The PHILCO Phorum

So where could one get simplified information about the function of each device in my model 70? As I prepare for the restoration, I'd sure like to have a better idea of what each part specifically does in the circuit. I was contemplating even painting the chassis, which would require a total stripping (gulp) I figure if I can't get it back together, I'll put a clock radio in behind the grill! Nah. I just would like to go through and restore the 70 and have a better understanding of how it works, then focus my attention on the other philco I have (model not known) so I can hear short wave! If anyone could steer me into some very simplified detail of the model 70 chassis though, I'd really appreciate it. Of course I will be consulting all of you too, along the way, for your input.

I'm rather new at radio restoration as well and I'm trying to relearn everything I ignored or forgot from school. There are plenty of sites on the web about electronic components once you learn what they are called. The hyperphisics web site is one of many. In a nutshell, resistors can be used to parse voltage. Electrolytic capacitors are used to smooth out bumpy, rectified AC. The other capacitors and coils can be used to filter AC or a modulating signal. Combinations of the two can work in unison and allow a narrow range to pass rather than just cutting out the highs or lows. Search the web for superheterodyne and read about how the radio works. You can also find out a little about vacuum tube cathodes, grids, screens and plates as well but some of the articles can get into the science that's interesting but hard for a novice to put to practical use. Old fashion books are great. I've got a beginning electronics book that my wife gave me as a Father's Day gift that I reference all of the time. I also bought a book on radio servicing by Marcus and Levy. The latter has a great deal of information but it will mean more once you get a better appreciation of what the circuit components do and how they work.

This takes a good deal of time to read through and a bit longer to digest. It's a great hobby that will give you much reward and appreciation for not only the technology involved but the skills and knowledge that people have.

Take a look at this site.

http://www.radioremembered.org/

Click thru to restoration... then to troubleshooting. It breaks down each stage of a 'generic' radio with some simplified diagrams and give a brief theory of operation for each stage that addresses most of the individual components.

I understand capacitors rectifier tubes and resistors, but am unsure what the other tubes in the set are doing, such as the #24 and #27 tubes. I could also use a better understanding of the tuning process for these AM radios. After experimenting with soldering on some piece of crap radio I have, I quickly realized that I certainly need practice here too. I might need to find an antique radio buff in my area, to hold a soldering workshop clinic! Thank you Exray, for the link. I always find this info so fascinating. By the way....What projects are you involved in now?

Here's a 'lightweight' explanation of the 70. I'm using the diagram at the top of the 3rd page at http://www.nostalgiaair.org/PagesByModel...013833.pdf with correlation to the radioremembered page

1st 24 to the left - thats a basic RF amplifier like the 6K7 at other other site.

2nd 24- Gets tricky here. This tube is a mixer. In the old days they usually called it a 1st Detector...read on...
The 27 below that one. Thats the local oscillator.

Before going further I'll point out that although radioremembered's block diagram shows two individual tubes for the 1st det (mixer) and the local oscillator, the schematic shows a single 6A8 tube. Long story short, the 6A8 is providing both functions. He provides as good of an explanation as I can. The 6, 5 and 8 pins on that 6A8 translate to your 27 in the older Philco. In fact, many texts will refer to that pin 6 grid as the 'oscillator plate' since that is its function compared to an older two tube scheme.

Using two tubes versus a single multi-purpose tubes is a subject in itself. Short version is that the multi-purpose mixer or converter tubes were not around at the time of the Model 70.

The next 24 is an Intermediate Frequency (IF) amplifier. It equates directly to the one at radiorem.

The last 24 is the equivalent of the detector/avc/1st audio stage at radiorem. There's a significant number of differences here. Older sets like the 70 are sort of 'crude' in comparison to later circuits.

The 47 is the Audio Output amplifier stage which is fairly straightforward.

So you see that even though the circuits are quite different, the basic building blocks are similar. This holds true for many superhet radios. Some have different frills, some have less, some have more, but the basic blocks will be there.

Soldering. First tip is to keep your tip clean. Second tip is to keep your tip clean. Third tip...you guessed it. Old radios often have a lot of oxidation on the joints and it takes a little extra effort to break thru it since it impedes the heat transfer. That also will nasty up your tip in a heartbeat. What are you using for a soldering gun/pencil? There might be some specific advice to make life easier.

Me, project wise, not much. I've got a 3-tube homebrew project going on and I'm diving into the realm of solid-state for the first time in 25 years with a little regenerative rcvr project.

Adios,
Bill

I don't trust the solder joints in these old chassis. They were made by unskilled assemblers using megawat welding irons with solder of a lower quality than is available today. An iron that size is only needed when it's necessary to resolder a grounded lead on the chassis. Old oxidized solder joints aren't a problem: add flux and suck the old solder off, then add flux and resolder it with 63/37. I bought a huge iron a few years ago at a garage sale but haven't found anything yet I couldn't do with a 30w iron. Get a bottle of rosin flux for about $8 and start using it. When you see how much better your joints turn out you'll never solder without flux again.
Dave

Thanks to both of you guys. I ran off a copy of this page. I recently used on loan, a very old Weston? I think. It was in the original case, and was so big and old, that a had a beard almost as long as mine! I was thinking of getting an older iron, seeings how I seem to love antique things. Bill, have you ever seen the inside of a Marantz model 7t? It's just a solid state preamp with no printed circuit boards, all point to point connections. They're a real work of art for the late 1960s, in the day of the beginning Japanese electronics. I have a real nice 7t and occasionally I remove the covers to check out the quality. I'm starting to get all pumped about going to work on my model 70. It works now, and when I'm done, I'm sure it wont!