Radio City Products Signal Generator Model 705A

[vortalexfan]

Hello, this is kind of a follow up thread to the Heathkit SG-8 Thread that was kind of getting really long and was also starting to become a wild goose chase so to speak with all the issues that I was having with it.

At the same time I bought the SG-8 I also bought a Radio City Products (RCP for short) Signal Generator Model 705A.

When I got it, I opened it up to see what the condition of the guts were and sure enough it was all original paper caps inside the unit.

There was only one repair done to the unit and that was the filter caps, which I'll give them this much, at least they took the original ones out of the circuit and out of the cabinet, but they did a terrible job with installing the replacement caps which was a cardboard tubular job that they unsuccessfully attempted to screw and solder the mounting bracket to the chassis, then proceeded to solder the ground lead to the capacitor's mounting bracket for the ground, and then attached the positive leads to their respective locations.

I'm surprised that the unit worked that way, and didn't short.

I replaced all of the old paper caps and the old improperly installed tubular filter caps which cleaned up the look of the underneath of the signal generator's chassis quite a bit from what it originally looked like.

This unit has a high and low RF output, with the Low RF Output being of the old Mic Jack variety like the Heathkit had, which has an adaptor with it that goes from the mic jack to an RCA jack which I was able to order a Phono to BNC adaptor from radio shack that I can use to make this unit work with my test cable I have.
The RF High output is a Banana plug that I was hoping to somehow devise a 50 ohm BNC cable that had the BNC plug on one end and a Banana Plug on the other end so I could use that RF high output as the monitoring output for my Frequency counter because the RF High Output on this unit isn't affected by the Attenuator according to the manual.

The AF Input/Ouput is also a Banana Plug and then there is a Ground connection which is also a banana plug which I'm assuming its for all of the banana plug connections on the unit.

below are some pictures of the unit in question as well as an ad for the unit that I dug up online, I also have a manual for it as well but its too big to upload on here so if you want to see it I can just email it to you.

Thanks.

[Notimetolooz]

Just post a link to the manual.
http://bama.edebris.com/manuals/radiocit/705a

Interesting that both this and the SG-8 use just two tubes.
You definitely need to use shielded connectors and cable on the high output source since otherwise it will broadcast to the radio. Switching out the banana jack for a BNC will probably require you to enlarge the hole in the panel. Something like this.
https://www.mouser.com/ProductDetail...zdb4bk0FGlA%3D

[vortalexfan]

Originally Posted by Notimetolooz

Just post a link to the manual.
http://bama.edebris.com/manuals/radiocit/705a

Interesting that both this and the SG-8 use just two tubes.
You definitely need to use shielded connectors and cable on the high output source since otherwise it will broadcast to the radio. Switching out the banana jack for a BNC will probably require you to enlarge the hole in the panel. Something like this.
https://www.mouser.com/ProductDetail...zdb4bk0FGlA%3D

I actually found some Shielded Banana to BNC cabling that could be used on this unit.

See link below

http://https://www.digikey.com/produ...2106-ND/737216

[Notimetolooz]

That is kind of expensive. Usually it is better to buy something that you can use for several different things. Changing out the jacks or adding adapters is more flexible.
Buying several of these is a good idea.

https://www.digikey.com/product-deta...256-ND/4481753

[vortalexfan]

Originally Posted by Notimetolooz

That is kind of expensive. Usually it is better to buy something that you can use for several different things. Changing out the jacks or adding adapters is more flexible.
Buying several of these is a good idea.

https://www.digikey.com/product-deta...256-ND/4481753

The only problem with converting this unit over to BNC from Banana Plugs and what not is that this unit was specially shielded and grounded from the factory for optimal precision and performance (aka no drifting or signal bleeding or what not), and if I disturb the shielding and grounding of the connections and what not then it may affect the performance and the precision of the signal generator (which the precision of this signal generator is ±2% with the ability to go up to less than 1% precision.)

So I would need to figure out what kind of affect changing out the plugs could have on the precision of the unit first before I go switching out plugs which is why I want to try and utilize the plugs that are already there so I don't risk screwing up the performance and precision of this signal generator, which unlike the Heathkit unit, is actually a highly precise instrument.


There is actually cheaper versions of the cable I showed you, that are just a red and black wire that are wired to a set of red and black banana plugs respectively on one end and on the other end they are wired to a female BNC jack which I could just connect a shielded Male BNC to Male BNC cable to.

See Links below:

https://www.digikey.com/product-deta...395-ND/5801070

https://www.digikey.com/product-deta...1558-ND/736733

I hope this helps.

[maxhifi]

Can't you use a dual banana to bnc adapter? That's how I get signal in/out of my tube type test gear. Is the banana spacing non standard?

