RCA AR- 8. 8 Series Receivers. High B+ on filter capacitors and audio output transformer when the. TRANS position - This is because the FUNCTION switch. B+ at the output of the triple filter of the power supply.
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Reducing the load on the B+ while in . Although there are bleeder resistors in the circuit their load. K A- B resistors have drifted quite high in value so they no longer. Some. overseas users.
B+ excursion is high enough to cause problems in the. I tested my original CR- 9. B+. at the first filter capacitor was +3. REC MOD and measured +4. TRANS (our AC line is 1.
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This is about 1. 00vdc of . It might be. possible that the 2. This may account for. B+ in some overseas locations where the house voltage. RCA rated the 2. 40vac primary to be able to go up to. Normal setting in the USA would be on.
One note also. switching transients when actively using the . All of that being said, it's probably a good idea to avoid.
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Safe operation of the AR- 8. T/R switch if your primary mode of operation is CW. This would have the. REC. CW position all the time with the T/R switch protecting the receiver. Electronic T- R switches were designed for . AM operation using an. T- R switch isn't really practical because of the audio feedback.
Remote muting can easily be accomplished and is detailed in the. Audio Output Transformer failure when using speakers other than the. Z ohm speaker - This is nonsense. An. important part of the impedance calculation involves frequency. So what. frequency is standard for speaker calculations? Well, actually the.
Suffice it to. say that speaker impedance is . All of that being said, the manual. Z ohms as the desired speaker nominal impedance.
Note that. when operating on Hi- Z earphones the phone jack connects a 5 ohm 4 watt. Apparently RCA thought that 5 DC ohms was a close enough match. AF transformer. Using a 4. Z ohm speaker is perfectly. If you can find a 3.
Z ohm speaker (fairly easy) you will find. AF gain control doesn't have to be advanced quite as much as. Z speakers. Although I have operated AR- 8. Z speakers and haven't had any trouble, I think the 3. Z. speakers work very well with the receivers and are worth looking for. One thing that is important relates to receiver operation exclusively on.
Hi- Z headphones (with no loudspeaker connected,) in which case the operator must be sure that the. This is a. two- position jack and when the 'phone plug is .
Half- way in was to allow the Hi- Z 'phones to. Z ohm speaker winding to allow both. NOTE: The first 3. AR- 8. 8LF receivers have a different audio output transformer that has a.
Z and 2. 0 Z ohm outputs. Audio Output. Fixed Bias Operation - The cathode of the 6. K6 AF Output tube is. CT of the B+ winding of the power transformer with resistors. Fixed bias. allows the AR- 8.
K6 to develop more power with better audio. The other option would have been.
This was the economical way to bias an audio amplifier and. However. there are dangers when operating fixed bias. If, for some reason, the. This can rapidly cause damage to the tube and sometimes.
Some AR- 8. 8 users advocate adding a 1. Theoretically, bias failure is possible. BUT, the resistors used in the bias voltage dividers are high rated.
Failure of this circuit is rare. However, if you are a. Most of the time. If a problem. developed it might be several hours before anyone noticed and the. In amateur use today, a. Insufficient BFO. Injection - Most versions of the AR- 8.
BFO into the grid circuit of the 3rd IF. This adds some gain to the BFO at the detector stage and.
But, along came. SSB which required a higher ratio of BFO injection to the signal level. BFO to substitute for the signal's missing carrier and. The normal ham modification approach added a. BFO. injection which seemed to give better results on SSB. However, it's much. AVC by switching to MAN, increase the AF gain to maximum and the reduce the RF.
Gain to a level where the signal to BFO ratio is correct. SSB. This then allows the receiver to. CW reception. However, listening to a SSB net can. RF gain for each net participant. If you don't do CW, then maybe. BFO injection makes sense. Incidentally, the final versions of.
AR- 8. 8 series, the CR- 8. SC- 8. 8 uses a fixed 1. BFO. Remove Negative Feedback for. This originally was done by. The audio response. AR- 8. 8 to provide relatively low. Using a close match to the 2.
Z audio output. probably does more to improve the audio than removing the negative. I have a feeling that this modification.
When the AR- 8. 8 receivers. VERY LOUD. Normal position of the AF GAIN control is about 2.
Z speaker. Reduce Value of RF Gain. Control Divider to Improve Sensitivity - The RF Gain.
K ohm potentiometer connected to the negative bias. RF Gain pot. The value of this resistor. RF Gain control and therefore controlled the maximum sensitivity of the. This resistor value was usually set to allow about - 2 volts. RF Gain control (minimum R of the. Some users feel that a bit more sensitivity can be had by lowering. R to ground and thereby reducing the minimum.
Generally, most users feel that around 2. K is a good. value. This will reduce the minimum bias to about - 1.
Before. changing the value of the resistor be sure to check its actual value as. K to 6. 8. K called out by the receiver design. If. the resistor value is within the proper range and you want to somewhat. K This is an easy. K. The. results are somewhat subjective but its an easy mod to remove if the. Tube Substitutions - There is a subtle difference between the metal 6.
SA7 (used as the mixer. AR- 8. 8) and the glass 6. SA7. GT tubes. Some users believe that the.
This is easy to try since it is just a. I have only found this substitution in one. Expect results from subtle to no apparent improvement at. Many users report better performance with all NOS RCA tubes. Certainly, a complete set of NOS JAN tubes will significantly. Miniature Tube Modifications - .
CQ magazine, notorious for publishing articles that advocated the. Lee, February 1. 95. The article details. CR- 8. 8A receiver that involved the replacement of. Most. of the article details replacing the 6. SG7 1st RF amplifier tube with a 6. BK7. A dual triode tube, claiming a.
Also, the article continues with the BFO change to a. BE6, acting somewhat like a product detector. Replacement. of the Mixer tube with a 6. BA7 nine- pin tube (it was the Mixer tube used. Collins 7. 5A- 4 so.
Additionally, all of the remaining 6. SG7. tubes were replaced with 6.
BA6 tubes. The article concludes with the. Venezuela was now installing CR- 8. As that. incorporated.
RF modification. The article is reminiscent of post- L. E. Geisler's. Hammarlund Super- Pro . Extensive modification.