Hello to all happy New Year.
I am still trying to fix this STC radio it is model 55 not 54 as I previously called it,
Not sure why I am persisting with it apart from the fact I do not want to be beaten by it. The valve line up is 2A7, 58, 57, 2A5 and 80 rectifier. There was a lot missing from it . I did manage to get it going it a sort of a fashion. The sensitivity is terrible , it does pick up local stations only. The volume could be better.
The circuit I have is terrible.
Does anyone have a good circuit with legible information including component values ?
I don't care if it is not an STC circuit . Another circuit with this valve line up would be good .
Regards Jimb.

Try this link out. it has the 55 listed

http://www.thebakeliteradio.com/Circuits/STC/

Good find.

Hello Flakes.
Thank you that is very helpful , the voltage tables for the valves is great , a lot of the resistors were missing and it looks like I may have missed one.
Kind Regards.
Jimb.

I did post comment on this on the American forum

Hi Jimb, don't give up! I had a look at the circuit and its a lovely straightforward superhet with a very minimal component count. I cant see it being a "ball of fire" as far as sensitivity goes and it would probably need an decent outside aerial to work well.

My approach would be to use the circuit as referenced and then work out what each valve SHOULD do and then measure if that is what happens. That means you need to input signal of audio or RF into the various stages and if a valve should amplify then check that it does.

Radios are pretty simple things when you break the stages down. All they do is take a microvolt RF signal from the aerial and amplify that from microvolts to millivolts in the RF stages and then from millivolts to volts in the audio stages so the output transformer can step plate volts down to low volts to push the voice coil back and forth. Measuring the AC plate output of each valve gives you a clue as to how the process is going.

There are clever things along the way to cut the carrier of the RF out and sieve out the audio from the modulation but basically the set is just an amplifer.

In your set, the 2A7 should function as an osclllator part to mix with the incoming signal part to produce a constant IF frequency signal. The 58 amplifies this IF signal as filtered by the resonant IF transformers. Again you will measure increasing signal level. This assumes you have a wide range meter or CRO so you can measure the HF signal and its an advantage having a signal generator so you are dealing with known level signals. But you can just fly with the aerial signal and common sense.

The 57 looks like a biased amplifier cum detector, I think its an "anode bend" detector, and now volts of RF and an audio sideband appear at the plate.

The choke in the plate lops the RF off and the left over audio progresses to the output valve grid.
The 2A5 output valve amplifys the audio AC grid volts to hundreds of AC plate volts and the output transformer steps that down to a lower voltage at high current to drive the voice coil back and forth.
The 80 rectifier just supplies a high voltage DC to run the plate voltage and current inside the valves.

The volume control is achieved by lowering and raising the bias on the front end valve cathodes, a particularly crude method but it works. If you can work back wards from the speaker and get plenty of audio level by injecting signal into the output grid then you can test the IF stage and once that is working play with the mixer and oscillator.

I guess its a bit more complicated than that but having done the process a few times it really tests your basic understanding or radio and valves!

Where you have a lack of signal or no amplification you then have to work out WHY and that is the whole art about trouble shooting. The WHY may be wrong bias, wrong plate or grid DC levels, poor coupling of signals, shunting of signal due to poor components, you have to know what each part does and if that part is doing what it should.

Don't give up, test, measure and understand!

Cheers, Fred.

I have made as said comment on this on the American forum. It is an Anode bend detector, best done with a Pentode. These are notorious for distortion and changes in line voltage do not help.

I have in a 59 chassis had to change the bias, as in the process of looking for lost signal, with an oscilloscope, there was a lead dress issue found; along with distortion. I use a CRO for the measuring instrument calibrating and where distortion was found, it was used to pinpoint the source.

This is not in any way an overly complex set. You just have to understand how it works and ensure that your test equipment is fit for purpose.

Do not forget when injecting a signal in to the signal grid (TC) a blocking cap in series with the generator is essential to maintain bias and keep the signal to a minimum.

http://antiqueradios.com/forums/viewtopic.php?f=6&t=311068&start=20

Marc

Hello Fred and Marc.

Thank you both for your helpful explanations, this circuit did confuse me as it is quite a bit different.
Flakes sent me a link with STC circuits on it .The one in this has had a few more resistors added to it, I need to print this out ,it is in TIF ,and if I try to print it I just get enlarged sections of it. I will try and download it on an old laptop and play with it. The circuit is still smudged and the component positions eg. R1,R9 or whatever I cannot read , and I can see there is another resistor that appears to have been added between grid 2 of the 57 and 58 valves, cannot read the component value, I am not worried at the moment, I will concentrate on trying to print out the whole circuit first.

I have renewed interest in this now because a friend has offered to help me out with the cabinet.
I did not understand what the choke did in the audio out section, so thank you for the explanation, I have a fairly good selection of test instruments being a retired electronic tech myself. I would be silly to abandon the project because I can learn much from it with the help I am getting from you guys. Getting time to work on it is somewhat limited , but there is no dead line,it does not matter If I take awhile longer it has been sitting around for quite a while, I have a tendency to be impatient at times.

Kind Regards.
Jimb.

TIF is often a pain in the jaxy as far as file formats go but there should be a way to 'print with preview' in the printing options of the TIF viewer you are using so you can select 'fit to page' or similar. If this isn't available in the TIF viewer, try to convert to BMP and go from there in Paint or the default Windows image viewer. Paint is an ancient programme but is sufficient for basic tasks and I sometimes use it in conjunction with Photoshop for one or two things that Photoshop doesn't do well or very quickly.

Jimb, if you use Control Panel to unhide your email address, I'll send you PDF versions of those TIF images, which may be more usable.

Hello GTC.
I have unblocked my email address , thank you for your kind offer.
Kind Regards.
Jimb

Okay, sent.

Hello GTC.
Thank you very much, printed out fine.
Kind Regards.
Jimb.