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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Mon 09, 2012 5:16 pm 
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noisebox wrote:
R 18 was open. I replaced it and now the sound is less garbled and the tone control works :D
There are still a few problems. There is quite a bit of hum and I think there should be much more power and bass coming from the speaker. With the volume turned all the way up it still doesn't have the punch and clarity it should have.
OK. Now I would highly recommend you don't turn the radio on any further until you have replaced a few items.

You never said that anything else has been replaced on the radio so based upon your comment about hum, as a minimum (if not already done), the power supply electrolytic capacitors need to be replaced before playing the set any further. The hum indicates these capacitors are already failing and if they continue to be subjected to voltage they are likely to fail further which could result in damage to other components (something to be avoided if possible).

For what it's worth, it is always best to mention all the symptoms of your radio up front. If you had mentioned hum earlier it would have been recommended to change power supply capacitors much earlier to avoid potential damage to the power transformer and speaker field coil (two items which can be a problem to replace or repair). Also, paper wax capacitor C41 may have been the source of damage to your old audio output transformer and may damage the new one if not replaced, just as C29 may have caused the damage to R18.

So, at a minimum, please don't turn the radio back on until the electrolytics and C41 have been replaced.

I would also highly recommend the careful replacement of all paper/wax capacitors in the radio since we know some appear to be failing to the point of causing damage. Proper precautions should be taken to make sure as you replace components you do so with the correct values and put them back in the same places where the old components were removed.

Curtis Eickerman

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Mon 09, 2012 7:42 pm 
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Curtis,
Thank you for pointing that up and yes to all of the above. This set has already been recaped. That's one of the first things I've learned from the helpful members of ARF. I started restoring radios around three years ago.

A brief history of this radio: I had restored the cabinet and record player of this 46-1226 and ended up buying the chassis from another ARF member, who had recapped it and had it running. Being one of my first bigger projects I hacked the original chassis and just parted it out. I ended up using the radio on and off for a year when it went funny on me.

I had to clean up the electrolytics though as they were hanging loose. I did stuff the dual C36 in a can and now I'm wondering if there' a short.
Ben


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Mon 09, 2012 8:05 pm 
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noisebox wrote:
I had to clean up the electrolytics though as they were hanging loose. I did stuff the dual C36 in a can and now I'm wondering if there' a short.
Based upon getting hum it may be more likely that something came loose in the "stuffing" process rather than a short, but that's mostly just a guess.

Now that I am less concerned about the possibility of damage to a field coil or transformer...

Before doing anything make sure R30 is not shorted or being short circuited by some other connection and don't turn it on until you do so (I'll explain later). This also applies to C34 which is in parallel with R30. In this case, don't bother to take either lead loose when you measure its resistance. If there is a short or resistance way under 170 ohms then you can start taking things apart to find who is causing the short. If you get to this point also check that the power transformer CT wire is not shorting to the case which will cause the same problem. If there is a short at this point don't power the radio on until you find and correct the problem (the transformer you save may be your own).

Then if R30/C34 is not shorted...

Before you take the "stuffing" job apart you might just temporarily parallel another electrolytic across the existing ones to see if the hum goes away. This might tell you which capacitor (if either) is not doing its job.

The voltages you reported seem a little low which sounds like inadequate filter capacitors or an overloaded power supply.

It seems like you are reporting voltages around 170 VDC at places I would have thought you would be seeing more like 240 VDC. This could be caused by a faulty power supply cap, broken connection, connection to the wrong place, etc. Alternatively it could be a sign that the power supply is being overloaded by something. If there is an overload one would expect that you would notice something getting unusually hot (perhaps smelling hot or burned). A third possibility is R30 being high or too low (shorted) in value which will also kill your B+ voltages, mis-bias the audio output tubes and make things sound pretty bad too. You said there was only slight voltage on the 6K6 pin 5 grids and I think they should be around negative 15 to negative 20 volts. If they are near zero the 6K6s will pull too much current and heavily load the power supply and might also blow the audio output transformer (this is why you need to check for a shorted R30 first).

So, check R30/C34 first. Then if it checks OK, probably should measure the DC voltage at 5Y3 pin 8, then at the positive terminal of each of the C36 power supply capacitors.

Is the hum still present while a strong station is playing? Does it go up and down with the volume control?

Are you pretty sure what you are hearing is not coming from florescent lights, light dimmer controls, or any motors in the house?

Curtis Eickerman

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 1:12 am 
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Tested R30, the candhohm and it's fine. I've tested it previously but wanted to make sure. Replaced C34 as well. However, though it seems to have improved in volume and power I get a horribly loud oscillation when I mess with the volume and tone knobs. Could that be a cap too close to something?

I think the last voltage readings were wrong because my variac was turned down. Here they are plugged into house current, 122vac.

6J5 pins
2 and 7 heaters 2.8 vac
3 = 105vdc
4= 105v
5 = 20vdc
8 = 34vdc

6K6
V6 pins
2 = 5.6 vac
3 = 153v
4 = 200v
5= slight - voltage
6,7,8 no voltage

V7 pins
2 = 5.6vac
3 = 186vdc
4 = 198vdc
5 = slight -voltage
6,7,8 no voltage

7C6 pins
2 =114vac
3 =-.5v
4,5,6,7 nothing or slight voltage
8=2.8vac

5Y3
2=322
8=322

Also have voltage problem--I think--coming out of the positive of the 10MF (C36) electrolytic at 323V. My Philco service manual shows it should have around 118V. The 25MF has around 200V, which is close to the recommended 225V. C37 has 272V, which I think is also close to the 300V recommended .


