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 Stromberg Carlson 68 Circa 1938
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 Return to top of page · Post #: 1 · Written at 9:11:53 PM on 18 November 2016.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 712

Hello from new member Jimb.

I am trying to repair a friends Strongberg Carlson 68 dual wave radio. It was professionally restored some years ago when it was purchased by my friend. It must have worked ok for a number of years.

The chassis work is neat all re-capped etc. The valve line up is as follows EK32, 6D6, 75, EL3G, 80 and magic eye 6E5 . It was dead when I received it due to faulty rectifier after replacement, IF and audio end ok, however still not receiving , oscillator works , the problem is around the EK32. I am very bad on front end faults it is the only valve I cannot be sure is ok however I think it is , I had two secondhand ones they all test the same in a Taylor 45C however this tester does not give an emission value for this tube. I normally use a Hickok 530B tester unfortunately it has no test data for this tube.

The AVC volts to grid 4 top cap are very high -32 volts I disconnected the resistor and left grid floating it tries to pull in a very strong local station that absolutely swamps most most radios.you could almost pick it up with a piece of wet string it is so strong, it was barely audible and goes into high pitched oscillation each side of station. I tried a power supply + to ground negetive to grid 4 and varied bias volts still no improvement. I have checked all resistor values and they are in spec.I have also rechecked the capacitors which are all good quality poly caps. The voltages around the EK32 are as follows Anode 249 v. Grid 3and 5.50 v. Grid 1. Gave two conflicting readings when measured four times It read -3 v and - 12 volts. Grid 2 . 232 v. K g6 0 v. G4 top cap reads - 32 volts.

I keep coming back to the wave change switch it does seem to be ok I find it difficult tracing through this bit and trying to work out where the aerial and oscillator coils start and finish. I have put a lot of time into this. If it was mine, it would have gone into the too hard basket however It belongs to a friend and I am desperate to repair it. The circuit I have is out of the 1938 edition of AORSM. The values are very smudged and almost impossible to read with many mistakes on the circuit diagram. I have previously failed miserably on another set with a similar front end fault where signals are very small.

Any help will be greatly received.

Kind Regards.

Jimb.


 
 Return to top of page · Post #: 2 · Written at 10:18:44 PM on 18 November 2016.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5385

-32 on the TC is too high, what is the 6E5 doing? It can give a guide to what the front end is doing and the AGC. 6E5 is the one that only requires -7.5V on the grid (AGC).

EK32 has the same pin out as 6A8.

I would try IF frequency into the TC of the Octode EK32 via a say 200pf series cap. Changing parts around the det audio (#75) can cause instability and an oscillating stage can generate a lot of AGC volts.

6D6 is electrically the same (other than base) as 6U7, with out shielding they will cause havoc. Notorious for it.

That has a long WW tapped resistor R21 -R24 Those are also notorious for going open.

If the oscillator is working & you have a digital meter some will short, or generally get confused with RF. DVM because of their high impedance 10M in some cases they will present less current draw to an analogue, & may read higher.

Beware: Apart from the RF issue some cheap meters will not withstand the voltages found in valve radios

Marc


 
 Return to top of page · Post #: 3 · Written at 10:49:40 PM on 18 November 2016.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1205

I don't have much technical expertise with these matters.

But I do have the original manufacturers service sheets for this set if that is of any help.

Stromberg Carlson 68 Service Data


 
 Return to top of page · Post #: 4 · Written at 8:27:08 AM on 19 November 2016.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1311

Hi Jimb,

Marcc has put his finger straight onto the situation, high AGC ( 30 volts is full blast) means high signal level, means IF stage is running full clipping output, means is UNSTABLE and probably at any freqency but the one (IF) it should be working at!. That's why the front end wont tune!
I have built superhets from scratch and done all the wrong things and not making sure the IF valve is properly shielded is one of them. A 6D6 or similar valve will just go berserk and the set will exibit all the symptoms you have.

Scoping or measuring the plate AC signal will reveal full swing oscillation which is exactly what the high AGC is telling you as it is a derivative of the IF signal.The front end will then not tune or function as the frequencys are all wrong. A simple cause in one of my sets that gave exactly what you have was the GOAT shield on the 6D6 was clamped over the valve but NOT connected to chassis despite having 2 fingers grounding it. I had sprayed the shield with paint and insulated it nicely. Grounding the shield (you DO have a shield on the IF valve I hope?!) with a clip lead immediately removed all the squealing choking. non tuning, wild stations and turned the set back into a meek superhet with normal AGC range 5 to 15 volts.

Scoping the signal points will comfirm all this.

Good luck.

Cheers, Fred Lever.


 
 Return to top of page · Post #: 5 · Written at 9:53:48 AM on 19 November 2016.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5385

Looking at the circuit. I don't think it is the issue: But it has a mechanical link pin in the PU socket. Rarely does a set with that configuration provide any radio noise if its missing.

