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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 10:37 pm 
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Location: Cleveland, Ohio
I am trying to sort through this.

You cannot check that 600 ohm pot while it is in the circuit. Also, when checking other resistors, one needs to study/understand the immediate circuit to determine if the targeted component can be checked while in circuit. Other components can affect that resistor's reading. In that case, one resistor lead has to be disconnected from the circuit or another component removed as required.

The 'theory of operation' in these manuals is not thorough but it is designed to help ensure proper use of controls and results are understood.

I believe that you need to understand how RF gain controls function as well as how AGC (automatic gain control or AVC, automatic volume control) designs function to prevent excessive gain/signal overloading. This overloading without AGC is normal. AGC circuitry automatically controls signal gain and reduces overload.

When the AGC is not switched in (i.e MVC, manual volume control), then the RF gain control must be used to reduce stage gain (manual operation) to achieve the same clear, non-distorted audio that AGC provides.

The above basic circuit gain/control operations are directly related to and reflected by s-meter readings.

There are no 'dual scales'- if I understand your comments correctly. I can only assume that you are switching between AGC and no AGC, and affecting stage gain additionally with the RF gain/sensitivity control. The s-meter will read higher when the RF stages are not controlled by AGC (AGC off). AGC reduces stage gain as noted and this is reflected in the reading.

So, the correct s-meter reading is with AGC on. Also, as noted before, the RF gain in this case should be advanced until you hear the outboard switch click in. In some receiver circuits, the s-meter does not function unless the AGC/AVC is on.

The typical engineering design goal is 50uv of signal at the antenna terminals is reflected as 'roughly' S9 on the carrier meter.

That being said, what signal strength reading are you typically getting from a local, strong broadcast station (external antenna attached)?

Todd
ka8gef

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Last edited by KA8GEF on Oct Sun 18, 2020 11:35 pm, edited 2 times in total.

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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 11:04 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Before I work on the S meter anymore I will calibrate. Not so easy as my Fluke needs work it was a cheap ebay buy.

I am excited because on the way is 50m which is about 164 feet of electric fence wire and 100 feet of Carol C3528 RG11/U Premium FFEP/PVDF Plenum Direct Burial Coaxial Cable. I am going to make my next dual dipole with an independent coax line for each leg of the dipole. The RG11U is pretty good for getting a clean signal and doesn't cost as much as Times Microwave

Check this coax attenuation chart

https://w4rp.com/ref/coax.html


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Mon 19, 2020 1:06 am 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
I was using these tubes from slovakia purchased on ebay.

Matched Pair 6F6S 6Ф6С 6F6G Figural Tube Röhre Valve Pentode NOS NIB 50's.

The voltage spiraled up past 50v and blew my filter cap.

I put back in the original Sylvanias and good to go.

I just ordered a back up set of sylvanias from ebay.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Mon 19, 2020 3:46 am 
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Joined: May Wed 20, 2020 1:33 am
Posts: 311
Location: Rockford, IL
I suspected this was the case :-)

KA8GEF wrote:
I am trying to sort through this.

You cannot check that 600 ohm pot while it is in the circuit. Also, when checking other resistors, one needs to study/understand the immediate circuit to determine if the targeted component can be checked while in circuit. Other components can affect that resistor's reading. In that case, one resistor lead has to be disconnected from the circuit or another component removed as required.

The 'theory of operation' in these manuals is not thorough but it is designed to help ensure proper use of controls and results are understood.

I believe that you need to understand how RF gain controls function as well as how AGC (automatic gain control or AVC, automatic volume control) designs function to prevent excessive gain/signal overloading. This overloading without AGC is normal. AGC circuitry automatically controls signal gain and reduces overload.

When the AGC is not switched in (i.e MVC, manual volume control), then the RF gain control must be used to reduce stage gain (manual operation) to achieve the same clear, non-distorted audio that AGC provides.

