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 Post subject: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Wed 10, 2021 11:57 pm 
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Joined: Aug Thu 06, 2020 7:46 pm
Posts: 117
Location: Morro Bay, CA USA 93442
Opened this radio up only to find not one, but two electrolytic capacitors in the expected spot. (see photo below)

The mystery cap appears to be a single value of 200 mfd and is connected as follows:
- one lead to a pin 8 on 35L6, which I think is just a ground terminal point.
- the second lead to pin 4 of 35Z5. Pin 8 and 4 are bridged, which I'm not seeing on the schematic. Also connected are the red lead to the main filter cap and a lead to the output transformer.

Though the chassis appears to be mostly original, I'm seeing some things that make it clear that at least one other person has been inside the radio. Connections that are wrapped with electrical tape, for example, and this 200 mfd, which I can't find in the parts list.

Is it a modification? Do I need to keep it in place?

Thank you!
Chassis view: https://www.dropbox.com/s/z07ka9cw60kj6 ... p.jpg?dl=0
Schematic: https://www.dropbox.com/s/lytl8fxfqgcfv ... c.pdf?dl=0
Attachment:
EXTRA CAPACITOR VALUE.jpg
EXTRA CAPACITOR VALUE.jpg [ 415.27 KiB | Viewed 1575 times ]


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Thu 11, 2021 4:42 am 
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Joined: Sep Mon 14, 2020 2:14 am
Posts: 97
200?? I have a hard time believing that's WAY over what came in a radio like that. I have never seen a 200 in a tube radio in my life. Unusual enough to even see a ONE hundred. And no, I wouldn't leave it in there even if it was original. Too old and dried up. I believe I'd get a copy of the paperwork for that radio and just go by that.


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Thu 11, 2021 5:46 am 
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Joined: Feb Fri 01, 2019 11:35 pm
Posts: 706
Location: Riverside Ca. 92504
I have worked on several radios that use a 200 mf cap, but it usually at ten volts and in the filament section. That has a higher voltage rating and could be a filter for something. It is highly unusual but I have seen some freaky installations when it seems people run out of the correct parts and sub something in. I had one where three large caps were hanging out of the chassis in parallel on a one foot wire. Check it out before you make any mods and go by the schematic in this instance since it is an aftermarket. Tom


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Thu 11, 2021 5:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1096
Location: 98802
In the pictures it looks like the positive lead is connected to the same place as one section of the other cap. I think the other cap had a bad section, and someone tacked that 200uf on in parallel to fix the hum. I'm fairly certain it does not belong.


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Thu 11, 2021 6:42 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 821
Location: Suburban Chicago
Filter caps are likely to have been replaced one or more times by now so changes are to be expected. A value of 200 uF is uncommon in tube gear and if that is the first filter cap after the rectifier it may violate the load capacitance ratings of the rectifier tube. I would replace both with the specified values. In fact I recently restored an S-38 and that is what I did. Both caps in mine were replacements and one was leaking. It wasn't hard to make the replace decision in that case! I used a capacitor kit from Hayseed Hamfest for that radio and it was fine but I often just source my own from Sals or Digikey.

I'm working on an SX-11 right now and that appeared to have a chassis mounted cap added by a previous owner. It was a dual section cap as it was supposed to be however both sections were half the required value and they were connected in parallel to the same point. The function of the second section of the original cap was provided by a different cap located at the wrong end of the radio. Once I looked at a few pictures of other SX-11s here I realized that the chassis mounted style cap was in fact the factory standard, not an owner modification, it just was not shown on the Hallicrafters chassis layout drawing. You never know what you will find in an old set but the filter caps are the components most likely to deviate from the original. On this radio I have restuffed the filter cap with modern caps from Sals. In fact I got a full set of replacement resistors and caps from Sals although now I realize that there were some errors in the Hallicrafters documentation and I may need to order two or three more caps....


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Fri 12, 2021 11:04 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1197
Location: Tokyo
Quote:
The mystery cap appears to be a single value of 200 mfd and is connected as follows:
- one lead to a pin 8 on 35L6, which I think is just a ground terminal point.
- the second lead to pin 4 of 35Z5. Pin 8 and 4 are bridged, which I'm not seeing on the schematic. Also connected are the red lead to the main filter cap and a lead to the output transformer.


Pin 8 on the 35L6 is the cathode, it's not a ground point. If a 200uF cap has been added to the existing cathode bypass cap, perhaps it was an attempt to boost the audio bass response.

Rob


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Fri 12, 2021 12:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1096
Location: 98802
In the under chassis picture it looks to be connected from the 35z5 cathode to b-, in parallel with the first filter cap. It does not belong.


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Fri 12, 2021 9:45 pm 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 5420
I think Rob may be right. Do a careful trace of the wire connections.

Under the chassis, find the connections for C29. You might find this "extra" capacitor is electrically parallel to this original/existing C29. You can confirm this by locating R14 (150ohm).

The connection you described on 35Z5 socket is not related to the tube; those pins are not used by the tube internally; the pair of terminals on the socket are used as convenient tie points.

People do stuff to their radios; it's entirely possible this was an experiment/hack.