Marc


 
 Return to top of page · Post #: 6 · Written at 11:39:17 AM on 19 November 2016.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 712

Hello from Jimb.

Thank you very much Marc, Monochrome TV, and Fred.

I am not too sure how to use this forum I would have like to individually thank each of you. Firstly Marc you have brought up several issues I had not even considered , Monochrome TV, that was an amazing manual you sent me the circuit I had was so bad I could not read much of the information and I had no idea of parts location, the manual is excellent thank you so much. Fred thank you to again I had not considered these points. I would have given up on this had it belonged to me, however I am trying to repair it for a friend.

I have messed around with old radios off and on for 57 years since I was 12. Most have not involved much deep thought. In fact, you have all renewed my interest in radios and given me some new hope of repairing this one.

My back ground is industrial electronics , broadcast television , and commercial electronics.
I will report back as I get back into this. The other problem with this it was previously restored by someone before my friend bought it and it's originality has changed somewhat component wise.

Thank you all.

Jimb


 
 Return to top of page · Post #: 7 · Written at 4:43:33 PM on 19 November 2016.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5385

My normal procedure is to check the thing to see if it is in a condition to power, & rarely do they meet the criteria. Where there is obvious tampering there is an obvious need to inspect everything. You would be amazed at the things that are often seriously wrong.

A lot of old paper caps were "outside foil" & replacing them (virtually mandatory) can cause instability. The most common now caps for valve radio are 630VDC. I do love the way this lot (albeit better than some) have omitted the rectifier from the voltage chart. It is useful to know what AC volts are on the rectifier plate? Beware of modern Electrolytics, these do not have a surge rating and with #80 and 5Y3 twice 250 in that set can surge closer to 500V. 450V I won't use with those rectifiers, nor Silicon Diodes: 500-600V better.

With these things you actually work backwards Power & Voltage, then audio and then RF.

An Oscilloscope & Signal Generator, and perhaps a counter to calibrate it, make a formidable combination for detecting dead oscillators, distortion, lost signal and signal that should not be there. Many forget to check all the resistors & that can bring you unstuck.

Marc


 
 Return to top of page · Post #: 8 · Written at 5:42:18 PM on 19 November 2016.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 712

From Jimb.
SUCCESS !!!!!!!!!!!
Thank you all so much for your contributions now working perfectly. You were so right. The shield was the problem on the 6D6 .Yes it had a can around the valve , however the cup piece around the valve base had become loose on the rivets and had corrosion under it . It had no connection to earth as you suggested . I scraped away the paint and soldered the cup to the chassis and to my amazement instant success. I would not have been able to fix this without your input Marc and Fred . Also a special thank you to Monochrome TV for the manual it was great to be able to locate which coil and trimmer did what.
So again a very big thank you.
Kind Regards.
Jimb.


 
 Return to top of page · Post #: 9 · Written at 10:55:36 AM on 20 November 2016.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1311

Hi JimB, well done!
I did give myself a quiet smile because having been right in your shoes I knew exactly what the problem was.
My advantage was that I have created the condition in a couple of other sets just to understand what a sitting duck a 58 ,6D6 or 6U7 IF valve is if it and the surrounding IF coils are not properly screened. The later valves 6K7 and so on have an internal shield over the works and don't seem to be so critical. You can actually ran such sets with the external can removed and only a CRO will show if there is a tendency for the valve to oscillate and usually you don't know about it except for strange audio distortion.
Typically if I take the shield off a 6D6 it will run with clipping positive feedback output. My 1938 Kriesler is a good example and the valve settles at about 1MHz and that goes right through any resonant filters! Then of course the derivative AGC from that signal can be anything up to -60 volt. As the IF valve is out of control that bias does nothing fed back to it via the AGC line but it does shut the convertor valve right to cut off. Thus the overall set cannot tune anything logically. As the IF valve is running at full gain the set then acts like a weird TRF with mangled modulation and the resultant audio can be screaming, distorted stations from anywhere, whistles and god knows what. Scoping the signals in this condition shows wall to wall RF at the IF plate of many frequencys hence totally no logical operation. My Kriesler will just sit there sucking air and crackling with the 6D6 shield off. If you lower your hand around the 6D6 you can "tune out" the feedback like a theremin, all the sproogles showing on the CRO attached to the plate disappear, a normal 455kHz top and bottom AM modulated signal returns, normal AGC levels reappear and the set tunes normally! Magic? No, just logic Jim!
Have a look at my rebuild of the Kriesler on this web site its a very similar design concept to your SC68 and I did some dumb things you will laugh at on the way. Its in the special projects section: restoring a 3K15 Kriesler.
Cheers, Fred Lever.


 
 Return to top of page · Post #: 10 · Written at 8:03:00 PM on 20 November 2016.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 712

Hi Fred.
Thanks again you saved me a heap of frustration , I have learned much.
I will check out your restoration.
Regards. Jim


 
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