Vintage Radio Restoration - STC A5130 Part 4 - Distortion Fix and Alignment

I continue with restoring the STC A5130. The distorted sound cured by an unlikely suspect and a final alignment carried out.

[@ibrahimkocaalioglu]

I watched most of your restorations, you did very good job. If I were you I would change all the caps, resistors valve sockets and cables, clean or change the pot controls and than test the radio. Restoration takes a lotvof time and effort. Not replaced parts may become proplem in the near future. All small parts replaced would give the radio another 50 year to go without servicing.

[@DavidTipton101]

Thanks for the tips!

[@Plons0Nard]

It is funny to watch this in August 2021. You have achieved a lot in 2 years 👍🤝🇳🇱

[@DavidTipton101]

It's a world away Nard, the beginning of a long journey 😄

[@erin19030]

I picked up a unique AC voltage measuring instrument today at a local thrift store. The item was an HP-403, the True RMS voltmeter. It is transistorized meter. I plan on using it as a dedicated alignment voltage indicator. With this meter I can measure any AC signal voltage , even the 455 Kc IF frequency. I can measure down to 10 microvolts. This will be good for measuring RF sensitivity in the AM broadcast Band. The meter is lab quality and several steps above an all purpose VTVM, that I can now utilize elsewhere on my bench. This meter was originally designed to run on Nicad batteries only. The previous owner devised a way to have it run on AC line current. That I am very happy with. The batteries were special and no longer available. I was studying schematics to see what I could come up with and nothing was simple. The previous owner was known as "Mr. Electron" a retired EE. Items from his estate found their way to a thrift store. The only thing I need to do was find a suitable three prong AC power cord. In the meantime I soldered an AC cord to the internal connections So I can calibrate the meter. I found a full copy of the HP manual on line. I hope your day was as exciting as mine was today.

[@DavidTipton101]

My day was not nearly as exciting by the sound of it Cosimo. It's a shame it ended up in a thrift shop, Mr Electron would be sad to see his life's work scattered about like that but by the same account it's great that you rescued it. Being able to read low signal voltages would be very handy for troubleshooting circuits. I'm very happy for you Cosimo, well done 😃

[@Greg-tz5kz]

Just found your interesting tutorial on the restoration of this old STC thanks David. Of particular interest to me was your method for aligning the radio using a signal generator and multimeter which was straightforward and easy to understand!

[@DavidTipton101]

Thanks Greg, glad to have been of assistance 😃

[@Martin-io4wc]

You sir are a master. Nice job.

[@DavidTipton101]

Thanks again Martin 🙂

[@schraubnix1583]

Hello David thank you for your fine restoration videos on your channel , very nice content , thumbs up greets Uli ( Schraubnix )

[@DavidTipton101]

Thank you Schraubnix 😃

[@widecast]

That 1 meg resistor has a 470k in series with 100k, across it when measuring it in circuit so it’s going to measure less that 1 Meg.

[@DavidTipton101]

Haha... yeah, I worked that out... eventually 😀

[@erin19030]

Single ply toilet paper and Duco cement. Spread an area wider than the tear then lay down a strip of TP over the tear. The TP will absorb the glue . Then spread another layer of Duco like paint over the TP. Like when you cut yourself shaving. The blood acted like glue as it saturated the TP.

[@DavidTipton101]

We can't get Duco cement down here 😏

[@erin19030]

The rubber paper glue is good idea too as it remains more pliable.

[@tubeDude48]

Thumbs-Up and Subscribed!

[@DavidTipton101]

Thank You Ted and welcome 👋😀

[@samhatch4665]

Awesome, thanks mate 👍

[@DavidTipton101]

😃

[@widecast]

It’s interesting to see a radio with a pentode 1st audio where a triode is the usual. Don’t think I have seen that before.

[@DavidTipton101]

All to do with the negative feedback I assume, been awhile since I did that one 🙂

[@michaelmacdonald3408]

I noticed you have the same generator lsg 11 someone in the past took out the 1k RF fine pot and replaced it with a function switch with varied resisters i might go back to the original.

