Hameg 408-1A

[tronde]

What kind of knobs are missing? Rotatating with double shafts?

A picture of what you have now and some mechanical dimensions would be useful.

OK, picture in the morning, but basically they are the typical small knob that you would find on controls like focus, position etc, rotating, single, 2mm shaft and the base of the knob is approx 9 or 10mm dia and approx 6mm at the top. More precise dimenions in the morning.

OK.
Something like this and a metal tube soldered or glued on the shaft should do.

http://no.farnell.com/elma/020-2220/knob-black/dp/1209749

http://www.ebay.com/itm/Brass-Pipe-Copper-Pipe-Copper-Tube2mm-3mm-4mm-5mm-Long-300mm-Wall-0-5mm-/222369005754

This knob is for 3.175mm shaft, but I think it sould clamp on 3mm as well. The tube with 2mm inside is 3mm outside.

The same knob is available in 4mm too, and if you can find 2x4mm tubes it might be a better choice.

[Specmaster]

+5V is the feedback reference for the SMPS and I think it should have some small load on it. Say a 6 or 12V auto bulb.

Tooth brush is what many use with IPA.
Have a look at repair vids by Defpom, he also uses some tough little wipes under the brush to mop up as his last cleaning action.

So the +5v rail controls the others then, without some form of a load, the others effectively shut down? I thought the +5 rail was the supply rail for all the IC's in it and there are plenty of those.

I tried the toothbrush idea, it does indeed make a far better job and much much quicker but also leaves the board with a white coating like a frosting over it. I'll give it another go today and see if a second cleaning removes the frosting effect.

As I work my way around this scope, I see there are a number of what seems like 2 pole testing points cropping up and I find no mention of these in the service manual so I'm guessing that either are test points used during the manuerfacturing process or there is somewhere in existance a proper workshop manual that their own service dept used to use when these machines were current, if so then I'd like to get my hands on a copy, if only to burrow it while I'm trying revive the beast.

[tautech]

+5V is the feedback reference for the SMPS and I think it should have some small load on it. Say a 6 or 12V auto bulb.

Tooth brush is what many use with IPA.
Have a look at repair vids by Defpom, he also uses some tough little wipes under the brush to mop up as his last cleaning action.

So the +5v rail controls the others then, without some form of a load, the others effectively shut down? I thought the +5 rail was the supply rail for all the IC's in it and there are plenty of those.

Yes but you said you had the PSU on the bench and without a load, SMPS don't normally like that.
You can see where the FB comes from the +5V rail to an op amp and then onto a circuit where all the other LV rails terminate too. This I believe is some sort of LV rail monitoring too and will affect the FB level in some way.
If they are all connected on the PSU board to this part and you apply some small load to +5V I think it will operate.

I see there are a number of what seems like 2 pole testing points cropping up

2 are marked on the schematic between the base of T454 and +UREF

[Specmaster]

Finally reassembled the scope and plugged it into the dim bulb unit, switched on, bulb glowed dimly for a second and went out, the power led and others on the front flashed about 5 times and went out then nothing. Switching off and on again, produced exactly the same results except that the 100w bulb now does not glow??

There was no arcing, bang or magic smoke at all.

As I had to rebuild the heater connections in the PSU it is possible that I have got the connections to the heater reversed bearing in mind that F14 connection to the heater also goes to the HV board?  Could this be the problem??

[tautech]

Finally reassembled the scope and plugged it into the dim bulb unit, switched on, bulb glowed dimly for a second and went out, the power led and others on the front flashed about 5 times and went out then nothing. Switching off and on again, produced exactly the same results except that the 100w bulb now does not glow??

There was no arcing, bang or magic smoke at all.

As I had to rebuild the heater connections in the PSU it is possible that I have got the connections to the heater reversed bearing in mind that F14 connection to the heater also goes to the HV board?  Could this be the problem??

Maybe, the heater supply is polarised and need be around the right way.

Otherwise sounds good. 
Try it with a lower wattage bulb and if all is well it should do similar but not go completely out.
With the tester in circuit you might like to check the SMPS is working with another scope......no connection ahla near field probe style.
Look for it oscillating in the 20 -40 KHz range.

[Specmaster]

Finally reassembled the scope and plugged it into the dim bulb unit, switched on, bulb glowed dimly for a second and went out, the power led and others on the front flashed about 5 times and went out then nothing. Switching off and on again, produced exactly the same results except that the 100w bulb now does not glow??

There was no arcing, bang or magic smoke at all.

