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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Wed 24, 2018 12:12 am 
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Nice work Richard, glad to see the original components are still good.

Those are neat looking radios.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Wed 24, 2018 2:19 am 
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Congratulations :D , it sure is an interesting little set.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Wed 24, 2018 2:22 am 
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Location: Utica, NY 13502 (USA)
Glad you got it working!

Dave


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Wed 24, 2018 2:59 am 
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Ditto to what Dave said !

Expecting a performance report once "tweaking" completed...

John


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Wed 24, 2018 3:48 am 
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Thx guys! According to the link Eddy provided, this is supposed to be a hot little receiver. We'll see...

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Wed 24, 2018 9:35 am 
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Well...I re-flowed all solder landings, and then proceeded with alignment.

The IF alignment went smoothly, with very little adjusting required. The instructions said to use a loop (for inductive coupling of the 455 Kc generator signal), which I did. Minimum signal was required for a good audible tone.

The problem began with RF alignment, as I had trouble getting a tone, until I connected the generator's hot lead to the radio's external antenna lead. 1600, 1400, and 600 Kc alignment procedures were followed, but in car terms, this set is a sled. I have to have an external antenna connected to it for any kind of selectivity, and if I want volume, six volts instead of 4.5. Also, it will only receive one blowtorch (KFI/640/50KW) below about 740 Kc, and really doesn't pick up in quantity until about 1070 Kc.

I measured the converter elements using a 4.5 volt supply; Collector, 4 volts, Base, .7, Emitter, .6. The print calls for 3.8, 1.7, & 1.8, respectively. So the Collector seems normal, but the Base and Emitter are each a good one volt below snuff.

In checking my Transistor sub guides, they listed a 2N212 and 2N1058, which also cross to an NTE (or ECG) 101. I just so happened to have several 101's, so I un-soldered the existing Sony 2T73, tacked in a Transistor socket, and tried them. None worked quite as well as the existing, and all had Base and Collector readings several tenths below, at .5 or so on both elements.

My experience with this type of situation is that usually it's the converter, either with tubes or Transistors, but I have a feeling that the decreased voltages and resultant lack of selectivity/sensitivity might be caused by something else

I've attached the front end circuit. What component(s) do you think would cause the decrease in base and emitter voltages, and sensitivity/selectivity?


Attachments:
Sony TR-712 front end.jpg
Sony TR-712 front end.jpg [ 186.75 KiB | Viewed 334 times ]

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Wed 24, 2018 1:02 pm 
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I think we determined earlier that the Sams schematic does not match the version of the radio in question. But I agree that the problem could be a leaky converter transistor. In my experience, any germanium transistor that I've tested has had some degree of leakage. There is a point when the amount of leakage begins to noticably affect performance.

Dave


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Thu 25, 2018 7:10 am 
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As I had previously posted, I tried several NTE 101's, and none exceeded the performance of the original 2T73. Also, their emitter and base voltage readings were each several tenths below the original, so I re-installed it. I noticed that R2 (10K) seemed to be the supply for the converter;
Image
So I experimented with a pot to see if I could increase apparent gain by raising the voltage. I was marginally successful, so jumpered R2 with a 470 ohm resistor, which raised the base voltage to 1.2 and the emitter to 1 volt. One problem though, is that with or without the added resistor, the emitter is less positive than the base, which ain't the way it's supposed to be.

The 2.8 volt source coming off of C6 that supplies this;
Image
is only putting out 1.2 volts. C8 shows 4.41, and C7, 3.96, so both are within normal range. I D/C R9 and measured it, and it's dead on. I tried jumpering it with a pot, to see if decreasing the resistance would increase the voltage, but no go.

So how can I get the emitter of the converter Transistor to be more positive than the base, and how can I get the 2.8 voltage source to produce what it should? Again, all Electrolytic's have been changed.

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Last edited by fifties on Jan Thu 25, 2018 7:27 am, edited 1 time in total.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Thu 25, 2018 7:25 am 
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Dave Doughty wrote:
I think we determined earlier that the Sams schematic does not match the version of the radio in question.

Even if we go by the print that Eddy's link displayed, the base and emitter voltages are still below what they are supposed to be, although I can't tell by the circled and diamond values what's what. If the circled values, then the base is more positive, but if the diamond, the emitter is.


Attachments:
Alt version of front end.jpg
Alt version of front end.jpg [ 129.33 KiB | Viewed 315 times ]

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Sun 28, 2018 5:27 am 
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OK, I finally got it working up to snuff.

The 2.8 volt source was delivering 1.2 volts, and I couldn't find the reason for it, with R9 and C6 testing within their specs. So I modified the circuit by shunting R2, the 10k resistor supplying the base voltage to the converter, with a 470 ohm resistor. Now, both the base and emitter voltages are each within about 1/10th of what the print calls for.

I also found that putting the chassis into the cabinet stopped the set from working, twice. I finally tracked it down to an apparently intermittent connection at the converter emitter, so I sucked up the solder at that landing, inspected the three connections, and then applied new solder.

I also added a nine volt battery snap connector, in case I'm out of 3 D cells whenever it might need batteries.

This set was somewhat the worse for wear, and I couldn't completely clean off whatever was on the transparent dial cover, but it didn't come out bad, and it does sound pretty good, what with the 4" speaker. Sensitivity and selectivity are on par with or better than most of the other 7 Transistor sets I've collected.


Attachments:
front.jpg
front.jpg [ 138.38 KiB | Viewed 279 times ]

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Sun 28, 2018 5:37 am 
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Location: Indianapolis, IN
Neat looking radio and glad you got it working !

How deep is that stain on clear plastic ? I've had good success carefully wet-sanding with 2000 grit, then polishing with Novus-2...

Good that it has a good home---followed the other thread also...

John


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Sun 28, 2018 6:14 am 
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Thanks John. You are more of a cosmetic expert than I am; I wouldn't dare using sandpaper on plastic, esp clear. I did use a Turtle Wax headlight lenses polishing compound, which helped out.

Edit; after playing with it last night and this morning, I can say that the sensitivity is excellent at night and in the wee hours. With the D cells, the volume can blast you out of the room on local stations. The little guy now has a permanent place in my kitchen. :D

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 30, 2018 12:02 am 
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Location: Palos Verdes, CA
fifties - Nice work on the Sony TR-712! I've also tried various car polishes on plastic transistor radios and found that Turtle Wax Chrome Cleaner works the best on cases that have seen a lot of use. - Mark


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 30, 2018 12:29 am 
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Location: Mount Pleasant UT 84647 (from Coventry, UK)
That is a neat looking radio. I am glad you got it working correctly.


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 30, 2018 9:50 pm 
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Location: Corvallis, Oregon
Congratulations fifties! Really pleased it has found a home in your kitchen. Thanks for posting on this radio.

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