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 Post subject: Need help troubleshooting a Sony TR-712
PostPosted: Jan Mon 22, 2018 9:21 am 
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I received the subject radio from member Bosch's Fault, generously for just the cost of postage recently, and proceeded to change out it's 8 Electrolytic caps.

Firing it up produced no audio, but using a small Rat Shack audio amp, first at the center wiper of the VC, and then through the ensuing circuitry, provided audio reception. I spec'd the resistances of each side of both audio transformers, and they proved within tolerance, and ditto for the voltages at the driver Transistor elements.

The base voltages at the two output Transistors however, are suspect. Per the print, they should read .1 on the Base, 0 on the emitter, and 4.5 on the collector. The emitters read as they should, and the collectors at 4.42 volts seem OK, but the base of each reads 4.27. I don't think both output Transistors are shorted, or it would seem that their base and collector readings would be the same. So what other component(s) would you suspect?


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Sony TR-712.jpg
Sony TR-712.jpg [ 201.99 KiB | Viewed 719 times ]

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Mon 22, 2018 1:39 pm 
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Hey Richard---have you checked M3, the "bias regulator" diode ? If open, would cause the base voltage increase...

John


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Mon 22, 2018 1:41 pm 
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I would also suspect an open M3 diode if you are certain that the output transistors are not leaky or shorted. What is the voltage across that diode? I also suspect that, with 4.27 volts on the bases and 0 volts on the emitters, you also have open output transistors.

Dave


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Mon 22, 2018 1:46 pm 
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For the germanium push pull pair of NPN output transistors if you have 4.27 on the base-emitter then the transistors have failed. In order to get 4.27V on the base then diode M3 must have failed open also since it should clamp the voltage at a diode drop.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Mon 22, 2018 8:26 pm 
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Thx for the analysis' guys!

I measured between the CT of T1 and ground, and got readings of 203Ω one way, and 1200Ω the other. Would that condemn or validate the Diode?
Now an interesting aspect is that the print calls for R20 to be 2200Ω, and yet what's factory installed is a 220Ω, 5% resistor. In fact, if you look at the PCB, and compare it to the SAM's pictorial, notice there is in reality another resistor alongside of it, not showing in the SAM's, nor the cap on the right side of them.

So, should I replace the Diode, and given that it's a Sony part #1T51 Bias Regulator that doesn't show in a Google search, is there a more common component, perhaps a resistor, that I could sub for it?


Attachments:
Sony TR-712 PCB.JPG
Sony TR-712 PCB.JPG [ 218.32 KiB | Viewed 678 times ]
Sony TR-712.jpg
Sony TR-712.jpg [ 231.47 KiB | Viewed 678 times ]

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Mon 22, 2018 11:25 pm 
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I would use a germanium diode which will have a low forward voltage drop of approx. 0.1V, you will need to replace both output transistors before the radio will work. Remove the transistors install the diode first and check that the diode voltage is now approx. 0.1V then add the new transistors if all is well.
You may have to adjust the value of R20 to get the correct bias current in the output transistors. Lowering the value of R20 will slightly increase the diode voltage and the transistors will conduct more current.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Mon 22, 2018 11:57 pm 
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You may find this article interesting:

http://messui.polygonal-moogle.com/valves/SC201703.pdf

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 2:04 am 
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I was a little confused by your Sams schematic at first, now that you posted an actual photo of your board I can see the two TO-5 black output transistors (2SB51) are definitely PNP and looks like there is a black thermistor just ahead of T1, not a diode.

So it makes more sense to go by Eddy's schematic (Fig.1) in the PDF file.

Looks like you have an early version of the TR-712

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 2:46 am 
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I agree with Roco, it looks like your board does not match the SAM.s schematic. You will need to check the voltages for the PNP.s the emitter is 4.47V, the collector at 15mV

This is the PNP version schematic

Attachment:
PNP.PNG
PNP.PNG [ 255.13 KiB | Viewed 644 times ]

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 9:22 am 
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Thx for the link and guidance, Eddy! The Transistors are 2SB51's, and the "Diode" is actually a Thermistor. I'll replace as you suggested, and report back.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 12:22 pm 
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With this new revelation, if you accidentally swapped the collector and emitter readings, your original voltage measurements appear correct. (15 mv might not be seen on an analog meter.)

fifties wrote:
....using a small Rat Shack audio amp, first at the center wiper of the VC, and then through the ensuing circuitry, provided audio reception.
Did you have audio at the output transistor collectors and/or at the speaker terminals?

