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 Post subject: Cap in output transformer
PostPosted: Dec Fri 23, 2016 10:52 pm 
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Joined: May Wed 31, 2006 6:41 am
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This is about the radio on this post
viewtopic.php?f=6&t=192966

The radio is working but i didn't like how it sounded. then the tone control stoped working and i thought the tone pot was open again. then i realized it was only a cold solder joint.

The schematic is here
http://digilander.libero.it/iw2dgs/boll ... /G-502.png

Because i already opened the radio i decided to recone the speaker because it had a crack.

With the speaker reconed it sounds far better but this is my problem.
when i first repaired the radio i noticed that some of the caps installed had different values than the schematic.

one of those caps was the 2000pf cap in the output transformer. it was replaced by one 50000pf cap. that made the set sound without highs. after several iterations i used the values in the schematic. that made the set produce more highs, but was harsh. finally i settled with an intermediate value for this cap.

Now with the speaker reconed the radio sounds a lot smoother, but lacks highs, so i will put a 2200 pf cap here.
The original cap was paper/wax.
I can get ceramics here for the replacement, or i can order online some polypropilene 1600V and wait several months for them to arrive.
Is there a big difference between ceramics and polypropilene?


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Fri 23, 2016 11:55 pm 
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A ceramic cap will be fine in this position.

Matt


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 1:12 am 
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I would not use a ceramic cap for this application,. Ceramic caps using an X7R or Z5U dielectric will change value with applied voltage, the change can be as much as 70% from 0V to full rated voltage. The change in capacitance can introduce distortion and change the frequency response. A film capacitor is a better choice. A ceramic with COG dielectric could be used but these are not as common as X7R or Z5U types.

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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 1:23 am 
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Just be sure it is not a metalized film. In this application fast rising spikes from QRM and QRN can easily exceed the dv/dt of conventional metalized caps and actually cause a loss of capacity and noise by separation of segments of the metalization. This cannot happen to a foil/film capacitor. Choose a buffer type, one rated for pulse service. Such caps are available from many manufacturers, such as Cornell-Dublier. Use at least 1200 volts, 1600 if available in the value. The dv/dt ratings for the Cornell line are published. This cap will not only cut highs but will protect the output transformer from an internal flash-over that would destroy it.

GL

Chas

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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 4:12 am 
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Eddy wrote:
Ceramic caps using an X7R or Z5U dielectric will change value with applied voltage, the change can be as much as 70% from 0V to full rated voltage. The change in capacitance can introduce distortion and change the frequency response.

While this is true in theory, the purpose of this cap is to roll off the response at very high audio and ultrasonic frequencies. Very little distortion will be introduced into the audio passband of the receiver. With a 2kV-rated cap, the plate voltage is only a fraction of the rated voltage of the cap, and the superimposed AC of the audio signal is only a fraction of that. I used a 2200 pF / 2kV ceramic cap in exactly this position in my Hallicrafters S-40B and the audio sounds great.

Matt


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 6:39 am 
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Thank you all,

Before i saw all the replies i found this caps
http://www.mouser.com/ds/2/315/ABD0000CE25-36779.pdf

and i purchased some.

now i am worried because they are metalized polypropilene.
They are rated for pulse. Can they be used in this application?


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 10:06 am 
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Well I will stick my neck out and say I would use one.

A good thing is the cap is across the output transformer and not from the anode to B- (ground here) and so would have virtually the full B+ across it. All it has is the drop across the transformer primary, say 50V and the AC component so is having an easy life.

Don't know where Chaz is finding these high voltage capacitor destroying spikes from? Would have to be massive to do that surely?

I see the circuit has a line decoupling cap that should catch some and would be the first to suffer; I would change that for a "Y" cap of the correct voltage.

Gary


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 11:43 am 
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The line decoupling caps are often there for un-grounded, or sets that cannot be grounded to provide an RF path for the antenna circuit, hopefully via neutral. With the audio, the plate bypass cap to ground historically had the highest probability of a failure.

