Is this a three leg capacitor?

Intermittent of some type.
Have you monitored the voltage on the variable resistance output?

Place a multimeter on it and check what it is when you have the prob as compared to when u don't.

Alternatively u can place a switch in parallel with a 10 k resistor, so when u have the prob just flip the switch off forcing a higher resistance and speed shutdown.

Mosaic said:

Intermittent of some type.
Have you monitored the voltage on the variable resistance output?

Place a multimeter on it and check what it is when you have the prob as compared to when u don't.

Alternatively u can place a switch in parallel with a 10 k resistor, so when u have the prob just flip the switch off forcing a higher resistance and speed shutdown.

Click to expand...

Thanks for the quick reply!

I have metered the two conductors that run down to the foot-pedal at the power connector with it disconnected. I get the readings that earlier posts indicate: Infinity at rest, as the pedal is pushed, the first reading is 20K ohms, and with the pedal all the way down, about 15 ohms (should be close to zero). I have not tried to make measurements with the board in place, as the machine is currently spread across my bench. What might knowing the voltage across those two leads at various stages of operation tell me? I can re-assemble the machine and then take the cover off the power connector to take those readings if that piece of information would be helpful. Are there readings I could compare to?

Since the problem appears to be with the footpedal, detecting if it is behaving intermittently is useful.

Thus you need to check voltage drop across it so that you can watch the voltage change as you press it.

In this way you can determine what the voltage should be when the pedal has stopped the machine and compare it to the voltage when the machine isn't stopping as described in your problem statement.

If the voltages are mains A.C. you need to exercise care when doing this as you can get a nasty shock from exposed leads etc.

See if the transistor labeled #56 in one of the links above. Use your diode mode on your DMM and look at the B-E drop especially. You might have to remove it to test properly.

OK, I reassembled the machine far enough to take some measurements. I get 18.5 VDC at rest (machine stopped, pedal up), 8 VDC with the pedal pressed just far enough to get the machine going, and 0 VDC at maximum speed. I tested these settings until the machine started to fail, where at rest, the voltage was 16.5 VDC. To determine if the voltage drop were caused by the foot pedal, I shut everything down and changed my meter connection. I found I could pick up one side of the foot pedal potentiometer by connecting to board connection #8. That way, when I got the machine in the failing condition again, I could pull that pin from the mains connector to see if the problem goes away. Then I tested for 30 minutes, but the machine would not fail for me. Gotta love intermittent problems.

I will test the transistor as recommended in the latest suggestion, let me know if my readings prompt any other actions, questions, etc.

Before testing the transistor, I did a little more testing and got it to fail. When it failed, it read 16.5 or less DCV, so while it was failing I disconnected one side of the potentiometer, and the voltage stayed the same. Remember, I found the other side of that circuit is common to connector #8 on the board, so I was able to watch the voltage even with the foot pedal disconnected. I also noted that the longer you leave the machine in the failed state, the lower the voltage goes. It was down to 14.5 VDC when I shut it off.

Well your voltage ref is sagging so your motor control is not able to respond properly.
On post # 42, there are quite a few components affecting that Vref, including the 33K resistors which are known to fail. Since they're cheap, I'd swap them out to remove two variables and then retest. One could be starting to fail open as it heats.
It's difficult to do detective work when you have intermittents so you have to do some isolation of known problem areas.

I combined several recommendations and went out to purchase the transistor, zener diode, and the two large resistors. As mentioned in my earlier post, I own the 110V version of this machine, so my large resistors are 2 watt 15K ohm. I figured that since we were talking about less than $5 worth of parts, there really was no reason not to replace them all. I replaced them, put the control back in the machine, and did about 20 minutes of testing. No problems! So I finished assembling the machine and set it up for use, leaving it turned on. About 5 minutes later it started running by itself. I am guessing I will need to spread the machine across the bench with the control board hanging out so I can hang meters across various points to see where the voltage sag originates. Does anyone have any documentation on what voltages I should see at each point on the board?

