Repair old Hurst tachometer - Page 1
Forum section: electronics / repair

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 21, 2022, 02:58:22 pm
content: Hi!Yesterday a guy came in with an old Hurst, two digit tacho meter from the mid 70's. No sign of light from the display he said that it smells burnt from one of the circuit boards. At a quick look I found it to be a small dc/dc converter because the display board has a two digit VFD. A tantalum electrolyte was found blown.At first I did not see that the transistor also was blown so I only swapped out the tantalum with a radial 47uF/25v which was part of the incoming low pass filter. Hooked up the tacho meter to my bench psu and limited the current to 100 mA and the current draw was found to be no more than 60 mA. The display did not lite up and accidentally, I took this board in my hand and I can promise you that there was quite a high voltage that measured 220 volts. I do not know why the dc/dc converter delivered such a high voltage thinking of the blown transistor.Anyone recognise this transistor? Marking is partly gone so I would say that it could be really hard to see.

author: fzabkar
author_rank: Super Contributor
timestamp: September 21, 2022, 07:41:14 pm
content: Could we see photos of the tacho PCBs?What are the 3 characters on the bottom row? EBC (emitter-base-collector)?

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 21, 2022, 07:54:20 pm
content: fzabkar, yes of course! I am not in my workshop at the moment, but I will take some photos tomorrow morning.

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 21, 2022, 08:07:47 pm
content: Yes fzabkar, it is EBC and a bipolar transistor.

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 22, 2022, 09:52:49 am
content: Here are som pictures of both PCB's - dc/dc converter and display board. I will make a schematic for the dc/dc converter as soon as possible; not that I have to, but just for fun.

author: Ranayna
author_rank: Super Contributor
timestamp: September 22, 2022, 01:53:55 pm
content: I can't help, sorry, but i have to appreciate the looks of that last image. With the pristine white/gold DIP package and the carbon resistors... So colorful

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 22, 2022, 03:03:16 pm
content: I agree fully Ranayna and no doubt that SMD components has no soul!

author: floobydust
author_rank: Super Contributor
timestamp: September 22, 2022, 03:42:56 pm
content: That's a rare collectable. In 1973-1977 Oldsmobile '442/F85 Hurst/Olds Digital Tach option WW-8, around 1,200 built per year I think. A nice big V-8 455ci. musclecar in Sweden?It's not a VFD, it is a Sperry PanaplexSP-352neon display which run off around 180VDC. There is a keep-alive cathode that should have a faint glow.Beckman had DD-700 and DD-702 IC's to drive them but 16 pin DIP. That looks like a custom IC MCC1510 it must need 5V or say 9V power?I would check the low voltage power for the big IC, where it comes from. The tantalum and electrolytic capacitors are surely old and need replacing.

author: Gyro
author_rank: Super Contributor
timestamp: September 22, 2022, 04:44:52 pm
content: Are you sure it's the transistor that has failed? That package looks as if it has had metalic material splattered onto it from the outside, the prime suspect would be the green Tantalum bead capacitor. Take a close look for any suspicious looking pin holes on that side of the bead.That's not to say that it might not have taken out the transistor when it failed, but maybe not. It's worth a closer inspection anyway.

author: floobydust
author_rank: Super Contributor
timestamp: September 22, 2022, 06:08:33 pm
content: I was going to say the Panaplex display looks kinda burnt, as if it was stuck displaying 11's or something. I did see them here surplus for salehttps://www.surplussales.com/Bulbs-Incan-Panel/VacDisplay.htmlThe tach electronics get beat up with transients on vehicle 12V power, as well as the ignition coil signal can be over 500Vpk and the clamp/filter circuit hopefully is good enough to protect the IC.

author: fzabkar
author_rank: Super Contributor
timestamp: September 22, 2022, 07:03:42 pm
content: Quote from: floobydust on September 22, 2022, 06:08:33 pmI was wondering whether those two TO92 transistors above the IC were handling the pulse shaping from the distributor contact points.

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 22, 2022, 10:58:05 pm
content: Thanks Gyro!You are probably right and guess that this happened when the Tantalum capacitor exploded because it is next to the transistor. Besides, the dc/dc converter would not deliver 220 volts if the transistor was blown; with other words: i did not think clear!

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 22, 2022, 11:05:53 pm
content: Thanks floobydust for all the very valuable information about this display that I thought was a VFD! Thanks also for the link to the surplus sales!As a matter of fact I have never come across a display like this and never heard of it before and I have been into electronics for nearly 35 years. I may have, but they where probably not at fault. I understood that the owner valuated this tacho meter a lot because he was very eager to have it repaired. I know that he has a real muscle car, but I will have to ask him what kind it is and for your question: yes, there are plenty of these cars in Sweden.I will have a look at the tacho meter and check all voltages on the display board tomorrow Friday. It seems that the only voltages coming from the dc/dc converter is 12 and 220. The 220 volts I measured may be that high because there is no load. I also saw that the display was burnt and hopefully it is not dead. I tried to google the MCC1510, but to no avail and probably, as you wrote, a custom IC.Yes, you get pretty high peaks on the primary side of the ignition coil when the field collapses. I have been teaching automotive electronics at an upper secondary school, but I retired mid August this year. The rpm pulses enter the dc/dc converter board through a ceramic capacitor followed by a resistor and a diode, sort of clamped to B+, and then one more resistor before it enters the display pcb.

author: fzabkar
author_rank: Super Contributor
timestamp: September 23, 2022, 12:56:39 am
content: I tried to find the MCC1510 in MOS Technology's old databooks, but no luck.https://mirrorservice.org/sites/www.bitsavers.org/components/mosTechnology/The "MCC" prefix doesn't seem to exist in MOS Technology's standard product range, so maybe that denotes a special order. "MC" is MOS Technology's ceramic packaged device (C is the package designator).I guess you could monitor the digit and segment driver outputs and test whether they are consistent with a "00" display. That would give you some confidence in the IC.

