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 Too high voltage out of the rectifier

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zenithguy



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PostSubject: Too high voltage out of the rectifier   Sun Oct 17, 2010 2:02 pm

This is my headache a Firestone R-307:
http://www.nostalgiaair.org/resources/153/M0006153.htm

My voltage out of the rectifier is measuring 350 volts instead of the 310 listed thus my voltage on the B+ line is 40 volts high across the board. SO this puts me in trouble right off the bat. This cannot be right. I was always under the assumption that if a tube goes bad the the voltage drops now I could be wrong but do I need a new tube with this high voltage issue.

There seems to be another issue on this schematic as resistor #45 going into the OSC. tube isnt even there and it looks like factory wiring

AAAARRRRGGGHHH Mad

Cheers
Mike
Comon Lions
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Johnnysan



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 2:42 pm

Is the radio working (ie, drawing correct amount of power)?

Is your line voltage the same as rated for the radio?
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 2:47 pm

Radio does work but very poor reception I found a bad connection on the antenna coil repaired that it picks up stations only when I touch my finger to the grid cap on the 6L7G grid cap.

I have measured the voltage going into the radio and it is 115 on my variac

Mike
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pixellany



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 3:31 pm

If the radio is working at all, then let's assume the output transformer and the field coil are both OK.

A bad tube (no emission) wiuld cause the B+ to be too high.

Have you checked the negative bias string (#s 35, 48, 36)? If the bias is too high, it will cutoff the output tubes and the B+ will go high because of the reduced load.
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Chas

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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 3:56 pm

Quote :
...measuring 350 volts instead of the 310...

Most schematics state where to measure the voltage AND what the characteristic of the voltmeter should be. Usually, it is 1000/volt, with a dvm or a vtvm the voltage will be higher as these are in the megohms input resistance. Not all B+ measurements are common to chassis. Be sure some other resistor in the B+ string has not gone open or gone up in value. Check the bias on the output tube and well as its condition.

#45 is a plate load resistor, has to be there or oscillations will shunt to B+. Be sure there are no metal tube substitutions.

If the signal improves touching the grid cap there is still a input rf problem, coil, capacitor, return circuit wiring poor contact, poor solder joint, even in the tube bases....
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 4:19 pm

Thanks folks, I will address all these questions over the next several days. Looks like I will be starting a new job on monday so that will take some of my time and I WILL follow up on all your questions. ARF is the best you all care

Cheers
Mike
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 4:40 pm

pixellany wrote:
If the radio is working at all, then let's assume the output transformer and the field coil are both OK.

A bad tube (no emission) wiuld cause the B+ to be too high.

Have you checked the negative bias string (#s 35, 48, 36)? If the bias is too high, it will cutoff the output tubes and the B+ will go high because of the reduced load.

First thanks Pix for the reply, Yes I checked those 3 resistors as that was my first stop all are good. Voltage across the field was 76 volts close enough. I am to the point of trying of looking for a new rect. tube (5W4G) as I cannot explain why the voltage is so high (+40v) at the B+ output.

Cheers
Mike
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 4:47 pm

Chas wrote:
Quote :
...measuring 350 volts instead of the 310...


Be sure there are no metal tube substitutions.

..

Chas by that you mean none of the small metal can "tubes"? If so both the osc and 2nd det have been subed with metal cans and 2nd det can was questionable on my friends tube tester. Should have said that in the opening message. So whats the issue with metal vs. glass tubes?

Thanks
Mike
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marcc

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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 5:17 pm

Metal tubes (octal) usually have pin one connected to the metal body; That's one reason that they do not need shields.

Under the heading of "expedience & cost cutting" As the majority of octal glass tubes do not use pin 1; Some manufacturers will use pin one as a tag strip... too which anything could be connected.

I would go with the general concensus. High B+ with correct AC input is most likely a tube not conducting. That could be a faulty tube, high plate resistor, or a failed resistor or dry joint on a cathode. The latter can be spotted with a high impedance meter. An open cathode resistor will move toward B+ potential at the pin. Yes do watch out for the B- reference point.

Marc
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Bob D.



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 6:01 pm

pixellany wrote:
If the radio is working at all, then let's assume the output transformer and the field coil are both OK.

A bad tube (no emission) wiuld cause the B+ to be too high.

Have you checked the negative bias string (#s 35, 48, 36)? If the bias is too high, it will cutoff the output tubes and the B+ will go high because of the reduced load.

Yes, I agree. If one stage is not drawing it's normal load current, the B+ will be a little high. If everything else checks out OK, I wouldn't worry about the B+ being a little high (in this case 13%). Some of the increase may be due to the 110VAC vs 120VAC issue. I've never worried about the line voltage myself, but some people are very anal about it using variacs or bucking transformers to knock the voltage down.

