Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Aug Thu 16, 2018 6:39 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 21 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Emerson 838 Squealing on Station
PostPosted: Feb Fri 02, 2018 4:51 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
I'm working on an Emerson 838 subminiature tube/hybrid radio. The issue I'm having is that I cant tune into any station without the radio breaking out into a horrible groaning squealing. I can hear stations faintly through the noise but not clearly, and the issue only happens when tuned into a station.

I assume the IF stages are breaking into oscillation. I have already replaced both electrolytics and the three paper capacitors and tried staggering the IF transformers. I also checked out the AVC bypass cap and the resistors in the AVC circuit. All of the other resistors that I can check seem to be within tolerance. Can anyone offer any suggestions?

Here is the schematic:

Attachment:
usa_emerson_56_838_sch.pdf [149.12 KiB]
Downloaded 32 times

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Fri 02, 2018 5:38 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 36741
Location: Livermore, CA
Looks like they knew of this problem? There is a 5 pf cap (C12) from plate of 1AH4 back to inverted grid signal. Also a .002 mf (C11) to ground. This caps limits action of the 5 pf.

Has the .002 mf value been changed? Try a slightly different cap value for the 5 pf. Maybe two pieces of insulated wire twisted together.

_________________
Norm


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Fri 02, 2018 8:51 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
Thanks Norm, The .002uF cap is still in the set, I did lift one leg and jumper the connections with a .0022uF film cap to no avail. I will attempt to alter the value of the 5pF cap with a gimmick in the morning.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 4:08 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
Thanks for your suggestion Norm, I went ahead and wired a gimmick capacitor in parallel with the 5pF cap C-12, this seemed to have the desired affect so suspecting that the 5pF cap C-12 had gone open I went ahead and replaced it with a NOS tubular ceramic capacitor. However, this replacement had no effect as the original problem persisted.

I then added a 1.5pF cap in parallel with the new 5pF cap which seemed to reduce but not completely eliminate the feedback squealing. Adding one more 1.5pF cap got rid of the feedback completely but also had the effect of reducing my volume of received stations to almost nonexistent.

Now if I understand this correctly C-12 is acting as part of a negative feedback network, correct? So as I increase its value I am reducing the gain of the IF stages to prevent oscillation, which would also reduce volume if increased beyond a certain level.

I will also play with the value of C-11 to see if anything can be accomplished as such.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 4:21 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 26592
Location: Detroit, MI USA
Hunter, try shielding the 1AH4 with a bit of aluminum foil grounded to the chassis. If you change the value of the 5pf cap, it will also slightly affect the alignment. I wouldn't expect that adding just a tiny bit more capacitance would drop the signal so much.

Also try adding a capacitor from the bottom end of the volume control to ground as there's some feedback happening in the filament circuit as well. Don't know whether it would need to be a film cap or an electrolytic.

Those sets were very fussy when they were new, it seemed like the ones we got in the shop never worked as well as other battery portables. Particularly when the batteries started to get a little low, operation was less than perfect.

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 5:02 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
Dennis, The 1AH4 is already shielded with a spray on metallic coating which is connected internally to pin 3 which is one side of the filament. If I were to ground the shield, it would likely burn out either the 1AH4 or the 1AJ5, as the full filament supply would be expended within the filaments of only these two tubes.

I tried a couple of capacitors from .001uF to 10uF between the bottom of the VC to chassis and perceived no difference, as of yet only changing the value of C-12 seems to have any effect.

I concur with what you are saying about these sets being fussy, I honestly don't think the build quality is that great either. The design of the chassis is almost a circuit board but with just lumps of parts soldered together on top of one another instead of traces. Very difficult to find components and I can just imagine the tech who found a bad resistor buried down deep. :?

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 5:34 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 26592
Location: Detroit, MI USA
Yes, we hated to see them come in for service. Many of the owners put them away and stopped using them after all transistor sets became affordable a few years later. Even the Emerson factory distributor in Detroit hated those sets and said they should never have been released for sale until the design was perfected. At the time Emerson seemed more interested in selling window air conditioners than consumer electronics.

