Please help in saving this WORKING power supply

[administr4tor]

Hello,



I have an old furnace in house and this is its working control module.

How can I collect information/date/readings/measurements from this working module so in case if something happens to this working module  I can cross check from the information noted down and find broken component easily.

Should I take diode mode measurements of all the soldered pins on the GREEN SIDE OF THE board?
Any other ideas?

I am not too good electronics troubleshooting





thankyou

[fzabkar]

If it were my device, I would trace the circuit.

The coil would be some kind of custom wound ignition coil. Thyristor THY1 would be the ignition switch.

The IC is made by Hitachi, but I can't find any information about it. I'm thinking it may be a custom uC, possibly 4-bit or 8-bit, but I can't remember if there was anything of this kind way back in 1975 (assuming 7518 is week 18 of 1975). I can't see a crystal, so that makes me think I'm wrong.

I see references to an NEC uPC3605CA, but this turns out to be a 3-terminal regulator. :-?

[james_s]

I wouldn't mess with it, if it does fail some day it's probably going to be fairly easy to troubleshoot, usually the issue is bad solder joints, bad capacitors and occasionally a bad relay.

That looks very old though, what kind of furnace is it? Personally if it's more than about 15 years old I'd just replace the whole furnace if it fails. Last year I put a new furnace in my brother's house, the old one was only 70% efficient, the new one is 98% efficient and has a much quieter variable speed blower and modulating burner, it's a vast improvement.

[administr4tor]

yes, its an old furnance and old house, I dont want to spend too much money for furnace now, I am ok with 70% efficiency until it fails. As cost of new furnace is insane now a days. I have upcoming expenses for roof next year

[administr4tor]

If it were my device, I would trace the circuit.

The coil would be some kind of custom wound ignition coil. Thyristor THY1 would be the ignition switch.

The IC is made by Hitachi, but I can't find any information about it. I'm thinking it may be a custom uC, possibly 4-bit or 8-bit, but I can't remember if there was anything of this kind way back in 1975 (assuming 7518 is week 18 of 1975). I can't see a crystal, so that makes me think I'm wrong.

I see references to an NEC uPC3605CA, but this turns out to be a 3-terminal regulator. :-?

how do I trace the circuit? I dont know how to trace it. can you help me?

[Black Phoenix]

If it were my device, I would trace the circuit.

The coil would be some kind of custom wound ignition coil. Thyristor THY1 would be the ignition switch.

The IC is made by Hitachi, but I can't find any information about it. I'm thinking it may be a custom uC, possibly 4-bit or 8-bit, but I can't remember if there was anything of this kind way back in 1975 (assuming 7518 is week 18 of 1975). I can't see a crystal, so that makes me think I'm wrong.

I see references to an NEC uPC3605CA, but this turns out to be a 3-terminal regulator. :-?

how do I trace the circuit? I dont know how to trace it. can you help me?

When he says trace the circuit means picking up a pencil and paper and drawing where each component is and to where it connects.

You can follow the connections by reading the traces on the PCB or using a DMM in continuity mode and probe component legs to confirm if they are connected between them or not.

More about it:

https://youtu.be/wkAp5x3Z_gc

[GigaJoe]

frankly , it useless

at first , chip are basic custom logic to manage furnace, it not a power supply at all ...

it turn exhaust motor, if exist
it manage gas valve on/off
ignite it
using a sensor to verify flame
turn on air motor
and do in opposite sequence if unit shut down

due to chip unavailability , the whole board non reparable at all

everything you can do to find same , or similar board.  you need to know what each pin responsible for and with some luck find a similar
for example: pin "sensor" most likely flame sensor, diff board and sensor may be not compatible.
 
would be a very cool ka-boom if timing to open gas valve and ignition will shifted by differ board.   But it your house anyway

[administr4tor]

frankly , it useless

at first , chip are basic custom logic to manage furnace, it not a power supply at all ...

it turn exhaust motor, if exist
it manage gas valve on/off
ignite it
using a sensor to verify flame
turn on air motor
and do in opposite sequence if unit shut down

due to chip unavailability , the whole board non reparable at all

everything you can do to find same , or similar board.  you need to know what each pin responsible for and with some luck find a similar
for example: pin "sensor" most likely flame sensor, diff board and sensor may be not compatible.
 
would be a very cool ka-boom if timing to open gas valve and ignition will shifted by differ board.  But it your house anyway

yes, if chip dies end of story, its getting thrown but how do we know if chip is not dead? Take diode mode measurements of chip legs? so if chip dies we know right away and dont troubleshoot remaining board?