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Sat 13, 2021 12:23 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1096
Location: 98802
On pin 8, the cathode of the 35z5, there is a domino mica cap, I assume that is c39, 470uuf, and that the other end is grounded. It is half buried by 2 .02uf paper tubular capacitors. The 22 ohm resistor also connects to pin 8, and heads across the socket where the other end lands on an unused pin. The output transformer connects here (red cloth wire) as does the first section of the otiginal filter cap (red rubber wire). The mystery cap also connects here. I assume, from the schematic that the negative side of the original cap is grounded.

Also connected at that same unused pin is R17, 680 ohm. It heads over to the 35L6 where the other end lands on pin 4, the screen grid. The second section of the original filter cap (yellow) connects here too. There is also a maroon cloth wire connected to that pin. It is a fair guess that the maroon wire goes to the IF transformers, but it is buried and you cant see it. Underneath R17 and partly blocked from view, the blue cloth wire returns from the output transfomer and lands on pin 3, the plate, proving that this socket is the 35l6.

Meanwhile over on the other side of the 35L6 socket we see R14, 150 ohms and the blue rubber wire from the original filter cap (audio bypass section) connected to pin 8, the cathode. The opposite end of R14 is connected to chassis ground, a ground it shares with the mystery cap. That puts the mystery cap directly in parallel with the first section of the original cap, C33, which uses the red rubber wire.


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Mar Sun 14, 2021 6:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10756
Location: Beaver Falls, PA. USA
That's an imported capacitor; the original is probably open. In the old days, it was common to "piggy back" replacement electrolytic capacitors across the defective ones, without disconnecting them. This is not a good idea, as it leaves a potentially leaky capacitor in the circuit.

_________________
Tim KA3JRT


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Sep Mon 13, 2021 12:25 am 
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Joined: Aug Thu 06, 2020 7:46 pm
Posts: 117
Location: Morro Bay, CA USA 93442
UPDATE
1. Rewired per well known Geojohn.org S-38 article to eliminate hot chassis problem. Performed related safety checks.
2. I replaced the Electrolytic with a 4 section prestuffed cap from Hayseed.
3. I've replaced a couple of wax type caps on the chassis. A handful more remain.
4. I used my isolated variac to apply a very small amount of voltage to the unit. My first attempt to apply any power to the radio.
Current draw was off the charts so I immediately removed power!!

Wondering: Where to start looking for a short of this magnitude??

Thanks, Bill A.

Schematic: https://www.dropbox.com/s/fhddr2qjoyms3 ... c.png?dl=0


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Sep Mon 13, 2021 6:42 am 
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Joined: Apr Sun 08, 2007 6:47 am
Posts: 4982
Location: British Columbia
Tim Tress wrote:
That's an imported capacitor; the original is probably open. In the old days, it was common to "piggy back" replacement electrolytic capacitors across the defective ones, without disconnecting them. This is not a good idea, as it leaves a potentially leaky capacitor in the circuit.


Yes, a Matsushita/Panasonic capacitor to be exact, no way an S-38, or any of it's successsors up to the S-38E would have used one, an S-120 maybe. From what I remember of my S-38B it used a "toilet roll" multi section cap under the chassis, I can't remember whether I restuffed the original one or used a terminal strip. It's easy enough to make a replica even if the original is missing, Saran wrap, or aluminum foil tubes are often of the correct diameter.
Regards
arran


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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Sep Mon 13, 2021 6:47 am 
Member

Joined: Apr Sun 08, 2007 6:47 am
Posts: 4982
Location: British Columbia
billalpert wrote:
UPDATE
1. Rewired per well known Geojohn.org S-38 article to eliminate hot chassis problem. Performed related safety checks.
2. I replaced the Electrolytic with a 4 section prestuffed cap from Hayseed.
3. I've replaced a couple of wax type caps on the chassis. A handful more remain.
4. I used my isolated variac to apply a very small amount of voltage to the unit. My first attempt to apply any power to the radio.
Current draw was off the charts so I immediately removed power!!

Wondering: Where to start looking for a short of this magnitude??

Thanks, Bill A.

Schematic: https://www.dropbox.com/s/fhddr2qjoyms3 ... c.png?dl=0


This is why I always suggest getting the radio working to factory spec before doing any mods, then you trace down the problem easier. If drawing excess current either you have a shorted cap, or made a wiring error. I have to go back through a National set where someone went through and did a bunch of mods (according to some article), they improved the set to the point it self oscillates at a low frequency, if you look at it the wrong way, and is only marginally better then an AA5.
Regards
Arran


Last edited by Arran on Sep Wed 15, 2021 8:38 am, edited 1 time in total.

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 Post subject: Re: Hallicrafters S-38 and the Surprise Capacitor
PostPosted: Sep Mon 13, 2021 2:01 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 821
Location: Suburban Chicago
Did you do the variac test before you started working and did it pass then?

If so then the short is in something that you did.

If not then you don't know where to start looking!

You can poke around with an Ohmmeter on the unpowered, unplugged radio and probably isolate the circuit that is shorted. A recent thread in the test equipment section here discussed using a DVM with a diode test function to try to get to the exact location of the short. Basically you poke around and look for the place where the "diode" voltage is at a minimum. That is likely to be the location of the short.

https://antiqueradios.com/forums/viewtopic.php?f=8&t=401925


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