Trying to repair an Onkyo T-25 Tuner - Page 1
Forum section: electronics / repair

author: Niklas B.
author_rank: Contributor
timestamp: November 21, 2024, 03:24:36 pm
content: I got an onkyo T-25 on my bench right now. It doesnt output any Audio on FM, i can't test AM since there are no Stations up anymoreI unfortunatly have 0 experience with FM Tuners, but there are some test points and one of the readings im getting seems wrongI have VT with about 2.9V thatchanges with the selected FM frequency marked in Greenand about 4.5V on the FM Osc out marked in blueI had a 10MHz sine wave on FM Osc out once but i never got that to reappearThe Ic marked in Pink is the FM Demodulator wich has only a 60kHz saw tooth on pin 16https://www.alldatasheet.net/datasheet-pdf/view/130601/HITACHI/HA1196.htmlThe Ic in red i can not find information aboutThe Ic marked in white is a quad bilateral switchhttps://www.alldatasheet.net/datasheet-pdf/view/130601/HITACHI/HA1196.htmlThe Ic in yellow is the Controller and a prescaler for the FM/AM Tunerthe Prescaler has 4.5V DC on the input, a 6.25MHz sine on the output and a 3.03MHz on the PSC pinThe Controllers FM pin has about 1V, but i dont have precise enouth equipment to measure he output, but concidering the VT test pin and the diagramm in the datasheet, i think this Ic is working as intendedhttps://www.alldatasheet.com/datasheet-pdf/view/125921/NEC/D1703C.htmlhttps://www.datasheet4u.com/datasheet-pdf/NEC/B553AC/pdf.php?id=613360I have no idea if the FM Tuner module needs an additional input or if its just borkedI´d appreciate any advice on how to proceed, since i have no idea on how to check an FM tuner module (i´d also like to avoid desoldering the module)

author: Phaseseeker
author_rank: Regular Contributor
timestamp: November 21, 2024, 04:20:34 pm
content: I'm having a bit of trouble understanding how the whole thing is supposed to work without having a schematic, but going from the datasheets you should have a (square?) wave on the prescaler input, with a frequency 16-17x times the frequency on the output; this is your local oscillator for the FM receiver, without which you wouldn't get any audio out.A DC tuning voltage changing with the selected frequency should be the right behavior, no idea which voltage should correspond to which frequency though

author: Niklas B.
author_rank: Contributor
timestamp: November 21, 2024, 04:34:31 pm
content: If you look at page 9 of the datasheet of the Controller, the prescalers output is into the Controllerso I´m expecting a signal into the controller, not out of itI´m expecting something inside the FM Tuner module (the metal can) to be broken, but i really have no idea where to start looking in there

author: floobydust
author_rank: Super Contributor
timestamp: November 21, 2024, 06:00:46 pm
content: I don't know this tuner. The frequency display works? Any audio? Even on AM should be noise at least.You can add the prefix uPD or uPC to NEC IC's part markings to better search for them.I would first check the power supply voltages are OK: +5V, -5V, +12V, +26V etc.I second look at the synthesiser PLL loop, that frequency is reasonable. You command say 88.5MHz but it can't go there. Varactor diodes acting up or the prescaler not working.Prescaler 6.25MHz *17 = 106.25MHz or *16 is 100.0MHz. What did you dial in? That seems OK, and I would expect audio hiss unless muting is on.Look for other tuners using the same basic chipset likeDual CT1450to get an idea of how it's done.You can look at uPD1701 datasheets which is same as the uPD1703 but without the clock feature.Some other IC info I dug out:Q402 - NEC uPD1703C, Synthesiser and clockQ401 -UPB553Prescaler divide by 16 or 17, but I can't see full p/nQ103 - NEC uPC1167C2, FM IF amp and demodulator+AFC+AGC. See alsoHA11225,Mitsubishi M51173PQ201 - HITACHI HA1196, FM Stereo demodulator PLLQ202 - TOSHIBA TC4066BPQ301 - SANYO LA1240, AM radio ICQ104 - SANYO LB1426, VU meter driverQ143 - NEC D4011BC (CD4011B)Q410 - NEC D4035C (could be CD4035)I would stay out of the can for now, all that is in there is a RF amp MOSFET and RF oscillator.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 21, 2024, 06:11:48 pm
content: sv manual on ebay or some sites like this who ask $$$https://www.servicemanuals.net/en/ONKYO-T25-Service-Manualfreehttps://audiocircuit.dk/onkyo/?ee=1&eeListID=1&ee=1&eePage=6check for the T22 and T33 they could share some designs and signal routing, attached here

author: Niklas B.
author_rank: Contributor
timestamp: November 21, 2024, 06:45:39 pm
content: After thinking about it for another second, it did make no sense that the prescaler had a stable output, i checked again and found that the FM OSC did have a signal, just way more faint that I expectedtuner set to 92.2MHz btw so i dont know if that FM OSC frequency is correct at allI´ll move on to checking Q103

author: Niklas B.
author_rank: Contributor
timestamp: November 21, 2024, 08:30:53 pm
content: I think i got it figured outThe Ic Q201 is getting the audio input and voltage rails, but is not outputting anythingthere is also no mute input or anything, so its just not working as far as i can tell

author: floobydust
author_rank: Super Contributor
timestamp: November 21, 2024, 08:42:14 pm
content: I don't see anything that would cause no audio output unless... it's something downstream after the IC that is shorted like a CMOS switch or MUX.DCV on the input and output pins would also be something to check.

author: Niklas B.
author_rank: Contributor
timestamp: November 21, 2024, 09:38:24 pm
content: yeah the input has the signal offset by about 3Vthe preamp output on pin 3 has an offset of about 7V and the output pins are both at about 8-9V

author: floobydust
author_rank: Super Contributor
timestamp: November 21, 2024, 10:07:16 pm
content: It looks like the chip is not working. I don't have suggestions for a decent source of them.If the FM Stereo lamp ever shorts or draws too much current (filament breaks over) it can damage the IC. I would check that too.

author: Niklas B.
author_rank: Contributor
timestamp: November 21, 2024, 11:34:40 pm
content: yeah i´ll have to look into where to get one, and the indicator lamp isnt even a lamp, but an led lol

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: November 22, 2024, 12:06:16 am
content: If there is a tuning meter or indicator this can be used to determine if the front end (tuner) is working if not then scope the MPX ic next for signals and audio in/out and work backwards to the tuner.Jeff

author: Niklas B.
author_rank: Contributor
timestamp: November 22, 2024, 05:16:42 pm
content: Yeah i´ve figured out where the audio signal is supposed to come from and where the FM signal goesthey intersect in the Q201 Ic wich isn't doing anything so thats my current suspect