The controller is no longer available so I really need to fix this one.

The controller has a 120v to 24v ac transformer and a circuit board. The circuit board has a fuse which blew. Other components physically look ok. Replaced the fuse plugged the unit back into 120v without the chair attached and the fuse blew again. So something is shorted and it isn't the motor powering the chair.

The circuit board has resistors, capacitors, transistors, diodes, 2 relays and a rectifier. The relays are used to change the direction of the DC current to operate the chair into recline or upright.

I've checked the transformer and the rectifier (KBPC1010). 23.9 v AC from transformer and 21.2v DC from rectifier using a multimeter. A new KBPC1010 rectifier measures the same output voltage. All of the resistors appear good, measured in circuit, they have the correct resistance according to color bands except that 2 are identical and read 740 ohms but the first 2 bands are not violet/purple and yellow. All of the resistors are blue and the colors of the bands are difficult to determine. They aren't burnt or anything. Maybe it is just my eyes (getting older). Since the 2 resistors are identical and the resistance is identical, I assume they are good. I also wouldn't think that if they were bad, they would be shorted and causing the fuse to blow. Could that be the problem?

If I get a photo attached here are the resistor measured values.
R1 0.99
R2 4.65
R3 0.74
R4 0.74
R5 4.64

I checked the capacitors with an ESR Meter. Meter says they are all good. They are not swollen or leaking.

The diodes. They all checked good except 2 showed voltage in both directions. I removed them and tested our of circuit. They only passed voltage the correct way when they were out of circuit. Maybe the test passed voltage back through the circuit and was read by the meter? I checked them multiple times in circuit and got voltage both ways so it wasn't user error. Can a false reading in circuit happen?

Now to the transistors. This is the first time I've ever checked transistors and I'm not understanding my results. Seems I should be getting OL at other measurements.

First letter was red lead and second letter is black lead

1st Transistor
2N551 B 331
C and B Meter had a blinking 3 and then .000 so I think that was equiv. to OL
E and B 0.745
C and E 1.083
B and E 0.751
B and C 0.748

2nd
BC547 B 011
C and B 1.539
E and B 1.106
C and E 0.751
B and E 0.704
B and C 0.700

3rd
BC547 B 011
C and B 1.435
E and B 0.909
C and E 0.752
B and E 0.687
B and C 0.684

It should be noted that pin C of transistor 2 is connected to pin B of transistor 1 by solder on the back of the board. Looks original but I don't know why that would be.

Any help is appreciated. I'm going to have to order the 2 diodes I removed and not sure what else I may need to fix this. Oh, the rectifier and fuse were removed in these photos. I have a new fuse (T5A 250V).

 

Does it blow the fuse without the motor plugged in?

 

Yes it does. Transformer and circuit board are on my bench. BTW, I can't seem to edit my post above and the Transistor I called 2N551 is actually 2N5551.

If the issue is with a transistor, how would I know which version of the transistor I would need? Seems the 2N5551's and BC547's have units with different specs each.

 

Transistors readings are ok. Can you draw the power supply as it comes into the board, it will help us to rectify it. The 2N5551 is a 600mA rating.

Why have you removed parts..?

 

The fuse was blown so I took it out. I took the rectifier out to test it.

The 2 IN4007 diodes that I removed ( D3 and D4) showed voltage in both directions in circuit. All caps had been discharged. When I took them out and tested them, voltage was read only in the correct direction. I'll have to order replacements as I broke 1 leg too shoot to reuse and the other I need longer legs to feed it back into position. It beside the red capacitor and under the black wire.

Hopefully this photo will help.

 

I don't see how you can get a voltage reading between the ends of a diode when it is out of circuit. You need to set your meter to the diode test position. On a good diode it should display the same as it does without the meter leads connected to anything In one direction. In the other direction it should display about 0.6 volts. The most likely failure mode is it will display almost zero in both directions. If your meter does not have a diode test position use a low resistance range. If the diode has failed short circuit (Which is what I expect.) it will read almost zero ohms in both directions.

Les.

 

The fuse was blown so I took it out. I took the rectifier out to test it.

The 2 IN4007 diodes that I removed ( D3 and D4) showed voltage in both directions in circuit. All caps had been discharged. When I took them out and tested them, voltage was read only in the correct direction. I'll have to order replacements as I broke 1 leg too shoot to reuse and the other I need longer legs to feed it back into position. It beside the red capacitor and under the black wire.

Hopefully this photo will help.

D3 and D4 are the bemf diodes for the relay coils, they won't be faulty, can you measure the resistance across the output of the bridge rectifier on the PCB for shorting.

Also check the bridge rectifier on diode test across each diode.

 

Yes. Meter was set on diode. + on anode and - on cathode reads about 0.61v. The other way around it reads 0. This was out of circuit and both removed diodes measure the same. In circuit, these 2 read about 0.6v with leads either way which is why I thought they were bad. All of the other diodes tested correctly in circuit.

"zero ohms in both directions" from your post?? I just went out and tested ohms/continuity across both and there isn't any continuity. Ohms was high and blinking suggesting resistance was too high to read.

 

The fuse and rectifier have been soldered back onto the board.

The resistance between + and - is 0 . Continuity beeps.

+ X < 0.580 > X ~
^ XXXXXXXXXX ^
0.580 XXXXX 0. 580
v XXXXXXXXXXX v
~ X < 0.580 > X -

So on all 4 sides I get 0.580 regardless of which leads are where. i.e. Red on ~ and black on - or vise versa modeled on the 4 sides.

