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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 6:38 pm 
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What voltage are you seeing on the collector of TR208?


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 6:47 pm 
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Did you confirm that you have DC Suppy volts at pin 24 of the IF MPX board? Check it and see if you have roughly +12 volts DC.

The AM/FM selector switch Ss-5 routes DC Supply voltage to this point when the receiver is in the AM mode. If you don't have voltage there while in the AM position, the mixer and locl oscillator transistors will not function (nor will you get normal voltages at those transistors).

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 6:50 pm 
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I'm seeing 13.2 volts on the collector of TR208. I'm just trying to figure out why the base has practically no voltage on it and why it is dropping from 1.5v to 40mV after R206. I did pull R206 and it measures around 66K ohms, so I don't think this is the problem. The only other thing I can see is C261.

Paul, I have 13.2 volts at pin 24, so I assume the switch is working properly in supplying the 12v to the board.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 6:55 pm 
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5Y4GT wrote:
I'm seeing 13.2 volts on the collector of TR208. I'm just trying to figure out why the base has practically no voltage on it and why it is dropping from 1.5v to 40mV after R206. I did pull R206 and it measures around 66K ohms, so I don't think this is the problem. The only other thing I can see is C261.

Paul, I have 13.2 volts at pin 24, so I assume the switch is working properly in supplying the 12v to the board.


You could lift one side (or remove) C261 and see if it is shorted.

Shouldn't hurt to operate the radio without
C261 long enought to see if the bias voltage on TR208 resume to something closer to normal.

You may also want to test TR208 out of circuit and verify that the junctions all test good.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 7:19 pm 
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Well, removing C261 made no difference. I'm really confused as to why the voltage on TR208 is low. I used the diode test function on my multimeter, and the transistor read between .5-.7v from the base to each element. Is the base on the transistor physically connected to the emitter, because I seem to be reading 1.8k ohms between the base and ground, as well as the emitter and ground. I believe the 1.8K I'm seeing is caused by R274, which is 1.8K ohms.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 7:30 pm 
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5Y4GT wrote:
Well, removing C261 made no difference. I'm really confused as to why the voltage on TR208 is low. I used the diode test function on my multimeter, and the transistor read between .5-.7v from the base to each element. Is the base on the transistor physically connected to the emitter, because I seem to be reading 1.8k ohms between the base and ground, as well as the emitter and ground. I believe the 1.8K I'm seeing is caused by R274, which is 1.8K ohms.


Other circuit elements can result in misleading readings when testing a transistor in-circuit.
Yes, removing the transistor is a pain, but is the only way to remove the interaction from other components.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 8:00 pm 
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A signal tracer with RF probe will be useful. Start at the AM antenna and follow the signal through the IF stages.

There is a ceramic AM 455kHz IF filter that’s part of T207. Sometimes those fail open circuit and no IF signal passes through. You might be able to substitute a similar 455kHz ceramic IF filter from a different Japanese receiver of similar age.
After T207, the rest of the IF chain is shared by both FM and AM, so that section is probably OK if FM is working.

-EB


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 8:43 pm 
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Yes, a bit of a strange one.

Your in-circuit measurement of the transistor would tend to indicate that transistor is not bd - but I agree it really is necessary to test it out of circuit to be sure.

You measured 1.8K from base to ground, so you do not have a shorted C261 or rod antenna.
You have +13.2 volts on the collector, so the transistor is not drawing any current (consistant with the low bias) but the first IF transformer, R276, C263, and the mode switch are probably OK.

R206 measured OK. C262 *could* be bad, but not likely - use the ohmmeter and check it for a short anyway.

Back to a defective TR208. Pull it out of circuit and do a full 6 way diode test on it.


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 11:13 pm 
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Don't worry guys, I'm still listening to all of your suggestions. Right now, I measure 24 ohms from the transistor's (TR208) base to its emitter, so I'm going to remove it from circuit and test it some more. While I have the transistor out of circuit, I will test the radio again to see if the voltage at points 22 and 23 comes back up to 1.05V.

UPDATE: Yep, TR208 is bad. I removed the base from circuit, and now have .006v from base to emitter using the diode test function on my multimeter. Thanks for all the help guys! This has truly been an enlightening experience. I'm kinda starting to like solid state equipment now.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 11:24 pm 
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Would a 122DL NPN transistor work in this application? The original transistor was a 2SC829 NPN transistor. The 122DL has higher maximum voltage tolerances, but does it also take more current to turn it "on" and make it operate than the original 2SC829 transistor?

122dl Datasheet: https://cdn.sparkfun.com/assets/home_pa ... asheet.pdf

Original 2SC829 Datasheet: https://alltransistors.com/transistor.p ... stor=17177

EDIT: The 122DL datasheet linked seems to show a thru-hole mount transistor. However, the 122DL I have on hand is a pcb-mount type.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 11:28 pm 
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One must be careful with meter impedance and ohms with transistors. Applying voltage can actually turn a transistor full on, or another in a chain, causing damage. Often transistor bases have a voltage divider. The source one can do your thing if the source one fails. Tracks on some boards can be followed, where there is no circuit, by shining light through it. Check wiring & board for over soldering or wire fibrils shorting.

Marc


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 11:35 pm 
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R206 should have roughly the same voltage on both ends. The voltage across R206 from one end to the other shouldn’t be more than 1V or so.

