Collins R-388 Receiver Explorations

I like to restore old radios.  So far of the 100 or so radios I've restored only a couple of those have been communications receivers.  Enter the Collins R-388.

A couple of months ago, the W7OS Museum at the Radio Club of Tacoma (which I am involved with) received as a donation an old R-388, circa 1950. 

At first glance the radio seemed to play well but had 60 Hz hum in the audio.  It was easy to replace the power supply filter capacitors which did not help.  Listening closer I noticed that when there was no signal being received then there was no hum.  This 60 Hz hum was riding on the B+ rail and measured at 1.0 VAC RMS.  In addition to the hum- the S-Meter did not work very well- no matter what receive level was received- the meter never rose more than 20 percent of its travel.  Investigation revealed a possible AGC (Automatic Gain Control) issue.  More on that issue later.

History Lesson:  In 1949 Collins began building the 51J which was intended for military or commercial users who required a drift free and very stable receiver with extremely accurate frequency readout.  The 51J was to be used for RTTY reception but could also provide good AM listening.  The cost of this receiver was $900.  The military bought the 51J and improved 51J-2 which evolved into the R-388.  This receiver has a 500 KCs IF output jack that provided a signal for additional equipment for decoding RTTY.

The World Wide Web has a couple of good R-388 sites with enough valuable information to get me started on the Museum receivers repair.  Note that this is a work in process… with progress being made.  The websites will be listed below.  I joined a Yahoo Group for Collins 51J, R-388, and R390 receiver aficionados to gather more information.   I think it is interesting that the rules of the Yahoo group state that a new member must post an introduction of them self and the (implied) purpose of joining.

I posted my introduction to the Yahoo group.  Some helpful members provided information on the most popular components to replace that might fix the Museums receiver.  C204 and C136 were the culprits.

  Geek talk follows:  C204 provides a signal from the last IF amplifier to the AGC detector and AGC amplifier which manages the gain of three IF amplifiers and the front end RF amplifier.  If C204 is open then no signal would be detected and the receiver would be at full gain.  C136 was reputed to become leaky and apply plate voltage from the Crystal Calibrator Oscillator tube to one of the input grids of the first mixer.  That would make a mess of the mixers amplification which would severely distort the received signal.  It was hard to find C136 as well as replace it… but perseverance prevailed.  Note to self:  Get some longer needle nose pliers.  After the cap swap-outs, the receiver was connected to an antenna and viola!  No more 60 Hz modulated audio.  High power AM broadcast stations sounded much better than before- much more intelligible audio.  But something was still wrong.

C136 and C204 are in there somewhere.  New terminal strip in lower left is the AGC mod mentioned later in this blog entry.  The cylendrical gizmo in the top-center is the PTO, which Collins is famous for.  The PTO is part of the VFO that provides a super linear and accurate tuning dial.

The receiver sounded weak, with more noise than signal.  The AGC was working but not on CW and SSB with the RF Gain turned all the way up.  On AM, the AGC worked pretty well but the audio sounded better with the RF gain turned down a bit.   The S-Meter still did not indicate a high reading on loud signals- like the 50KW AM station 6 miles away.

The Army TM854-25 instruction manual is a very good document that explains in detail the operation of each of the radios stages.  There's even a section on how to destroy the radio so it will not fall into the hands of the enemy.  But some internet enthusiasts have posted that "The manual has errors.  Trust the schematic inside the radios cover”, they say.  I did find an error, BTW.

  The TM has a section on the S-Meter that reveals the higher the amplification of the IF stages then the higher the meter reading- which means that the S-Meter works with the AGC OFF if the RF Gain is all the way up.  Also, low IF amplification could be caused by defective AGC circuitry.  Armed with the Tech Manuals tube voltage chart, simplified schematics, and my Fluke VOM… I set out to measure the front end RF amplifier and the three IF amplifiers (all gain controlled by the AGC) tube voltages looking for discrepancies. 

Tubes V101, V107, V108, and V107 pins measured pretty close to the chart, EXCEPT pin 6 of V107, which is the screen grid of the first IF Amplifier.  I measured +3 V instead of the required +47 V. 

