Help with Realistic QTA-720

[G.Wess]

Bought one of these just to hear what Q-vox sounded like and found the front right channel
either squeals or just hums depnding on other settings. But no sound on that channel. Good Stereo sound through rear speakers with headphones and speakers. Anyone able to help point me to a service maunal or a board layout/schematic.

If/when I get this solved, then have an STA-77A with distorted right channel to fix.

Thanks in advance-
Gale W.

 

[Lavane]

If your getting good sound from the B speaker terminals but not the A I would be looking at switches. A and B are both tied to the same amplifier outputs. So if your getting normal sounding stereo from the B or rear I would look at the A speaker switch first.

 

[G.Wess]

Good thought and I will check back through to the switch. Should clarify original post where is said "no sound" on the front right. Should say "no audio". There is plenty of sound but it's all hum (60 hz) or loud squeal that increase in pitch the longer you let it run.

 

[BruceRPA]

The QTA-720 is a Quad receiver. There are four amplifiers.

 

[G.Wess]

Identifying the amp circuit for front left would be next then. Found service manual for QTA-770, guess it would have a good
hint about 720's layout. Squeals when set to 2-channel, 60hz humj when set to 4-channel, makes me think something shorted
in that amp circuit putting current where it ought not be?

 

[Lavane]

He stated Q-Vox so I figured stereo with the pseudo quad. If it has four amps then there very well may be a problem with the front amp.

 

[G.Wess]

It appears to do both, true quad and Q-vox. May never get to hear the true quad for lack of a source, but the psuedo quad is supposed to be pretty cool effect on 2 channel sources.

 

[G.Wess]

Posting solution for this issue hoping to help next poor bastard. Never found Schematic. But realized that being a quad amplifier unit gave me three good amp cicuits to compare component measurements against the bad amp. Third time through, I finally found a transistor bad with the emitter leg literally snapped loose at the transistor body. Damn thing checked out OK from the bottom with a multimeter, but channel started working while I was pushing around on components from the top side. Could not see it snapped because it was wrapped in a bit of a tin heat sink(?). So Q-vox effect is pretty cool in separating frequencies front-back on the same channel, like very early surround sound. Wish I had some true quad channel sources to check out the unit's decoder operation.

 

[tcdriver]

... Wish I had some true quad channel sources to check out the unit's decoder operation.

Great news that you were able to repair your quadraphonic receiver. A multi-channel SACD player can be an excellent source for vintage quadraphonic receivers.