[GigaJoe]

to know if chip NOT malfunctioned you need a detail description how it work , sequence of signals, then compare with actual work. 

at least what you can do with multimeter to make a detail schematic and layer of voltage map, when furnace working , so late you can compare.

[james_s]

yes, its an old furnance and old house, I dont want to spend too much money for furnace now, I am ok with 70% efficiency until it fails. As cost of new furnace is insane now a days. I have upcoming expenses for roof next year

I haven't looked lately but a year ago I paid $2100 retail for a top of the line 60k BTU variable speed modulating furnace, with what natural gas costs these days the payback time is not too bad. Of course if you pay a HVAC company to come out and install it they charge a fortune but it's not all that difficult. 

As far as repairing that one if it fails, the electrolytic capacitors are the part I'd most expect to fail, one of them looks like it's gotten pretty hot, I suspect that's a zener diode next to it. 

[J-R]

Find the controller part number and buy a spare on ebay?

[administr4tor]

to know if chip NOT malfunctioned you need a detail description how it work , sequence of signals, then compare with actual work.

at least what you can do with multimeter to make a detail schematic and layer of voltage map, when furnace working , so late you can compare.

On which pins should I take these readings?

[coromonadalix]

First i would say this board heat a lot  seeing all the burned spots on the pcb

I would add  more ventilation  with holes in the casing

Capacitors should be serviced  or checked .... mostly the small ones around the ic

Check the parts solders for cold joints, cracked solders

It will fail sooner or later

The coil seem's to be the igniter IE high voltage pulse outputs ??


And the question about  monitoring this device :

Without a schematic, i would not even try any external device additions or modifications or sensors  ... i would need to probe many spots,  one misshap and you could blow a part or create some problems

Mostly due to ac lines going into it  too...

and finally  your sentence:   I am not too good electronics troubleshooting


Get or find a spare in case of ...


Furnace brand / model, and the pcb green side markings   you have some who may tell the model  ending with 37F 

Seems an old Hitachi logo ??

[james_s]

to know if chip NOT malfunctioned you need a detail description how it work , sequence of signals, then compare with actual work.

at least what you can do with multimeter to make a detail schematic and layer of voltage map, when furnace working , so late you can compare.

On which pins should I take these readings?

Frankly even with experience reverse engineering things I would not attempt to take voltage readings off he IC. All it takes is a slip of the probe tip to short something out and release the magic smoke. You could easily damage the board beyond economical repair by attempting to do this.

[GigaJoe]

" slip of the probe "
so true , better use a sharp needle like probes, with isolated ends

[fzabkar]

What is the model number of the furnace? I notice that there is a 24VDC input, but both relays are rated for 12V. This seems odd. :-?

[administr4tor]

This is the model number of Furnace

Lennox Furnace Model G14Q3-60-13

https://www.repairclinic.com/ProductDetail/1236445



https://www.completeheating.ca/images/pdf/LennoxG14.pdf

[james_s]

Oh that's a Lennox Pulse, I haven't seen one of those in years. Those are actually super cool, they're essentially a pulsejet engine, completely different design than any other furnace out there. They're one of the first 90%+ efficiency furnace, IIRC some are prone to cracked combustion chambers and I don't think you can get the leaf valves for them anymore but if it's working you may as well keep it going.

[administr4tor]

Oh that's a Lennox Pulse, I haven't seen one of those in years. Those are actually super cool, they're essentially a pulsejet engine, completely different design than any other furnace out there. They're one of the first 90%+ efficiency furnace, IIRC some are prone to cracked combustion chambers and I don't think you can get the leaf valves for them anymore but if it's working you may as well keep it going.

Yes, its has not leaked carbon monoxide yet because of known crack issue, I have put 4 carbon monoxide detectors around it.so I want to save  the board of this furnace as much as i can

[coromonadalix]

Well i gave you some tips,  change the capacitors for a start  with 105 degree ones, sames values for voltages and capacity

Check the solders on the board as i wrote earlier it could have some defects on them, mostly the big parts who heat a lot

Venting the board would help cooling it

Once again without no schematics i would not play in it,  when or if it become defective  you will know fast enough

Try to get a spare part, this one seem old enough to became rare on the electronic side of things


attached document for some wirings and tips

Yes its not what you want to hear BUT  you need something to work on before doing any attempts who could result in a dire situation if you do not have a backup plan

Searched  tons of websites for  the IC, never found it   some references for near number match but not the good one,  it could have been a custom part in the past

[Jetecnet]

There's one module on Ebay
https://www.ebay.com/itm/285059921681
JC

[administr4tor]

you all are AWESOME

[coromonadalix]

Where you the buyer  loll