Spikes from the mains (Utility line) are not the issue, it is Audio transients. We are not looking at DC we are looking at the PP AC Waveform of the Audio, at high volume setting with a strong signal. Worth a look on an Oscilloscope as it can measure the voltage on a properly set graticule.

Marc


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 4:26 pm 
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Find it hard to believe that audio transients, in an old radio, are ever going to be high enough to damage a metalized film cap, by a top maker, rated at 1000V or more Marc. I use film caps in this position rated at 630V and have never had a failure.

Don't see how you would see these transients on a normal scope either and few play old radios at full volume.

Gary


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 5:09 pm 
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Radio Fixer wrote:

I see the circuit has a line decoupling cap that should catch some and would be the first to suffer; I would change that for a "Y" cap of the correct voltage.



This cap was the reason why i first repaired the radio. I always put a safety capacitor here.


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sat 24, 2016 11:50 pm 
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I rarely use more than 630V but that depends on the set, when you have four 6F6's in a PP output you are looking at a reasonable degree of noise, but never forget that the tube is not shut off at low volume, all that you are doing is reducing the signal into the grid. The OP tube itself, or tubes, can still be running almost flat out, so anything including RF that gets into the tube/s will be amplified in proportion to its initial amplitude.

Spikes an RF riding on the mains here is a constant problem, lightning being a contributor. Most Tube radios here are transformer and can be grounded, so Line caps are rare. It it is the modern stuff that is at the greatest peril, from spikes & surges. The mains (utility) power & the transients in the radios audio are two different animals & the tubes tend to be able withstand spikes & lighning up to a geater point than anything Solid State, not withstanding that overall quality / tolerances in some of that stuff is marginal.

As I have said previously, I have MOV's and caps in fireproof (metal) boxes, strategically plugged in to get rid of the RF and actually kill the circuit if the surge is a big one: They have actually done that, so I know they work in practice.

Marc


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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sun 25, 2016 3:50 pm 
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Radio Fixer wrote:
Find it hard to believe that audio transients, in an old radio, are ever going to be high enough to damage a metalized film cap, by a top maker, rated at 1000V or more Marc. I use film caps in this position rated at 630V and have never had a failure. Don't see how you would see these transients on a normal scope either and few play old radios at full volume. Gary
This suppression cap is only for plate circuit of last audio QRN & QRM will cause eventual failures of a metalized cap in that location and put OPT's at risk, the cap absorbs the spikes, protects OPT's but must be able to take it and stay alive. Forensics done on both OPT's, metalized misapplied and battery set transformers, confirmed pulses from QRN & M opened from internal arcing. Forensics done on metalized in repeated pulse application failed often in one or two hits, loss of capacity and become noisy. Metalized O.K. for other areas of radio, except for across line and in auto buffer. OPT cap also functions to some degree on tone, just as it (absorbs) high frequency spikes it will absorb high frequency audio so actual value in MF should not get larger than specified. Dv/dt is an important issue in caps as much as ESR. There would be no problem if all caps were foil/film. Re-fitting OPT's is a pain at least, for vintage potted ones, a project. Some folks swear by CL-50 thermistors, O.K. one can restore a radio however they please. BTW forensics was done some 15 ya when I ran e-lab, developing pulse circuits for portable use. Had the microscopes and adjacent wet lab for analysis, fun stuff...

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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sun 25, 2016 4:42 pm 
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The problem you may be having could just be from the reconing. Sometimes a speaker wont sound exactly the same without the original cone material. Were all the resistors within tolerance as well? I would double check that everything is as it should be in the tone control section too.

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 Post subject: Re: Cap in output transformer
PostPosted: Dec Sun 25, 2016 9:11 pm 
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Radio Fixer wrote:
Don't see how you would see these transients on a normal scope either ...

To see QRN spikes you would wait for QRN-type weather and set the 'scope triggering to a point just above the peak audio. You would have to keep your eyes glued to the 'scope as a burst of QRN would likely result in a single sweep.

A QRN spike can be much higher than the peak audio, and is too brief to affect the AGC, which has a relatively long time constant.