Remembering my Dark Ages vacuum tube electronics training… I knew that the tube will not conduct (and therefore not amplify) without a large value positive voltage present on the SG.  It turned out that R126, a 27K ohm resistor, in between the B+ and the SG, had changed value to many meg-ohms.  I calculated the voltage drop across this resistor to determine the wattage needed I substituted 3 various values in parallel to come up with 27K at .7 watt.  Knowing Ohms Law has benefits, me thinks.  I'm excited... let’s take it for a drive.

The R388 now connected to an antenna works hundreds of times better.  Stations were loud, proud, and the noise was lower.  And the S-meter worked as it should.  With the RF Gain all the way up still caused SSB and CW signals to distort.  It seemed like the AGC wasn’t fast enough to deal with the quickly changing signal level of these modes.  Whilst listening to AM Broadcast… the receiver sounded pretty good and the S-Meter tracked the more solid signal pretty well.   Again, R388 aficionado’s websites mention that the R388 AGC is lacking.  The search is on for AGC Mods.

I knew a small bit about AGC from restoring many 30s, 40s, and 50s entertainment radios… but after reading the 388s Tech Manual I was a bit puzzled over attack and decay times and how it happened. I found a nicely written article by the modern day Flex Radio folks that was enlightening.  Also, the website www.neidlinger.us/R388.htm reveals there are four different AGC mods to consider. 

The AGC Modification I settled on was written by Dallas Lankford in an obscure publication called HSN (Hollow State News) Summer 1990 edition.   My friend Google found HSN #26 which contains info on the R388s Band 1 sensitivity, the PTO, and the AGC Mod.  Mr Lankford’s modification is dependent on the AGC no signal voltage being -1.60 to -1.80 volts.  The Army Tech Manual says this voltage should be -1.40 volts.  Other AGC articles want the voltage to be 1.40.  Mr Lankford’s article reasonably states that if the AGC voltage is too positive then the dynamic range of the receiver will suffer.   The R388’s AGC voltage is derived from R149, R148 Pot, R147, and R146.  I replaced R147 because its value had changed 40 percent.  This resulted in idle AGC voltage of 1.7 volts. 

Photo shows the terminal strip I added to add the AGC mod components to the AGC amp circuitry.  With this set up I can easily reverse the modification.  More info in the Dropbox folder listed below.

I performed the Lankford AGC mod with low expectations and was greatly surprised with the results.  Now the R388 copies CW and SSB quite well with the RF Gain all the way up.  AM Broadcast works quite well also.  Of course with a really loud CW or SSB signal it is prudent to turn the RF Gain down for more comfy listening.  The AGC attack time is quite rapid and decay is slow in comparison.  I suspect that this AGC would not work well for QSK CW when listening to my own transmit signal in the receiver as it would take a while for the amplification to ramp back up in order to quickly hear the far station.  I have been using the R388 with my circa 1950s Globe Chief transmitter with manual T-R Dow Key type non-QSK operation.  Slow decay AGC is not a problem with my station. 

AGC Movie:
https://www.dropbox.com/s/9xhqd29u3hrxuxa/R388%20AVC%20Demo-Lankford%20Mod%20Movie.wmv

The next step is to figure out if I want to do the product detector mod.  The suggested mod on the neidlinger website seems pretty easy, but it involves removing the front panel.  On this radio I’d have to swap the AGC SPSP switch with the Noise Limiter DPDT switch to make the product detector work.  Before I do this I will be seeking opinions and personal testimonies.

https://www.dropbox.com/sh/6i5za0ozryys290/LbEc5w97Vn

This dropbox link is to a folder that contains the documentation that I acquired during this receiver repair.  I found most all of it on the web... so it's not new news.  But it is all in one place.  

What's in the folder...
+ Photos of my attempt of the AGC mod
+ A movie of the AGC in action
+ The Army Technical Manual (two parts)
+ Annotated schematic
+ PDF files that contain the content of the web pages I found useful.
+ An article on AGC that I found useful
+ The Hollow State News collections in PDF (the entire set of HSNs are worth reading if you are a boat anchor kind of hobbiest)
+  Something I am forgetting to mention

But wait, there's more... in another blog entry.
That's all for now.  Regards,  KR7W