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 1:36 am 
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You are going through a lot of work measuring voltages. As Curtis mentioned voltages are lower than we would expect. You don't need to measure pin #2 each time but it is low. Reads 5.7 when filament voltage is usually 6.3. Sometimes higher now with line at 120-125.

What is the actual voltage on pin #5 of 6K6's? Should be around -15 volts. It it's not tubes will draw too much current, distort and could be a reason for low voltages & hum?

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 2:15 am 
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That's a relief knowing I don't have to measure all the voltages each time :oops:
Both pin 5s on the 6K6s are around .05v.
R25 and R26 have both been replaced.
I replaced C32 and the previous owner has replaced C33.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 3:32 am 
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noisebox wrote:
Both pin 5s on the 6K6s are around .05v.
R25 and R26 have both been replaced. I replaced C32 and the previous owner has replaced C33.
That voltage is way too low. So lets find out why.

With the radio turned off an unplugged please place your meter at the indicated spot below and measure the resistance and let us know what it is.
Attachment:
Bias.jpg
Bias.jpg [ 80.89 KiB | Viewed 181 times ]
Then switch your meter to DC volts, plug in the radio and turn it on. When you begin to hear audio measure the voltage then turn the radio off and let us know what that is.

Please refrain from running the radio for any extended period of time until we figure out why there is no voltage on the control grid of the 6K6 tubes (needs to be about -15 volt for proper operation).

Curtis Eickerman

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 5:21 am 
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Resistance is .1 ohm. DC voltage is 1.3mv


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 5:30 am 
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noisebox wrote:
Resistance is .1 ohm. DC voltage is 1.3mv
OK. There is a short circuit. Now to find out where it is, first disconnect the "green" wire that comes from the transformer and goes to R30. With this wire disconnected, measure the resistance from the green wire to ground. If it is 0.1 ohms the short is in the power transformer (don't worry just yet).

If the green wire to ground does not measure 0.1 ohms, put the meter back on R30 where it was and see if that measures 0.1 ohms.

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 6:21 pm 
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The green wire back to the transformer has zero continuity to ground when disconnected and measured.
R30 to ground from where the green wire was measures 0.1 ohms.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 6:37 pm 
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noisebox wrote:
The green wire back to the transformer has zero continuity to ground when disconnected and measured.
R30 to ground from where the green wire was measures 0.1 ohms.


Not necissarily bad news yet. You should be getting infinite ohms when green disconnected. And you should get approximately 95 homs between "green" center tap and the two yellows that go to the plates of your rectifier. That means the short is the capacitor which establishes the bias for the output, or possibly the output tube itself. Measure again (unppluuged please.)


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 7:11 pm 
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Here is a possibility. C37 should be insulated from the chassis. Could this cap be a can mounted directly to the chassis?

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 7:21 pm 
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The ground to C37 goes to the non ground side of R30.

The green CT to 4 and 6 of the rectifier is at 87ohms and 95ohms.

What did I need to measure when powered up?


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 7:36 pm 
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You can measure voltage across R30, like Curtis shows in the schematic. As mentioned earlier don't let the radio on very long when measuring voltage. (Measuring voltage is just in case resistance reading didn't turn out right.) Reading should be around 15 volts across R30. Much less and something is shorted. Much higher and this voltage isn't getting to 6K6 tubes.

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 7:46 pm 
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Just to clarify, should I keep the CT green disconnected from R30 while powered up for testing?


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 8:01 pm 
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noisebox wrote:
Just to clarify, should I keep the CT green disconnected from R30 while powered up for testing?
Don't power anything up with the green disconnected AND don't power anything up until we find the reason for the 0.1 ohms across R30.

noisebox wrote:
The ground to C37 goes to the non ground side of R30.
I found this statement to be a concern because neither end of C37 should be "ground".

The negative side of C37 should go to R30, but should not go to ground in any way. If it is in a can it should have been insulated from ground (typically a phenolic insulator).

So please take the connection from R30 to C37 loose. Then again measure R30 resistance to ground. Then measure the resistance to ground of the C37 connection you just took loose.

Do not apply any power to the radio.

Curtis Eickerman

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 8:47 pm 
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When I measure from the end of where C7 is disconnected from R30 on R30 to ground I get 73ohms. Since the other end of R30 is grounded to the chassis I get no resistance between that and the chassis. And from ended to end of R30 I get the same 73 ohms.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 9:05 pm 
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noisebox wrote:
When I measure from the end of where C7 is disconnected from R30 on R30 to ground I get 73ohms. Since the other end of R30 is grounded to the chassis I get no resistance between that and the chassis. And from ended to end of R30 I get the same 73 ohms.
73 sounds a little low but at least not shorted to ground.

How about resistance to ground of that C37 connection that is now no longer connected to R30?

Curtis Eickerman

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 9:14 pm 
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That would be infinite resistance on that one.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Tue 10, 2012 10:48 pm 
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noisebox wrote:
That would be infinite resistance on that one.
OK, so now nothing is measuring 0.1 ohms to ground?

Time for a good visual check because something that was connected to the ungrounded end of R30 was shorting to ground. Look at every wire and every component that once was connected to this spot to see if something was touching the chassis or something that it was not supposed to.

After that connect C37 and the green wire back to the ungrounded end of R30 and again check that point's resistance to ground to make sure the 0.1 ohms has not returned. Let us know what resistance you measure.

If the 0.1 ohms is not back it would be time to power up the radio again briefly and measure the voltage at R30 just like before and then turn it off.

Curtis Eickerman

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