Many people are interested in Part 15 AM transmitters because they can be used with antique radios and this is one of my interests also. My current project is to "recondition" an antique radio which I obtained while still a kid. I bought it at a community auction house and as I recall I paid $2 for it. It was working at the time and it has given me many hours of enjoyment for many years.
A couple of years ago it stopped working and I found the problem was a chassis mounted power resistor which had opened. To get it working again I made a sloppy repair by adding some power resistors and it came to life. This is the only repair I have ever had to make to this receiver in the 55 plus years that I have had it.
It is a 1934 vintage Crosley Model 61 with a 6H2 chassis. I decided that since it is over 80 years old it might be a good idea to replace the capacitors even though, remarkably, it still works. While surveying the unit and preparing for the surgery I noticed that someone before I got the radio had already done some repair. One of the original electrolytic cans had been disconnected and another dual electrolytic was installed. It was this way when I bought it but I didn't realize it had this fix and it appears that this and my resistor fix are the only repairs since the radio was new. I assume but cannot verify that some tube replacement was done, yet I know that none have been replace since I have owned it. This is truly remarkable reliability for such an old radio.
This is a work in progress and I have decided to also replace the resistors as well as the caps and the parts are on order. The wiring in such a radio is a concern since it is cloth covered rubber yet the wires appear to be in good condition and I probably will not replace them unless the insulation fractures with handling.
Why, you may ask, bother doing this? The first answer is that the choke coil used to filter the power supply is also the DC electromagnet for the speaker. If an electrolytic fails short it will most likely damage this coil as well as the transformer and the radio will immediately turn to junk since replacements are probably not to be found. The second reason is, despite the fact that it works, the wax dipped paper capacitors are most likely leaky and the radio will perform much better with new modern film capacitors which do not leak.
Here's a photo of the bottom of the chassis: (Click to enlarge)
It looks like quite a mess but with the help of the service manual and schematic obtained on line it is not nearly as bad as it appears to figure out the parts values and functions.
The receiver is very sensitive and selective and I plan to align the IF as well as to adjust the oscillator tracking and RF peaking with the hope of getting it back to its original specs.
When this is done I will provide a report with more photos. I am doing so to encourage others who might have old receivers which may or may not work to at least consider a recondition.
Neil
See how good things were made back then! Hand wired and lasts forever.
Can't get things like this now.
And made in USA! Or with makes like Marconi made in Canada!
Mark
The "old" equipment was made to the appropriate size for humans to work on it.
Equipment today is made by robots and is so small that trained ants would have trouble doing repairs.
I had a Robert's Reel-to-Reel Tape Recorder with 5-tubes and electronics made in Japan or South Korea, but it was very sloppilly constructed, so I used the schematic and neatly replaced every resistor and capacitor with new parts.
Right at that time a fellow happened to give me an Ampex direct-drive capstan motor, 15 IPS and 7.5 IPS, so I gutted the rubber-belt driven capstan from the recorder and got the Ampex motor installed. The original Robert's motor remained, because it supplied rewind and fast-forward.
Actually that machine still exists, down in the archives. It handles up to 7" reels.
Interesting story, Carl, about the recorder motor. Now that's major surgery!
The parts arrived yesterday and I spent about 8 hours last night replacing the power supply electrolytics. What sounds like a simple task wasn't so simple after all since one thing led to another such as removing parts to gain access to the circuit areas needed to replace the caps.
Another gotcha was that the circuit didn't exactly match the published schematic and some of the resistor values did not match the parts list. Crosley is known for making design changes during the life of a production model so this is not surprising but it does slow down the process since the circuit changes had to be reconciled with what I know about what works and what doesn't in tube equipment.
Most of the resistors and dipped wax capacitors were way off values marked. For example, a 15K resistor measured 22K, a 150K resistor checked as 90K. The caps were so leaky that a reliable measurement with my meter was not possible. These are being replaced.
One of the major problems concerned the three terminal wax caps which have two leads and a metal band around the body of the part (you can see one in the first post picture at the lower left.) According to information on web sites about this these were built with two caps in series with a common terminal at the strip connected to the junction and most of them were, but comparing this type of connection with the schematic and prior knowledge showed that some of them were wired differently internally. At least half of the time was spent pondering the schematic and reconciling the wiring to accommodate the new parts.
