Advent Model 400 - No Reception

[earache]

Hello

I'm fooling around with an Advent Model 400 mono table radio from the 70's. It is unable to pick up any radio stations, whether there is an antenna wire attached to the screw leads or not. I can switch it to "Aux" and hooking up an external signal source (IPod), and it plays the supplied signal OK.
How to go about troubleshooting this? I am unfamiliar with troubleshooting radios or anything else with an antenna or tuner.

The schematic
Advent_400_radio_schematic_only.pdf

http://www.davidreaton.com/PDFs/Adve...matic_only.pdf

Thanks In Advance!

[soundguruman]

This forum deals with guitar amps and pro sound equipment, mainly.
Not home stereos and radios, for the most part.

You would probably have better luck here, with vintage tube equipment such as McIntosh, or HK Citation...
Because we love our tube equipment.
Solid state home hi fi, not so much. We are pro sound techs.

Pro sound meaning
Equipment that is used on live sound stages and recording studios, by musicians and sound system / recording studio engineers.

However there are forums that are more home hi fi oriented like these:
http://www.diyaudio.com/
http://www.audiokarma.org/forums/sho...d.php?t=537474
http://www.antiqueradios.com/forums/...?f=15&t=210089
http://www.avsforum.com/t/1425373/newbie-question-advent-baby-ii-speakers-repair-or-replace

[Enzo]

Try over at www.DIYAudio.com That forum is all about all manner of audio.

[earache]

Try over at www.DIYAudio.com That forum is all about all manner of audio.

Ok Folks, thanks for the re-direct!

Earache

[Jazz P Bass]

A classic case for the 'Divide to Conquer' troubleshooting technique.

Scope Q5's base for signal.
Check IC 1 & 2 for signal.

You need to ascertain where the signal is dropping out.

.Check the jfets for a signal at the gate.
If it is there, then check that the drain/ source pins are reacting to the gate signal.

[earache]

Thanks Jazz, will do!

[km6xz]

Do you get any hiss from output even if there is no signal? FM has high gain in the IF amplifiers so noise will be seen at a constant amplitude if there is a signal or not. The IF is 10.7 Mhz so your scope should be capable of seeing that in the 10-50 mv p-p level at IC1 and IC2 pin 4. If you do not see that, and you have a wide bandwidth scope, look for local oscillator output at C32. It will be 10.7 + the frequency of the dial, so if tuned to 100Mhz, the local oscillator will be 110.7 Mhz. If nothing there, check Q3. If you have broadband noise at pin 4 of IC2, move on to the ratio detector. The signal at C26 will be recovered audio and not much if no station is tuned in. If all these look OK it might be as simple as someone messing with the alignment of the IF and detector slug tuned RF transformer/detector transformer.

[earache]

This will be the motivation to learn to use the scope I have access to.
I will likely have time this weekend to try what has been suggested.
In the mean time I will download the spec sheets for the JFets.
Thanks for the input!

[km6xz]

The IC IF amplifier chips are more suspect than the rugged FETs. Given the time, they might be uC703 ICs which were not terribly reliable but were used in many products that needed gain bandwidth product over 30Mhz.
The scope is a wonderful tool for visualizing what it going on a circuit, learning a bit about interpreting its display, vertical attenuators and x axis sweep will come in handy in any type of repair or design/building you are interested in.

RF circuits are really nice to trouble shoot because they are pretty precise compared to the flexible tolerances of audio gear. The benefit in that is a help in trouble shooting. If a circuit is supposed to emit a specific power or spec sensitivity, one measurement pins down whether there is a problem or not. If spec is 0.25 microvolts for sensitivity for 20 db SNR+Noise+distortion and it is only measuring 0.35 microvolts for that SNR, it is not working right. Not many audio applications worry about 1/10th of a millionth of a volt so when someone says they gain is down, it is not always easy to determine if that is normal for that particular unit, specs mean almost nothing in audio.

[earache]

Hi all - decided to bail on this project when the owner of the radio (my sister) decided that she would just get another table radio.

I also had trouble getting spec sheets for the IC's, which were round 8 legged beatsies.

Thanks for your comments regardless, I do so very much appreciate them.

[km6xz]

Yeah, the 8 leg metal can ICs are probably uA703 which are not common now and were not terribly bullet proof. Keep around because it will be good practice in troubleshooting and not hard to repair. They were Mono only but still sounded good and easy to convert to stereo with a MC1310P or AN7415 IC and a small handful of parts and 12volts.