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 STC Model 562 Restoration Chassis.
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 Return to top of page · Post #: 16 · Written at 2:09:09 PM on 3 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1490

What you've measured so far sounds about right. The anodes on the 83V are AC volts so if you measure them DC you'll get a wrong answer.

Same with other valve heaters, but if they light up it's OK! You can move on!

It sounds like the B+ at the rectifier filament is 400 volts. About right if measured during warm-up.


 
 Return to top of page · Post #: 17 · Written at 2:31:00 PM on 3 July 2017.
Captgogo's Gravatar
 Location: Sydney, NSW
 Member since 22 May 2017
 Member #: 2114
 Postcount: 120

Ian ,
All measurements were AC on the 83V, I thought the Heater voltage was meant to be much lower.
The Anode on the 2A5 was about 413V AC, with the Heater at 1V AC

Have been called away so I will finish measurement tomorrow if I get a chance.
But you think at this stage all is ok, all the valves do light up and some faint static comes from the speaker but not much luck in tuning anything.
George


 
 Return to top of page · Post #: 18 · Written at 2:59:57 PM on 3 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1490

Put your meter on DC when measuring any of the voltages I've asked for.

You don't need to measure the heaters or the 83V pins.

Put them in a table, like this:

2A5:

G1
G2
Anode
Cathode

57 (detector)

G1
G2
Anode
Cathode

etc. etc.


 
 Return to top of page · Post #: 19 · Written at 4:50:47 PM on 3 July 2017.
Captgogo's Gravatar
 Location: Sydney, NSW
 Member since 22 May 2017
 Member #: 2114
 Postcount: 120

Ian, I have made the following DC measurements
2A5
G1 Pin 3, 241V
G2. Pin 4, 0.04V
Cathode Pin 5, 8.5V
Anode Pin 2, 200V

57
G1 Pin 3, 230V
G2 Pin 4, 8.8V
Cathode Pin 5, 9.0V
Anode Pin 2, 11V

58
G1 Pin 3, 242V
G2 Pin 4, 85V
Cathode Pin 5, 85V
Anode Pin 2, 250V

57
G1 Pin 3, 252V
G2 Pin 4, 12.24V
Cathode Pin 5, 12.2V
Anode Pin 2, 275V

Let me know your thoughts and next steps.

Thanks
George


 
 Return to top of page · Post #: 20 · Written at 5:09:19 PM on 3 July 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3967

59E chassis is one that neither I, or HRSA could find & was only used in one model. The chassis I had was hacked and needed to be fixed as the person who started on it never finished it, as with his hack he could not get the bias right & I had a hell of a job finding out what the missing volume pot value was. So after I sorted it out I drew up that circuit. It was only by accident that I spotted the chassis & realised what it was as I had actually made a part for it, for him ages ago.

I still think yours is 56.

I would put a 470K resistor (voltage divider) across each of those series Ecaps especially if you put the proper 80 in it. Seen several fail from not doing that.

I should have photos of the 59 filed

I will have a look if your email is un-hidden if it is email me & I will send what I have on 56.

Marc


 
 Return to top of page · Post #: 21 · Written at 5:23:54 PM on 3 July 2017.
Gandhn's Gravatar
 Location: Windella, NSW
 Member since 5 November 2010
 Member #: 770
 Postcount: 327

Radiomuseum.org does have a circuit for the 562 chassis 56 which shows some voltages.

This may be useful.

Harold


 
 Return to top of page · Post #: 22 · Written at 5:33:35 PM on 3 July 2017.
Captgogo's Gravatar
 Location: Sydney, NSW
 Member since 22 May 2017
 Member #: 2114
 Postcount: 120

Hi Marc
I have unhidden my email, did not relise it was hidden, must be the default. Anything you have on 56 would be most welcomed.
George


 
 Return to top of page · Post #: 23 · Written at 5:45:19 PM on 3 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1490

Yes George, Marc, I thought that looked a bit familiar too.

