Realistic STA-225 - Printable Version

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+--- Thread: Realistic STA-225 (/showthread.php?tid=15952)

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RE: Realistic STA-225 - Ron Ramirez - 01-16-2017

At home now, putting together a Mouser order and can now look at the schematic...

Mike, the original transistor in question is 2SC828. VCBO and VCEO of 30V, VEBO of 5V. Your original recommendation of BC547CTFR has ratings above that of the 2SC828 and should work well. I have enough resistors to be able to increase the values of R703 and R702.

Edit: Ordered. Should have the parts close to the end of the week. I'll let you know what happens...

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RE: Realistic STA-225 - morzh - 01-16-2017

Try to first increase both 3-4 times in value at the same time. While not gaining any voltage gain you will gain input impedance so if you lack in it when for whatever reason your input is low it will give you some extra edge. If it is some other effect then...
Also depending on the result increase the top one twice as much as you increaee the bottom one. This will add voltage gain on top the impedance gain. Do some experiments.

Oh. R704 might need to change proportional to the R703, the bottom one. It is the feedback, and if kept the same, the total gain will drop with R703 increase.

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RE: Realistic STA-225 - Ron Ramirez - 01-18-2017

Since we're on page 5 now, and I had posted the schematic fragment back on page 1, I think this is a good time to post it again so anyone reading this thread will understand (click to enlarge):

[Image: http://www.philcoradio.com/images/phorum/STA225_014sm.jpg]

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RE: Realistic STA-225 - Ron Ramirez - 01-18-2017

Parts have arrived. I'll try to dive back into the STA-225 this weekend.

In the meantime...

What do you guys think of increasing the value of C701 from 0.47 uF to 1 uF?

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RE: Realistic STA-225 - morzh - 01-18-2017

You will decrease the cap's impedance from 5.6k to 2.8k. Compare this to the input impedance of at least a 100k. Likely more.
Try it, won't hurt.

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RE: Realistic STA-225 - PhilcoMike - 01-19-2017

Ron,

Could even temporary in a potentiometer (with a series resistor to keep at least a minimum value in the circuit) and play around with the settings to get the sensitivity you want standing on the flooring of the room where it will live.

Thanks,

Mike

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RE: Realistic STA-225 - Ron Ramirez - 01-19-2017

Thanks, guys...

Rather than wait until the weekend, I pulled the 225 out this evening, pulled chassis from cabinet, and put it on the bench.

I replaced TR701 with a BC547CTFR from Mouser.

I decided to triple the values of R702, R703, and R704.

Resistor - Original value - Calculated value - Actual value
R702 - 220K - 660K - 680K
R703 - 1K - 3K - 3.3K
R704 - 820K - 2.46M - 2.2M

As you can see from the list above, I used the closest standard values to the calculated values.

I did not replace the 0.47 uF C701 with a larger value - I left the ceramic unit in place that I had put in previously to replace the original electrolytic.

Soldered everything back into place, and performed the first of two tests by carrying the chassis upstairs to my home office desk, plugging it in, plugging a set of headphones into the unit, and turned it on.

Auto Magic worked as it is supposed to!

Next, I carried the chassis back to the basement, put it back into its cabinet, and brought the complete unit back up to my home office.

Plugged in headphones, plugged the until into the power strip and...

Yes! It works properly!

That's right, the tuning meter now turns white and stays white (and AFC stays off) as long as I have my hand on the tuning knob. As soon as I let go, it turns green again and AFC kicks back in.

So, with these slight mods, Auto Magic works as designed.

I'm not going to leave it in my home office - I've already switched my Sherwood S-7900A with a Lafayette LR-3500 that I recently recapped and converted all lamps to LED. When I decide that it is time to switch receivers in here again, I'll bring up the STA-225, knowing that it will work as designed.

That is it for this STA-225. Many thanks to Mike (morzh) for his suggestion of the transistor change and of increasing the values of R702, R703, and R704. It is just what this receiver needed! Thanks also to Mondial, and to everyone else who commented.

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RE: Realistic STA-225 - Eliot Ness - 01-20-2017

Congratulations

I suspect that "Его так просто не сломать" doesn't actually translate into "it is not so easy to break"???

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RE: Realistic STA-225 - Ron Ramirez - 01-20-2017

Thanks!

The literal translation, according to Mike, is "he is not easily broken". It's a rough translation of the phrase "You can't keep a good man down."

I remembered late last night that I still need to replace electrolytics in the tone control board. I'll do that later. It's back on the shelf for now. Last week I bought parts for my next big project - and it is going to be yuuge - rebuilding a Marantz 4230.

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RE: Realistic STA-225 - morzh - 01-20-2017

I will have to rebuild

1. A Thorens TD-124 turntable, and
2. Two Eico HF-22 amps.

Building a second stereo system

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RE: Realistic STA-225 - morzh - 01-20-2017

(01-20-2017, 10:18 AM)Ron Ramirez Wrote:  Thanks!

The literal translation, according to Mike, is "he is not easily broken". It's a rough translation of the phrase "You can't keep a good man down."

Yes, it is. Of course one got to keep in mind that Russian not only has grammatical gender but also uses personal nouns to refer to inanimate objects, unlike English that has "it" for it :
This way a table is "he" and a bed is "she" and a tree or a window is "it" as they are of the neuter gender.

So if I refer to a table that is built very well, I could say "его так просто не сломать" - "he is not easily broken". Although with proper English it should be "it".

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RE: Realistic STA-225 - Ron Ramirez - 04-26-2020

Three years + on...

Finally, I have replaced the electrolytic capacitors on this receiver's tone control board.

2.2 uF and below were replaced with WIMA MKS2 film; larger ones with Nichicon electrolytics. I would have replaced the 3.3 uF units with WIMA as well, but I had a dozen or so 3.3 uF electrolytics and I thought it wise to use them rather than buying more caps.

It does sound better now. I have read that the Realistic receivers of this era were heavy on bass when the "Loudness" button is pressed, and this one is no exception. Still, I'm happy with it.

It is going into service for now as my workbench receiver. The Fisher 432 I was using as my workbench receiver has dropped its left channel. It was a gradual thing in that the left channel became weak, and finally all but went away completely. I'll have to look into that problem.

But the STA-225 is finally all finished.

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RE: Realistic STA-225 - morzh - 04-26-2020

Woo-hoo!

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RE: Realistic STA-225 - Ron Ramirez - 04-28-2020

Mike

I just noticed...you've passed me! You now have the greatest number of posts on the Phorum!

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RE: Realistic STA-225 - morzh - 04-28-2020

Not my goal.....and I think I do not post as much as I used to...if I am the most prolific keypincher here, does that mean we all slowed down?

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