Denon TU-680NAB Tuner Repair - Page 1
Forum section: electronics / repair

author: dkmthecook
author_rank: Newbie
timestamp: December 15, 2015, 10:24:52 pm
content: Hello. I am new to this forum as well as watching the EEVBlog videos, which I just love. I thought I would ask for a little help. I am a radio station engineer, for over 20 years now, and being in radio I get to work on some old stuff (radio station owners are notoriously cheap). I have a Denon TU-680NAB AM/FM tuner that is defunct. I have trouble shot the thing down to the point where I am positive that the main Toshiba microcontroller is bad. It is a TMP47P870N. I called Toshiba and all the people they suggested to see if I could find another of these that was programmed for this radio. No luck. One place had them listed for $90 US but they said they no longer had access to them. Since Dave and so many others on this forum somehow find parts that seem to be made out of unobtanium on a regular basis can anyone suggest where I might get one of these or if there is some way I might be able to get a blank one, read the data off the old one, presuming it still will do that. The data sheet I have says this:"The 47P870 is the OTP microcontroller with 64 Kbits of EPROM. For program operation, the programming is achieved by using with EPROM programmer (TMM2764D type) and adapter socket (BM1107A). The function of this device is exactly same the 47C670/870"That is complete with jenglish. I checked and the programmer is more or less a universal but the adapter is for 64 pin ICs.Here is the link to Toshibas data sheet.http://pdf.datasheetcatalog.com/datasheets/700/315381_DS.pdfSince this receiver is about the only one that has a good enough AM, yes AM, front end to be useful as my off air monitor I really would like to get it fixed. Any help would be appreciated.

author: mzacharias
author_rank: Frequent Contributor
timestamp: December 15, 2015, 10:27:36 pm
content: Front-end related problems are more common on this type of gear. PLL chips and the like. What are the actual symptoms?

author: dkmthecook
author_rank: Newbie
timestamp: December 15, 2015, 10:41:36 pm
content: The symptoms, as far as visible, are that only a couple leds light up on the front panel and those are dim. There is no display and none of the buttons do anything. The only thing I could find is a bad voltage at the reset pin, #21. It is supposed to be 5.4 and it is about 2.2 (I have an actual service manual for this thing). I checked the transistors and diodes around it, out of circuit, but it still stays right there. All the VCO voltages and other important stuff comes out of this chip so there is nothing I can check as far as the radio part goes.

author: dkmthecook
author_rank: Newbie
timestamp: December 15, 2015, 10:53:24 pm
content: Here is the schematic for this thing:The LEDs that light up are Bandwidth Wide and Narrow and NR/NB ON. That is about all it does.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 16, 2015, 04:51:25 am
content: Is the reset pin (pin 21) still low when you remove TR502 and the 1uF capacitor (is it C303? Pixely schematic...) from the circuit?

author: dkmthecook
author_rank: Newbie
timestamp: December 16, 2015, 08:09:47 pm
content: Ok here is what I found. If I remove TR 502 and C 503 the Reset pin goes high. The Hold pin stays at 3.4 . If I cut the traces going to reset and hold and then short out reset and hold (force hold high in other words) the whole receiver works. The audio is awful but it tunes, changes bands, bandwidth and everything. TR 502 still is about 1/2 turned on. This is because there is 1.2 volts on the collector of TR 605 with .685 volts on the base (shouldn't the transistor be turned on and no voltage on the collector since the emitter is grounded?). Because there is 1.2 volts on the collector of TR 605 there is .357 volts on the base of TR 606 and 3.4 volts on the collector going directly to HOLD on the IC.I am a little confused.

author: dkmthecook
author_rank: Newbie
timestamp: December 22, 2015, 03:07:09 pm
content: OK. I give up. I just can't find what is wrong with this thing so I cut the traces to both reset and hold and tied them both high. I also cut the emitter of TR312 to stop the half on condition that put a DC voltage on the line output. The whole thing works fine and has been running since last Thursday. I am going to call it "repaired" and move on. If anyone else has any ideas please let me know. I don't like giving up but this one got me. I is probably something simple but I sure can't find it.