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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 3:49 pm 
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Location: Annapolis, MD
Your previous post is encouraging, but...

ken/sc wrote:
It is cutting in and out getting stations and motorboating really bad

Fix this first...
With no antenna or signal generator connected, see if it is stable. If not, check carefully for loose or missing ground connections---or ANY bad connection. One common issue is mechanical grounds to chassis that are loose or corroded.
If inspection doesn't turn up anything, then it's time for "tap, wiggle and poke". With the radio on, use a non-conductive tool to manipulate things, tap on things, etc.---you are looking for any response to a mechanical disturbance.

Does the set use any shielded tubes?--If so, make sure the shields are installed and well-grounded.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 4:27 pm 
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Location: Mountains of Mourne. Ireland.
Fred Rice wrote: Believe it or not, Philco didn't shield the converter tube on a lot of the 1937 Philcos. This is contrary to tube manuals of the day that state that the 6A8G and its battery equivalent, the 1C7G should be shielded. My 37-650 has no sign of the shield base on the 6A8G. My next year Philco battery set, 38-38 has an unshielded 1C7G. On the other hand, my 6 volt tombstone Philco 37-624 has a shield on the 1C7G and the base to hold the shield. Yet my 37-611 AC-DC set has an unshielded 6A8G.

Not sure what the Philco engineers were thinking.

------------

I recall the B voltage is 135 volts in this set. There may also be a 67.5 volt tap. It may require two C voltages, -3 and -9 volts. Easy to do with 1.5 volt cells. The electrolytic in mine was leaky but in a battery set, since the B voltage was supplied by battery power, the electrolytic was there for decoupling stages from each other. An open electrolytic will usually cause oscillations.

One of the original batteries for this set was a large battery pack that had a socket for that 7 prong plug off the cable. It supplied B and C voltage. Philco marketed a dry battery for the filament at 3 volts with a regulator tube that plugged right into the battery to keep it at two volts.

I just run my 38-38 off a modern A-B-C battery eliminator I built a few years back. These sets use a 1J6 output tube which runs class B, and puts out close to 2 watts. Pretty good for a battery set.

Post subject: Philco 37-38 viewtopic.php?p=850678#p850678


Greg.


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 4:33 pm 
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Quote:
[Not sure what the Philco engineers were thinking]

Can't speak for Philco, but i can hear someone saying: "it work's fine--let's start production."

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 5:30 pm 
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Joined: Nov Sun 20, 2011 1:10 am
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Location: South Carolina
I got it playing on one station. If I turn the volume knob I get motorboating.


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 5:44 pm 
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Location: Mountains of Mourne. Ireland.
NRI No.23 How to fix a receiver that squeals and motorboats.
http://canadianvintageradio.com/wpsite/wp-content/uploads/2015/03/How-To-Fix-a-Receiver-That-Squeals-and-Motorboats.pdf

Greg.


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 6:39 pm 
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ken/sc wrote:
I got it playing on one station. If I turn the volume knob I get motorboating.

Which way do you turn the knob to get motor-boarding?

Did you do the other things I suggested?

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 10:01 pm 
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Location: South Carolina
I checked all the connections with my chopstick. I found nothing.
I opened the volume switch and sprayed some contact cleaner in it and worked it good.
If I turn the volume control higher I get motorboating. I have no way to check the tubes they are too
Old for my tube tester. I checked the resistance on pins 2 and 7 on each. I also checked the heater circuit getting 2v all the way to the pilot light

Could this be a resistor? I have recapped the entire radio.


Thanks for the help all
Ken


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 04, 2019 10:27 pm 
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Tack solder** a 0.01uF cap across the secondary of the 1st IF transformer. See if that stops the oscillation.
Repeat for the 2nd IF. (Only shunt one at a time)

Better yet, get a scope on it and find the frequency of oscillation.


** no clip leads and make the wires no more than about 1"

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: May Tue 21, 2019 12:34 am 
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Location: South Carolina
Replaced a couple tubes. Did not stop the motorboating.

Ready to try something else. Have not wired cap across the secondary of 1st I.F. Transformer, not
Really sure about how to do this.


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 Post subject: Re: Philco 37-38
PostPosted: May Tue 21, 2019 1:34 am 
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Here is a snip from the schematic showing where you would connect a capacitor for trouble-shooting. By shunting one of the secondary winding, you greatly reduce the IF signal, which in turn might stop the oscillation. If shunting one of the secondaries does NOT stop the oscillation, then you know it is happening downstream from that point.

