Coffee maker control board - no heat - Page 1
Forum section: electronics / repair

author: bogart219
author_rank: Contributor
timestamp: December 27, 2018, 12:10:54 pm
content: Hello fellows. Hoping someone can point me in the right direction. I have a few control boards from a commercial Bunn cdbcf coffee makers. The only problem with these boards is the heating element is not getting energizerd. There is a power relay on the board for the heating element that is not getting the 12 volts it needs. Something on the board is not letting it happen. Can't figure out if its a optocoupler, triac or something else. I've enclosed some pics .Any help would be greatly appreciated!

author: Armadillo
author_rank: Super Contributor
timestamp: December 27, 2018, 12:23:36 pm
content: Firstly you check Q2, measure the voltage BCE for example.In return, tell us what equipment you have so that we tailor answer towards using those equipment to troubleshoot.

author: bogart219
author_rank: Contributor
timestamp: December 27, 2018, 08:02:45 pm
content: Hello. By equipment I assume you mean test equipment. I have a digital fluke meter among other vom meters, ampmeter, and a 30 volt dc power supply. I just bought a hantek ds05102p oscilloscope, but I have to learn how to use it. Q2 is that smd transitor. As you suggested, Im going to try to find the specks on it (number on hard to read) and try to test it then compare with Q1, then compare with a Q2 on a new board I have. Thank you for your time!

author: Armadillo
author_rank: Super Contributor
timestamp: December 27, 2018, 08:10:08 pm
content: Its quite an arsenal of test equipment you having there. More than sufficient to deal with this board.Yes, it's the SMD transistor Q2 I am talking about.Do you have the whole coffee maker of just the control boards? because there is interlocking water level to deal with? Otherwise need to work on bypassing it.

author: bogart219
author_rank: Contributor
timestamp: December 27, 2018, 08:23:23 pm
content: Hi Arm, That Q2 Transistor is a 1N SMD NPN Transistor. Enclosed more pics. I got the harnesses and plug-ins for the board along with all probes attached. I have to leave right now but will get back with you in the morning. Once more, thanks for taking the time to help me!

author: Armadillo
author_rank: Super Contributor
timestamp: December 27, 2018, 11:15:57 pm
content: Well you can jumper level sensor but the temp sensor is missing.I suggest if you want to test safely, wire DC to the 1000uF cap instead of turning on AC power.I have no experience otherwise it will be easy logic even without the membrane switches or the sensors.Many experts here, no worries.

author: bogart219
author_rank: Contributor
timestamp: December 28, 2018, 09:41:06 am
content: Arm, the temp sensor is there (enclosed another pic). I will have to jumper out the water level probe like you pointed out. Normally, when you power one of these units up, after it fills with water the heating element will turn on. Those temp sensors ring out around 14K ohms at room temp. I figure if I jumper that water probe, the temp sensor (just hanging there at room temp), should tell the board to turn on the heat. I do have a membrane switch that I can hook up if necessary. I'm going to try to test that 1N smd and will report back. Thanks!

author: Armadillo
author_rank: Super Contributor
timestamp: December 28, 2018, 10:02:48 am
content: Oh I see, LOL, harness flying around...I am confused with this connector.

author: Armadillo
author_rank: Super Contributor
timestamp: December 28, 2018, 12:26:45 pm
content: Hi bogart;if you have the time , scratch out the IC numbers, so that we can guess the short cut troubleshooting methods.That thing looks awfully like a pic micro-controller. Guessing.

author: bogart219
author_rank: Contributor
timestamp: December 28, 2018, 03:54:55 pm
content: Hi Arm. Here are the IC numbers I believe are correct. I tested Q1 and Q2. I compared to the ones on a new board and they appear to have the same readings. Two questions:1)What exactly would the purpose be of that 1000 uf capacitor next to the transformer.2) Why do they put that clear coating over the board? To protect it from heat? Talk to you later. Thanks!

author: Armadillo
author_rank: Super Contributor
timestamp: December 28, 2018, 04:28:16 pm
content: Hi,The HI P49SB MM74??? is not completeThe clear coating is for water/moisture/evaporation from the hot water protection.The 1000uF capacitor I supposed is the LM317 voltage regulation DC power supply voltage filter.Is not enough to say old board and new board measure the same. You should measure the Base to Gnd, Collector to Gnd, and Emitter to Gnd voltage. This will tell alot of things from it example whether power is available to the transistor, the driver is driving the base etc....The best would be also to measure all the pins voltage of this IC "P49SB MM74???" between the old board and the new board, and you will know exactly why the relay is not driven. Lucky you have a new board for this easy comparison.Yes you have to power it up. Best using DC voltage to be safer. Firstly you power up the new board with AC  and measure the voltage developed across the 1000uF cap. Then you off the AC and hook DC wire across the capacitor with the correct DC voltage from your bench power supply. Remember not to mix AC and DC power together. From here onward, test with DC power supply only.In all the above tests, the level sensor must be jumped and the temp sensor must be hooked up. [in all cases]."P49SB MM74???" Bad BoardPin 1 = ??Pin 2 = ??Pin 3 = ??Pin 4 = ??Pin 5 = ??Pin 6 = ??Pin 7 = ??Pin 8 = ??Pin 9 = ??Pin 10 = ??Pin 11 = ??Pin 12 = ??Pin 13 = ??Pin 14 = ??"P49SB MM74???" Good Board with Relay EnergisedPin 1 = ??Pin 2 = ??Pin 3 = ??Pin 4 = ??Pin 5 = ??Pin 6 = ??Pin 7 = ??Pin 8 = ??Pin 9 = ??Pin 10 = ??Pin 11 = ??Pin 12 = ??Pin 13 = ??Pin 14 = ??

