HAMEG HM-305 O-Scope

[Bewildered]

Hi, I'm bewildered.
My trusty HM305 has packed up.

I have the service manual and have done a visual check and test of the PSU board.

F1001 630mAF has blown. Testing the input to the fuse from the bridge gives me supply voltage at DC.
(one probe on fuse tab, one on the metal chassis)
On the other side of the fuse get around -130V.

No visible damage to the supply, but replacing the fuse gets another blown one as paypack for being silly.

Anyone give any tips on removing PSU board from the scope. (Sensible ones please)
Any troubleshooting assistance gratefully accepted.

TIA
Tony

[Bewildered]

Update.

Got the magnifying light on the top of the board (didn't want to remove from chassis yet). No visible blackening or sweating of any of the components. I measured the switching NFET and found it was shorted.
As the scope isn't working decided to go for it and snipped the leads to put on the cheapo ebay tester. Lo and behold the NFET is two 15 ohm resistors against a known (but not the same NFET) that read as it should.

So I have a broken switching FET and have ordered a replacement.

Anything else that it might have taken with it?

I'm tempted to solder up a FET out of the junkbox in lieu of the spare on order to see if the scope operates.

Any thoughts?

[dave cox]

I can't find a manual / schematics for this model (for free). The service manual for the305-2 is available so I had a quick glance at that in case it's a bit similar. It would be good to post a schematic so more people will get a look at it.

Anyhow, the 305-2 has a switch-mode PSU with the usual rectified mains / cap / chopper and HF transformer. Assuming a fault in the hot (read lethal) side then the chopper transistor (mosfet?) would be a candidate or something wrong with the feedback control ...

Not knowing your level of experience I must strongly advise extreme caution working on the live side, preferable using a floating supply or isolation transformer. Personally, I use a 350V / 1A stabilised DC supply (with current limiting) to feed the reservoir cap (after testing the bridge and cap) and then keep one hand in my pocket

dc

NOTE TO SELF - I must type faster !

[Bewildered]

Thanks for your reply.

Fairly experienced but not with SMPS.
I'm treating the job as if it is valved equipment, using an plastic rod to peek and poke etc with one hand etc.
I'm working on the principle that the N-MOSFET has blown taking the TDA4605 with it as there is a direct connection from the gate to the chip.
The board with the caps discharged without the mosfet is now not measuring short circuit with tick from the meter when the caps give up their charge from the ohmmeter battery - which is a good start.

I'll post the PSU schematic once I'm back at base.

Getting the scope working again would be nice - the component tester feature on the Hamegs is useful.

[dave cox]

The hameg scopes always look like good value for money, if I didn't have a lots of old Tek gear I would buy one!

dc

[Wendymott]

Hi Tony.
The TDA 4605 was used on a number of TV's in the SMP. If the Source resistor has been stressed, that could cause it to shift value or burn out. In the SMPs that the NEI 32" used, the Source resistor was a 1% tolerance and was around 0.22 ohms. If it deviated, then the inrush current could destroy the chopper fet. The startup resistor to pin 5 I think. was usually 150K or there abouts. This should be rated as a High voltage type, NOT a standard CF resistor, as these are rated at 250V DC.
It is wise to look at all the resistors in the "hot" side of the SMPs, and measure them
Regards
Wendy G8BZY.. ex NEI TLO

[Bewildered]

Thanks for the replies all,
I'm now back home so here's the PSU schematic.
Replaced the TDA9605 and FET and got a brief flickering of the power on light and guess what the fuse went. My fault for being too confident.... (I'm sure these things can detect if you are trying to be too clever)
Anyway - will be looking at the 0R22 Resistor. Any more ideas from the schematic appreciated.

[maninashed]

Switch mode psu's can be very frustrating to repair, a complex mix of SMD and wired components performing the seemingly impossible!. If you have found the chopper fet s/c, then its likely that there is a short somewhere else on the board. If the short was on the secondary side, then its likely that you would hear a ticking noise as the SMPS tries to strart. So from your description, I would suggest that the fault lies in the primary (mains) side. Its important that you check all the silicon components for faults, because when the chopper fails, usually other components are damaged along with it. This includes the diodes. Some you may need to remove from the board to test properly. Its pointless just replacing the chopper and i/c without thoroughly checking the rest.
Bill

[Bewildered]

Will be checking more components but I'm trying not to remove the board from the unit as it involves dismantling a lot of hardware to remove the connectors.
Hope the avalanche diodes didn't go pop with the FET as direct replacements are expensive (mouser quote 728p). Possible use of modern equivalents where the price comes down to 22p each.

It may turn out to be an SMPS repair practical training session and if the scope is mended it would be a bonus.

So I might bite the bullet and remove the SMPS board from the scope so's I can remove some bits to put on the component tester. (Cheapo one from eBay as I'm not professionally equiped).

Keep the ideas coming - they are keeping my spirits up.

[evingar]

Yep, I'd go along with all that Bill says. I repair a fair few SMPSU's commercially and you need to check all components that may have received a "jolt" from a failed MOSFET channelling HT through its gate.

It's often moot point what comes first, a fault causing the MOSFET to fail or a MOSFET failing due to line spikes (etc) causing mayhem elsewhere.

Specifically in your case, aside checking for destroyed silicon and resistors, make sure that the primary side decoupling caps are up to spec (with an ESR meter) and check out the opto-isolator in case any of these were the original cause of the problem.