[@DavidTipton101]

Hi Michael, they are a good basic generator, I wonder why the pot was replaced with resistors 🤔🙂

[@erin19030]

The AGC voltage is delayed by the RC time constant of the AGC filter. The voltage is smooth and filtered . I'll give it a try next time.

[@DavidTipton101]

It works well but you need a high impedance meter, I now use the output valve plate so as not to influence the IF/RF section.

[@user-tc5nk6oq7u]

@David Tipton 📻📻📻🌹 I lave you. My frand

[@DavidTipton101]

Haha... thanks Abdul 😄🌷

[@KorAllRBare]

🐨 In a previous episode " I forget which episode" I made mention that you may have to replace the very Capacitor you were forced to replace in this here episode, so please ignore my previous Post in regards to this capacitor.. APU I dropped a 👍

[@DavidTipton101]

Haha... Ok, thanks KorAllRBare 😄

[@erin19030]

While recapping a 1948 Philco today I came across a .2uf cap bulging at one end and the wax plug oozing out. The odd thing was a coil of #20 wire was wrapped around the ground end of the cap. The coil was 10 turns of insulated wire on a 3/4" diameter cap. One end if the wire attached to the cap lead on the low end the other end of the coil was open ended. I have no idea what the purpose of the coil is for. The inductance was too small to measure, it measured .81 Ohms D.C. Resistance. I guess I could calculate the inductance by know physical values , the figure out the LC resonance of the tank circuit. Have you ever seen a cap configured this way? Anyone have ideas. So for now I just replaced the cap and made a note in my journal to revisit later on.

[@DavidTipton101]

I believe it is a wave trap as it's only connected at one end Cosimo, I had one on a previous radio and did a little searching about them, see link at the end. There seems to be some conjecture as to it's purpose, it seems it's either to stop external signals penetrating or stop feedback from within the set itself. It's position in the set may answer that. Either way it may be redundant with modern capacitors. Have a look at this link Cosimo: antiqueradios.com/forums/viewtopic.php?f=6&t=190080

[@erin19030]

David Tipton Thanks ...yes , that's it exactly. I figured it was some kind of gimmick inductor. The measured inductance was 10 micro-henries. It had a stray capacitance in the nano- farad range. I kept the old part in tact , just in case it's some RF Black Magic device. The schematic shows it to be a physical coil in series electrically from the cap to ground. There you go , I learned one more thing and I knew it all until I didn't.

[@erin19030]

So it turns out that in the schematic this .2uf cap is the virtual ground connection in series with an open coil. The cap. has an Xc of 10k at 60 Hertz. The coil is a phantom inductor. It reminds me of small inductances that were used to suppress parasitic oscillation in RF circuits. Or maybe it's some king of ground loop magic.

[@widecast]

Did you measure the agc before and after tuning in a station to see if it’s responding to signal levels. It looks like it was overloaded due to low RF bias.

[@DavidTipton101]

I thought I tuned a station using the voltmeter after I aligned it, the voltmeter is connected to the AGC 🤔

[@michaelmurray3422]

I have had good luck with tissue paper (wrapping type) and finger nail cement. (that women use ) You can get it quite wet and put the paper on, and when it dries, it is the same consistency of the cone to begin with.

[@DavidTipton101]

Hi Michael, thanks for the tip, I'll give it a go 👍🙂

[@widecast]

The oscillator coil should be adjusted at the bottom of the band, like the aerial coil.