Is your filament voltage also high?

Bob D.
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burnt fingers



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PostSubject: Re: Too high voltage out of the rectifier   Sun Oct 17, 2010 6:50 pm

I usually find that the anal ones are those who really dont understand the consequenses of high line voltage yet feel an overwhelming urge to condemn those who do.

In any case, if you actually read the OP's comments, you would see that he is using a variac set to 115V as specified in the Riders info.

So now its a matter of taking socket voltage and resistance measurements as the next step in finding the problem. Id also be concerned why resistor #45 is missing altho Id suspect it is hidden in the oscillator coil and bandswitch area. It is a 2W so its going to be larger than the other carbon resistors in that set and not very far from the 6C5 plate pin.

The finger on the 6L7 grid cap is telling you that there is an open circuit between the antenna connector and the 6L7. If all bands are dead its likely one of the antenna side coils since they are in series. This is often a failure right at a solder connection due to corrosion from the solder flux used and is often an easy repair....but not always. It could also have been from a nearby lightning strike some time in its history and it was taken out of service.

Get back to us when you have time and the combined gray matter on ARF will lead you thru it.

Carl
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marcc

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PostSubject: Re: Too high voltage out of the rectifier   Mon Oct 18, 2010 5:02 am

On actually looking at the circuit, I note that this thing is back biased via 3 resistors (Candohm?) B+ will increase if they go high. Some times a heater cathode short will contribute to their / its demise.

Marc
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pixellany



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PostSubject: Re: Too high voltage out of the rectifier   Mon Oct 18, 2010 5:58 am

zenithguy wrote:

First thanks Pix for the reply, Yes I checked those 3 resistors as that was my first stop all are good. Voltage across the field was 76 volts close enough. I am to the point of trying of looking for a new rect. tube (5W4G) as I cannot explain why the voltage is so high (+40v) at the B+ output.

Cheers
Mike

Did you actually check the grid bias voltage (see the other comment about a short in the tube affecting bias).

I do not see how a bad rectifier tube would cause the B+ to be too HIGH. I would think if the tube had low emission, the B+ would be low.
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Mon Oct 18, 2010 6:24 am

Pix, I am not real sure what that is or how to check that. So, more learning to do, But its off to work.

You all are great to offer your help

Cheers
Mike
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marcc

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PostSubject: Re: Too high voltage out of the rectifier   Mon Oct 18, 2010 6:52 am

Rectifier fail causes low voltage.... Methinks you are barking up the wrong tree.
This a problem that seems to be related to lack of loading on the circuit.
Replacing old leaky caps will often cause a slight increase in B+

The biggest hassle here is that voltages are high & the variac may change the emmission. If you pull the voltage down too low you can poison the cathodes.

Did I see a reference to the AC voltage going into the rectifier? Should be 325VAC plus or minus nominal is usually 20% and as suggested check the heater voltages. If they are high as well as the secondary wind the variac back until they are correct & then check voltages as per schematic.

Also pay attention to R43 & R45 feeding the screens of the Converter & first RF. These are the screen divider and will impinge on the operation of both of those tubes. The voltage quoted is using a 1000 opv meterI would expect 100V with a digital. The resisors should be "floating" at one end to check with an ohm meter, not unusual for one of these resistors to fail.

Marc


Last edited by marcc on Mon Oct 18, 2010 7:25 am; edited 1 time in total
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pixellany



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PostSubject: Re: Too high voltage out of the rectifier   Mon Oct 18, 2010 7:22 am

zenithguy wrote:
Pix, I am not real sure what that is or how to check that. So, more learning to do, But its off to work.

You all are great to offer your help

Cheers
Mike

OK--thanks to someone for including all the voltage info on the schematic. From the numbers, we see that the total B+ current is ~ 71 mA. (75 volts across the 1060 ohm field coil)

I make the total of the bias string to be 240 ohms--thus the bias applied to the 6K6 would be 71 X .24 = 17 volts. This matches well with the tube specs if the plate is at 230 volts. (Plate current ~30mA)

Ideally, you should check the grid voltages right at the grids--using a high-impedance voltmeter (eg VTVM). Other wise, you can measure at the top of the bias string (the HV centertap) with a VOM.
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Mon Oct 18, 2010 5:39 pm

pixellany wrote:
zenithguy wrote:

First thanks Pix for the reply, Yes I checked those 3 resistors as that was my first stop all are good. Voltage across the field was 76 volts close enough. I am to the point of trying of looking for a new rect. tube (5W4G) as I cannot explain why the voltage is so high (+40v) at the B+ output.