[vortalexfan]

Originally Posted by maxhifi

Can't you use a dual banana to bnc adapter? That's how I get signal in/out of my tube type test gear. Is the banana spacing non standard?

Yes, spacing is not standard, the ground connection is on one side of the signal generator and the RF Hi output banana jack is on the opposite side of the signal generator that the ground connection is.

Take a look at the picture of the front panel of this unit I posted above at the top of this thread, and you'll see what I mean about the spacing of the RF Hi output banana plug and the banana plug ground connection.

[Notimetolooz]

It wouldn't effect the accuracy of the frequency and besides you intend to use it with a frequency counter. According to the manual the high output connector would have been used if a radio was in bad enough shape that it would be "deaf" to the normal signal levels. It was not intended to be used with a counter, which would have been a rare beast in 1946 outside of a research laboratory. You can adjust the the frequency fit of the dial using the frequency counter after you get the generator re-capped and running. The RF amplitude is not well controlled on this service grade generator anyway. If you were able to measure the signal strength you would find that it probably varies with frequency across the dial and from band to band.
I would guess the hole for the banana jack is 1/4 to 5/16 inch. The BNC jack I linked to would need the hole increased to 3/8 inch. You could do that with a drill. Stick a lump of modeling clay or "tent" some masking tape on the other side to catch most of the metal fragments. You also could do it using a tapered reamer instead of a drill.

[vortalexfan]

Originally Posted by Notimetolooz

It wouldn't effect the accuracy of the frequency and besides you intend to use it with a frequency counter. According to the manual the high output connector would have been used if a radio was in bad enough shape that it would be "deaf" to the normal signal levels. It was not intended to be used with a counter, which would have been a rare beast in 1946 outside of a research laboratory. You can adjust the the frequency fit of the dial using the frequency counter after you get the generator re-capped and running. The RF amplitude is not well controlled on this service grade generator anyway. If you were able to measure the signal strength you would find that it probably varies with frequency across the dial and from band to band.
I would guess the hole for the banana jack is 1/4 to 5/16 inch. The BNC jack I linked to would need the hole increased to 3/8 inch. You could do that with a drill. Stick a lump of modeling clay or "tent" some masking tape on the other side to catch most of the metal fragments. You also could do it using a tapered reamer instead of a drill.

Ok, well, seeing as the banana plugs didn't originally have a ground connector to them I'm gonna have to add a ground connection if I do install a BNC jack.

[Notimetolooz]

Sure. The metal case should be the RF ground, but running a short wire from the nearest ground terminal is a good back up addition. BNC jacks usually come with a ground lug on the lock washer. You occasionally will have to make some modifications to thing to improve performance or ease of use. Test equipment units are tools to help repair other things, they rarely are museum pieces.
Obviously you will have to re-cap this generator. I wouldn't be surprised that some of the resistors will have to be replaced also. The switches should be cleaned. The grease in the tuning capacitor may need to be replaced. I think I see some sort of wire-wound trimmer resistor in your picture, there is none shown on the schematic. That may mean someone has made a modification. Unfortunately the manual does not have any info on adjusting the generator or troubleshooting it. You might get a general idea from the Heathkit manual info. From the schematic it looks like each RF coil has a trimmer capacitor connected to it so you can adjust the generator to one frequency on each frequency band. Also typically the frequency pointer can be aligned to one end of the scale by loosening the setscrew holding the pointer to the tuner capacitor shaft.

[vortalexfan]

Originally Posted by Notimetolooz

Sure. The metal case should be the RF ground, but running a short wire from the nearest ground terminal is a good back up addition. BNC jacks usually come with a ground lug on the lock washer. You occasionally will have to make some modifications to thing to improve performance or ease of use. Test equipment units are tools to help repair other things, they rarely are museum pieces.
Obviously you will have to re-cap this generator. I wouldn't be surprised that some of the resistors will have to be replaced also. The switches should be cleaned. The grease in the tuning capacitor may need to be replaced. I think I see some sort of wire-wound trimmer resistor in your picture, there is none shown on the schematic. That may mean someone has made a modification. Unfortunately the manual does not have any info on adjusting the generator or troubleshooting it. You might get a general idea from the Heathkit manual info. From the schematic it looks like each RF coil has a trimmer capacitor connected to it so you can adjust the generator to one frequency on each frequency band. Also typically the frequency pointer can be aligned to one end of the scale by loosening the setscrew holding the pointer to the tuner capacitor shaft.

Well I recapped this unit already, and I got an adapter that adapts RCA jack to BNC and I was able to use the existing Mic jack to RCA jack adapter that came with the unit, and it works fine.
I was even able to use that same jack to hook to up my frequency counter and it worked fine, I just disconnected the frequency counter and then hooked up the test cables and it was fine.