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 Post subject: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Fri 04, 2020 4:49 pm 
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Location: Toledo, Ohio
Hello, I was just finishing up on a 66VW karmann ghia restoration and pondering what radio to use.

Looking over the schematic of a Saphire 1 radio by Bendix made for VW (all transistor) I was wondering if one could safely run the 6V transistor radio on 12V if I change out the electrolytics rated at 8V to 12V+. Otherwise all of the components (other then the dial lamp) are probably rated for well over 12V. Would it be possible? What are the factors that would cause this to fail? I am guessing bias of transistors etc perhaps?

Which leads to next question, running a 6V radio off 12V. Best method for power drop? I have seen DC-DC converters on Amazon etc...what would be best?

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Fri 04, 2020 8:23 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
Your assumptions are correct. You can't run a 6V radio on 12 Volts. The electrolytics are the obvious issue, but the radio was designed for 6V and would need to have the bias networks examined and probably changed. Getting 2x the audio power output maybe appealing, but the radio probably wasn't designed with any margin.

You can buy a DC-DC converter, rated at enough amps to run the radio, but be wary that RF noise from the converter might be a problem. Don't try a dropping resistor. It gets very hot and won't ensure you have 6V all the time. Your car runs at 14.4 V; the battery will be close to 13V with the engine off. On a cold winter morning, depending on your location, it could be 10 or 11 volts.

A linear regulator will give you the best DC quality/no EMI, but will run just as hot as a dropping resistor, so will need a large heatsink.

Rich

https://classiccarradiodoctor.com/6to12.htm


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Fri 04, 2020 10:42 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
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Location: Montreal, Quebec
Back when it was topical, there were various projects for using 6v radios in 12v cars. I have no specific dates. And I suspect mostly for transistor car rsdios.


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Sat 05, 2020 1:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: dayton oh usa
how much does it draw at high volume?
less than 1a?
then a 7808 with the normal caps for stability but also a series diode on the input and output.
input to protect from negative going spikes and drop .7v so the reg doesnt get as hot.
and output to drop .7v.
expect up to 8v on a 6v system anyway.
it ought to be happy on +- 7.5v
heatsink it to case.


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Sat 05, 2020 8:36 pm 
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Joined: Jan Thu 16, 2020 12:29 am
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The voltage regulator suggestion is a good one.

If you need more than one amp, you can shunt the regulator with a resistor to carry some of the load, but it cannot be too low in value. Or use more than one regulator with small value balancing resistors on their outputs.

There is another way that works, you can knock 6 to 7 volts off your 12V supply feed with a power zener diode. like this, in series with the 12V feed:

https://www.ebay.com/itm/Mil-Spec-Micro ... Sw4O1eXFla

With one of these on a heatsink you could easily have 5A or more current provided it was on a decent heat sink.It is rated to 50W dissipation. The power dissipated in it will be 6.8V x the current, then you can calculate the temperature rise from the thermal resistances (Deg.C/watt) of the device (die to body & body to the heatsink & heatsink itself) and aim to keep it below 90 deg C is usually safe for silicon power devices, but it could handle higher temperatures. If you don't know how to calculate it, put a fixed load on it and measure the heatsink temp rise above ambient with a thermometer.

When the power supply in the car was 14.2V as it often is on charging, then the radio would receive about 7.4V and with the batter sitting at 12.8V, you would have about 6V for the radio , say with the car parked & engine off.

I would agree with the remarks of avoiding a stepdown switching regulator, due to the RF noise, that could be difficult to suppress.


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Sat 05, 2020 11:29 pm 
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Install a separate 6-V battery to run the radio. Setup a circuit so it trickle-charges from the main 12-V system

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"Voltage is fun to watch, but it's the CURRENT that does the work."


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Sun 06, 2020 10:25 am 
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pixellany wrote:
Install a separate 6-V battery to run the radio. Setup a circuit so it trickle-charges from the main 12-V system


Hmmm........very good suggestion ! Never doubt the value of 100 year old electro-chemistry to solve your problem.


Also if the battery in the car was an old fashioned tar top, with external lead inter-cell straps, you could just tap off three cells for a 6V radio, but batteries like that are much less common, though I have seen some vintage replicas for the world of vintage cars.

Some people (not many) have issues with gutting old car radios and putting modern electronics and bluetooth in them (I am one) but for most people its ok and doesn't bother them. For my vintage car I kept the AM radio, added a wired input for an iPod and use an Audiovox converter on its front end to get FM stations. These were a good FM receiver, but have a terrible diode modulator, so I replaced the modulator with my own circuit. This modification to an Audiovox converter got published in Silicon Chip.


Last edited by ACORNVALVE on Sep Sun 06, 2020 10:25 pm, edited 1 time in total.