Presently, I have replaced about two thirds of the caps and resistors but I just had to fire it up (maybe I shouldn't say "fire") and check things. I installed a permanent line fuse just in case of problems and it held with the power on. The B+ and screen supply voltages came up nicely and settled close to spec and held steady indicating no apparent problems.
However, there was absolutely no sound from the speaker, not even the slightest hum. Applying an old radio man's trick I touched the grid pin of the final amplifier and heard the reassuring 60 cycle hum (remember....old radio men think cycles since Hertz is what happens when you touch the wrong pin...don't want that). Checking further revealed that one of the three terminal caps wasn't built for bypass to ground but rather to function as a feedthrough. This is how I discovered they are not all built the same way. Fixed this and was treated to glorious audio which sounds better than it ever had despite that most of the audio circuit caps have yet to be replaced. This is very encouraging since it can only get better when these are replaced.
It is written that Powell Crosley Jr. followed the David Sarnoff model and constructed both a radio station (WLW) and a factory to build radios so people could listen to his station and would buy his radios. It was a successful strategy. Crosley developed a modulation method which gave very good fidelity for WLW. I recall they used to identify as "The nation's highest fidelity radio station".
For this fidelity to be important, the receivers had to have good audio systems and an IF bandpass wide enough to give good frequency response. The receivers also had to be sensitive since in the 1930s there were very few local stations in a given area and listeners needed to tune to distant stations for programming. Selectivity was also needed to keep the distant stations separated from those nearby and both characteristics are evident in this receiver. Such quality was necessary to induce buyers to spend one to two week's pay for such a receiver. After WW Two the price and quality of many receivers was cut as I witnessed while repairing late 40s and early 50s radios.
Enough rambling for now but I expect to have this finished in a couple of days and will report more then.
Neil
A part 15 radio station would do well to operate in Neil's neighborhood now that we know he is a DX radio listener.
Of course he is also a part 15 station, so the neighboring station across the street would need to work overtime trying to get Neil's attention.
Thanks for the comments about the Roberts tape recorder. I will dust it and photograph it out in the sunlight.
There is great satisfaction in repairing or building electronic equipment.
If only more people had those Radio's. yes it might make me want to try and run the Talking House AM Transmitter as it could be heard quite far on one of those Radio's I'm quite sue even with the inside antenna wire next to a window. I'm quite interested to hear about the sensivity of this Radio when it is done.
The recapping is complete and several resistors were replaced. The radio sounds great despite not being realigned after disturbing parts and wiring in the RF and IF sections but I tried to maintain the original lead dress and it must have worked. The underneath of the chassis looks a lot better with the new and smaller parts.
Click photo to enlarge
Here is a view of most of the removed parts. I don't know whether to museum them or trash them but probably will do the latter.
Click photo to enlarge
More to report after alignment and installation in the cabinet.
Neil
P.S. While testing I tuned to a couple of non part 15 stations and spent several minutes waiting for commercials to end. Found one spewing forth sports banter that I can't believe anyone cares about. Fortunately, it is now tuned to Radio8z and some great music is filling the lab.
It might be fair to claim that radios of the early years when radio was blooming were genuine ahcievements of quality, before mass production turned cheapness into a science.
We were never meant to keep them going so far into the future. It might violate some federal law.
I'm listening to relatively new radios that are in various stages of falling apart.
Magnificant job, Neil.
Well, it is in the eye of the beholder. I was fooling around taking some photos of the radio for documentation and a couple look rather nice.
This view of the naked (gasp!) chassis is illuminated only by the pilot lights which have some tape placed to keep from blinding the camera:
Click to enlarge
Here's a view of the fire that makes it work. Two tubes are shown, the AF power tube and the type 80 rectifier. Only the filaments are seen.
Click to enlarge
To a tube guy this is beauty. For those needing an explanation, none is possible.
Onward with the alignment!
Neil
What can I say? Everybody likes it,
including me.
Very Best Wishes,
Brooce
Never met a tube I didn't like.