Here are your measurements, commented in (brackets):

Ian, I have made the following DC measurements
2A5
G1 Pin 3, 241V (Pin 3 is G2 and that's OK)
G2. Pin 4, 0.04V (Pin 4 is G1 and that's OK)
Cathode Pin 5, 8.5V (OK)
Anode Pin 2, 200V (OK)

The 2A5 circuit is OK.

57
G1 Pin 3, 230V (Pin 3 is G2 and that's high)
G2 Pin 4, 8.8V (pin 4 is G3, connected to cathode. OK)
Cathode Pin 5, 9.0V (same)
Anode Pin 2, 11V (Much too low.)

58
G1 Pin 3, 242V (Pin 3 is G2 and that's high)
G2 Pin 4, 85V (pin 4 is G3, connected to cathode. Much too high)
Cathode Pin 5, 85V (same)
Anode Pin 2, 250V (about right)


57
G1 Pin 3, 252V (Pin 3 is G2 and that's high)
G2 Pin 4, 12.24V (pin 4 is G3, connected to cathode. About right)
Cathode Pin 5, 12.2V (same)
Anode Pin 2, 275V (about right)

Conclusion: There is a problem in the circuit around the voltage divider and volume control, as well as the anode of the detector 57.

Check the 250k and 47k resistors connected to the plate of the first 57 you measured. Also check the capacitor that goes from the plate to the chassis.

If you tell me what voltage you have on the junction of the 47k and 250k resistors, I'll be able to tell you if its the capacitor or what resistor is bad.

That's one fault.

Trace the circuit around the voltage divider (shown 15k and 12k on the circuit) You should just be able to measure resistance with no power applied to work out what's wrong. It looks like the volume control might be open circuit or not connected to chassis.

That's the 2nd fault.

If all this is too much, drop it in to me and I'll fix it while you wait!


 
 Return to top of page · Post #: 24 · Written at 9:15:33 PM on 3 July 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3967

That voltage divider is one of a type notorious for failing. The 59's pot was, as said, missing but its 6K (similar to an AWA). If the one in that one is cactus its going to be a 5KWW and you can be guided then by what I did to 59. I did bother to write ohms on the bias train.

Note on 59 the comment re the bias on the IF Amp. That, I was obliged to alter as the Oscilloscope (calibrating) picked up distortion (over and above a typical Plate detector).

That should not have the same factory fault on the Mixer as 59.

Photo's of 59 chassis sent to Brad.

Marc


 
 Return to top of page · Post #: 25 · Written at 10:06:02 PM on 3 July 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3967

In light of the current situation I have emailed photo's of 59 to show difference & circuits of it and the 56 chassis. If anyone else wants these circuits for reference advise.

If the pot is cactus its 6K and that will require a modification to the bias circuit as the replacement will be a 5K WW. I note that one has its Local DX switch. It was missing on 59 and what I ended up doing was using a conventional switch, where the off position became Local.

Using the strongest station signal here (8Km away) I found after upping the 6D6 bias that 68pF was the best for the strong signal to avoid distortion & overload.

Modifications have ramifications and that's why my 59 chassis circuit comes with the warnings it has on it and why the mods were done.
It also ended up with a new speaker as its was poling.

Marc


 
 Return to top of page · Post #: 26 · Written at 10:52:56 PM on 3 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1490

Regards mods, it was apparently very common during the 1930s for "radio mechanics" to change out the cranky autodyne converter for a 2A7 and the nasty anode bend detector for a 55 (dual diode triode) - all still 2.5v valves so no big deal except that the 2A7 needs a 7 pin socket. It was a simple matter then to get rid of the wirewound volume control and add "AVC".

I think I've seen an old Radio and Hobbies article entitled "updating old sets" or something similar which had instructions for doing just this.