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Sun 06, 2020 5:49 pm 
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Hi Bastardbus,

You are correct, the transistors need to be re-biased if you want to run the radio from +12V.
Probably that would be the preferable solution compared to dropping the supply voltage by some means.
It is rather simple (in theory), only four resistors need to be changed, and the low voltage electrolytic caps changed, as you pointed out.

I am enclosing the schematics, with the updated values, based on simulations.
Currents are marked in red, those are the values calculated from the original +6V setup.
Those are the currents you want to maintain.

Let me know if you are enterprising enough to do this, then I can get into the details.

Regards, Peter


Attachments:
Sapphire_I_sch.pdf [478.56 KiB]
Downloaded 18 times
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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Sun 06, 2020 8:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1509
Location: Shelton, WA
I deal with a lot of conversions with the sapphire I radio for those that have switched over to 12 volt. I'm not the only one doing this and we use the Aurora Design board which operates at 12 volt so no voltage drop is required. In addition you get 4 channel stereo 45 watts per channel so convertuibles are no problem and you can add bluetooth and USB to the am fm tuner. The boards fit nicely inside those Bendix radios which were not great to begin with and the new electronics are guaranteed for a year. By the time you get through adapting the current radio you probably have spent as much as or more then the conversion.

Look under tech-retro.com for a dealer near you and to see more.

billn


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Mon 07, 2020 3:36 am 
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Joined: Jan Mon 25, 2010 7:51 am
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Location: Toledo, Ohio
orbanp wrote:
Hi Bastardbus,

You are correct, the transistors need to be re-biased if you want to run the radio from +12V...

Let me know if you are enterprising enough to do this, then I can get into the details.

Regards, Peter


Peter I would VERY interested in hearing the details and learning how to properly do this. I know the first year for 12V was the Saphire 5 (or V) that is supposedly very similar circuit to the Saphire 1 as well. I wonder how the schematics would compare. But thank you and I look forward to reading your posts.

T

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Mon 07, 2020 4:00 pm 
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Joined: Jul Wed 22, 2009 3:07 pm
Posts: 866
Hi T,

Looked at the schematics of the Sapphire V, it is similar, but different enough that it is not much help here.
For starters, it uses Si transistors...

Start by changing all the electrolytic capacitors, irrespective of voltage ratings, all of them are old enough to be replaced without any question.
Before you modify your radio do measure the sensitivity for before and after comparisons, if you have the test equipment.
Do not forget to adjust the antenna trimmer cap!
Make a dummy antenna as described in the notes on the schematics.
Take a sensitivity measurement e.g. at 1MHz.

Did run another set of simulations, this time with the Ge transistor models that I had and used them before.
Previously I used the Russian Ge transistor models that are are now part of LTspice XVII.
Resistor values are a bit different, will comment on them.

First do measure the current draw of the final stage. It is set by R28 transistor. Do change it to 100 kOhm.
Measure the current as instructed between the collector of Q6 and the ground. The current meter effectively shunts the resistance of coil L6, that is the reason that coil does not need to be removed. The current should be between 0.78A and 1.06A.
Do make this measurement with +13.4V supply voltage.
If the current is too small, decrease the value of R28, the second simulation gave 82 kOhm value.

Measure the current of Q3, it should be 2.4mA. Measure the voltage across R11, 270 Ohm, it should be 0.65V.
If the current is significantly less than that, use at R10 location 39 kOhm instead of 47 kOhm from the first simulation.

Measure the current of Q2, it should be 0.52mA. For this the voltage across R6, 2.2 kOhm, should be 1.15V.
Again, for R4 use 10 kOhm, or 8.2 kOhm, depending on the current (the voltage measured). Lower resistor value gives higher current.

Measure the current of Q1. That is the collector voltage, or the voltage across R3, 2.2 kOhm, it should read 3V. For R1 use 150 kOhm or 100 kOhm.

The reason to maintain the rated currents in the RF-stages is that a BJT-transistor generally has less gain with lower collector current.
Make a sensitivity measurement of the modified radio for comparison!

I hope this all makes sense!

Regards, Peter


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Mon 07, 2020 8:51 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 637
Location: Montreal, Quebec
If you're going to modify the radio, why not separate the power amplifier stage. Modify it for 12v, then a relatively low wattage resistor and zener to supply 6v to the rest of the radio.

Thus you don't have to use a large current voltage dropping scheme, and the amplifier stage gives more output at 12v.

Maybe the output transistors need replacing, I don't know what kind of voltage they handled back then.


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Mon 07, 2020 9:14 pm 
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You'al makin' this hard...

All that's needed are 10 1N5401 rectifiers in series, walah drop 7v.

Six volt charging systems were rated at 7.2v & 12v 14.4. At worse you may have a extra .2-.6v.

