Problem with 70s radio/cassette recorder- high pitch sound - Page 1
Forum section: electronics / beginners

author: RiRaRi
author_rank: Regular Contributor
timestamp: March 27, 2025, 11:28:20 pm
content: Hello. I successfully repaired few of my boomboxes but this one is giving me troubles so i am begging for help. This is Sanyo mono radio with cassette recorder from 70s. Problem is when i turn the radio on, or cassette motor (without tape) high pitch sound start to hear. As i turn the volume up, high pitch sound goes up. There is no radio reception at all but there is tiny change in sound as i rotate variable capacitor for radio tuning. All pots and switches has been cleaned. E-caps are still strong, made by Rubycon. Please, take a look at video and let me know if you know where the issue might be. Any help is welcomed. Issue is there when using with batteries and mains.https://www.youtube.com/shorts/_hKaibecGvYNote that i am not actually tuning the radio on the video.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 28, 2025, 09:40:49 am
content: IDK, the pitch sounds remind of microphony. My guess is the boombox might have an electret microphone somewhere, and somehow the microphone is on during normal playback, maybe because of bad contacts in the play/rec switches.

author: Xena E
author_rank: Frequent Contributor
timestamp: March 28, 2025, 06:05:42 pm
content: I am inclined to agree with George, it definitely sounds like audio feedback, caused by defective switching.Try speaking into the microphones and see if it is that... if not, using an insulated tool tap around the circuitry to see if you can find any thing else that is microphonic.X

author: floobydust
author_rank: Super Contributor
timestamp: March 28, 2025, 09:03:59 pm
content: Move many times back and forth by hand, clean with PPE... the RECORD/PLAY switch. It's that longggg switch.You can, door open, holding the REC lockout tab lever up/in, manually press and release the REC button a thousand times, to actuate the switch and maybe clear a bad connection there. You don't need power on to do this. Some decks want the door closed to do this, so you use a junk cassette with the REC lockout tab taped over, in the transport.The REC/PLAY switches very commonly oxidize and can cause oscillation/feedback squeals. The switches rarely get moved, who does recording on these anyway?With the PPE or contact cleaner, don't spray chemicals on the rubber parts.If it's not the REC/PLAY switch then it's an old electrolytic capacitor that has dried up and gone low value.

author: RiRaRi
author_rank: Regular Contributor
timestamp: March 31, 2025, 09:00:21 pm
content: Thank you for helpful answer. I disassembled, cleaned and tested all switches including rec/play. Also, on preamp/amp/power supply board i changed two e-caps which were out of spec. Only two caps were bad. Both were 0.22uF 10V. Every other one including big ones were good (ESR and capacitance). Also, i did diode check on all transistors and they are good. After all of this i noticed a change: things were great with sound and radio reception when MIC is turned off, on button as shown on video. Note that this is unrelated with REC/PLAY switch. This is only button for recording inputs: built-in mic or mix (mic/source). When the switch is on MIC ON, high pitch sound starts to happen (still on play mode, not recording mode). As i think, when rec/play switch is on play all off this should be off by default and built-in mic should be off. In this case, even if recording mode is not activated, built-in microphone is on and working. Please, check the video and let me know what you think. Additional PCB photos are also included.https://youtu.be/Exp4shXV-Hk?si=u2qLeXgncccb_XT5Arrows on photo points to MIC inputs. On this side is plus and on other side are minus and ground.