IF and audio module. Note the presence of test points and adjustment pots available through the top of the module. I haven't really poked around with it to figure out what they are for, but there's a high chance that it is typical adjustments such as AGC, audio limiter, mod depth, etc.Top of module opened. Lots of point-to-point components and wires despite the use of an etched PCB. I guess multi-layer PCB construction did not exist at the time.Other side of PCB. Lots of Collins branded hybrid modules of some sort. No idea what's actually in them, and I'm definitely not going to destroy it to find out.Synthesizer module. For those that aren't familiar with radio theory, this module produces the necessary local oscillator frequencies for the radio to operate. A frequency synthesizer is a way of obtaining a large range of precise frequencies from only one reference crystal oscillator. In modern transceivers it is often a one-chip solution, but in 1967, it needed a whole lot of components to make one, as you will see!Connector porn! Gold plated high-reliability mil-spec goodness. These must have cost a fair chunk of change!

author: TheMG
author_rank: Frequent Contributor
timestamp: May 07, 2014, 04:18:24 am
content: Top off the synthesizer module. Again, lots of point-to-point wiring on top of PCB. Note the gold fingers barely visible near the top edge, to provide good RF shielding to the housing.Side view of synthesizer. Visible is one of 3 daughter PCBs with a large number of surface mount ICs (yes, SMT tech in 1967!). These chips are most likely the programmable dividers in the synthesizer PLL feedback loop.Bottom side. You can see the 3 programmable divider boards, and a bunch more of those hybrid modules.Other side view. Each hybrid module has its own part number and individually serialized. Again, no idea what each one does exactly.Receiver module.Inside top of the receiver module. All point-to-point construction!

author: TheMG
author_rank: Frequent Contributor
timestamp: May 07, 2014, 04:18:38 am
content: Bottom side of receiver module, showing the air variable capacitor which is coupled to the gear mechanism for the tuning knobs. This is very likely for the front-end RF bandpass filter. In a modern receiver, the same job would be done electrically instead of mechanically by varactor diodes. Note also if you look closely, the moveable capacitor plates are sectioned into petals. I noticed some of the petals were bent a little, but realized this is done intentionally, as a way of fine-tuning the characteristics of the variable capacitor.Modulator and RF power amplifier module. This module also has a variable capacitor much like the RX module.End view of the TX module. Note the shaft of the variable capacitor has a cam on it, to engage a couple of switches depending on selected frequency range. Not entirely sure what they do though.Inside TX module. More point to point construction.

author: TheMG
author_rank: Frequent Contributor
timestamp: May 07, 2014, 04:19:27 am
content: That's all for now folks!Unfortunately, no schematics. I have not been able to locate the service/technical manual for this.One thing is for sure, this transceiver must have cost a FORTUNE in its day! Components and technology that were not yet available in consumer gear, combined with almost entirely hand-assembled construction. Imagine how many hours of labor went into assembling each unit! It's insane! Truly a work of art.

author: SeanB
author_rank: Super Contributor
timestamp: May 07, 2014, 04:48:59 am
content: Nice, I saw a PRC77 at the local hamfest last weekend. Was tempted to buy it ($15) but decided not to. Way too many boat anchors, plus it was in worse condition.

author: Vgkid
author_rank: Super Contributor
timestamp: May 07, 2014, 05:16:36 am
content: Thanks for the teardown, it is truely beautiful inside.

author: 128er
author_rank: Regular Contributor
timestamp: May 07, 2014, 02:09:01 pm
content: Thank you for the pictures!Old military stuff is the best. It is built so rugget and the designers put a lot attention to detail to this devices. If I have time, I will show you my SEM 35 (Mid 60's german military radio). Total discrete analog technology, not even one IC. And from a mechanical view totally overengineered, but very beautiful.http://rigreference.com/en/rig/5168-SEM_35

author: Fank1
author_rank: Regular Contributor
timestamp: May 08, 2014, 02:02:07 am
content: I still have and use a set of Collins Radios made in 1958.Still work great, you just can't beat Collins quality.