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 Post subject: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 6:04 am 
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I acquired this 1961 era Tube VHF Hi receiver. It is in very nice shape and seems to mostly work. All the tubes tested good, but it has very weak receive audio. I would expect a clean signal at about -95 dB input, but even at -70dB, the signal is very weak and distorted. Since all the tubes are good, capacitors are next on the list to check. I'm not familiar with oil capacitors or whatever the other ones are with wires around them. None of them actually look like electrolytics, do they all need to go? There is also one multi-pin 'can' capacitor mounted next to the tubes. Where to start? I have not been able to locate a schematic as these seem to be somewhat rare.
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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 7:29 am 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 457
Location: San Jose, Ca.
Hi Brett, Nice and clean. Let us know the tube lineup and it would be easier to give better suggestions. Distortion can be a leaking coupling cap between the audio amp and the final audio control grid. Or, other issues. Does the B+ seem ok?

Regards, Larry


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 12:32 pm 
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Joined: Apr Wed 01, 2020 7:22 pm
Posts: 364
I see a lot of paper in oil Tokai caps that are highly suspect for breakdown and leakage...the caps "with wires wrapped around them..." are tubular ceramics and chances are they are fine...I'd be checking the resistors for drift too...

It is about as pristine as one could hope for and appears to be unmolested...!

I hope a schematic shows up for you...!

73..Jordan...VE6ZT


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 2:06 pm 
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Joined: Mar Sat 27, 2021 3:00 am
Posts: 6
Tubes are:
3 Toshiba 5B (6AU6) Vacuum Pentode RF/IF-Stage
Toshiba 6A (6AQ8) Double Triode
Toshiba 5B (6AQ8) Double Triode
Toshiba 5B (6AL5) Double Diode Detector
Toshiba 2K (6X4) Full-Wave Vacuum Rectifier Power-supply
Toshiba 4J (6AQ5) Beam Power Tube Power/Output 
Toshiba 5B (12AX7A) Double Triode   Audio Frequency

I do suspect audio stage a bit more than the RF stage since with no signal applied, full volume with open squelch is extremely low volume.
I do see a schematic for sale for the VHF Lo model, which shares a lot in common with the VHF Hi model, but there is are some differences.
Transformer output voltages seem correct.


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 3:15 pm 
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Location: Sayreville, NJ 08872
What are you using for an antenna?

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Pete, WA2CWA - "A cluttered desk is a sign of genius"
https://www.manualman.com


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 4:02 pm 
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I have not tried an antenna yet. I'm using a signal generator on 154.740 with an FM modulated tone varying the signal from -100 to -70db.


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 5:34 pm 
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Joined: Apr Wed 01, 2020 7:22 pm
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Is there a Realistic catalog number on the back...? eg. 20-xxxx or something similar...??? I've found manuals and schematics are easier to find for the more rare Realistic gear using the catalog number rather than the model number...

I'd expect that the major differences between the 30/50 and the 148/175 are the single channel crystal control capability of the VHF-Lo version, and some of the tubes used at the higher frequencies in the 148/175 version...Much of the remaining circuit should be about the same or very similar....

73...Jordan...


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sat 21, 2021 7:43 pm 
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No luck on the RS catalog number R94LX597 - From 1962 RS catalog
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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sun 22, 2021 1:21 am 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 457
Location: San Jose, Ca.
Thanks for the tube lineup. Check the voltage on the control grid of the audio output, pin 1 of the 6aq5 with a high impedance vtvm. It needs to be 0 or below. If above, the coupling cap from the previous audio plate is probably leaking. Also check the cathode bias voltage on pin 2, it should be around 5 to 10 volts.

If both stages of the 12ax7 are in series, then the plate to grid coupling cap needs to be checked in the same manner.

After checking those, if the distortion is still present, swap the 6al5, 12ax7 and 6aq5, one at a time.

Regards, Larry

Edit 20:10 If the distortion is still present, you can feed the audio from the volume wiper to a separate high impedance audio amp input and see if it's ok there. Use a .01 mfd cap for coupling.

Regards, Larry


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Aug Sun 22, 2021 6:07 pm 
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Joined: Mar Sat 27, 2021 3:00 am
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Thanks Larry for that very helpful information! That gives me quite a few things to start checking.


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 Post subject: Re: Realistic RP: 148/175 Repair Help
PostPosted: Sep Tue 14, 2021 4:40 am 
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Joined: Mar Sat 27, 2021 3:00 am
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Just as a follow up: after a bit of procrastination, I tested a few of the oil caps and a few were not fully blocking DC. I decided to replace the 5 oil caps with film caps.
I'm happy to report the receiver is working great now.

Thanks for all your help!


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