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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Sun 13, 2019 10:44 pm 
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Location: Indianapolis, IN
Hello All,

Thanks to Huesby's info on subbing silicon transistors for audio outputs in another thread, finally got this radio back into service after a couple years !

Had already installed sockets on pigtails to experiment and decided to leave them in for now for the 2N4123 silicon transistors in case I wanted to try others in the future. Replaced R18 (82 Ohm) with 200 Ohm in series with a 1N4148 silicon diode---see pic below... Determined the resistor value using a pot---adjusted for best quality audio and took measurement with meter for fixed resistor. Tried this a couple times and value stable at almost exactly 200 Ohms. Audio seems good throughout volume control range and this set puts out a lot of it ! Have not checked current level with new transistors yet and am curious about what it will be...

This radio is another great performer all across the band with excellent sensitivity & selectivity and am grateful that it's now working ! Thanks again, Huesby !

Mod below which will hopefully be of help to others...

John


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Sun 13, 2019 11:46 pm 
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Kewl!

I have to ask, did you monitor current? I've done a bit of experimenting with Ge to Si(no not on GE that beating me up) and found it's difficult to tell a difference at say 10ma idle current and 20-25ma.

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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Mon 14, 2019 1:49 am 
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Location: Indianapolis, IN
Hey Tom,

No, haven't checked current yet as time ran out. Simply used a pot to determine the series resistor by listening for point of best audio quality while slowly adjusting---and it was a fairly tight spot where fidelity was clearly best by ear.

Am listening to Big Band Sunday Night right now on Zoomer 740 and man, does it sound good ! And the radio definitely looks more '40's than '50's...

Would really be neat to compare it to another in person with original germanium outputs...

John


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Mon 14, 2019 2:14 am 
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Joined: Oct Mon 26, 2009 10:02 pm
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Location: Indianapolis, IN
Hey Tom,

Curiousity got the best of me, so made a short trip from family room to shop-room, opened 'er up, and checked the current---around 20 mA idle (minimum volume with no signal) and stays constant at low volume on station. Rises to 25 mA when turned up uncomfortably loud arms length away... Guess that's almost twice the typical idle current, but not too bad considering how well it's working !

John


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Mon 14, 2019 3:41 am 
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LOL, I'm bit by the curiosity thing myself once in a while.

The added current should increase output, improving low freq reproduction(probably why it sounds better). Excepting for reduced battery life, I doubt there is a downside.

I dunno if there is a difference in audio between Ge & Si, still more than pleased with the SK3010 installed in the 780. Play it almost every day. It's so good, I tweaked on my transmitter to bring it more in line with sound of commercial stations. Just the nature of the dual control pentodes to slightly attenuate low freq, I've added a graphic Eq to kick up the bottom end approx 4db.

I've been trying Si outputs in a Roland 66(has plug in transistors), only needed to alter the voltage divider slightly, mod you used for Arvin would no doubt work. Problem is the 4" speaker isn't exactly Hi-Fi.

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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Tue 15, 2019 1:46 am 
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Location: Indianapolis, IN
Hey Tom,

Since the current is only high for "idle" state and remains pretty much the same at normal listening level where it's now on par with germanium outputs, battery life shouldn't be different unless you are in the habit of leaving radios on and at minimum volume for long periods of time.

Think the audio from silicon devices is a bit more "crisp"---need to try the silicon output mod in an older radio that I have duplicates of for better comparison...

How good of a performer is that Roland "66" ? I have a later Bi-Fidelity which uses same basic chassis with an additional transistor and it's an excellent performer with plenty of good audio...

John


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Tue 15, 2019 12:44 pm 
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I agree, even at double current battery life probably won't be noticeably affected.

Until I connect a better speaker to the Roland, I'm not sure if the sound is any different using Ge vs Si transistors. When biased for around 20ma the Si do seem to give a fuller sound, especially at low volume. If bias adj makes a difference with the Ge, my ears aren't good enough to tell.

A note on the original Ge transistors, all but one output were at least somewhat leaky. The RF/IF transistors extremely leaky, with one being shorted. I don't know if transistors now installed are correct enough to function properly. The osc is a Workman BE6-M that should be OK(listed on package as replacing original GT760), a transistor borrowed from a older Royal 500 was no better. The IF are a pair of International Rectifier general replacement RF types. They test good for leakage and gain, but the osc would not function with one installed in that position. At present it has a intermittent squeal and doing good to receive the stronger locals(yes electrolytics all replaced). I suspect it has a out of spec resistor or maybe cap, yet to get into that aspect of troubleshooting.

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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Wed 16, 2019 10:38 pm 
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Hi XRHONDA91

I'm glad that my research has inspired you to dare to make some mods into your radio.
I need to tell you that I'm an admirer of your restoration work, your are a genius man! I love those picture you upload about your work.

Regarding transistors, I always substitute Ge transistors with Si ones because they are hard to get, expensive (sorry but I'm cheap :oops: ) and poor performers in current gain .