Cheers
Mike

Did you actually check the grid bias voltage (see the other comment about a short in the tube affecting bias).

I do not see how a bad rectifier tube would cause the B+ to be too HIGH. I would think if the tube had low emission, the B+ would be low.

OK so to check the grid bias voltage its simply follow the connections as stated in the schematic is that correct? again something new for me. The filament voltage is 6.2 volts so thats good. It sounds like I need to get some glass tubes to replace the metal cans that have been put in so I will put a request for that in the classified. As I sai I had the tube tested but I know that does not duplicate "real world" and the osc. tube was marginal on the tester

Cheers
Mike
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marcc

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PostSubject: Re: Too high voltage out of the rectifier   Tue Oct 19, 2010 5:08 am

I don't think that tube idea will solve all the the problems, in fact if you replace the RF metals with glass & have no shields on them; Glass tubes especially 6U7, as an example, will quite often break into oscillation.

Better too make sure that the shield pins only go to ground (earth) and you have good tubes. There is nothing wrong with putting the tubes in another radio that takes that tube, (if you are lucky enough to be able to get at one,) to see if they actually behave in it?

Marc
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pixellany



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PostSubject: Re: Too high voltage out of the rectifier   Tue Oct 19, 2010 6:36 am

zenithguy wrote:

OK so to check the grid bias voltage its simply follow the connections as stated in the schematic is that correct? again something new for me. The filament voltage is 6.2 volts so thats good. It sounds like I need to get some glass tubes to replace the metal cans that have been put in so I will put a request for that in the classified. As I sai I had the tube tested but I know that does not duplicate "real world" and the osc. tube was marginal on the tester

Cheers
Mike

What kind of voltmeter are you using? You basically want to follow the schematic and verify all the voltages that you can---my earlier point was that, to measure the grid voltage directly, you need a high-impedance meter.
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pixellany



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PostSubject: Re: Too high voltage out of the rectifier   Tue Oct 19, 2010 6:46 am

As a minimum, tell us the voltage at the top of the bias string (junction of R36, C22, C21, tone control, and the HV centertap)

I don't remember if you said you replaced all the caps---regardless, be sure the polarity of all electrolytics is correct. C22, for example, should have the positive end to ground.

PS: You said earlier that a resistor was missing. Have you installed it?
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marcc

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PostSubject: Re: Too high voltage out of the rectifier   Tue Oct 19, 2010 8:39 am

It would be interesting to measure the resistance from the plate pin (3) of the 6C5 to B+ (Pin 4 of 6K6). That would indicate if the plate resistor was missing. I note two 45 resistors.

The one on the screen dividers is doing a bit of work that's why I suggested checking it & the one marked 43.

Midwest in particular had a habit of putting components inside the IF's
The Ceramic type bases were made to take parts inside at the base. Great idea until you need to replace the bits. This may be the same? Do not use modern 1/2 & 1/4 watt resistors as a lot of these now, do not have the voltage rating. use 1 watt.

Marc
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FrankB



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PostSubject: Re: Too high voltage out of the rectifier   Tue Oct 19, 2010 3:50 pm

IIRC, the voltages in any set are typically noted as +/- 20%; also, a newer voltmeter with higher input impedence will typically read a higher voltage than the original 1K input impedence of the old VOM's

FrankB
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Johnnysan



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PostSubject: Re: Too high voltage out of the rectifier   Tue Oct 19, 2010 4:04 pm

There is also the real possibility that the listed voltage of 310 is simply wrong.
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Tue Oct 19, 2010 5:13 pm

Thanks guys I want to let you know it will be a few days before I get to this as I am starting a new job and working both jobs for now. I will print all these notes out and work on them and report back all my results. FWIW I have a fluke digital meter.

I cant say this enough but thanks for holding my hand thru this. Its a long learning process for someone that has limited electrical radio experience.

Cheers
Mike
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zenithguy



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PostSubject: Re: Too high voltage out of the rectifier   Wed Oct 20, 2010 6:12 pm

pixellany wrote:
As a minimum, tell us the voltage at the top of the bias string (junction of R36, C22, C21, tone control, and the HV centertap)

I don't remember if you said you replaced all the caps---regardless, be sure the polarity of all electrolytics is correct. C22, for example, should have the positive end to ground.

PS: You said earlier that a resistor was missing. Have you installed it?

Yes both c22 and c21 negative are tied to the base of the old 30 mfd cap along with the center tap:


Now as far as measuring the voltage at the junction... I assume you meant from the junction to chassis ground as R36,C22,C21, tone control and the HV center tap are all hooked together. If that is what you meant it is -19.6 v

Mike
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