[@DavidTipton101]

Thanks widecast, I have finally worked it out 👍

[@jonka1]

Cellulose paint thinner usually sees off any glue, (and paint too)

[@DavidTipton101]

Hi jonka1, we don't have cellulose thinner in Oz, we do have lacquer thinner which may be the same thing, I didn't think to try that though 🙄

[@MrHBSoftware]

@@DavidTipton101 here we have cellulose, synthetic and acrylic thinners, should be the same all around the globe i guess. cellulose is very strong and mostly used for cleaning brushes and paint guns..most paints are synthetic or acrylic and for mixing we use the respective thinner...there is also the water based paints that are being shoved into our mouths due to environmental concerns

[@DavidTipton101]

@@MrHBSoftware the only name I recognise is acrylic but no doubt different names for the same product.

[@jamesmdeluca]

Greetings: RE: MINUS supply R

[@DavidTipton101]

Thanks James 🙂

[@KorAllRBare]

**ADDENDUM** The IF's have capacitors in them and guess what type they are? and what's the bet they have shifted out of spec ergo, no doubt why you needed to realign the set Err-Radio, So if the Radio or for that matter any radio with much the same IF's fail to properly tune in one day keep in mind that it may just be those capacitors inside.

[@DavidTipton101]

Yep, I've learned a lot since then KorAllRBar 👍🙂

[@Martin-io4wc]

Hello again David. I wanted to ask you something about the STC A5130 table top AM radio. I noticed that in order for a consumer to operate this radio they would need to add a long wire antenna. I imagine that may not be practical in all locations such as a condo or city located townhome. Would you be able to add a generic ferrite loop antenna to the radio so that the user would not have to have a long wire antenna placed outside their apartment or condo? Is it possible to add a ferrite loop antenna to a radio that was not designed to have one and still obtain decent RF reception? If yes, would you make a short video on how to add a ferrite loop antenna?

[@DavidTipton101]

Hi Martin, it is possible to add a loopstick antenna and that's a good idea for a video, thanks. It would basically involve replacing the existing antenna coil with the loopstick coils. I would have to do a bit of research though. I'll see if I can come up with something in the new year, chhers 🤔🙂😃

[@rogerbird6151]

Silvered Mica capacitors from my 50 year experience in radio and TV are most unreliable when dc is applied to one end, e.g. a plate to grid coupling. The silver tends to migrate internally.

[@DavidTipton101]

When I first started my radio repair hobby I was told mica caps are very reliable, don't worry about them. That proved to be a fallacy Roger, I now suspect any mica with a high voltage on one side just like you. It took me a while to work it out though 🙄 Thanks Roger 😃

[@Greg-tz5kz]

Hi David, can I ask where you got your capacitance meter from that you use in this video (at around the 10 minute mark) or is it a kit?

[@DavidTipton101]

Hi Greg, I don't have that anymore after I dropped it 🤦‍♂️ That one comes as a assembled board, the case is extra and comes as a flatpak. The replacement I got was fully assembled and in a nice case. There are similar ones on eBay and are very cheap, $20 - $30 here 👍🙂

[@Greg-tz5kz]

@@DavidTipton101 Awesome thanks David......off to eBay I go!

[@ricobass0253]

I've never seen that before; the mains transformer secondary centre tap connected to both an inductor to ground and a capacitor to the HT! How does that produce a negative bias wrt ground? I have a degree in electronics but I don't understand that. Did you ever work that out?

[@DavidTipton101]

I don't have access to the circuit at the moment Ricobass0 but the reason the choke is on the centre tap is to reduce stress and corrosion on the windings by reducing the potential between the windings and the former and core. When a choke was mounted in the positive side of the B+ some manufactures connected the body or former of the choke to B+ as well to achieve a similar result. The choke itself provides the back bias. Because the output valve cathode is directly connected to chassis ground, connecting the filter caps directly to ground also would induce a hum in the cathode. By connecting it to the centre tap it is essentially in parallel with the back bias resistor or in this case the choke.

[@ricobass0253]

@@DavidTipton101 I get what you are saying about the voltage difference between the inductor windings and the metalwork but don't see how any of this generates a -ve voltage wrt ground. The bias resistor chain is across the inductor and both the resistor chain and the inductor have one end grounded. They are both effectively in series with the HT output, albeit through the transformer windings, but without a third diode somewhere I don't see any way a -ve voltage can develop.