No matter how impractical it may be, an AM tube transmitter would make life better than finding a billion dollars in unmarked bills fallen in a remote part of the yard.
100 mW to the grid.
The alignment is finished and I am pleased with the results. There are two methods that can be used to align the receiver. According to the Crosley Service Note the recommendation is to peak each of four IF can adjustments at 465 kHz. Doing so revealed that the IF was very much off specification since it was centered at 452 kHz but tweaking brought it to 465 kHz and a very good peak was found. Unfortunately, the audio sounded rather muffled and it was confirmed that this method of alignment sets the IF bandwidth to 4 kHz which is too narrow for good frequency response.
A better method is to apply a swept frequency to the IF input and measure the output via a scope. The generator was set to 465 kHz center with a 20 kHz sweep range, thus the frequency swept from 455 to 475 kHz. The oscilloscope was set to trigger from the generator SYNC which flags the start of the sweep. Doing so produces a display of the spectrum on the scope.
The technique used is known as "stagger tuning" whereby the four adjustments on the IF cans are set to produce a wide and flat frequency response rather than a narrow peak. There is a bit of a trade between IF bandwidth and sensitivity but for my purpose audio response is more important. This is a bit of a tricky adjustment in that there are two IF cans each with two trimmers so four trimmers need to be set to get the desired response. It is necessary to repeat each trimmer adjustment since they interact but this was accomplished and the resulting IF bandwidth is a reasonably flat 10 kHz and the audio now sounds crisp and clear.
A screen shot from the scope is below with the top yellow trace being the IF frequency spectrum and the bottom blue trace being the sync from the generator. The sync indicates the beginning of the sweep at 455 kHz and the span was set so each horizontal division is 2 kHz in frequency. The peaks at the left and right are from the sweep restarting at the sync pulses so only the middle peak on the trace is important. The vertical cursors were set with S at the top level and E down 3 dB from this. This allows an easy way to measure the bandwidth. The plot shows a 3 dB bandwidth of 10 kHz with the center slightly low by about a kilohertz but this is good enough!
Click to enlarge
Following the IF alignment the oscillator and TRF tracking needed adjustment and this was comparatively easy without incident.
Some testing shows that the receiver needs about 10 minutes from a cold start for the IF response to stabilize. There is truth to the idea that tube equipment needs to warm up. Though some sensitivity was sacrificed it is still very good. It can receive the TIS station which is about 8 miles distant using a three foot wire antenna. The BCB is filled with signals as the receiver is tuned from end to end.
It remains to recondition the cabinet since it has survived my childhood through present with several different homes. It shows many nicks and dents and now that the electronics is up to par the next project will be to recondition the cabinet. The paint used appears to be a water based type which was applied by spray and highlighted by hand with an air brush. This is going to take some research but I want to do it right. I don't like to see old radios with modern urethane finishes which may be pretty but somehow just don't fit for an old radio. So, here's how it looks at present:
Click to enlarge.
It sounds much better than it looks but, given time, I will get it looking good as well.
This has been a good project but it is not one for a beginner. The biggest problem was reconciling the schematic with the custom capacitors mentioned before. The replacement resistors needed to be selected for the proper power and voltage rating. I was lucky in that most of the wiring insulation was good which is rare for a radio of this age. Soldering required use of a 150 watt gun since a pencil iron is inadequate for most of the connections. Some of the parts replacement was difficult because of the three dimensional construction which meant working behind and beneath things with cutters and soldering.
Even if you don't plan on such a project I hope you enjoyed this one and if you do plan on such an adventure perhaps you have learned something from this.
Neil
Wow is all I can say!
Very best wishes
Brooce
And thank you too.
!
I'm sure you wouldn't be thinking of a transmitter with a large sum of money turning up in your backyard!
Check out this site http://www.vcomp.co.uk/iTx/iTx_Kit.htm
A two tube AM transmitter, not PLL and don't know how good it is....
but TUBES!
Mark
Yes, Mark, that little circuit with two tubes is better than all the framed art and statues that people clutter their houses with.
Should be able to propose to a woman with a tube circuit instead of a diamond ring.