author: Armadillo
author_rank: Super Contributor
timestamp: December 28, 2018, 04:45:39 pm
content: Please in return also describe the life of the board.... example the LED is lighted up, blinking?? to show that the CPU is clocking or dead.

author: bogart219
author_rank: Contributor
timestamp: December 29, 2018, 02:40:47 pm
content: Quote from: Armadillo on December 28, 2018, 10:02:48 amThat connector has the pink water porbe, a ground wire and a black & white wire to the Temperature sensor.

author: bogart219
author_rank: Contributor
timestamp: December 29, 2018, 02:50:10 pm
content: Quote from: Armadillo on December 28, 2018, 04:45:39 pmArm, when unit is first plugged in, the not ready light (comes on right away, steady led 9) if water level is up, relay will make turning on element. On the new board, led 24 was lit also (this led does not show on the display. On the dut , this led does not light up!

author: Armadillo
author_rank: Super Contributor
timestamp: December 29, 2018, 03:02:37 pm
content: Hi, your original question, the relay does not get the 12V.And now, the Relay is energized.So problem solved?You know without the schematic and the front panel, I don't know what LED 24 is for also.

author: bogart219
author_rank: Contributor
timestamp: December 29, 2018, 03:54:39 pm
content: Quote from: Armadillo on December 29, 2018, 03:02:37 pm----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Arm, I had all this info typed out that took me a half an hour,, got distracted , hit a wrong key on the keyboard and it all went away. I'll give you the condensed version now. If you need more info let me know.The full number for that IC isP 49SBMM74HC14MI tested the voltages on Q1 and Q2. only difference was on Q2. Dut - C=.569 New C= .717. all other readings were the same.When testing the voltages on the dut, I tested the coil voltage and had 12 volts all of a sudden. WTF? I then took the dut and soldered the relay back on the board. I hooked up a 50 watt lamp to the relay.Hooked everything back up. Powered up. led9 lit up (not ready) .Led 24 (over next to Q1, hidden by the membrane switch, never noticed it before. this led never came on.After 4 or 5 seconds, the lamp came on! Its working! Why now, I have no idea. With the lamp on (calling for heat), I got my heat gun and heated up the temp probe. After 10 seconds led 9 turned off and the ready led lit up . The lamp shutoff at this time. Ok, appears to be working. Let the temp probe cool down after a few seconds the ready lamp goes out and the not ready lamp lit back up. At this time the lamp should of came back on (calling for heat again, but it never did. Unplugged it a few times. Never could get the relay to make after this. Confused more than ever.What do you make of this Arm

author: Armadillo
author_rank: Super Contributor
timestamp: December 29, 2018, 04:10:38 pm
content: It is common for the Level sensor to be coated with calcium carbonate.Lightly sand paper it or change a new one.I suppose you now jumper the level sensor, so its working.Also, re-solder the joints, sounds like moving the board you get 12V?

author: fzabkar
author_rank: Super Contributor
timestamp: December 29, 2018, 07:32:54 pm
content: MM74HC14M = hex Schmitt trigger inverterhttps://www.fairchildsemi.com/datasheets/MM/MM74HC14.pdf[/url

author: bogart219
author_rank: Contributor
timestamp: December 30, 2018, 11:49:44 am
content: Quote from: Armadillo on December 29, 2018, 04:10:38 pmArm, I replaced that level sensor with a new one a long time ago. One of the first things I checked. Thats not the problem. I will however re-solder the components I was messing with. Maybe a cold solder joint.

author: bogart219
author_rank: Contributor
timestamp: December 30, 2018, 11:50:23 am
content: Thanks for the info.

author: bogart219
author_rank: Contributor
timestamp: December 31, 2018, 11:40:20 am
content: Arm, just a follow up. I removed the coating around the IC, transistors and plugs. I re-soldered, or I should say touched up each connection. I did this to two "bad" boards that I had and now both appear to be working like hey should, at least on my bench which they never did before, I will test in them in an actual unit when I get back to the shop. (on vacation now). That #24 LED on the new board that lights up when power is applied still doesn't light up on either one the dut's. this led does not show up on the front panel (membrane). Will have to see about that. As of right now, I have to say that the issue with these boards were cold solder joints. I found a place on the web that fixes these boards for $109.00. Maybe they just do what I did? Will see. I asked to be pointed in the right direction and you did that. Thank you for that. Will keep you updated. Happy New Year to you and your family!!!

author: Armadillo
author_rank: Super Contributor
timestamp: December 31, 2018, 11:57:11 am
content: Dear Bogart;Glad you found the fault.Happy New Year to you and your family too.Best Regards;Armadillo

author: bogart219
author_rank: Contributor
timestamp: May 27, 2019, 11:13:26 am
content: Good morning. Revisiting an older thread. figured this was best since all the info and pics are here. Concerning this Bunn circuit board, it appears that by re soldering cold joints and changing a relay, I have been able to fix the board of its not heating problem.I have a different question now about a problem I run into alot with these boards, inconsistent water levels. To re calibrate the board on how much water to dispense in the pot, you have to hold the on button down until you hear Three clicks. Once the level is where you want it you shut off the unit. This is supposed to save the settings. On a number of boards I work on these levels are inconsistent. Some times the levels are a inch short in the pot and other times it might over flow. What components on this board determines how much water this machine dispenses? No doubt it is some kind of timing circuit. There is a company on the internet that repairs these boards. What could they be doing to repair them? Thanks for any input!

author: bogart219
author_rank: Contributor
timestamp: June 03, 2019, 09:43:27 am
content: Would one of these IC's on this board be the timing circuit? What would cause it to vary from one pot to another?