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 Post subject: Stromberg-Carlson Model 61-H Restoration
PostPosted: Mar Fri 02, 2018 10:54 pm 
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Joined: Jul Sun 09, 2006 5:18 pm
Posts: 1778
Location: Dandridge, TN 37725
The radio was purchased on eBay and was said to power up with noise and static, but no reception. As acquired, the cabinet, grille cloth and knobs were original and in good condition. There were scratches, finish loss and veneer damage on the cabinet, but it appeared that the radio should clean up quite well without refinishing. The best schematic I could find was a scan from my set of Riders Manuals. All of the usual on-line sources were barely readable.

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File comment: eBay Photo - Front
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File comment: Riders Schematic, Cropped
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The radio had seen some repairs. The 10K Candohm resistor R-13 had been replaced by two 12K ohm 2 watt resistors in series on terminal strips (not a good replacement - 24K vs. original 10K). The main filter capacitor C-43 and C-44 (dual 8mfd) had been replaced by a 10mfd/500 volt screw base capacitor for C-43 and an 8mfd/450 volt cardboard box type capacitor mounted under the chassis for C-44. In addition, both sections of a dual 10mfd/450 volt tubular capacitor was placed in parallel with C-44 (the input capacitor), perhaps to reduce hum, or the cardboard box capacitor may have failed. Both the volume control and tone control had been replaced. C-39, the audio amplifier coupling capacitor, had been replaced. The remainder of the paper capacitors appeared to be original. For some reason, a 270K(!) resistor had been placed in series with L-29, the 6A8 oscillator plate choke. I could not see how the 6A8 could possibly oscillate with this modification.
Attachment:
File comment: Chassis Bottom Before Restoration
P2.jpg
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Before starting repairs I did a complete survey of the condition of all parts. I assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original cases if possible).

SURVEY RESULTS
The power transformer, speaker field, and output transformer were OK. L-28, the antenna choke, was open (likely due to lightning damage). The RF transformers, the IF transformers, and all RF chokes except L-28 were OK. The 6F6 cathode resistor (400 ohm flexible) was open. Only five of the 17 dogbone resistors were in tolerance. Based in inputs from ARF members, the antenna choke L-28 could be replaced by a 2.5 millihenry RF choke, or simply omitted along with the wave trap L-1/C-1 if the radio behaves without those parts.

RESTORATION PLAN
The missing dual filter capacitor C-43/C-44 would be replaced by a more suitable dud capacitor and restuffed. The other filter capacitor C-15 (original) as well as the dual 10mfd/25 volt cathode bypass capacitor C-36 and C-40 would be restuffed. All original paper capacitors would be restuffed. The line cord and other rotten wiring would have to be replaced. The original Antenna and Ground metal labels were still present.

The missing Candohm R-13 would be replaced by a 10K 5 watt wire wound resistor on terminal strips. The actual power dissipation was calculated at 1.44 watts. I usually try to replace out of tolerance dogbone resistors with similar parts, if available. I have a stock of NOS as well as used dogbone resistors of various sizes. Of course, very few of these are in tolerance. My usual strategy is to look for a replacement of the same size, with suitable lead length, that is in tolerance but not necessarily marked with the needed value. If needed, I will repaint the resistor with the correct value markings using hobby enamel paint.

Next steps are to test the tubes and round up or order needed parts.

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Dave McClellan, W6SQV(/4)
http://mcclellans.com/RadiosPage2015.htm


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 Post subject: Re: Stromberg-Carlson Model 61-H Restoration
PostPosted: Mar Sat 03, 2018 5:07 pm 
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Joined: Jul Sun 09, 2006 5:18 pm
Posts: 1778
Location: Dandridge, TN 37725
Main Filter Capacitor C-44/C-44
I found a Philco part number 30-2013 filter capacitor in my dud stock which was a screw based chassis mount unit with the required three lugs – similar to the original (based on photos found on-line). The original was a dual 8mfd unit – voltage unknown. It was restuffed using two 10mfd/500 volt electrolytics. The Philco dud was chucked into my small Unimat lathe and cut almost through the case. The cut was finished using a razor saw and the edges of the cuts were smoothed using an Exacto knife. The innards were removed by heating the case with a heat gun. The case was then cleaned out. Holes were drilled into the hard rubber screw base of the capacitor near each external solder lug using a #61 drill and Dremel drill motor. The wire leads of each capacitor were extended using #22 tinned bus wire. The bus wires were routed through the holes in the case, insulated with spaghetti tubing, and soldered to the external lugs. The two parts of the case were then reattached using a PVC plumbing coupling and epoxy cement. Masking tape was used to take up any slack between the coupling and the capacitor case.

