Hi, I have an STC 734 vintage radio and I am looking for a circuit diagram so that I can refurbish it. Has anyone heard of this model and if so, can anyone help me with a circuit diagram and any other related documentation on this radio.

As a matter of interest, it looks identical to the STC 830, which is shown under a different article in this forum, but my one has a label on the back which refers to the radio as a STC 734.

Terry

AORSM's list only 730 &731 and these are all different to one another and 830.

May pay to list valves

Marc

G'day Terry, an STC 734 is listed in Vol 3 1939 AORSM page 271 if you have access.
It is listed as a 7 valve vibrator model but the diagram shows a transformer & 5Z4G as rectifier!!!

I think they muddled the labelling with the other diagram on the same page being a 732.

This link shows the vibrator model schematic
http://www.radiomuseum.org/r/stcaus_734j_ch_734.html.

Have emailed a copy to the addy listed in your profile..

Regards Paul

Paul

Thanks so much for providing me with the circuit diagram. That is great. Looking at the valves on mine, you are right. The circuit diagram for the 732 refers to all the same valves as those on mine. As a matter of interest, and as I do not have access to the AORSM, do they supply a parts list for all of the components that you can help me out with? Regards Terry

Hi Terry,
No parts list or any other info is supplied other than the diagram for most of STC models in 1939.
A quick flick through the book shows most manufacturers seemed to supply a parts list as well so it seems a bit odd.

Someone out there may have a service bulletin etc you would think.?

I usually take a photo of the under chassis before I start to do anything on a radio,- print it, measure, label & list the components so I've got at least something to go by if it goes belly up somewhere!!.

Regards Paul

Hi Paul,

Thanks for the suggestion of taking a few photos before I start. It's a shame that STC don't seem to do a B.O.M. No matter, I will struggle on now that I have a schematic. I've been looking for one for many years now, and it was by chance that I discovered the vintage radio URL after I spotted a similar one for sale on ebay. And to think, I almost got to the point of putting it up for sale twice, but both my wife and I kept deciding against it as its been in the family a very long time now and was passed to us almost 15 years ago.

Another member, Grahame, kindly pointed me to some of the valves on ebay. Just like to thank him as well if he is reading this article.

Hopefully, I have enough to go on with now. In time, I'm going to need to rebuild the speaker as the paper cone has been fishmoth eaten through on the outer periphery over the years. I may have to see if I can find someone who does them professionally as I want to get a good finish.

One question that I would like to confirm, what type of capacitors do you recommend to replace old paper capacitors with. Electrolytic, poly or what?

Now that I have the schematic I am inspired to get the refurbishment underway.

Regards
Terry

G’day Terry
I am only a relative newby into this area myself, but my experience has shown polyprop are great for all the NP caps as they come in yellow & axial... looking the part & are easy to get in the required voltages. For larger polarised electros I bought mostly from an Ebay seller as at the time most parts suppliers were to expensive...I could get 5 for the price of one generally. Mica dipped will generally do the rest. Jaycar - RS components.

I not long ago finished a HMV for a friend & found that there were caps inside of the IF cans that also required replacing. Drilling out the rivets to gain access was scary as I had never done anything like it before & was rather afraid of drilling that bit too far.....

Surprising to me though was that the majority of resistors were way out of tolerance & required replacing. The oldest ones which look like Dog bones were OK but the newer cracked carbon type were all stuffed.
Speaker cones can sometimes be saved with time & effort using Neutral cure silicon thinly until a better alternative comes along.
Pots can be temp re-vitalised using a harder grey lead pencil on the track until you can source a replacement.
Plenty of tricks & tips out there, some of the more experienced can probably offer better solutions but in the absence of instruction we get creative!!!
All the best with your rebuild, time & patience get the best results
Cheers Paul

Hi Paul
A tip I read about in the HRSA magazine for the use of neutral cure silicon for repairing speakers is to thin it with acetone (but steer clear of nail polish remover which is acetone but may have oil in it) and paint it on. It works quite well but takes quite abit of stirring to get the silicon to dissolve. Then you need to be pretty smart painting it on as the acetone evaporates pretty quickly.
The main point of thinning the silicon is to reduce the thickness of the repair to maintain the flexibility and lightness of the cone so it can react quickly to the force of the voice coil and produce a good sound.
Neutral cure silicon must be used because conserving a speaker cone is like conserving an old book and acid attacks the paper.
This works especially well for the threadbare areas of the suspension corrugations at the edge. It can also be used to glue on pieces of paper to patch larger holes that can't be bridged with silicon. The suspension gets the most flexing and I have avoided patching with paper here and have used muslin type fabric painted with the silicon.
It is surprising how well speakers can work with quite big holes as long as the voice coil isn't rubbing on the magnet or there isn't rubbish in this space. So the repairs don't seem to need to be super strong. After all the wattage is low so they are not pumping air like a doof-doof car.

Good luck
Graham