There are 2 connections on the PC board that go to wires that travel through the back panel of the receiver into a coil on the AM ferrite rod antenna. The PC board terminals are labeled 22 and 23 on the schematic. I suggest temporarily desoldering these 2 wires and checking for either wire having a short circuit to chassis ground. There might be a crimped or smashed wire in the wiring harness that goes from the PC board to the AM rod antenna. When both wires are disconnected from the PC board, either wire should have infinite resistance to chassis ground. The resistance measured between these 2 wires should be <1 or 2 ohms because there are only a few turns in that coil on the rod antenna.

Also while these 2 wires are disconnected you can use a DVM in “diode check” mode to verify 600mV from base to emitter of TR208. TR208 is a generic small-signal silicon NPN transistor and can be replaced with any other similar small-signal NPN silicon transistor. There’s nothing special about it that would require an exact replacement.

Another way to confirm that everything on the PC board is OK, including TR208 itself, is to temporarily connect a jumper wire between terminal 22 and 23 on the PC board while the wires to the rod antenna remain disconnected and out of the circuit. With this jumper in place, all voltages measured on TR208 should be close to the values on the schematic.

I’ve worked on hundreds of Japanese receivers of 1968 to 1990 vintage. Broken and/or shorted wires on back panel AM ferrite rod antennas are very common. In fact I often find the AM rod antenna assembly torn off or missing. Evidently many people think the AM rod antenna is a handle for carrying the receiver.

-EB


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 11:40 pm 
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Marc, the meter indicated the transistor was bad when it was out of circuit under the diode test. When I put the positive lead of the DMM on the base and the negative lead on the emitter, I got .006V. Previously, I got a good reading when the transistor was in circuit. Also, the transistor still measured 24 ohms from base to emitter when it was removed from the circuit. My meter also puts out less than a volt on the resistance scale, so I don't think I have ruined the transistor by testing it with the meter. The transistor's max emitter-base voltage is 5v, so I don't think it was harmed and was bad originally. This receiver has been in my family since it was purchased new. The only work I saw done to it when I opened it up was a replaced output transistor and a replaced resistor, both in the amplifier. When I received the unit, it had a note in it indicating that the AM band was dead. I'm guessing this is what put the set out of service and up in the attic, where it had been for the past 20-30 years, until it was given to me.

Here's the definite information so far: TR208 is measuring .006V from base to emitter with diode test, measures 24 ohms on resistance scale. Voltages at both 22 and 23 are the same, and there is no evidence that the antenna was yanked (all wires are still in place and not pulled. I have removed C261 from circuit, as well as R206. R206 is still within tolerance, and C261 is not shorted. C262 is also not shorted. I believe the root cause of failure is defective TR208, with short between base and emitter.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Tue 12, 2019 12:27 am 
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As a further sanity check on TR208, measure the base-emitter junction with the pos lead on the emitter and neg lead on the base. Then compare that reading with the neg lead on the emitter and pos lead on the base. A good junction should read high resistance or infinite in the reverse biased condition. A shorted junction will read low resistance in either polarity of the meter.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Tue 12, 2019 12:32 am 
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Yep. Same result, which is .006v regardless of meter lead orientation. Guess I'll be buying a new transistor. I found a cross-reference sheet, so I should be able to hopefully find a modern equivalent. If not, I've found a few NOS transistors.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Tue 12, 2019 1:14 am 
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How about a NTE123AP? That 122 you have would be way to noisy I would think.

http://www.weisd.com/test/WEISD_TBL_view.php?editid1=NTE123AP

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Tue 12, 2019 1:20 am 
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I am not a great believer in testing transistors & some things "in circuit". There are some gadgets that claim to be able to test these things in circuit, however the standard hobby type stuff may well be found wanting. It is always a good idea to check around a failed transistor, to see if it just wore out, or was actually "caused to fail". If voltages are on the schematic, in similar fashion to tubes, the accuracy of the reading is influenced by the type of meter used now and its suitability; relative to the type that took the original reading.

If the testing indicated bad, that shows a lack of confidence in the test gear: I would have never put it back. There are pitfalls in measuring & interpretation.

All part of learning. The little under $A15 (even after buying a battery) multifunctional LCR-T4 has already told me what a transistor was & failed it. German technology nicked by China.

I have brochure. Not selling, or promoting, but be aware that such things exist and are helpful.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Tue 12, 2019 1:37 am 
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Even better and cheaper KSC1815YTA. You want a low noise audio amplifier transistor.

https://www.digikey.com/product-detail/en/on-semiconductor/KSC1815YTA/KSC1815YTACT-ND/3908203

* edit: has same pinout as your 2SC829

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Tue 12, 2019 2:37 am 
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Roger that, tbone8. I have ordered the KSC1815YTA transistor that you have recommended. It should hopefully arrive in a few days, so I should have the receiver up and running by the end of the week if I'm lucky. Thanks again, everybody, for your help!

Best,
Sam

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Tue 12, 2019 2:39 am 
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I suspect finding a satisfactory substitute for this application won't be overly difficult or critical. Its a broadcast AM receiver, after all.

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Last edited by processhead on Nov Tue 12, 2019 3:34 am, edited 1 time in total.

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