As I had to rebuild the heater connections in the PSU it is possible that I have got the connections to the heater reversed bearing in mind that F14 connection to the heater also goes to the HV board?  Could this be the problem??

Maybe, the heater supply is polarised and need be around the right way.

Otherwise sounds good. 
Try it with a lower wattage bulb and if all is well it should do similar but not go completely out.
With the tester in circuit you might like to check the SMPS is working with another scope......no connection ahla near field probe style.
Look for it oscillating in the 20 -40 KHz range.

I would love to do that but as I don't have a near field probe, I can't.

I have also bypassed the bulb with same results and talking with 9a4wy he suggested that I measure the voltages each time I switch on and here the results
Measured at the test socket for voltages (P2700} and had to break out my analogue Avo for this as my DMM was unreadable with the constant voltage changes and I got the following results

Expected        Actual
-12v               -3v
-5v                 -2.5v
+5v                +2.5v
+12v              +5v
+115v            +60v
+57v              +20v

All of these were only present for a second and decayed to zero with in a couple of seconds.

So its common on all rails.

We have also checked the resistance to ground for each of the rails at the same location and the results are as follows:-
-12v     75.5ohms
-5v       257ohms
+5v      440ohms
+12v    202ohms
+115v   36.92k
+57v      32.93k

so we can rule out short circuits.

He also recommends recapping the whole PSU so that is my next task but I need to order up another couple 470uf 25v as I'm short.

So thats where I'm presently at.

Picked up a lovely Hitachi V525 scope this week almost mint condition apart from a (rusty handle, been in its carry bag all its life and the probves still in sealed bags), and a slight problem on channel 2, the volts per division are not changing on screen, timebase and both perfectly ok on channel 1, but that one can sit while I refurb this Hameg.

[tautech]

OK, cool.

Ahla near field is just using an ordinary scope probe close to some EMI source.......try it on a wallwart or something. 

[Specmaster]

I'll try that then, thought you was talking a higfh impedance probe for high voltages for a minute.

[tautech]

It might seem that the SMPS is going into UVLO. Check that it's VCC pin is getting full voltage as spec'ed in the datasheet as needed to continue running.
Check C422 on the VCC supply too, I've seen them leaky and their current source not strong enough to maintain voltage for the SMPS IC VCC.

[Specmaster]

OK, cool.

Ahla near field is just using an ordinary scope probe close to some EMI source.......try it on a wallwart or something. 

I got the scope back on the bench tonight and tried the probe as you suggested, it does indded seem to be oscillating ok, holding the probe near the transformer and switching the scope does indeed produce waveforms and then after a couple of seconds disappears as the smps shuts down.

Tomorrow I'll start the recapping process, albeit I'll be 1 cap short (470uf) until my order is delivered, but at least I'll be able to replace the C422 and see if that cures the problem.

[Specmaster]

OK.
Something like this and a metal tube soldered or glued on the shaft should do.

http://no.farnell.com/elma/020-2220/knob-black/dp/1209749

http://www.ebay.com/itm/Brass-Pipe-Copper-Pipe-Copper-Tube2mm-3mm-4mm-5mm-Long-300mm-Wall-0-5mm-/222369005754

This knob is for 3.175mm shaft, but I think it sould clamp on 3mm as well. The tube with 2mm inside is 3mm outside.

The same knob is available in 4mm too, and if you can find 2x4mm tubes it might be a better choice.

This looks like a good solution and one that I'll try out once I get the power supply working correctly, thank you.

[tronde]

2x4mm tubes

http://www.ebay.com/itm/Inner-Diameter-2-12mm-Brass-Round-Tubes-1mm-Wall-Project-Pipe-Tube-500mm-Long-/282447547848

The knobs are known as "collet knobs" on ebay.

[Specmaster]

It might seem that the SMPS is going into UVLO. Check that it's VCC pin is getting full voltage as spec'ed in the datasheet as needed to continue running.
Check C422 on the VCC supply too, I've seen them leaky and their current source not strong enough to maintain voltage for the SMPS IC VCC.

All electrolytics have now been replaced, did find one that was essentially a resistor, 139k to be precise. switched it on and its the same, power there for approx. 2 seconds and off.  SMPS is always bit of a mystery to me so I'm poking about in the dark really Is there a really fool proof method of deciding if the unit is oscillating etc without any need for specialist equipment?

Any idea on what to check on next?

[tautech]

It might seem that the SMPS is going into UVLO. Check that it's VCC pin is getting full voltage as spec'ed in the datasheet as needed to continue running.
Check C422 on the VCC supply too, I've seen them leaky and their current source not strong enough to maintain voltage for the SMPS IC VCC.