Dave


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 2:08 pm 
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fifties wrote:
Thx for the link and guidance, Eddy! The Transistors are 2SB51's, and the "Diode" is actually a Thermistor. I'll replace as you suggested, and report back.

I would not replace anything just yet , if the voltages on your board matches the PNP schematic then the problem may be elsewhere.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 5:34 pm 
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Dave Doughty wrote:
With this new revelation, if you accidentally swapped the collector and emitter readings, your original voltage measurements appear correct. (15 mv might not be seen on an analog meter.)

I just measured them again, this time verifying by the tab as to which lead was the collector's, and the readings in my OP are accurate; voltage on the collectors, zero on the emitters. I use a Fluke DMM.

Dave Doughty wrote:
fifties wrote:
....using a small Rat Shack audio amp, first at the center wiper of the VC, and then through the ensuing circuitry, provided audio reception.
Did you have audio at the output transistor collectors and/or at the speaker terminals?

Dave

I did, but there appeared to be no amplification from them. When I tested with just a speaker, the volume was extremely low, and the audio seemed garbled.

Eddy wrote:
fifties wrote:
Thx for the link and guidance, Eddy! The Transistors are 2SB51's, and the "Diode" is actually a Thermistor. I'll replace as you suggested, and report back.

I would not replace anything just yet , if the voltages on your board matches the PNP schematic then the problem may be elsewhere.

OK, here's where the confusion lies; the Transistors are indeed PNP units, BUT, they are wired as per the SAM's print I displayed in the OP, even though that print shows them as being NPN, with a different part number. I just measured each emitters resistance to ground, and they were like 6 and 7 ohms each, and infinite when measured against battery positive.

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Last edited by fifties on Jan Tue 23, 2018 6:39 pm, edited 1 time in total.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 6:35 pm 
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The tab on a 2SB51 indicates the emitter, not the collector.

Attachment:
2SB51 data.JPG
2SB51 data.JPG [ 164.99 KiB | Viewed 607 times ]


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 6:47 pm 
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Thx Dave, nothing like them making it more confusing, tabbing the emitter instead of the collector!

OK, then the collectors are tied through 5 ohm resistors to ground, and the emitters have pretty much the supply voltage. So how do we reconcile the base voltages being so far off?

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 8:36 pm 
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You previously measured the base at 4.27V which matches what it should be for the PNP configuration which shows 4.3V on the schematic. Could your speaker or earphone connector be the problem?

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 10:11 pm 
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In the Sony schematic, the emitters are tied to the + battery voltage through 5 ohm resistors while the collectors find ground through the output transformer primary whose center tap is grounded. If it's not a problem with the speaker or earphone jack, I would remove the output transistors and test them in the usual manner with a VOM or DMM diode test function. In-circuit testing can lead to erroneous results in this circuit because of the low resistances of the driver and output transformer windings.

Dave


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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 10:43 pm 
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Agree with Dave, at this point I would pull the transistors and check them.

Have you tested the 5 ohm resistors? one or both could have drifted off spec.

Those type do not age well.

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 10:56 pm 
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Eddy wrote:
Could your speaker or earphone connector be the problem?

:D Step to the head of the class, Eddy!

I had D/C the speaker from it's leads to make it easier to recap the PCB, so by itself, it was out of the equation.

The circuit has two jacks, for both earphones, and another speaker. Measuring between the two connectors of each revealed a very small resistance between ground and the transformer's output.

I D/C the lead going from the transformer's solder landing to the jacks, and patched in the speaker. Voila, picked up a local station, clearly but at low volume.

In handling the PCB, the volume increased dramatically, temporarily. In feeling around the PCB (this is one reason I like these battery powered circuits-you can't get a shock), I located a landing that when pressed, increased the volume, so I reheated it, and now the set works reasonably well. Still have some tweaking to do with RF and IF alignments, but the hurdle is over.

Thanks out to Rocco, Dave Doughty, and, umm, umm, oh yeah, Eddy for sure! :wink:

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 Post subject: Re: Need help troubleshooting a Sony TR-712
PostPosted: Jan Tue 23, 2018 11:40 pm 
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Mwhaa haaa haaa haaa haaa

Beware forum members with radios for the cost of shipping...

Glad you have it pretty well fixed. Enjoy!

Bosch

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