Attachment:
File comment: C-43/C-44 Rebuild Part 1
C-43-44 Rebuild 1.JPG
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File comment: C-43/C-44 Rebuild Part 2
C-43-44 Rebuild 2.JPG
C-43-44 Rebuild 2.JPG [ 34.87 KiB | Viewed 284 times ]


C-15 Oscillator Plate Supply Filter
C-15 was a 5mfd 200 volt wet electrolytic. It was rebuilt in a similar fashion as C-43/44 using a 4.7mfd 450 volt electrolytic. In this case, the positive lug was attached to the original center lug using a solder lug attached to the center lug using a 4-40 screw and nut. The negative lead was extended using #22 bus wire and routed through a hole drilled in the bottom of the original case. This wire was wrapped around the screw base of the capacitor and clamped between the bottom of the capacitor and the chassis when the capacitor was mounted.

C-36/C-40 Audio Amp Cathode Bypass Capacitor
C-36/C-40 was a dual 10mfd 25 volt tubular electrolytic. It was restuffed using two 10mfd 25 volt electrolytics. The original guts were removed by first mechanically removing the tar from each end and then heating the case with a heat gun until the guts could be removed. New lead wires were attached to the new electrolytics, with the junctions insulated using shrink tubing. The capacitors were then wrapped in strips of paper towels to retain them inside the original case. The ends of the case were then resealed using brown hot melt glue.

Attachment:
File comment: Restuffed Filter Capacitors
Restuffed Filter Caps.JPG
Restuffed Filter Caps.JPG [ 190.58 KiB | Viewed 284 times ]

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Dave McClellan, W6SQV(/4)
http://mcclellans.com/RadiosPage2015.htm


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 Post subject: Re: Stromberg-Carlson Model 61-H Restoration
PostPosted: Mar Sat 03, 2018 5:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7928
Location: Cleona, PA
The schematic shows C43 and C44 with both negatives going to ground. In the picture, it almost looks like the negative of one is about to short to the positive of the other. Say it isn't so!

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 Post subject: Re: Stromberg-Carlson Model 61-H Restoration
PostPosted: Mar Sat 03, 2018 7:33 pm 
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Joined: Jul Sun 09, 2006 5:18 pm
Posts: 1778
Location: Dandridge, TN 37725
The caps were placed negative to negative when mounted. A lead runs from that junction to the common negative terminal (black) on the base. The positive end of each cap runs to a positive terminal (red or green) on the base. Each lead is insulated using spaghetti tubing. No danger of a short!

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Dave McClellan, W6SQV(/4)
http://mcclellans.com/RadiosPage2015.htm


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 Post subject: Re: Stromberg-Carlson Model 61-H Restoration
PostPosted: Mar Mon 05, 2018 10:44 pm 
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Joined: Jul Sun 09, 2006 5:18 pm
Posts: 1778
Location: Dandridge, TN 37725
Radio Cabinet

The radio's cabinet was cleaned using GoJo (white) Hand Cleaner and 000 grade steel wool. Most of the scratches and paint spots were removed or at least reduced. I suppose I could try Howard Restor-a-Finish to see if the scratches could be made less visible. Also, the areas of finish loss on the sharp edges should be better hidden. And I usually end with a coat of Johnsons Wax. But the cabinet looks OK to me, so I may leave it alone since it is all original at this point.

Attachment:
File comment: Cleaned Cabinet
P4.jpg
P4.jpg [ 114.33 KiB | Viewed 244 times ]


Replacement Dogbone Resistors

I managed to find suitable replacements for all of the out-of-tolerance dogbone resistors. Some were NOS or used originals that were in tolerance (+/- 20%). Others were the correct size and in tolerance with respect to the resistor to be replaced, but marked with a difference value. In these cases, I repainted the replacement with the correct color codes using hobby enamel paint (flat).

Attachment:
File comment: Replacement Dogbone Resistors
Replacement Dogbone Resistors.JPG
Replacement Dogbone Resistors.JPG [ 162.81 KiB | Viewed 244 times ]

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Dave McClellan, W6SQV(/4)
http://mcclellans.com/RadiosPage2015.htm


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 Post subject: Re: Stromberg-Carlson Model 61-H Restoration
PostPosted: Mar Sat 17, 2018 8:30 pm 
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Joined: Jul Sun 09, 2006 5:18 pm
Posts: 1778
Location: Dandridge, TN 37725
All of the tubular paper capacitors as well as the metal enclosed dual line bypass capacitor were restuffed using 630 volt axial film capacitors. All of the original paper capacitors were rated at 400 volts. Each capacitor was first held in needle nose pliers by one lead, heated using a heat gun, then wiped clean of dirt and beeswax using a paper towel. The capacitor was then again uniformly heated using a heat gun until the wax end seals began to bubble. At this point the entire contents could be removed cleanly with virtually no residue left. The new replacement capacitor was then wrapped in a narrow strip of paper towel in order to hold and center it in the original case. The ends of the case were then sealed using brown hot melt glue.