I'm aware some people refers them as they sound better but, in my opinion, that happen because they are poor performers in high frecuencies and that makes the sound mellow which is more pleaseable, specially for us old people.... but The same effect could be acomplished with tone controls, well that is just my opinion.

Another interesting thing is that by substituting the Ge for Si transistor, the stage get more amplification because any good Si transistor has about 200 to 300 gain (hfe) vs 30 to 40 of Ge transistor.

About the quiescent current (when there's no signal amplification) in Si transistors, that could be lowered with different values of resistance but there's some correlation with crossover distortion, you see the transistors are not ideal switch so they require some voltaje before it could begin to conduct (amplify) and ln Ge case it's about 0.2 to 0.3 v between base and emitter (Vbe), and in Si is about 0.5 to 0.7 Vbe and that's the job of those resistors in the base, to provide such voltaje.

Well I hope I could make me understand, English is not my native languaje but I give my best, dont be hard on me please.


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Thu 17, 2019 3:22 am 
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Location: Indianapolis, IN
Hello Huesby and thank you very much for the kind words ! And I understand you fine...

I had tried resistors for the biasing and still could not get these silicon transistors to work well in the past---the silicon diode you mentioned was what solved the problem.

The sound does seem "cleaner" than with germanium and the tone control functions well with the modification.

I'm not concerned at all about the current draw at no signal since it's nearly the same at normal listening level---and I don't leave radios on for long periods at minimum volume anyway...

I have used silicon transistors in RF/IF stages of a couple older sets with mixed results---probably need to play more with the bias levels and it's also possible that there were problems due to AGC action as you mentioned in other thread...

Thanks again for your information as I'm more confident now about using the silicon devices for audio outputs when I don't have proper replacements...

John


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Thu 17, 2019 6:07 am 
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xronda91

I don't know about you but I would play with the maximum volumen this radio can get, you see the resistor R19 has a 10 ohms value and that limit the maximum current (Ic) and hence the maximum volume.
As it is now the max collector current is 7.5/10= 0.75 Amp which is suitable for Ge transitors but it's nothing for power Si transistor (I like to substitute them with Si transistor with higher current like tip 31). Lets say you change the R19 with a 4.7 ohms value and now your new Ic current would be a maximum of 7.5/4.7 =1.6 Amp. Now due to the transformer internal resistance and the drop of voltage in Vce the Ic won't be 1.6 Amp but we surely double it which means quadruple power output, remember P=i**2*r (I squared times r)... every time you double current you get 4 times more power..
Anyway this is a great opportunity to have fun , enjoy !

pD the power rating of R19 should be higher than 1.6x4.7=7.5Watts
Pd2 This is how I spend my free time... Experimenting with old radios !


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Thu 17, 2019 11:29 pm 
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Location: Indianapolis, IN
Hello Huesby,

With the recent silicon transistor mod, the radio now has more clear audio than could ever be used when turned up. The 2N4123's are not power transistors, but seem perfectly adequate in this radio.

Am looking forward, though, to more experimentation with using silicon transistors in "marginally good" radios and their RF/IF circuits---will post any "exciting" results...

Thanks again !

John


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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Fri 18, 2019 5:33 am 
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R19 is there to reduce crossover distortion produced by transistors when switching. The value is determined by amount of current the transistors draw. When I rebiased the outputs on the Roland to use Si, I jumped it out completely and never noted any change in amount of audio.

Also a note on the Si ECG 159 transistors I was using, out of six NOS, only two were reasonably matched. Of course the 159 is for general replacement and not touted as audio output types.

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 Post subject: Re: Arvin 9562 project...
PostPosted: Jan Sat 19, 2019 5:23 am 
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xrhonda91 wrote:
Am looking forward, though, to more experimentation with using silicon transistors in "marginally good" radios and their RF/IF circuits---will post any "exciting" results...

Thanks again !

John

John, if you haven't visited this page, scroll to very bottom for links to check out the authors replacement of Ge with Si transistors in a GE P-805A radio. After all his efforts makes one wonder.

http://tomschumelectronics.com/radio-rd.html

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 Post subject: Re: Arvin 9562 project...
PostPosted: Feb Wed 06, 2019 4:46 am 
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You could wire sockets to the circuit board and then mix and match a few identical transistors until satisfied.
My old ears would not hear any differences.

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 Post subject: Re: Arvin 9562 project...
PostPosted: Feb Wed 06, 2019 5:26 am 
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westcoastjohn wrote:
You could wire sockets to the circuit board and then mix and match a few identical transistors until satisfied.
My old ears would not hear any differences.

LOL, I have so-so hearing as well but when I'd try to listen to my P-780 that transistors were badly mismatched I'd cringe(no doubt one was failing, had very low gain). Using scope, it was obvious one transistor wasn't contributing it's share of work.

The Ge transistors in these old radios are now failing from old age, probably mostly due to early mfgring issues, impure material, etc. I'd suspect replacement transistors mfg'd in the '70s & '80 will have a better track record than the ones of '50s & early '60s.

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