[@DavidTipton101]

I think I see what you are asking. How do you get a negative voltage by grounding the CT through a Choke? Consider the CT is the most negative point in the power supply. if a resistor (choke and some resistors in this case) are placed between the CT and the chassis, the chassis is now positive WRT the centre tap. Connect the control grid to the Centre Tap and the cathode to chassis and the grid is now negative WRT the cathode. You are right, the entire HT current flows through the choke/resistors to chassis so variations in current flow will affect the bias voltage.

[@ricobass0253]

@@DavidTipton101 If you consider the transformer as a voltage source and the double diode as just a single rectifier, then there is a complete loop from ground through the inductor, the V source, the rectifier and then finally C24 back to ground. If you define ground as zero volts and the +HT is developed across C24, how can the voltage across the inductor be anything other than positive? I know I must be wrong - you doubtless measured a negative voltage and if it were otherwise, all the valves would have been biased wrong. All I can think of is that the role of C20 from HT to the top of the inductor is crucial, creating some sort of crude switching supply, using the (100Hz) ripple voltage to build up a negative charge on its negative terminal.

[@DavidTipton101]

@@ricobass0253 The transformer and rectifier are an AC to DC converter and produce a DC voltage, DC will not pass through C24. In a simple circuit containing a 9 volt battery, a bulb and a resistor. If you measure the voltage drop across the resistor the negative end (negative battery side) of the resistor will be lower than the positive end. Same with the radio, the CT is zero, there is a voltage drop across the inductor to the chassis, the chassis is more positive than the CT meaning it is at a lower voltage than the chassis. With the output valve cathode grounded to chassis and the control grid connected to the CT there is a negative grid bias across the two.

[@andrefernandez8414]

I used some cigarette paper to repair loudspeakers; it worked, sort of...

[@DavidTipton101]

That doesn't seem to be a very confident recommendation André 👍😄

[@andrefernandez8414]

@@DavidTipton101 yes, it worked! I did this on a couple of Schaub Lorenz transistor receivers, and it sounded fine!

[@DavidTipton101]

@@andrefernandez8414 Ok, Cool 😀

[@iuliancosman637]

👏👏👏 .

[@DavidTipton101]

🙂🙂🙂

[@984francis]

I bet coca-cola would work well as a solvent to remove crud. You forgot to cut out the circle in the middle😬

[@DavidTipton101]

Hi 984francis, Coke will remove anything 😉 Did I? Which circle? 🤔

[@984francis]

@@DavidTipton101 I was pulling your leg. The circle where the speaker is!

[@BigJack1]

@@DavidTipton101 Coke contains Phosphoric Acid. That will cut anything. The reason it doesn't cut our stomachs is because we have anti-acids in our digestive system that neutralizes it. Look at the ingredients sometime.

[@GeorgeWMays]

Have you ever tried Rubber Cement on the speaker cones?

[@DavidTipton101]

Hi George, no, I haven't, the glue I use does stay pliable though but I will try and get some rubber cement for next time 🙂

[@MrHBSoftware]

hi David,toilet paper or kitchen paper soaked in a mixture of wood glue and water works great and is pliable, paint it black and its an invisible repair

[@DavidTipton101]

@@MrHBSoftware I use that on the cone area but not mixed with water, worth a try though, thanks 🙂

[@tubeDude48]

ALL the *Roundy* resistors should be changed.

[@DavidTipton101]

Hi pi-duino, I'm not sure which are the roundy ones, they all look round to me 🤔

[@anthonykeach1788]

WD40 is great at dissolving glue

[@DavidTipton101]

Hi Anthony, I didn't know that, thanks 👍😃

[@erin19030]

Belt sander.

[@DavidTipton101]

That could have worked Cosimo 🙂

[@984francis]

Thevenin.

[@DavidTipton101]

Thévenin's theorem?