A similar process was used to restuff the dual line bypass capacitor (two 0.01mfd/630 volt capacitors). The original lead wires were reused. The two capacitors were insulated from each other and the metal case using heat shrink tubing. The case was again resealed using hot melt glue.
Attachment:
File comment: Restuffed Paper Capacitors and Line Bypass Capacitor
Restuffed Paper Caps.JPG
Restuffed Paper Caps.JPG [ 87.27 KiB | Viewed 151 times ]

The restuffed paper capacitors and replacement resistors were then reinstalled. I prefer to remove most components and make detailed notes plus take photographs, vs. replace one component at a time. There were a couple of mica capacitors that were way out of tolerance. I measure the value of any mica capacitors any time one lead is disconnected. These were replaced using vintage (used, tested) mica capacitors or new dipped mica capacitors. The 400 ohm flexible audio output cathode resistor R-22 was replaced using a 400 ohm 5 watt wire wound resistor, since I did not have a suitable flexible resistor in stock. Here is the rebuilt chassis.
Attachment:
File comment: Restored Chassis Bottom
ChassisBottom.JPG
ChassisBottom.JPG [ 207.3 KiB | Viewed 151 times ]

Several tubes tested weak or bad and were replaced. Here is the completed chassis. Unfortunately, there’s not a lot I could do about the rust on the chassis.
Attachment:
File comment: Chassis Front - Finished
ChassisFront.JPG
ChassisFront.JPG [ 211.88 KiB | Viewed 151 times ]

Attachment:
File comment: Completed Chassis - Rear View
ChassisRear.JPG
ChassisRear.JPG [ 220.85 KiB | Viewed 151 times ]

SMOKE TEST
The speaker leads were connected using clip leads. I did not wish to solder the connections at this point since I would have to move the chassis around during testing and alignment. There is not much slack in the speaker leads, and no speaker plug is used in this radio.

AC power was supplied by a fused Variac through a watt meter. A DVM monitored B+ as the power was slowly applied. Amazingly, the radio came alive and worked well on all bands. The radio was then aligned. There were some issues during alignment. The RF interstage broadcast band trimmer (shunt aligner) and the broadcast band series padder ran out of adjustment range – either too tight or too loose. This indicates that the performance of the broadcast band may suffer somewhat due to poor tracking. There was not a lot I could do about the RF interstage shunt aligner trimmer being too loose short of removing turns from the RF interstage transformer secondary. The oscillator series padder appeared to need additional capacitance, since the signal level was still increasing with the adjustment screw all the way in. I first measured the range of the padder with everything disconnected. It was 250pf to 470pf. I placed a 220pf silver mica dipped capacitor in parallel and repeated the 600kHz tracking and 1400kHz adjustments. This time there was a clear peak within range of the padder capacitor.

Before buttoning up the radio I disconnected the substitute antenna choke coil L-28 (an old TV peaking coil) and the wave trap L-1/C-1 as suggested by ARF members who felt that these parts may not be needed and may reduce performance if the choke is the wrong value. With no DC path to ground, there was a strong hum modulation (gravely) distortion. There was no obvious reduction in signal level with the choke in place. So I left both parts connected. In this radio the wave trap is NOT adjustable.

Finally I checked all the voltages documented in the Riders documentation and wrote them down for future reference. All of the voltages were close to specification with the exception of the 6F5 audio amplifier plate voltage. That voltage is measured through a high resistance, and the original meter used would have been a 1000 ohms/volt or so rather than a DVM. Also the voltages were low due to the fact that all my tests were done at 112 volts line voltage.

The chassis and speaker were then installed in the cabinet. Here is the completed radio.
Attachment:
File comment: Completed Radio - Front
Front.JPG
Front.JPG [ 166.37 KiB | Viewed 151 times ]

Attachment:
File comment: Completed Radio - Rear View
Rear.JPG
Rear.JPG [ 170.68 KiB | Viewed 151 times ]

_________________
Dave McClellan, W6SQV(/4)
http://mcclellans.com/RadiosPage2015.htm


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