All electrolytics have now been replaced, did find one that was essentially a resistor, 139k to be precise. switched it on and its the same, power there for approx. 2 seconds and off.  SMPS is always bit of a mystery to me so I'm poking about in the dark really Is there a really fool proof method of deciding if the unit is oscillating etc without any need for specialist equipment?

Any idea on what to check on next?

Read and study this:
http://www.nxp.com/documents/data_sheet/NE_SE5560.pdf

Monitor VCC on startup, it must remain above UVLO.

[Specmaster]

I already have downloaded this data sheet but I have to say that it does not make a wonderful lot of sense to me. A lot has changed in the last 50 years since I was last active in the field of electonics so certain aspects leave me struggling a little. I have scoured the data sheet looking for any reference to UVLO which I know is Under Voltage Lock Out but not seen any.

What sort of voltage should I be expecting on Pin 1 and what other pin (pin 16?) would I be monitoring for UVLO?

[tautech]

I already have downloaded this data sheet but I have to say that it does not make a wonderful lot of sense to me. A lot has changed in the last 50 years since I was last active in the field of electonics so certain aspects leave me struggling a little. I have scoured the data sheet looking for any reference to UVLO which I know is Under Voltage Lock Out but not seen any.

What sort of voltage should I be expecting on Pin 1 and what other pin (pin 16?) would I be monitoring for UVLO?

The NE5560 is an old chip now, the datasheet says 1994 and some of the terminology is not the same as used today.
They refer to UVLO as: Low supply voltage protection and it means the same thing. On P3 it's mentioned as:  low supply voltage protection thresholds and the value to use is 10.5 V on pin 1 (VCC) relative to pin 12 (Gnd).
10.5V must be present for the chip to start and continue running and SMPS will sometimes start then fail as there are generally 2 sources of current to supply the SMPS IC and the second (run) supply might not take over. For the start supply there is generally some high value resistor or chain from rectified mains, in this case I believe via R422 and then another supply from a feedback winding on the SMPS transformer. This supply (run) I believe is via R421 and fed from winding 2.
The components in each supply chain need be checked as each must be OK for the IC to start and continue running.

Monitor pin 1 and if it falls below the UVLO after start, there is your problem. 
They (the IC) can be coaxed to run with a bench supply and output then checked with a scope but this should be done without mains connection.

[Specmaster]

The NE5560 is an old chip now, the datasheet says 1994 and some of the terminology is not the same as used today.
They refer to UVLO as: Low supply voltage protection and it means the same thing. On P3 it's mentioned as:  low supply voltage protection thresholds and the value to use is 10.5 V on pin 1 (VCC) relative to pin 12 (Gnd).
10.5V must be present for the chip to start and continue running and SMPS will sometimes start then fail as there are generally 2 sources of current to supply the SMPS IC and the second (run) supply might not take over. For the start supply there is generally some high value resistor or chain from rectified mains, in this case I believe via R422 and then another supply from a feedback winding on the SMPS transformer. This supply (run) I believe is via R421 and fed from winding 2.
The components in each supply chain need be checked as each must be OK for the IC to start and continue running.

Monitor pin 1 and if it falls below the UVLO after start, there is your problem. 
They (the IC) can be coaxed to run with a bench supply and output then checked with a scope but this should be done without mains connection.

Thank you, it makes sense now, I was looking for the term ULVO. I came accross the section on page 7 where it discuss the low voltage protection being active when V(1-12) is below 10.5v. This I thought was saying that the voltage between pins 1 and 12 must be above 10.5v for the psu to function.

The same section also mentions that the circuit can be fed from a DC supply between 10.5v and 18v maximum, so do I take it then that I could connect say 12v to this SMPS via Pin 1 and ground, leave the mains disconnected, and the scope should fire up?

If so, I could also do this, I have a bench supply capable of providing upto 30v @ 5A with built in current limiting. If I have understood this correctly, it would allow me to see if there are any other issues apart from the SMPS?

[tautech]

The same section also mentions that the circuit can be fed from a DC supply between 10.5v and 18v maximum, so do I take it then that I could connect say 12v to this SMPS via Pin 1 and ground, leave the mains disconnected, and the scope should fire up?

No it doesn't work like that.
12V on P1 and Gnd will only (hopefully) get the SMPS IC to start PWM output and at: When the supply voltage surpasses the 10.5V level, the IC starts delivering output pulses via the slow-start function. (soft start)

When powered appropriately from a bench supply you then check the output waveform.....this only confirms the IC is operating as it should.
When mains powered you need next confirm VCC on pin 1 maintains 10.5V +.