Attachment:
If_shunts.jpg
If_shunts.jpg [ 37.06 KiB | Viewed 551 times ]

_________________
-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: May Thu 23, 2019 1:30 am 
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I shunted both secondaries. See attached pics. It still motorboated after I shunted each.


Attachments:
5BC52F0B-B67E-4D86-8741-ABB5D871E678.jpeg
5BC52F0B-B67E-4D86-8741-ABB5D871E678.jpeg [ 89.35 KiB | Viewed 529 times ]
402C9C1D-8D71-4502-8E54-64DCA5B2BB66.jpeg
402C9C1D-8D71-4502-8E54-64DCA5B2BB66.jpeg [ 86.54 KiB | Viewed 529 times ]
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 Post subject: Re: Philco 37-38
PostPosted: May Fri 24, 2019 2:26 pm 
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That suggests that it is not the IF stages oscillating. That leaves us with the audio stages.

First, try shunting the AVC cap (item 7--0.05uF) with a .01 or.02 cap) This will probably not stop the motorboating, but will lower the frequency. This is to verify "classic" motorboating**

Next test: Instead of shunting across the secondary of the 2nd IF, put the shunt from the top of the secondary (grid of the 1H4 det. tube) to chassis ground. Also try shunting the grid of the 1E5 (1st audio) to ground. (In both cases, use a cap anywhere from .01 to .05uF)


**The typical motorboating is a side effect of any oscillation that affects the AVC action. The oscillation increases the AVC voltage until one or more tubes cut off, then the cycle repeats. The time constant of the motorboating is set by item 7 and the 1meg resistor that feeds it.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: May Sat 25, 2019 10:31 pm 
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Location: South Carolina
I shunted all three in order. Nothing really changed on the first or second one.
On the third one 1E5 the motorboating changed to a high pitch squealing motorboating.
I have also noticed that the motorboating gets faster the higher I turn the volume control up.
If the volume control is turned all the way up the motorboating stops.


Thanks for your help, if I don’t get it fixed, I have learned a lot.

Ken


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 Post subject: Re: Philco 37-38
PostPosted: May Wed 29, 2019 11:50 pm 
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Location: South Carolina
Bttt


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 Post subject: Re: Philco 37-38
PostPosted: May Thu 30, 2019 12:51 am 
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I replaced the volume control with a small one I had on hand. No more motorboating. Bad volume
Control switch.


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 Post subject: Re: Philco 37-38
PostPosted: Jun Wed 05, 2019 12:33 am 
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Location: South Carolina
I had replaced the volume control with a small one I had on
Hand. It played for a while and then it quit getting stations. Put
The old volume control back on and my motorboating is back.
The highest volume is not that loud either, would it be worth a new volume
Control?


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 Post subject: Re: Philco 37-38
PostPosted: Jun Wed 05, 2019 1:15 pm 
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I'm not seeing the cause and effect here. With the other control, it played and then stopped getting stations? It's not clear to me how that would be fixed by putting in the original control.

Have you checked the original control with an ohmmeter? Check the following:
--total resistance end to end, and to any taps (by convention, these are given with respect to the "low end" of the control. (When the wiper is at the low end, that is minimum volume.)
--resistance from the wiper to either end as the control is turned. This must be a smooth continuous change in resistance as the control is turned.

If you get back to the condition of "no stations", turn up the volume to see if your hear any background noise---or touch some grids to see where the signal is getting lost.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: Jun Sat 08, 2019 2:17 pm 
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Joined: Nov Sun 20, 2011 1:10 am
Posts: 384
Location: South Carolina
I disconnected all three terminals.
I get no reading between the two outer terminals.
Between the center and one outer terminal my reading will
Sweep higher as I rotate the knob. I get no reading between the center and the other
Outer terminal.

The only place it will not motorboat is when the volume is turned
All the way up. Also I get motorboating if I touch the wire from
the tuning capacitor to the top of 1C7G tube.


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 Post subject: Re: Philco 37-38
PostPosted: Jun Sat 08, 2019 6:41 pm 
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If the control is open, then that could be the whole issue. If you don't have any similar controls to try, then just put in 2 fixed resistors that add up to ~ 500K. To start, try something like 470K and 36K. This gives a voltage division of about 14. If that's too loud, put another resistor in parallel with the the 36K.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Philco 37-38
PostPosted: Jun Sat 08, 2019 7:40 pm 
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Joined: Nov Sun 20, 2011 1:10 am
Posts: 384
Location: South Carolina
If I solder the resistors between the end terminals and the center one on the volume control
Will I still be able to control the volume or does that just bypass the control?


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