Do you think there's continuity from that spray shield to the filament wire? I remember we had to change tubes in those sets sometimes to get them to work properly. If you wanted to try adding another shield, you could use paper or plastic wrap to insulate it from the spray coating.

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 7:02 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
I checked and there is indeed continuity between the outer coating and pin 3 (filament) of the tube. I can try another 1AH4, as I have one to substitute from my Motorola pixie.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 7:30 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
It's getting late but I think I have it to a point where I like it. Changing the 1AH4 made no perceived difference so I went ahead and started playing around with the value of C-12 in small increments. With the equivalent of 8.9pF in its place I am able to peak the IFs without them breaking into oscillation, and without decreasing the volume to an unsatisfactory level. I think I'll make the modification permanent and then do a full alignment tomorrow.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 10:48 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28198
Location: SoCal, 91387
Hcompton79 wrote:

I concur with what you are saying about these sets being fussy, I honestly don't think the build quality is that great either.

I have to disagree with you, little brother.
I have eleven 747/838/856 models, and these are reasonably hot receivers. They used to be available in abundance on eBay, which seems to indicate that they were decent sellers. I would venture for good reason.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 6:47 pm 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
fifties wrote:
Hcompton79 wrote:

I concur with what you are saying about these sets being fussy, I honestly don't think the build quality is that great either.

I have to disagree with you, little brother.
I have eleven 747/838/856 models, and these are reasonably hot receivers. They used to be available in abundance on eBay, which seems to indicate that they were decent sellers. I would venture for good reason.


I'm not saying that they can't be good performers when working properly, from what I have seen so far, the set seems to have fairly good sensitivity. However, from a technician's point of view, the layout of the set leaves a lot to be desired. The component density on the top of the phenolic board is extremely high and makes the set difficult to troubleshoot.

Obviously I don't think its a bad set, I bought one and want it working right, however I think the design could have benefitted had PC board technology emerged a little earlier.

As collectors we get to see both the perspective of the consumer and that of the service man. In this case I believe the Emerson 838 is a set that would have been quite popular with the public because of the battery life increase afforded by the output transistors but something that, like Dennis said, service shops didn't like to deal with.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Sun 04, 2018 7:03 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28198
Location: SoCal, 91387
Hcompton79 wrote:

Obviously I don't think its a bad set, I bought one and want it working right, however I think the design could have benefitted had PC board technology emerged a little earlier.

Keep in mind that it's predecessor, the four tube 747, came out in 1953, and the only difference between that and the 838/856 models was the deletion of the audio output tube and inclusion of the two Transistors, with but little change in the chassis layout. It's simply a primitive PCB mounting/point to point wiring conglomeration.


Hcompton79 wrote:
As collectors we get to see both the perspective of the consumer and that of the service man. In this case I believe the Emerson 838 is a set that would have been quite popular with the public because of the battery life increase afforded by the output transistors but something that, like Dennis said, service shops didn't like to deal with.

I agree that it can be a PITA to work on, but these are some of the pitifully few models from the mid '50's where I haven't had to change capacitors, except in I think one set.
Given that, I would imagine they were primarily trouble free when they were in use in the '50's, and that aspect may have also contributed to their popularity.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 12:15 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
Alright, a slight problem with my plan occurred. After reassembling my set I found that I had no discernible reception below 700Kc. I confirmed the oscillator is not dropping out, but the signal level of received stations drops to almost unperceivable levels with any capacitor above 5pF in place of C-12. I'm starting to get to my wit's end with this set.

One other thing I have noticed however, is that the frequency of the oscillation changes as I move the B battery harness, however I don't quite see how this would be related.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 1:36 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28198
Location: SoCal, 91387
IIRC the osc coil has an adjustable core, so you might consider slightly re-tuning it, esp if the front end circuit has been changed with the addition of a small value capacitor.