There's absolutely no question that you would end up with a radio that is much easier to live with!
Without changing the appearance in any way because these are contemporary valves.
Distortion and fading gone, better dynamic range, less noise, better sensitivity.
The distortion from an anode bend detector is much worse with modern AM transmitters that run 110% modulation...

While I was searching for a 1H5GT I noticed I have two 55s and two 2A7s....


 
 Return to top of page · Post #: 27 · Written at 12:03:15 AM on 4 July 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3967

A lot of that modifying depends on the willingness keeping it as close as possible to original. The guy that hacked the 59 tried to put the volume control into the grid of the OP valve. This all came unstuck in the bias train & I kept the pile of resistors used intact, on the tag strip used (souvenir) & proceeded to put it back as best I could to the original concept using a 58a for reference as that whole series was similar & a fairly common plot with that valve combination (screen grid radio)

Patience is a virtue in a situation like this as it failed initially to fire up. With a Sig Gen & CRO (formidable combo for tracing lost signal) it was swiftly pinged to the Plate detector & I had missed an issue with a hidden resistor. In the process of using the CRO as a signal tracer, it showed a second oscillation at the mixer? That was a factory oversight with the wiring of the plate, screen,& a wire from the oscillator coil. I ended up moving the screen wire totally & putting it in shielded cable. That quashed that little bit of unwanted coupling & feedback.

Plate & screen wire were parallel beside each other & the other wire did not cross ninety degrees: Lead dress is important.

If they cut the modulation percentage & actually played real music, that would help things sound better & not risk stressing things.

Might have a 1H5. Actually found a 6K8 in a set a while back: So what! Well it should not have been in that set? One of those fixes when valves were scarce I assume?


 
 Return to top of page · Post #: 28 · Written at 8:12:22 AM on 4 July 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 878

I see that your STC paper label is in a parlous condition. I have made reproductions of a couple of these and for this purpose collected images of labels from STC radios on offer on ebay. From these can lift words, letters, numbers etc to build up a label in Photoshop type software. You are welcome to this stuff if you need it.

Note that the serial number that appears on the label should be stamped on the back of the chassis.


 
 Return to top of page · Post #: 29 · Written at 8:34:44 AM on 4 July 2017.
Captgogo's Gravatar
 Location: Sydney, NSW
 Member since 22 May 2017
 Member #: 2114
 Postcount: 120

Hi Marc and Ian,
After reading Ian's response this morning on the further diagnosis on the initial suspected faults and prior to reading the post that followed, I was about to respond to his generous offer of " if it is all to much bring it into me and I'll will fix it while you wait" with a thank you Ian, but why should you have all the fun, I will give it go......

Then I read the dialogue between Marc and Ian in the post that followed, regarding some of the challenges with circuit mods and as I am new to this, my head started spinning and I began to think about my initial response to Ian, and the word "Fun" morphed in to word like "Frustration" Lol...

Now even though, I am still trying to understand some of the jargon used here, I think I should continue to take the challenge of trying to get this radio up an running with as much of the orginal design in tact.
If you guys are still willing to put up with my basic questions etc, I am hoping also that others who did not grow up with this field but have a passion like I do to learn might benefit from this thread.
I do not have a CRO, so I am not sure if proceedIng with this project this will be essential or not.

Ian, I will take the measurements you suggested regarding the two initial faults detected and will get back to you with what I find.

Will I also need to measure capacitance? as I do not have this function on my current multimeter, I am happy to upgrade to one that does if required.

George


 
 Return to top of page · Post #: 30 · Written at 8:40:34 AM on 4 July 2017.
Captgogo's Gravatar
 Location: Sydney, NSW
 Member since 22 May 2017
 Member #: 2114
 Postcount: 120

Hi STC 830
Yes, a very little bit of the label, could just read 2 of the valves noted. The S/N stamped on the back is 56-4216 and yes if a reproduction label is possible that would be great to include on the restoration.
Thanks George


 
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