The above basic circuit gain/control operations are directly related to and reflected by s-meter readings.

There are no 'dual scales'- if I understand your comments correctly. I can only assume that you are switching between AGC and no AGC, and affecting stage gain additionally with the RF gain/sensitivity control. The s-meter will read higher when the RF stages are not controlled by AGC (AGC off). AGC reduces stage gain as noted and this is reflected in the reading.

So, the correct s-meter reading is with AGC on. Also, as noted before, the RF gain in this case should be advanced until you hear the outboard switch click in. In some receiver circuits, the s-meter does not function unless the AGC/AVC is on.

The typical engineering design goal is 50uv of signal at the antenna terminals is reflected as 'roughly' S9 on the carrier meter.

That being said, what signal strength reading are you typically getting from a local, strong broadcast station (external antenna attached)?

Todd
ka8gef


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Thu 22, 2020 6:13 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
On order are all new 2 watt resistors for where the old 2 watters went. It is actually hard to find nice 2 watt resistors on Ebay. I even ordered a few military surplus US ones with 1% tolerance and some Russian military ones with a nice price.

When the new 500 ohm potentiometer comes I think everything will be kicked up a notch and the weaker resisters I have now won't hold as I already found out along with my blown audio tube from Europe.

New antenna parts on the way.......


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Thu 22, 2020 8:25 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 311
Location: Rockford, IL
I am lucky enough to have a shop where we have literally 10s of thousands of capacitors and 10s of thousands of resistors - the bulk being the vintage and NOS carbon comps. I want to start selling them off and out of the inventory as I like to order new resistors and caps for no-hassel-later repairs. I don't do restoration: just repair. Putting a 40 to 70 year old resistor or capacitor in a repair is just tempting the rework-at-no-charge gods. My own stuff I'll put in NOS but that's because if it starts sputtering I can fix it :-)

mk2020@ wrote:
On order are all new 2 watt resistors for where the old 2 watters went. It is actually hard to find nice 2 watt resistors on Ebay. I even ordered a few military surplus US ones with 1% tolerance and some Russian military ones with a nice price.

When the new 500 ohm potentiometer comes I think everything will be kicked up a notch and the weaker resisters I have now won't hold as I already found out along with my blown audio tube from Europe.

New antenna parts on the way.......


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Thu 22, 2020 8:46 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
for everything under 2 watt there seems a huge selection. Once you get into 2 watt and above it seems there is the generic made in china stuff which is endless and then there is some military stuff like Allen Bradley and most of the not china generic stuff is very rare.

There is only 6 or 7 two watt resistors on the SX25 but it took a bit to find some nice quality ones in different values.

Locally I have two shops here in Vancouver but I am 100 percent sure they just sell the cheap generic stuff with low tolerances and a big markup.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Thu 22, 2020 10:49 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 311
Location: Rockford, IL
I order from DigiKey or Mouser in bulk. And for resistors I look for metal oxides.

For little orders (under bulk) and most repairs, I order from tubedepot.com or Hoffmanamps.com. eBay is full of the China crap and I have an assortment of 2 watters I am slowly pulling out of gear I put them in before I stopped buying them altogether.

In canada try tubestore.com - They are a mirror image of tubedepot and have great selection and service for parts.

mk2020@ wrote:
for everything under 2 watt there seems a huge selection. Once you get into 2 watt and above it seems there is the generic made in china stuff which is endless and then there is some military stuff like Allen Bradley and most of the not china generic stuff is very rare.

There is only 6 or 7 two watt resistors on the SX25 but it took a bit to find some nice quality ones in different values.

Locally I have two shops here in Vancouver but I am 100 percent sure they just sell the cheap generic stuff with low tolerances and a big markup.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 25, 2020 5:38 am 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Think I may have found an issue. I was looking for a short to ground and I think I found one or two.

There are two solder points that I circled in the pic that are measuring short to ground.

they both are connected to wires going into T2. I think this is called an IF transformer.

