 Porsche 914-6 tachometer repair (914 641 301 10), sharing helpful information to simplify tachometer repair |
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| 914Gelb |
 Jan 13 2026, 03:55 AM
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#1
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Member  Group: Members Posts: 105 Joined: 7-July 11 From: Vienna Member No.: 13,288 Region Association: Austria  |
Hello 914 enthusiasts!
My tachometer of my 914-6 wasn't working. I decided to repair it by my own. I don't like to explain how to open ore how to remove the needle, this can already be found in other threads. I just like to share the circuit diagram and how I found the defect device (transistor). 3 Transistors are used in the device, 2 of them(BCZ10) are used for a monostable multivibrator. The very smal input puls from termial "-1" is used to trigger the monostable multvibrator. The smal puls time is extended and goes to terminal "Inst" (instrument). Transistor 3 (AC128) is used for voltage regulation. I added the circuit diagram I made. The wires for the Transistors are colored (R=red, G=green, S=black). Without any test bench and oscilloscope the simpliest way is to check the transistors via multimeter. I disconnected the base of the tranistor and measured the break voltages beteween base-emitter and base-collector. for BCZ10 the break voltage is arround 0,6V and for AC128 arround 0,2V (germanium). In my case on Transistor (BCZ10) was broken. I replaced both BCZ10 with BC177B which is still available. This solved the issue, tachometer is working again!    [attachmentid=964368]    Attached image(s) 
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Replies(1 - 2)
| Superhawk996 |
Jan 13 2026, 06:49 AM
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#2
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914 Guru Group: Members Posts: 7,625 Joined: 25-August 18 From: Woods of N. Idaho Member No.: 22,428 Region Association: Galt's Gulch 
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(IMG:style_emoticons/default/smilie_pokal.gif) thanks for the write up and schematic. Good job fixing your tach.
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| rgalla9146 |
Jan 13 2026, 07:02 AM
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#3
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Advanced Member Group: Members Posts: 4,906 Joined: 23-November 05 From: Paramus NJ Member No.: 5,176 Region Association: None
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QUOTE(914Gelb @ Jan 13 2026, 04:55 AM)  Hello 914 enthusiasts! My tachometer of my 914-6 wasn't working. I decided to repair it by my own. I don't like to explain how to open ore how to remove the needle, this can already be found in other threads. I just like to share the circuit diagram and how I found the defect device (transistor). 3 Transistors are used in the device, 2 of them(BCZ10) are used for a monostable multivibrator. The very smal input puls from termial "-1" is used to trigger the monostable multvibrator. The smal puls time is extended and goes to terminal "Inst" (instrument). Transistor 3 (AC128) is used for voltage regulation. I added the circuit diagram I made. The wires for the Transistors are colored (R=red, G=green, S=black). Without any test bench and oscilloscope the simpliest way is to check the transistors via multimeter. I disconnected the base of the tranistor and measured the break voltages beteween base-emitter and base-collector. for BCZ10 the break voltage is arround 0,6V and for AC128 arround 0,2V (germanium). In my case on Transistor (BCZ10) was broken. I replaced both BCZ10 with BC177B which is still available. This solved the issue, tachometer is working again! [attachmentid=964368] Wow ! Thank you for sharing. Where there's a will... |
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