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 Post subject: Realistic STA-77 Receiver Dead AM Band *Fixed!*
PostPosted: Nov Wed 06, 2019 3:54 am 
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Hello everyone,
I recently got a Realistic STA-77 receiver in for repair. When received, the receiver functioned well on FM, but AM was completely dead. Due to the function of FM, I figured I could rule out the audio amp and power supply from being suspect, so I went ahead and started replacing the 40 + year old electrolytic capacitors on the front end of the radio, hoping that would cure the problem. Unfortunately, it didn't. The radio is the same as when I received it, only receiving on FM. I also sprayed all the controls with contact cleaner, so I don't think dirty contacts would be the problem. Any ideas on where to begin? I have worked on several radios before, but this solid state receiver is a first to me.

Schematic here: http://akdatabase.org/AKview/albums/use ... ematic.pdf

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Last edited by 5Y4GT on Nov Mon 25, 2019 3:20 pm, edited 1 time in total.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Wed 06, 2019 4:06 am 
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No hum or anything with the volume cranked up? Did you try wiggling the selector switch and related switches? Did you try injecting a strong AM signal in? That's my thoughts... :D

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Wed 06, 2019 5:10 am 
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Greetings to 5Y4GT and the Forum:

I would do the standard local oscillator sniff trick. Get another AM receiver and tune it to around 1455 KHz. Bring its antenna into close proximity to the Realistic chassis. Tune the Realistic around 1000 KHz and see if you can hear the L.O. in the other receiver. If not, then start there. If you can, then look at the RF amp (TR701) or the mixer, which is TR208. If you can't hear the L.O., that function is provided by TR703.

You might also wish to find a signal generator that covers the LF band and see if that band is working. If it doesn't work either, then the problem is most likely in the mixer or the IF.

Good Luck,

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Wed 06, 2019 3:10 pm 
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Thanks for the information, guys! To clarify, I get soft static that I have to crank the volume up to hear. I also hear a faint hum in the background that sounds like an open audio line. I do have a signal generator, so I should be able to inject a signal. I don't know why I forgot all about the second radio test, but thanks for reminding me! I'll look at it again tonight.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Wed 06, 2019 3:32 pm 
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Faint hum can be deceiving, as it could arise from a bit of cathode leakage(tube set). Audio working on FM, is a reasonable assumption, but only from a certain point. My tactic is always to attempt to calibrate the IF, once I have some semblance of working and I did comment on a Philco recently where the oscillator was dead. Its IF was calibrated as it does not rely on the oscillator. That proved the set from the Pentagrid to the speaker.

It was then a case of fixing the known cause relating to the oscillator(socket). One of the traps here is that you may get a perhaps tuneable (not in most cases), strong local station, but weak. That weak signal is being fed into the 2nd detector via the AGC. Often creating the illusion that the oscillator is actually running, when nothing can be further from the truth.

Marc


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Thu 07, 2019 12:52 am 
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I think it’s great that the Schematic has all the voltage readings around the transistors. This will give clues if the stage is working as well.

I am guessing there is a loop antenna hooked up somewhere as well. It’s not mentioned, but I think that’s what they used.

Transistors look menacing if you are used to tubes, but it’s not a big deal. Just take your time.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Thu 07, 2019 1:18 am 
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As Tbone infered... You do have an exxternal AM loop attached, right? There is no built-in AM antenna and an external AM loop is required. https://www.amazon.com/Fancasee-Antenna ... 3K4QEAHWKJ


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Thu 07, 2019 2:23 am 
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Aside from failing to connect antennas etc. With the Ferrite rods, it is not uncommon for the coil to move on the rod and throw it off. I am also finding with a lot of, in particular Germanium, oscillator, and oscillator mixer transistors: They are failing.

Many used electrolytic caps for coupling, they are drying out & failing, plus there are a plethora of caps around half inch long & three eigths round (axial) that crack & fail regularly. Early transistor sets also had some ceramic cap types that were far from relaible.

Marc


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Thu 07, 2019 3:00 am 
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Well, the Realistic passed the local oscillator test. I get oscillation in the test radio around 1455 kc with the realistic at 1000 kc. My receiver has a ferrite rod antenna on the back of it. See pic here: https://www.lakemichigancomputers.com/p ... 0988316714 (This isn't mine, but it has the same antenna. It's the black bar in the upper right hand corner on the back.) When I first got the set, I tried it with a loop antenna, and it still didn't receive anything. The tuning meter has no action on AM, but works fine on FM.

Just to clarify: this is a mid-70s (1976, I think) set. It doesn't use germanium transistors, and only uses the modern black-plastic half-moon shaped types. However, I think there are quite a few glass germanium diodes in the receiver.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Thu 07, 2019 4:37 am 
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Germanium diodes as small signal diodes I don't have a lot of trouble with but I think 1N914 will replace. Both as noted Silicon & Germanium transistors can fail. My tactic Tube, or Transistor is the same. Whist I often use an Oscilloscope as a signal tracer for nasty ones, there is still merit in using a cap of 100pF, or less in series with the Signal Generator @ IF frequency (modulated) & inject signal into the transistor at the start of the IF or each IF stage starting with the last. That should give a quick indication of where the signal has gotten lost.

The fact that the tuning indicator is not working does (without checking the circuit) suggest dead front end, no signal and therefore no AGC voltage to it. Where there are printed circuit boards cracks can appear & dry joints were never discontinued with any new technologies: Still popular :mrgreen:

Marc


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Fri 08, 2019 3:54 am 
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Greetings to 5Y4GT and the Forum:

Look at the voltages around the mixer (TR208). If they look reasonable, try injecting a 455 KHz modulated signal at TR208's collector. You should hear it if the IF and detector stages are good. If you can't hear it, work your way back through the IF toward the detector until you can.