If anyone thinks this is worth investigating and maybe what I should do next.

Thanks in advance.

Mike.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 25, 2020 1:25 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 311
Location: Rockford, IL
Suspect any dark grey blobs of solder or 'goober pea' soldering. Always remember that solder is not wire, nor is it glue. It is a poor conductor and is not suitable to hold connections that are under tension. I remove solder gun looking work in many items I repair. Carefully melt with the soldering iron and suck the excess solder out and use a soldering pencil or a decent Weller station to solder. Leave the soldering gun for reminiscing how soldering was done in the 1950s. Use a good 60/40 type solder (or similar), 0.32" diameter, and make sure its rosin core so it flows nice. I use a 35-40 watt setting normally. Make the connection to clean contacts, heat both the wire and thing it is being attached to, then apply solder and let it flow on and around connection to hold it.

A bad solder connection can cost many hours (over many weekends part-time) of checking everything else only to find it was something that was broken from the get-go.

Remember: you don't get extra points from the Hallicrafters gods for the amount of solder you use :-)


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 25, 2020 4:52 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Im going to check resistances across the wires and compare them to the other rf towers. Any other input from anyone else? What should the capacitance reading be of the mica caps?

thanks in advance.

Mike.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 25, 2020 5:09 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
rf tower inside.

Looks like the SX 25 rf towers are not plastic like ones that came after.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 25, 2020 5:44 pm 
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Posts: 2884
Location: Westminster, CO, USA
T2 secondary should measure as ground because it is grounded in both positions of the selectivity switch.

Tony

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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 25, 2020 7:40 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
all is looking good the two caps are measuring 200 pf and they look like they are labelled brown green brown so 150pf.

the mica trimmers add another 25 pf to each side give or take.

there is one coil of windings on the top of the can and two at the bottom that are not connected to each other.

I have a feeling there is shorting from the soldered on wires to the sides of the aluminum housing.

I am adding a few layers of thermal tape to protect the soldered wires.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Mon 26, 2020 3:51 am 
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Posts: 311
Location: Rockford, IL
The transformers can - and do - die from age. I replaced both 1st and 2nd IF in my S20 because both were kaput. I used one I bought online from some other brand radio and one that was a NOS 2nd stage IF I had in stock. I spent way too long on those IFs only to find they were just crumbling and degraded on the inside.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Mon 26, 2020 4:11 am 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
I was able to use my BFO for the first time to make some ham dialogue understandable.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Mon 26, 2020 12:59 pm 
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Location: Rockford, IL
I found the BFO on the SX-25 works very well.

mk2020@ wrote:
I was able to use my BFO for the first time to make some ham dialogue understandable.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Wed 28, 2020 5:38 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Anyone have trouble with these if cans like sparking and shorting. I painted nail polish enamel on the top of the inside of the can to insulate. I noticed black carbon deposits at the top where you tune.

I am also thinking of getting a Johnson Transmitter to go with my sx25. I am a complete newbie at this any ideas.

Mike.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Wed 28, 2020 6:55 pm 
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Posts: 361
Location: Dallas,TX
Mike, A Johnson transmitter is an excellent choice. I would recommend a Viking I or II. They are made with heavy duty components, they're well designed, easy to work on and just about bullet proof. I use a Viking 1 along with a Johnson 122 VFO. It will run key down all day putting out about 110 watts. It seems like hams that have them and sell or trade them always regret parting with it. I have a lot of Johnson equipment and my Viking 1 is my favorite AM transmitter.

73, Mike / KE5YTV

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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Wed 28, 2020 7:33 pm 
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Posts: 1965
I liked the successor to the Viking II, the Valiant. I was happy with mine for many years. I don't miss it; I had my fun and today's rigs are better than either it or the SX-25, which I also used for many years.

But for nostalgia you can't beat that combination. I just acquired a new to me TS-520S for my own nostalgia fix.


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