By testing the control voltage (thanks for insisting) I proved the machine only goes amok when the control voltage drops below 17 VDC. I am hoping this proves that the SCS (BRY39) is operating as designed, as it works correctly for the most part, and only runs the motor when the voltage drops below 18V. I will make a list of the remaining original resistors, and complete the upgrade I started. Hopefully, one of them is veering off spec causing the voltage drop.

I am reading the schematic in previous posts to verify the values of the resistors. The values are different than the schematic, but I am pretty sure that is the difference between the 120V version and the 220V version of the machine. Unfortunately, I have never seen these square (chicklet) type resistors before, so I need some help with identifying replacements. I can make out the resistor values, but what wattage are these square resistors? And what can I replace them with?

Hy folks,

First, thanks very very much for all this interesting and helpful posts. Found this page because of the same problem the original poster had, blown filter capacitor...

GoDale: If still of interest: this squared resistors had a power rating of about 250mW. I also used them in my young childhood (about 1985), but, they've been old at this time already. ;-) Just replace them by modern metal film or also by older coal types. But usually they don't get damaged, if they're not high-power types like the 1W-resistors with 33k (or sometimes 15k according some posts).

Thanks for that information! I had a couple of other projects in the queue, so I was finishing those up to make space on the bench for the sewing machine. I will tear it down this week and see if I can find my sagging control voltage. It would still be great to know if anyone has voltage readings at measurement points for the previous schematics, it would be good to have something to compare to.

OK, I replaced all remaining resistors on the board which made no difference. I have now made several voltage measurements with the machine in both normal mode and in the failing condition. I started with the voltage found at the common side of the two 15K 2W resistors. Measured from that point to connection #4 (I am presuming this is a decent voltage reference point) I get 22.5 to 22.8 VDC, with a minor drop when the machine goes into the failing condition, but that is most likely caused by the current draw of the motor. Measuring across the Zener diode, I get 19.3V DC in normal mode, and 19.0 when failing. When I measure between the foot control lead (where R6, R7, and R9 all connect to the blue wire soldered to the board) and board connection #4 I am reading 18.9 VDC when operating correctly and 15.2 VDC when failing, roughly a 3.7V difference.

Can anyone tell me what Thyristors are in use on the US version (110v 60Hz) board? The part on the board says BBC CS1 05g03, but nobody can translate that for me. I tried ordering the BStc1040 as shown in some of the drawings, but my supplier subbed that to an NXP BT151-650C, which did not work.

GoDale , been a recent post re this machine you may be able to assist each other...
https://www.electro-tech-online.com/threads/control-pedal-issue.147534/#post-1256222

With +18k views perhaps this sewing machine thread needs it's own tab !

Hi all. The information on all threads is just great to increase the knowledge bank. I have a 1222 in Canada with no motor operation. Done all the obvious pre-checks and traced to PCB. I have 110 voltage between terminals #1 and #3. Went to check AC voltage on #4 and #5 terminals with power connected and on and foot control depressed, and zero AC voltage there. Any thoughts?

Ziedar said:

Hi all. The information on all threads is just great to increase the knowledge bank. I have a 1222 in Canada with no motor operation. Done all the obvious pre-checks and traced to PCB. I have 110 voltage between terminals #1 and #3. Went to check AC voltage on #4 and #5 terminals with power connected and on and foot control depressed, and zero AC voltage there. Any thoughts?

Click to expand...

You dug up two old threads about an old Pfaff sewing machine? Do you have different questions or just trying to insure an answer?

gophert said:

You dug up two old threads about an old Pfaff sewing machine? Do you have different questions or just trying to insure an answer?

Click to expand...

I am looking for a solution to my issue on #75

Ziedar said:

I am looking for a solution to my issue on #75

Click to expand...

Post 3 in the other thread showed one option to get it running...

gophert said:

Post 3 in the other thread showed one option to get it running...

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Click to expand...

Yes I saw that one and questioned how he went about bypassing. Have not had a response yet.

Ziedar said:

Yes I saw that one and questioned how he went about bypassing. Have not had a response yet.

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Post a nice clear photo of both sides of the board. We'll figure it out.