If you do hear a 455 KHz signl injected into TR208's collector, try injecting a 1000 KHz signal into TR208's base with the radio tuned to 1000 KHz. If you don't hear this, tune the Realistic around a bit (or the signal generator) to be sure that you have 455 KHz difference between the L.O. in the radio and the sig gen frequency. If you still can't hear it, change TR208. If you can hear it, the suspect is TR702.

Good Luck,

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Sun 10, 2019 6:30 pm 
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Thanks for all the help, guys! I will measure voltages, try injecting a signal, and report back. I assume that I measure voltages from the point being measured to B-, which is circuit ground in this case, unless otherwise specified or I am measuring AC.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Sun 10, 2019 7:58 pm 
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You are correct. The voltages expected are DC and expected readings are found in the boxes by each transistor element. For example, TR 701 has 1.05V on the base, 11.2V on the collector, and .4Von the emitter.

If you suspect one of the transistors is bad, you can use the diode function on the multi meter to test. On an NPN transistor you put the + lead on the base and the - lead on either of the elements. Expect to see a voltage between .5 to .7 volts. If not, then there is a bad transistor. Most of the time, in circuit testing is fine as long as the radio is powered off.

Here is a little info on transistors.

https://www.watelectronics.com/difference-between-npn-and-pnp-transistor/

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Sun 10, 2019 11:18 pm 
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Do not over think this. Try, as has been suggested by more than one , feeding in modulated IF signal. The detector diode (as an example, may have failed), or the switch. Therefore signal into the last IF would not get through (keep the level down). If you are not sure about this; step back, get a working radio & try the process on it to get a feel on how it all goes.

You can calibrate the IF, should it be working? I say that as it is never unusual when a radio dies, for some Monkey to move on anything that looks like an adjustment & moves. So one looks for signs of tampering. Fatal damage can be done breaking a ferrite slug in a transistor set IF.

Dead radio's & dead stages should not be treated as frustrations: These are the ones sent to educate you.

Marc


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 1:58 am 
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Don't worry Marc, I am listening to everyone's advice here. I just want to make sure I know how to do everything so that I don't screw anything up, seeing that this is my first time working on solid state equipment. I haven't done anything to the radio since the second radio test because I haven't had much time. I'm feeling a bit tired right now, so I'm going to hold off on working on the radio until tomorrow. It seems I often break or ruin something when I'm tired and not thinking clearly. Also, thanks for the article on transistors, Tony! It's been helpful in understanding the overall function of solid state equipment compared to tube equipment. Lastly, thanks everyone for the help you have given me so far! I hope to report back tomorrow with some better results.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 5:57 am 
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This is rather why I suggested getting a working solid state radio that works, to play with. By following the instructions given & injecting into the base / antenna circuit, using again a pF cap in series you can get an idea of the methodology needed to find where the other one has gone awry.

You may not get much of a response using IF frequency at the front (antenna coil). First priority is to get an IF signal through.

Marc


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 4:52 pm 
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Well, I'm starting to think the schematic I'm using is wrong. First of all, I was only getting about 40 milivolts on both the base and emitter of TR208. When I went to trace the base of the transistor on the schematic, I found it went to point D on the selector switch. However, when I tried to locate the components connected to it, they all had numbers starting in the 700s, like C719 and R719. The highest numbers the components on my chassis have are in the 600s, so I might just have to trace the circuit board out by eye and figure out what it causing the lack of voltage on TR208.

EDIT: Yep, just realized I've been using the schematic for the Longwave version of the receiver. Mine only has AM and FM.

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 5:39 pm 
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Your receiver was called the STA-77A, I think. The schematic is available online, or I can send it to you if you need it.


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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 5:57 pm 
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VideoJunkyard wrote:
Your receiver was called the STA-77A, I think. The schematic is available online, or I can send it to you if you need it.


I uploaded it to docdroid for you. :D A nice hi-res schematic.

https://www.docdroid.net/KRPPm4B/hfe-realistic-sta-77a-schematic.pdf

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 Post subject: Re: Realistic STA-77 Receiver Dead AM Band
PostPosted: Nov Mon 11, 2019 6:19 pm 
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Thanks! I also made an account at HifiEngine, and was able to download the schematic there. I'm only getting about 40 milivolts on the base and emitter of TR208, so I'm assuming that the low voltage on the emitter is due to the lack of current on the base of the transistor. I know that voltage and current are two different things, but I would expect that the full amount of current is not on the base of the transistor since the voltage is low. I measured the voltage on R206, and it measured 40 milivolts on the side connected to TR208's base, and 1.56V on the opposite side. The odd thing is that the resistor measures 16K ohms in circuit, so why would the resistor let less current through if it went down in value? (It was originally 68K). I'll measure it out of circuit to confirm it has gone out of tolerance. I also checked C261, and it doesn't appear to be shorted. Do I possibly have a short to ground somewhere, or a possible bad transistor?

EDIT: Yep, the base is shorted to ground somewhere. I'll have to start pulling stuff until the short goes away to see where it is.

EDIT #2: Well, my multimeter beeps with the continuity tester for a few seconds, then stops. When I measure the point where the meter sees continuity to ground with the ohms scale, I get around 1.7K ohms. Is there a short here, or is this just C 261 screwing around with my meter readings as it charges and discharges?

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