Repair of Hameg HM604-2 - Page 1
Forum section: electronics / repair

author: steveod21
author_rank: Newbie
timestamp: February 08, 2017, 05:37:11 pm
content: I've just had to go inside my venerable Hameg HM604-2. These old analogue scopes are a dying breed, but they offer some advantages including very low noise performance as some (this one) still use linear PSUs.The fault was that the couplings to the two vernier amplitude controls, which are concentic to the amplitude switches, had both let go (one was so loose that when I tilted the scope to move it, it slid out of the switch centreAs this scope has been so reliable, I hadn't needed to go inside previously, but when I saw where and how the preamplifiers were situated, it looked nigh on impossible to get inside them without everything coming out of the chassis first!The Hameg service manual, although it contains the all essential circuit diagrams, has no procedures to guide you in a tear down. I searched on the net, but couldn't find much to help. I did find one thread where someone had a similar problem, but I couldn't find whether or how it was resolved.Removing just 2 rear panel screws allows the cover to slide off. I could now see the 2 preamplifiers in their steel cans. Take any actuating rods which are in the way and set them aside. Removing all the knobs etc and then the front panel via the 6 self tappers into the front panel plastic moulding exposes the nuts that hold the amplitude switches to the metal sub-front panel. Also there may be a wire soldered to the front panel inner metal layer which needs to be desoldered before you can lift away the plastic and metal composite front panel.Then the 2 metal strips which come from the sub-front panel which are soldered to the bottom section of the preamplifier can are desoldered, you might as well at this stage desolder the bottom section of the preamplifier can too, and remove it from the main preamplifier can, you need it off later anyway. Be aware you need a lot of heat and maybe a 100W iron would be better for the job. My puny 50W Ungar really struggled! (See photo 1).Next desolder the short flexible wire from the input BNC card from the preamplifier pcb (the blue wire in photo 2).Press down on the white multiway connector at the end of the preamplifier to remove the ribbon and Channel 2 can be jiggled up and out of the chassis. The main preamplifier can has 4 indents which are soldered to the pcb. Desolder these (photo 3) and release the nut holding the switch to the can, and the preamplifier pcb can be removed.Now I could see the issue, as if I hadn't guessed: the plastic coupling had split. You can take the bracket that holds the potentiomer off the preamplifier pcb, it's just 2 nuts and bolts and you need to desolder the resistors on the pot too.(photo 4) I took some time in glueing and refitting the coupling to ensure that the inner shaft was now tight enough.Channel 1 got the same treatment but there was also, on mine, a very thick wire connecting the Channel 1 main preamplifier can to the chassis below, and it needs that big boy iron again! (photo 5) shows channel 1 refitted and channel 2 will be next.Refitting the main preamplifier cans by resoldering the 4 tabs, and the smaller bottom can which needs resoldering to the main can gets the preamplifiers ready to be refitted. I couldn't decide if fitting the bottom can after installation might be better, as at least you can get to the input wires to resolder them to the preamplifiers.(photo 6).The last hurdle is the refitting the front panel with all those knobs and switches to align - what a pain, but take your time and don't force it.

author: CJay
author_rank: Super Contributor
timestamp: February 08, 2017, 07:15:18 pm
content: Well that looks 'a bit of a bugger' to fix.I'd have suggested you scrap it and send it to me as it was going to be such a PITA to do if you hadn't fixed it already.Nice teardown pics and repair, I have a soft spot for those old Hameg 604 'scopes and would love to find one at a decent price

author: steveod21
author_rank: Newbie
timestamp: February 09, 2017, 12:16:37 am
content: Yes, you are right; although I wasn't sure if the word 'bugger' would be acceptable on the forum, it IS appropriate!I think you shouldn't have to pay more than £100 or so. But often they are sold by people who haven't a clue and you could buy a complete dog!Still if you have the time and are sufficiently dogged, you can most often get them working unless the tube or some other irreplaceable part has perished.All the bestSteve

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: February 09, 2017, 11:05:41 am
content: Thanks for the teardown. I have the 605 which has the same layout. I actually managed to replace the two caps on the end of both boards without disassembly but it was very tricky! Its interesting to note that the 604 has tantalum caps whereas my 605 has alu caps on those boards - at least those that are visible. Having replaced the smoothing caps and a handful of others which seemed opportune to replace at the time I have to say that mine has so far been very reliable and I haven't had the problem with those couplings, but I will file this information with my notes for future reference in case I ever have to go inside and remove those boards.One curious thing I noticed is that mine will display a signal up to around 85-90MHz - well beyond its specified 60MHz bandwidth, although with my sig gen I can't reliably measure amplitudes at those frequencies. The curious bit is that although the signal fades out at around 85-90MHz, I subsequently discovered that it can display signals upwards of around 98MHz-100MHz. I'm not sure how far it can go and again, I can't be exact with those frequencies, since my sig gen only goes to about 110MHz and is not that precise. I could definitely get a 100MHz signal displayed although I cannot explain the gap.For whatever reason. these do seem to command a decent price. I was fortunate to get mine as spares/repairs and it only required getting rid of the cobwebs, evicting the spiders and a fairly minor repair.

author: CJay
author_rank: Super Contributor
timestamp: February 09, 2017, 12:23:47 pm
content: They're a good piece of kit, reliable and still very usable.I've never measured the -3dB bandwidth of my 303 but I know it's more than the claimed 35MHz, it still displays and triggers on signals (at much reduced levels) past 50MHz but it's to be expected of any half decent 'scope.Where I think it's better than a lot is in the triggering, they're just a doddle to set up and use, I also find the component tester incredibly useful for repairs on analogue equipment.