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 Post subject: Re: Cap in output transformer
PostPosted: Dec Mon 26, 2016 12:10 am 
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The quickest & easiest way, after ensuring that the set is in spec and before jumping to conclusions, is to see if the problem is the speaker itself. As in this case it has been tapered with, and cones pole & often suffer damage in various ways.

That is merely a case of getting a known good permanent magnet speaker /speaker box of the correct impedance and disconnecting the OP transformer secondary from the VC, then tacking in the good one. If it suffers the same symptoms, then its a case of perhaps getting out the oscilloscope & see what the waveform looks like.

I have had occasion to adjust the bias to sort out distortion & wrong components, modifications, & parasitic oscillation can also cause issues. But in saying that....In, by far the majority of case, the set worked when it left the factory, so in the absence of modification information from the factory (or perhaps other reliable sources), one goes by the circuit diagram printed.

Some repaired sets just to make them go and used anything that would make them go, that was handy at the time.

Marc


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 Post subject: Re: Cap in output transformer
PostPosted: Jan Sun 01, 2017 5:44 pm 
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The speaker is good, I tested it before installing it on the radio.

Also when i went to remove the cap, i discovered i already put a 2000pf 2000V film cap.

And also i am an idiot. Because the AM reception is poor where i live, i used my old iriver connected to the phono input to test the radio. The eq function was on and with lots of bass.

i discovered this, because i connected my chromecast audio and it sounded far better.

I don't expect lots of highs, because according to the service manual the response is flat from 100Hz to 8000Hz.


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 Post subject: Re: Cap in output transformer
PostPosted: Jan Sun 01, 2017 11:27 pm 
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Remember he one that never made a mistake never did anything and achieved a lot less. One does need when adding test signal, to note things like isolation, matching and signal amplitude etc. There is a lot more to servicing than just changing a few caps & resistors. When it all goes awry that is what keeps this forum running.

One could always get a Function Generator / Audio Generator & Oscilloscope & have a look at different signal frequencies passed through the audio. Sometimes you get some nasty surprises, when you realise that the instability is bad design & the Audio tubes are happily amplifying RF, which should not be there, and radiating it.

Marc


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 Post subject: Re: Cap in output transformer
PostPosted: Jan Sun 08, 2017 6:00 am 
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I checked ally connections. and i found again a cold solder joint on the tone control. I resoldered it, and now i am sure all is ok. All components are on spec.
I plugged the radio on the dim bulb tester and i got only hum. also the dial lights where brighter than normal.

Disassembled it again, and found the 6v6 sounds like is something loose inside.
I think is time to get another 6v6. i think i have another somewhere to test the radio.
I can get locally a jj 6v6s . How is this tube?


I have a signal generator and an oscilloscope, but i want to sort everything elese before using them.


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 Post subject: Re: Cap in output transformer
PostPosted: Jan Sun 08, 2017 7:59 am 
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I never use a dim bulb on a transformer set. If the lights are too bright that could be over voltage?. That is a multi tap primary so there is no real excuse for that. The other reason is that there is no B voltage or the 6V6 being the hungriest tube is not drawing current.

The logical sequence is to investigate the voltages. Number one is your utility supply voltage: What is it, and how does that line up with the taps on the transformer. If you have more than 110 V it should be on the 125V tap if you say, have 120V.

The signal generator & Oscilloscope are formidable for finding lost signal, alignment, finding distortion & calibrating the IF. However, at this point I would be checking voltages and rechecking your work. The low voltages & AC output of the transformer can be checked with the 5Y3 removed as that will stop B voltage being generated.

I would try the 6V6 in another radio that works, Unless you can see debris in the tube? sometimes its just insulation & rubbish in the base. I suspect you may have a solder short, or a miss wire. Ones own mistakes are hard to spot: Leave it awhile.

Marc


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 Post subject: Re: Cap in output transformer
PostPosted: Jan Sun 08, 2017 7:27 pm 
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Here we have 220V 50Hz, but actually is no more than 215V.

The radio worked before with the dim bulb tester.
This time i only messed up with the solder in the tone control. didn't changed anything else.

The tube sounds like there is a piece of metal loose inside the glass. maybe this happened when i transported the chassis on my car.


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