help with vacuum fluorescent display filament drive on hitachi stereo - Page 1
Forum section: electronics / repair

author: c32a38
author_rank: Newbie
timestamp: June 08, 2025, 12:59:36 am
content: I have two identical Hitachi car stereos from the early 1980s both with the same problem: the vacuum fluorescent display works only intermittently. Seems to be a common problem with this model but I haven't found a solution.I've observed this behavior with both stereos:-Powering the stereo on, only about 10% of the time the display lights up, and the remaining 90% of the time the display stays blank. Regardless of the display, the other functions of the stereo always work when it's powered on. The issue seems specific to the display. When the display is working, all the letters and digits show correctly, so it's not a problem with that controller.-I have not found much of a pattern to what makes the display turn on. I can repeatedly power on and off the stereo and eventually the display will work.-However, if the display is on and then the unit is turned off and on again immediately, the display will continue to work. But wait 30 minutes, and it seems random again.This led me to look at the drive to the VFD filament. On both stereos, the filament always correctly reads around -21V DC when the unit is powered on. But when the display is not functioning, there's no AC voltage across the filament. When the display does happen to turn on, there is AC voltage. The problem seems to be with the circuit that generates the AC.I have included a circuit diagram from the service manual for the radio. From what I can understand, the AC is generated by an oscillator circuit involving the four transistors Q603-Q606. That drives the transformer T601, the other side of which has the -21 V DC offset. I have checked the -21 V supply on the filament side, and the 13.7 V supply to the oscillator circuit (right hand side of R606), both of which always read correctly.I've measured the pins of T601 in the display working and display not working conditions (on both stereos). When the display is working, the readings are consistent with what's indicated in the diagram, and there's AC on both sides of the transformer.However, when the display is not working, there's no AC, and the right hand side of T601 is sitting at 4.8 V instead of 9 V (a larger drop across R606). The DC voltage drop from base to emitter of Q603 and Q605 is ~1 V rather than the 8 V in this condition.My questions:1) Is there a name for the circuit involving the 4 transistors Q603-Q606?2) Does anyone have suggestions for circuit elements that might be at fault?

author: pqass
author_rank: Super Contributor
timestamp: June 08, 2025, 03:58:35 am
content: 1)It looks like aZVS driverexcept using a darlington arrangement vs the usual FETs. See attached andsimulation here.2)If T601 right-side center-tap with respect to GND isn't a stable 9V (is sometimes 4.8V) then check/replace ZD602 (HZ9C Vz=9V) or C615 (220uF/10V).I'd also check "M" cap (to right of T601)It's usually a bad (electrolytic or tantalum) cap that's the problem.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: June 08, 2025, 11:25:44 pm
content: Hitachi/Nissan? or aftermarket, Model number?Solder connections and capacitors very common in OEM/Nissan units.Aftermarket, never saw many.Jeff

author: c32a38
author_rank: Newbie
timestamp: June 09, 2025, 12:16:34 am
content: pqass: Thank you for the very helpful reply.Jeff eelcr: Yes, from the 1981-1983 Datsun 280ZX. One's labeled Hitachi KMS-3601B and the other KMS-3601ZW.I replaced C615 on one of the radios and now the display reliably shows. Will try with the other one. Thanks a lot for the help.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: June 09, 2025, 11:36:57 pm
content: 10V caps fail a lot, replace with 16V 105DEG.Jeff

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: June 09, 2025, 11:44:29 pm
content: Note: When replacing units be very careful with screw lengths, NEVER longer than noted on unit.Jeff