The other thing to check would be the converter; lack of low end reception is generally symptomatic of a weak tube.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 3:34 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
Further adding to the confusion is the nature of the IF transformer adjustments, in that they are not behaving normally. With the exception of the top core for the second IF, I am unable to find the definite peak for each.

I am able to find two peaks for each adjustment, with a trough between each of the two peaks. As I adjust one core in or out I can hear a gradual increase until it breaks into oscillation and then the 400hz tone cuts out. If I keep turning, it breaks into oscillation again and then a peak with the 400hz tone audible again. Hm....

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 4:53 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28198
Location: SoCal, 91387
I would suggest recognizing the set for what it is; a consumer grade item over 60 years old, and therefore no longer capable of exacting measurements. Simply adjust the IF cores to the point of oscillation, and then back them down to just before it's occurrence. Sometimes you have to settle for "good enough" with the older stuff.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 5:41 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
I finally found the issue, it was a bad solder joint on one of the oscillator coil leads. I couldn't see it because it was buried under a mess of other components but I was holding the chassis and bumped the oscillator coil which caused the set to work fine for a few seconds before the squealing returned. I checked the oscillator coil leads and found that by wigging it the primary resistance would vary between a few ohms and a few hundred ohms. I resoldered the lead which connects to .047uF capacitor C-8 and the problem has yet to return. :mrgreen:

Sensitivity and selectivity seem quite good as well. This is a radio which I really want to like based on what I've heard you say about it Richard, hopefully this sticks.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 6:12 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28198
Location: SoCal, 91387
I have a good size variety of 4 tube and hybrid portable sets, and I have to say that this model from Emerson is undoubtedly one of the best AFA sensitivity and audio output, compared to other coat pocket size sets.

If you want it to be self-contained, get a dollar store three AAA battery powered LED flashlight, remove the battery holder, and install it in the A battery cavity. It's just about the size of a C cell, and will fit perfectly. The extra half volt (4-1/2, when the radio calls for 4 even) won't harm anything.
For the B supply, I use a triple A dual holder, cut in half and lengthened, to contain six A23 twelve volt cells. You can use only 4, for 48 volts, but using six gives the set extra volume, and with that apparent better sensitivity.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 6:43 am 
Member
User avatar

Joined: Sep Sat 27, 2014 4:24 am
Posts: 1581
Location: Oxford, MI
fifties wrote:
If you want it to be self-contained, get a dollar store three AAA battery powered LED flashlight, remove the battery holder, and install it in the A battery cavity. It's just about the size of a C cell, and will fit perfectly. The extra half volt (4-1/2, when the radio calls for 4 even) won't harm anything.
For the B supply, I use a triple A dual holder, cut in half and lengthened, to contain six A23 twelve volt cells. You can use only 4, for 48 volts, but using six gives the set extra volume, and with that apparent better sensitivity.


Already done, though I got my 3 AAA holder from harbor freight. Are you able to fit six A23s in the 838? The six A23 holder I made for my Motorola pixie is about 3/8" too long to fit in the bottom compartment and still be able to shut the door. I'm working on one right now for four A23 using some dual A23 holders I bought a while ago.

_________________
Caretaker of Portable Tube Radios


Top
 Profile  
 
 Post subject: Re: Emerson 838 Squealing on Station
PostPosted: Feb Tue 06, 2018 7:44 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28198
Location: SoCal, 91387
I wouldn't tell ya to do six cells if it couldn't be done;


Attachments:
Emerson 747 B supply.JPG
Emerson 747 B supply.JPG [ 94.12 KiB | Viewed 690 times ]
Pixie PS.JPG
Pixie PS.JPG [ 133.14 KiB | Viewed 690 times ]

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////
Top
 Profile  
 
Post New Topic Post Reply  [ 21 posts ]  Moderators: Marcc, Norm Leal Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: DenisN, Google [Bot] and 11 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB