HS39 grounding issue and hum

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    • HS39 grounding issue and hum

      Hi guys,

      First of all, I hope you don't mind that this post is in English. My German is not good enough unfortunately. (What we Belgians call "Jean-Marie Pfaff- Deutsch)

      So I repaired an old Dual 1224 for my dad and I decided I wanted a Dual idler drive for myself. I pickuped a lot of three players with two HS39 bases for repair for €25 euros.

      One of the HS39's is broken with a short circuit, but the other one works! I've had to replace the 'Knallfrosch' but other than that it works at both channels.

      But now my problem:

      There is a constant hum at the speakers/headphones that gets louder with the volume. Some hum is to be expected with these old amps, but I've never heard as much as with this one.

      More importantly: I could hear a change in the hum when I touched the faceplate or the volume dial of the amplifier. To me that sounds like a grounding problem so I measured the voltage from the chassis to the earth ground in my house and I got 85 VAC!
      I know some old amps are designed with a 'live' chassis, but 85VAC seems to be high and potentially dangerous. Especially since the dials and buttons on the front are connected to the chassis ground.

      Does anyone know what could be causing this issue?
      So far I've replaced the elcos in the power amplifier, but not yet anywhere else. all ground connections work as shown on the schematics.

      Any help is appreciated and thanks in advance.

      PS: You can answer in German, I read it a bit :)
    • Hi !

      Gertjan schrieb:

      More importantly: I could hear a change in the hum when I touched the faceplate or the volume dial of the amplifier. To me that sounds like a grounding problem so I measured the voltage from the chassis to the earth ground in my house and I got 85 VAC!
      I know some old amps are designed with a 'live' chassis, but 85VAC seems to be high and potentially dangerous. Especially since the dials and buttons on the front are connected to the chassis ground.
      I would guess that this is "false measuring".

      The HS39 -by design- has a 2-wire, non-grounding mains cable. So it *may not* be connected with the green-yellow protective ground. It has been designed as an encapsuled device for dry indoor rooms. Therefore it has no "Schutzkontakt-Stecker" but most likely a formed plastic plug with no outer contacts.

      So - there is some sort of AC-transformer "crosstalk" when you have various devices attached to the mains from which some do use the protective ground and some don't. Essentially the mains transformer isolates the HS39 on its own and the output voltage is attached to the chassis ground via the "Minus" (GND) from the power supply only.
      Unless someone cut off the original cable and attached a 3-wire with outer grounding contacts on the plug. And with the protective ground wired to the HS' metal chassis. Now: this is potentially dangerous, since isolation faults *on other parts on the mains* could cause a dangerous voltage on the metal parts of that unit, which is designed to be floating above the mains voltage.

      Coming back to the hum:
      Basically the HS39 is pretty much quiet. There is a known problem with the relatively long wiring from the source switch towards the input sockets on the rear. This might cause some stray hum due to some soldering problems on the shielding part of the connecting cable. The source switch is another problem. It is an open type dial switch, which tends to contact oxydaten.

      Other than that the power supply filter capacitor tends to decay over the centuries as well as the supporting capacitor on the pre-amplifier board. Particularly if these are one of the solid plastic "ROE"-cylinders in wine-red, orange or light grey.

      Look at
      dual.pytalhost.eu/hs39s/
      as long as our server is still maintained.

      The main capacitor is C36 with 2200µF / 20V on the main amplifier board. It is the big axial "roll" in the middle.
      There is one supporting capacitor C38 with 47µF / 16V as a buffer to both channel input transistors T30 sitting on the main amp board. Another one is on the preamp board. It is C18 with 47µF / 16V buffering both channels volume control output buffer T10.

      While being on it: the two output capacitors for the main amp should be replaced.
      They are 470µF / 10V and when they reached a point of internal aging they do a short circuit, which -at least- blows the internal fast 0.5A fuse, but fairly often destroys the final (germanium) transistors as well before the fuse blows.

      If you replace the caps: higher operational voltages are okay.
      So if you can't get 47µF / 16V you might use one with 35 or 40V as well.

      Hope that helps a bit.

      ^^
      Peter aus dem Lipperland

      Solo mio, vendro unscrupuloso, custombres sansaclu.
    • hello,

      Thanks fot the reply!

      The HS39 still has the original two-prong plug.

      So far I've replaced all the electrolytic capacitors, and I notice some improvement.

      I occasionally still get the hum and distorsion in the right channel, but it comes and goes when I fiddle with the knobs, especially the volume knob.
      All knobs have been cleaned with Kontact cleaner, but I'll try replacing the volume pot with a spare one tomorrow.
    • small update:

      I've switched the volume pots but now it gets worse. the hum is still there in the right channel, but now it wont go away no matter what I try..

      Is it common for these switches to fail completely or might i have to look at something different?

      The right channel still plays, but with a lot of distortion..
    • Hi !

      I guess there is something essentially wrong with the entire unit.
      Normally the HS39 chassis (as well as its cousins with the same base design) are rather straight-forward and cause little problems.
      I myself revived one, which has been sitting unused in a shelf since 1978. The 50nF tar-capacitor died on a first attempt and caused some internal cleaning and the main capacitor showed only 50% capacity, both output capacitors were shot. After replacing them the unit worked - apart from a very hard to operate On/Off switch. It needed a good cleaning and that was it.

      If it makes distorted noises you have a good chance that the amplifier is damaged already.
      Even with a pretty bad volume pot it should put out a clear sound on both channels. You might try unsoldering the thing again and directly apply a signal to the middle point on each channel. You need a regulateable sound source however since with no volume controls it starts at "full steam ahead". This test should make clear if the amplifier stage itself is working or if there are more issues.

      hifi-archiv.info/Dual/hs39s/hs39-02.jpg

      If there is still distortion you'll need to sort out if it is the pre-amplifier / tonecontrol causing the problem or the final amplifier(s).
      For that purpose unsolder the two wires from the pre-amp to the main amp going to C30 (L) / C30' (R) on the board and apply a signal on the contact at the final amplifier.

      hifi-archiv.info/Dual/hs39s/hs39-03.jpg

      If the sound is still distorted you have an issue with the final stage. Check the fuse and with the unit powered the voltage on the + end of the output capacitors C35 (between T32 and T33). It should be around 7.3V. Depending on that voltage you might identify the cause of the problem. It might be C34 being damaged or a shot T32 if the voltage is far above 7.3V. It could be a damaged C32 or damaged T31 if is significantly lower. T33 barely dies, because a shot output capacitor connects the two final transitors towards GND over the loudspeaker and T33 is already connected with one end to GND. Therefore the fuse sits on the T32 leg only, which connects to the operating voltage of about +17.2V

      ^^
      Peter aus dem Lipperland

      Solo mio, vendro unscrupuloso, custombres sansaclu.
    • Hello again!

      The problem probably has nothing to do with the volume switch, because yesterday I found out that when I tap the print board on the right side of the power amp, the hum disappears! Especially when I tap on the zener diode there. I suspect a bad solder joint or maybe failing diode. The SM says D30 is a BZX62, and I measure 0.75V 'zener voltage', could this be correct?

      When I get home I'll check for bad solder joints.
      Does the power amp board susposed to have a little protective plastic board under it, like the power supply board? because mine might be missing.

      By the way, when the amp plays normally, the sound is quite bass-heavy, is this normal?
      It doesn't bother me too much, but I've had to turn down the bass on an old amp before.
    • Hi !

      Gertjan schrieb:

      The SM says D30 is a BZX62, and I measure 0.75V 'zener voltage', could this be correct?
      It is a "temperature compensated" silicon diode. Essentially it can be replaced with a 1N4148.
      Its purpose is a defined voltage drop for quiecence current, which is not adjustable on that amplifier. The only thing you could do is tune the *gain* of the leftside main amplifier for even level.

      When you are examining the board please check the NTC resistor R41. You might need to unsolder one leg and check its resistance - and while blowing on it - if it changes resistance. The "cold resistance" at about 20°C should be 40 Ohms - +/- 3 or 4 Ohms does not hurt, if that of the other channel is in the same range. They were in the circuit to turn down the quiecence current when the amp gets too hot to avoid damage of the final transistors. Well ... nice try ... but when the overall resistance along with the paralle resistor R38 get too high then the amp runs at too high idle current. Both should not exceed 24 - 26 Ohms at best.

      ^^

      Gertjan schrieb:

      By the way, when the amp plays normally, the sound is quite bass-heavy, is this normal?
      This is an effect of the permanent, not switchable Loudness function within the volume control. Down from about 2/3rd of the dial way the loudness effects the playback. Down from about 1/2 of the way it is *pretty* fat and more than neccessary.

      On this amp the Volume knob can be pulled to switch between stereo and mono. Later / bigger HS had a "Contour / Linear" switch at that place. It has been chosen to give the comparable simple broadband speakers a tad more "sound", but it is - as you noted - a bit overdone with some better speakers.

      It is also a reference to the 1970s listening habits. Records back than generally lacked subsonic bass and the deep tone spectre was at above 100 Hz for a bassguitar and a piano in best cases. With the advent of massive electronic gear and bad mixing habits modern records sound quite stuffed and over-instrumented with this kind of old equipment.

      ^^
      Peter aus dem Lipperland

      Solo mio, vendro unscrupuloso, custombres sansaclu.
    • ...

      Gertjan schrieb:

      Does the power amp board susposed to have a little protective plastic board under it, like the power supply board? because mine might be missing.
      ...

      I don't think so. It is latched with bended metal pieces stamped from the chassis as far as I recall and the heatsink part of it is bolted down to it. The distance between board an chassis should be sufficient.

      You'd meant the transformer connection and fuse board most likely.That is a different story.
      The regulations required an isolating layer between mains conducting copper and wires and the metal chassis.

      ^^
      Peter aus dem Lipperland

      Solo mio, vendro unscrupuloso, custombres sansaclu.
    • Hi!

      First of all, thanks again Wacholder for the extended replies!

      I'm getting very close to the source of the hum.
      I can eliminate the hum by unscrewing the metal 'hood' that covers the right side output transistors. So I guess there may be a problem with one of the transistors touching ground?

      I'm going to try and isolate the transistors with some mica isolation plates and heat-transfer-paste.

      Oh yeah, I should have known about the Loudness/contour feature. I used to have a little Philips amp with this feature, although it was a lot more subtle than the Dual. I'm going to pair the HS39 with some Philips 4 Ohm speakers from the sixties, should be suitably vintage :)
    • Hi !

      Gertjan schrieb:

      I'm getting very close to the source of the hum.
      I can eliminate the hum by unscrewing the metal 'hood' that covers the right side output transistors. So I guess there may be a problem with one of the transistors touching ground?
      Hmm. No. The transistor cases on AC180 / AC181 are not connected to any of the pins.
      But maybe a) there is a bad solder spot / cracked copper on one of them or b) one is damaged.


      Gertjan schrieb:

      I'm going to pair the HS39 with some Philips 4 Ohm speakers from the sixties, should be suitably vintage
      Philips made some surprisingly good speakers back then.
      They sometimes looked a bit uncommon (not to say: cheesy) but the sound was fairly often a real surprise.
      Could be a good combination. The Philips wide-range chassis (Philips / Valvo with the octagonal outer frame) did sound very good.


      Gertjan schrieb:

      First of all, thanks again Wacholder for the extended replies!
      It's fun. I like that sort of stuff.
      Fortunately I have a good IBM Keyboard and can type quite fast. That allowes me to throw in my stuff in a few minutes ...

      :D
      Peter aus dem Lipperland

      Solo mio, vendro unscrupuloso, custombres sansaclu.
    • I found it!

      The base lead on the right channel AC180 was broken inside the insulation wrap.. so sometimes there was contact and sometime there wasn't, so hum.
      The wire has broken very close to the transistor, so soldering was impossible. I probably broke it while trying to fix the hum in both channels caused by the aged capacitors. Fortunately I have a spare set of AC180/181 transistors from the broken HS39. I assumed these were shorted because the unit blows a fuse when I try to power it up, but the outputs turned out to be ok!

      I swapped the AC180 and everything works as it should. So now I'm never taking it appart again for fear of breaking another wire :D

      I got the HS39's with a couple of turntables, a 1216, 1215 and 1214. They all work (after some cleaning and re-greasing) so I decided to put the 1215 in there (original 1214 was rusted and doesn't look as nice and the 1216 got a seperate base hooked up to my 'main' amp)

      I'm a bit disapointed with the X-tal 650 cartridge so I'm going to install a cheap pre-amp board into the amp and try it with a spare Shure M75D catridge.
      Do you know if this is feasable? I've heard the original 1214 2-pole motor doesn't allow for a MM cartridge but the 1215 has the 4-pole motor. (I've already put the 1215 on 220V instead of 150V and that works)

      I hope to build the HS39/1215 into a fine bedroom unit :)

      Thanks again for the help!
    • Hi !

      Gertjan schrieb:

      I found it!
      :thumbsup: :thumbup:

      Gertjan schrieb:

      The wire has broken very close to the transistor, so soldering was impossible.
      Unfortunately on most Germanium transistors the wires are rather stiff, thin and brittle. Bending them a couple of times will surely break them. Moo !

      Gertjan schrieb:

      I'm a bit disapointed with the X-tal 650 cartridge so I'm going to install a cheap pre-amp board into the amp and try it with a spare Shure M75D catridge.
      The CDS650 suffers on hardening and deforming needle holder. This results in a rather limited sound spectrum. It is known for that sort of pitfalls. You could easily swap it for its successor CDS660 and then buy an aftermarket needle with a diamond tip. The sapphire tips are run down after maybe 200 hours latest and are weak and prone to break when you accidentially drop the arm on the record or against the record edge. The tip then splinters *and you won't probably notice* at first - and ruin the records. The original DN85 diamond tipped needle is rare and quite costly, but there are dealers which sell clones, which are fairly good.

      (Original)
      thakker.eu/tonnadeln/dn-85-nad…-tks-671-original/a-7900/

      (Substitute)
      ebay.de/itm/Ersatznadel-DN-8-f…8d45b4:g:mfoAAOSw~bFWNerH

      (CDS660 with old sapphire needle)
      ebay.de/itm/Dual-CDS-660-Tonab…b72a42:g:N7YAAOSwm8NbCq3h

      Gertjan schrieb:

      Do you know if this is feasable? I've heard the original 1214 2-pole motor doesn't allow for a MM cartridge but the 1215 has the 4-pole motor. (I've already put the 1215 on 220V instead of 150V and that works)
      It can be done and I did it with a portable P44 unit - featuring the rare 1210A and almost the same internal amplifier. I managed to get a magnet preamp from a crapped KA25, which runs on 12 - 19V (which is, what the amplifier uses), added a 1K5 Resistor and another 220µF / 25V capacitor to reduce power supply hum - and that was it.



      It got a stock M75D with beige N75-6 needle and standard TK and it runs - on the spring-balanced arm of the 1210A - at 2.5p.
      No problem.
      It *might* probably hum a bit an the motor *may* insert some stray signals - but honestly: with that kind of amplifier and speakers you don't await high-endish behaviour.

      With the 1215 you've got a nice player and the over-all-results might even be better than on mine. Particularly if you don't use these "Cover Speakers" that form the upper cover an transports, but some more serious speakers.

      The "lesser work"-approach on a unit used only occasionally (and in the background) would be the CDS660 + DN85 needle.
      You could do the swap within minutes.

      ^^
      Peter aus dem Lipperland

      Solo mio, vendro unscrupuloso, custombres sansaclu.
    • That P44 looks nice!

      Did you install the pre-amp in the chassis?

      I got myself a Velleman K2573 pre-amp kit, which runs on anything from 10VDC to 30VDC, but 18VDC is recommended.
      The plan is to take the power supply from the HS39 amp. after the rectifier diodes, there is 17.2V on the board.
      Could I simply hook up the 17.2V to the + of the pre-amp and run - to ground to power it?
    • Hallo Gertjan,

      yes, this will work. I did this in the past with my HS39 as well.
      I have allready refurbished my one witch I have since 1973.
      It was a Christmas gift from my parents to my brother and me.
      I snatched two CL35 as a bargain for 30€ at ebay. This speaker fits very well. It is a simple two way speaker.
      I hope you are satisfied with your one. It is a nice little pice of its day.
      I will have a look for pic's from mine.

      Viele Grüße
      Frank
    • Hi !

      Gertjan schrieb:

      Could I simply hook up the 17.2V to the + of the pre-amp and run - to ground to power it?
      As Frank already noted: Yes - that will work.

      One way or another.

      It *might* get a little hum on the supply, that's why I put an extra 1.5K / 0.5W resistor between the HS39 power supply and that of the preamp *and* added a 220µF / 25V capacitor on the preamp board to get around potential problems. But if I were you I would test it out "as is". If it is sufficiently quiet it will do.

      The P44 has been in use when my girl-friend and I were sitting out on the veranda on sundays a few years ago. Anke had a box of mainly 80's singles which we used for that purpose. Anke died in August 2015 and I plan to pull out the P44 for one summer event anyway. Just as a reminiscense on what fun it was. ^^

      .
      Peter aus dem Lipperland

      Solo mio, vendro unscrupuloso, custombres sansaclu.
    • Hello again,

      I'm sorry to hear about your girlfriend, Wacholder.

      I wanted to give a little update:
      Installing the preamp was easy enough, and it worked. but.. it was so full of noise i couldn't listen to it..
      Some (50Hz?) signal was overpowering the music and it made my speaker sound as if they were about to explode.

      So tonight I tried an external power supply for the preamp and now it works beautifully!
      An old laptop charger was sacrificed, I pulled the wires out the case through the backplate and soldered a plug at both ends.

      Now I have a nice little bedroom unit!

      I still have some plans to mod a spare HS39 case I have with cinch-plugs for phono-out and -in, so I can use the preamp or use a X-tal cartridge if I choose.

      Anyway, once I get it cleaned up a bit and install everything I'll update this post with some pictures :)