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Tue 08, 2020 4:59 pm 
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Joined: Jan Mon 25, 2010 7:51 am
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Location: Toledo, Ohio
35Z5 wrote:
You'al makin' this hard...

All that's needed are 10 1N5401 rectifiers in series, walah drop 7v.

Six volt charging systems were rated at 7.2v & 12v 14.4. At worse you may have a extra .2-.6v.



I looked into the many various "forced" voltage drops, resistors, diodes etc. and I recall there was a reason why that wasn't optimal for this set up I can not recall off the top of my head.

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Sep Wed 09, 2020 12:23 am 
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The 1N5401 is rated 3A, I doubt the radio even 2A, would be fine.

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Oct Thu 22, 2020 2:10 am 
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Location: Toledo, Ohio
startinagin wrote:
I deal with a lot of conversions with the sapphire I radio for those that have switched over to 12 volt. I'm not the only one doing this and we use the Aurora Design board which operates at 12 volt so no voltage drop is required. In addition you get 4 channel stereo 45 watts per channel so convertuibles are no problem and you can add bluetooth and USB to the am fm tuner. The boards fit nicely inside those Bendix radios which were not great to begin with and the new electronics are guaranteed for a year. By the time you get through adapting the current radio you probably have spent as much as or more then the conversion.

Look under tech-retro.com for a dealer near you and to see more.

billn


I find these interesting and would love to install in my radios....BUT all I can find is "see a dealer near you!" and they want a bazillion dollars for installing one. Would love to buy and install myself, they should really make these available to general public.

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Oct Thu 22, 2020 3:45 am 
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Location: Belrose, NSW, Australia
Quote:
Also if the battery in the car was an old fashioned tar top, with external lead inter-cell straps, you could just tap off three cells for a 6V radio, but batteries like that are much less common, though I have seen some vintage replicas for the world of vintage cars.

Do NOT do this! Battery life will be greatly shortened due to half the battery being over-charged and the other half under-charged.

I used to convert 6v radios to 12v when I was a kid in high school. Most of the solid state car radios had a fairly simple system of jumpers that you cut or soldered. You should chase up the schematic for the radio - you will probably find it's pretty easy.

Or search Aliexpress for this (slightly incorrect description):

LM317T DC-DC Adjustable Converter Buck Step Down Circuit Board Module Linear Regulator Power Supply with rectifier filter

Less than a buck! How can you go wrong?

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Oct Thu 22, 2020 5:58 am 
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irob2345 wrote:
Quote:
Also if the battery in the car was an old fashioned tar top, with external lead inter-cell straps, you could just tap off three cells for a 6V radio, but batteries like that are much less common, though I have seen some vintage replicas for the world of vintage cars.

Do NOT do this! Battery life will be greatly shortened due to half the battery being over-charged and the other half under-charged.



In principle I agree, but the power consumption of a transistor radio at least is too small to create a significant imbalance in the group of 3 cells with respect to the other 3, and it does not affect the charging process to any extent because the internal resistance of the series cells & their interconnects is much much lower than the internal resistance of the radio, so on charge at least, the radio will not cause any significant imbalance.

On discharge, with the car off, the radio would be ok for less than an hour, if you were parked as the group of 3 cells would only be mildly discharged, but it would not be good say if the radio was left on all night and the group of three cells powering it got significantly discharged, that would then upset things during charging. So if you only use the radio when the battery is charging, mainly while driving, it would not be a practical problem.


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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Oct Thu 22, 2020 7:48 am 
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Location: Belrose, NSW, Australia
Automotive lead-acid batteries use pasted plates so as to be able to supply heavy starting currents. With this form of construction even minor cell imbalances become cumulative and shorten the battery life due to plate sulfation in the 3 cells on the ground side. The weakened cells then suffer greater IR loss heating in both charge and discharge.

The imbalance is small I agree but in practice it's significant.

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 Post subject: Re: Question, is it possible to run 6V car radio on 12V
PostPosted: Oct Thu 22, 2020 8:56 am 
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irob2345 wrote:
The imbalance is small I agree but in practice it's significant.


But in the case of an alternator charging system in a car, all the cells will reach full charge on charging in most cases and the energy dilivery above that full charge simply results in the evolution of heat and each cell can no longer acquire additional charge, it is a charge saturation issue. And the the current drawn by the radio in this case is not only dwarfed by the charging current, but will not result in a charging imbalance of any significance at all. It is only the discharge issue that could start to result in an imbalance.

The only possible issue occurs if the radio is allowed to run down 3 of the cells over an extended time frame, and that group of 3 cells does not reach a full charge in the subsequent charge cycle which could happen on intermittent use of the car.

So if the radio is used with the car running, as it often is, it won't present a problem, so we have to disagree on this point.


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