Did I blow this rectifier when the 2nd fuse blow? XXX is a place holder to keep the symbols and values separated.

 

I would say the rectifier is blown if you're getting Zero ohms across +/- terminals.

 

Did you measure the resistance between + and - on the board before re fitting the bridge rectifier ? This is a link to the datasheet on the relay so you can see the pin out for tracing the schematic.

Les.

 

I would say the rectifier is blown if you're getting Zero ohms across +/- terminals.

Did you measure the resistance between + and - on the board before re fitting the bridge rectifier ? This is a link to the datasheet on the relay so you can see the pin out for tracing the schematic.

I just ordered some of the 1N4007 diodes. They should be here Monday.

I removed the rectifier and it tests fine out of circuit. Per LES's question, with the rectifier removed I'm getting 0 resistance between + and - points on the board where the rectifier connected.

Checking the relays - for each relay I am getting continuity between its COM and its NO as well as its COM and its NC pins. So both sides of the relay are connected. That can't be unless both are internally shorted - correct? Or is there a short elsewhere back feeding all the pins? And what would cause 2 relays to short at the same time?

I have a few of the following relays. Can they replace the ones that are there?
TRD-12VDC-FB-CL
6A/250v ~ 12A 120VAC
12A/28v 12A 28VDC

 

From the part of the schematic I have traced out the only things that look like they can short out the raw DC supply is the large electrolytic capacitor E1 and the contacts on the relays. I have never seen an electrolytic capacitor fail with a total short. Also a relay would have to fail with a short between the NC and No contacts and I have never seen that happen. I would first try removing the solder on the NC an NO connections on the relay with a solder sucker. There may be enough clearance between the pins and the track so they no longer make contact with the tracks. If this is the case then you should be able to see if there is still a short between the + and - tracks or between the NC and NO contacts on one or the relays.
If the short is still between the + an - tracks try unsoldering capacitor E1.

Above is a mirrored version of the track side to make schematic tracing easier.

Les.

 

Here is part of the main motor drive circuit, the NO and NC maybe reversed as I can't tell from the relays, but the circuit is the same.

So check the black capacitor for shorts on ohms , lift one leg out and test the PCB also, if it still measures short, then remove the relays,.

 

Ok, the relay close to D3 (which is closest to edge of board) - NO and NC pins desoldered. Pins aren't touching the tracks. No short between NC and NO.

Pins on the other relay (center of board) desoldered but still touching the tracks due to some residual solder the sucker didn't get. Short between NC and NO.
E1 not shorted
+ and - on the PCB still shorted.

Relay in the center of the board was completely removed. NC and NO are shorted. Tested multiple times
E1 not shorted
+ and - on PCB not shorted.

So it seems the relay failed shorted. I envisioned these things to be a T shaped piece of metal where one end touched a contact (NC) and the other end touching nothing. When the electro magnet was energized, the T moves to the magnet disconnecting from the NC contact and connecting to the NO contact creating the circuit. How is it possible to fail and connect both??

The relays I have will fit the board but are a lot shorter. Specs are:
TRD-12VDC-FB-CL
6A/250v ~ 12A 120VAC
12A/28v 12A 28VDC

The one on the board is:
SRU-12VDC-SL-C
10A/250vac 15A 120vac
10A/30vdc TV-5 120VDC

Per data sheets they are both normally closed with the closed pins in the same position. I have no idea what the TV-5 means.

If mine aren't up to the task, I'll have to order some. I've been unsuccessful finding a US source (i.e. faster shipping) for the SRU-12VDC-SL-C but have found SRD-12VDC-SL-C. Can the SRD version be used?

 

So you found the relay faulty, now you can put the other semiconductors back.. hopefully when the new relay is fitted it works.

 

I really appreciate your help. Thanks
Will the relay I have be a good replacement or should I wait weeks (shipping from China) and get the exact SRU relay? Or will the SRD be a good replacement?

 

Yes they're 10A rated.

 

I agree with Dave that the SRU-12VDC-SL-C relay looks to be a suitable replacement. This is a link to the data sheet.
www.szdahao.com/uploadfile/machine/relay/tti/pdf/ttitrd.pdf
I gave a link to the datasheet on the original relay in post #12
Here is a link to some information on TV-5 approval.
https://www.fujitsu.com/us/products/devices/components/relays/information/safety-std-bkgd.html
It would be interesting to see inside the failed relay to see how it has failed. I have seen the common contact weld to either the NO or NC contact but never a short between the NO and NC contacts.
Just as a matter of interest Q1 and Q2 form a latching circuit. When triggered Q1 breaks the negative feed to the relay coils.
Q3 detects when the voltage across the 0.33 ohm current sense resistor exceeds about 1.3 volts (Current of about 4 amps..) and this triggers the latch formed by Q1 and Q2.
If your DMM had better resolution for measuring low resistances you may have noticed that it was not a dead short between the power rails as it would have measured 0.33 ohms.

Les.

 

You said:
"I agree with Dave that the SRU-12VDC-SL-C relay looks to be a suitable replacement."

Did you mean my TRD-12VDC-FB-CL relay looks to be a suitable replacement? That is the spec sheet you linked to. Or did you mean the SRD version would be suitable.

Any suggestions on how to open the relay without damaging it in the process? I'd also be very interested in how it failed. Use a dremel and cut it on the sides close to the bottom? Or is there an easier way?