We need find a better datasheet, one that the typical application schematic diagram is legible. 

[Specmaster]

As I look round the schematic there are at least 3 different symbols being used to represent a ground connection. I always thought that ground was ground, but is that not the case here, is there like a circuit ground for the DC and an earthed ground for the AC mains on this chassis?

I just took a voltage reading from pin 1 on the NE5560N and the actual chassis and I actually got -115v. According to the schematic pins 14 and 12 are strapped together and grounded. I cannot get a path between them and the chassis??

[tautech]

As I look round the schematic there are at least 3 different symbols being used to represent a ground connection. I always thought that that ground was ground, but is that the the case here, is there like a circuit ground for the DC and an earthed ground for the AC mains on this chassis?

I just took a voltage reading from pin 1 on the NE5560N and the actual chassis and I actually got -115v. According to the schematic pins 14 and 12 are strapped together and grounded. I cannot get a path between them and the chassis??

Good catch.
Yes it appears there at least 2 circuit Gnd's, one of which seems exclusive to the PSU...........use it. 

[Specmaster]

OK, found a common point for the ground in the PSU primary and VCC is +12.5v ref pin 12 but is only there for the same period, approx 1 to 2 seconds and goes to zero.

Resistors R421 and 422 located and seem to be in spec at R421 = 6.8ohms and R422 = 2.4k ohms. D415 also checks out OK.

[tautech]

OK, found a common point for the ground in the PSU primary and VCC is +12.5v ref pin 12 but is only there for the same period, approx 1 to 2 seconds and goes to zero.

Resistors R421 and 422 located and seem to be in spec at R421 = 6.8ohms and R422 = 2.4k ohms. D415 also checks out OK.

Do you mean D416 ?

There's a few things I'm having trouble getting my head around, T403 for instance. It seems to be in the current path for IC VCC and the base is driven when the mains switch is closed. 


We need to study up on theory of operation from the English SM.
There's a bit of nutting out to do, eg I think I've got pin 10's circuit sorted, it's remote switching configured to ON from the output of winding 11.

Time to take a step back and understand how all these bits interrelate and might affect start and run.
You could just check all these bits (circuits) one by one but the more challenging is to diagnose the faulty part/area.

You need to be a bit careful sticking a scope in this SMPS while mains powered as any probe reference point is not mains ground referenced like a scope is.
Gotta go.

[Specmaster]

OK then, I started looking around and was not able to find anything around the area that was wrong, so I started to revisit everything that I had done previously and found that a pad had been disconnected from the trace on R434 3.3k 4w which one side is connected to pin 13 on NE5560N, DVR, the trace on the other side was the affected one before it connected to anything above the resistor. Would the fact this connection was open circuit cause the smps to shut down? I thought I'd ask for your thoughts before re-assembling the boards and psu again to test it as it is such a pain to do each time.

[tautech]

OK then, I started looking around and was not able to find anything around the area that was wrong, so I started to revisit everything that I had done previously and found that a pad had been disconnected from the trace on R434 3.3k 4w which one side is connected to pin 13 on NE5560N, DVR, the trace on the other side was the affected one before it connected to anything above the resistor. Would the fact this connection was open circuit cause the smps to shut down?

Yep, that might do it but I think you're reading the schematic wrong, it's R433 1M5.
R434 is part of a snubber that protects the power FET switcher.

I thought I'd ask for your thoughts before re-assembling the boards and psu again to test it as it is such a pain to do each time.

No problem. You've invested some time now so you need to be careful not to loose this time, do things methodically and don't work on it when fatigued.

[Specmaster]

.p
Yep, that might do it but I think you're reading the schematic wrong, it's R433 1M5.
R434 is part of a snubber that protects the power FET switcher.

I thought I'd ask for your thoughts before re-assembling the boards and psu again to test it as it is such a pain to do each time.

No problem. You've invested some time now so you need to be careful not to loose this time, do things methodically and don't work on it when fatigued.

Oops you're right, fatigue has set in in that case my information was all wrong, it was the connection to the 330pf cap that was broken and that wouldn't cause the problem I doubt?

I checked the components around T403, that is R432, D423, R417, T403 and R422, all checked out OK but while checking that leg, I also checked L401 which shows as a 270uH but measures 64uH on my LCR bridge. I checked R433 as well and it is 1M5 so that OK.