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 23, 2022, 04:41:30 am
content: I may have one display at home (I need to find it at first, check the type, test and de-solder). It had worked when I checked it several years ago. But since it works like Nixie tube, you can try to revive ithttp://www.tube-tester.com/sites/nixie/different/cathode%20poisoning/cathode-poisoning.htm- I'm not sure where the deposits are in your case. It can be that the deposits are on the outer lens.

author: CaptDon
author_rank: Super Contributor
timestamp: September 23, 2022, 02:14:22 pm
content: That neon plasma display is absolutely toast. I worked with them in pinball machines in the late 70's and early 80's. I think both Bally and Williams used them. Bally for sure used the 6 digit versions. When you see those blackened areas you know the display is going to look poor. The character shape won't be 7 segment lines but more like a dot or missing all together. If the rest of the unit checks out you can easily replace the display with a good solder sucker and TLC. Best wishes!! (I think the 2 digit versions were used to show 'credits' on the pinball machines)

author: TheMG
author_rank: Frequent Contributor
timestamp: September 24, 2022, 03:07:22 pm
content: Quote from: floobydust on September 22, 2022, 03:42:56 pmNote that the keep-alive cathode is not necessarily always used. Most often it's used when more than one display module is being multiplexed but in the case of a single module it's usually not used from what I've seen.

author: floobydust
author_rank: Super Contributor
timestamp: September 24, 2022, 09:02:44 pm
content: Apparently the displays have a keep-alive cathode for each digit, so there are two of them and 50uA each. I'm used to seeing them glow when working on machines, to see if HV power is present. There would be one or two high value resistors feeding them, they are helpful in cold weather, night-time.HeathkitGC-1005digital clock also used the Sperry SP-352 displays but did not use the keep-alives.HeathkitGC-1093is a car clock using Panaplex display (not the SP-352) and fed one keep-alive with a -ve voltage.

author: Gyro
author_rank: Super Contributor
timestamp: September 24, 2022, 09:31:33 pm
content: According to the Sperry datasheet, "a keep-alive cathode provides an internal ionization source that reduces reionization time to less than 30us, allows zero supression, and improves the operation of the display in dark environments and at low temperatures". I'm pretty sure I read in a later Beckman datasheet that they are only needed to provide predictable operation in multiplexed displays (with blanking intervals).On early Datron DMMs that use these same displays, the keep-alive cathodes are only used in the multiplexed display models (1051,1055,1057,1059), the static drive ones (1030,1041,1045) don't connect them.Once the display has segments lit, the keep-alive cathodes become redundant, it's hard to say whetherthey areit is used / needed in the OP's unit (it looks static drive). Itwillstart up at low temperatures and in the dark, just not instantly [Edit: It wouldn't anyway, the keep-alive is just another cathode].

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 25, 2022, 08:57:39 pm
content: Mine had been produced by Beckman. They are placed in the sockets.

author: 4cx10000
author_rank: Regular Contributor
timestamp: September 25, 2022, 09:25:40 pm
content: Thanks all for great information on the Panaplex display!Have not done so much because of other, more or less, urgent repairs. Since this guy uses his car only during summer time, he said that there is no need to stress with the repair. I just wanted to say that I am most grateful for all the help and rest assured that I will be back as soon as anything has been done on this tacho meter.

author: james_s
author_rank: Super Contributor
timestamp: September 25, 2022, 11:50:50 pm
content: Quote from: CaptDon on September 23, 2022, 02:14:22 pmIt looks worn, but I wouldn't go so far as to say it's definitely toast. The segments behind the blackened glass will be dimmer but I would bet the display will still work. I have revived similar displays where only part of many of the segments would illuminate, I did it by driving the segments at several times their rated current for anywhere from several minutes to a couple of hours. Nixie tubes can suffer from similar cathode poisoning.

author: Gyro
author_rank: Super Contributor
timestamp: September 26, 2022, 06:22:24 pm
content: You need to be really careful not to overdo that treatment. The reviving is achieved by sputtering off cathode surface material, which has to go somewhere - onto the other cathodes and insulation in the case of Nixies and additionally, the front glass of a Panaplex. It can certainly work but I never go above maximum specified current, or a higher current is bursts of a few seconds, and cycle around the cathodes to ensure that they also remain clean. The first thing I do with any Panaplex containing equipment is turn down the cathode currents to the minimum specified, they last a lot longer then.

author: 4cx10000
author_rank: Regular Contributor
timestamp: October 13, 2022, 11:10:44 am
content: Finally the Hurst tacho meter is fixed, at least the display, and thanks to DavidKo that sent me a working SP-352.Indeed a very nice display! Now I have to check if it counts RPM.

author: fzabkar
author_rank: Super Contributor
timestamp: October 13, 2022, 04:33:11 pm
content: If you need a signal source, and if your computer has a legacy COM port, then you can transmit an appropriate data pattern at 300bps. The output swings between +12V and -12V. You may need a clamping diode to protect the tacho input from negative transitions.You can create a large binary file with a hex editor such as HxD.

author: james_s
author_rank: Super Contributor
timestamp: October 13, 2022, 04:39:00 pm
content: That looks really good. Given how badly the original display aged I would consider modifying it to drop the current a bit, you might be able to get away with dropping the anode voltage depending on how high it is to begin with. These things often ran decades 24/7 so in a car it should last indefinitely.

author: DavidKo
author_rank: Frequent Contributor
timestamp: October 19, 2022, 09:07:23 am
content: One additional idea. Maybe you can replace electrolytic capacitors with MLC to prevent failure due to the heat in the car in the summer.