harman/kardon A30K recap: final stage help

[onanysunday]

Hello everyone. Just looking for a little help buttoning up a restoration. I've mostly recapped this gem with green ERO MKT1822 and red Audyn Q4 caps. Sounds much better than it had before. However, I'm at the final stage tackling those 3 large multi-section capacitor cans on top. I've increased capacitance and voltage as per prior AK recommendation and ordered suitable replacements that are somewhat "mocked" in place (see notepad photo) They are C23, C24, and C25. C23 is soldered in place and DONE. I need help determining if I should just twist the negative leads of the new caps together with the the black wire(s) for C24, C25 and what the yellow line is for which connects these caps to tubes that I don't see listed in the schematic? Where do the yellow leads need to be re-attached after I have disconnected them from the underside of the original, multi-section caps?

I'm also looking to replace those unusual, heavily oxidized and worn RCA input jacks. Ideas?

Still getting a 60Hz hum which may be remedied after completing the full recap. But..the hum is interesting in that it diminishes about 5-10% when I touch the chassis. And, interestingly, my hum actually goes away intermittently on both channels and then comes back very faintly on its own while listening to music. I think there was once a suggestion somewhere about splitting the bus bar into two parts to make it quieter? Any help with the C24 or C25 multi-section caps or hum is appreciated. Thank you.

* Here is a link to a larger, more legible schematic that was saved mislabeled. https://manuals.harman.com/HK/Service Manual/A500 schematic.pdf

 

[onanysunday]

This is a nice sounding piece, I'm excited to finish the project. There a couple other things I thought worth mentioning. It has a (new) quad of the Genalex Gold Lion driver tubes. The 12AX7 are all old Telefunkens and the one 12AU7 is a NOS W. Germany Amperex. I like this tube selection compared to others I tried. Many of the original ceramics are the 80/20 type. I'd like to upgrade the pair that runs parallel with the two diodes (circled in yellow) which I'd also like to switch over to a fast-recovery type. Generally speaking, does there need to be looser tolerance 80/20 ceramic caps in a circuit -would it always be better or worse to switch them out with 1% COG/NP0? There is a lot of slop in the tolerance of some of these old circuit designs; is that, in part, perhaps what helps give these older designs their beautiful, characteristic sound? I'm just looking to clean up the hum while adding some more oomph and body to the low-end and midrange with higher capacitance/voltage electrolytic replacements. Also, the larger 470 ohm carbon composition resistor from C23 -- to C24a + appears burnt and measures 390 ohm. The HK has excellent imaging, harmonics, stage, depth etc. In its current form, weak areas remain: hum and softer bass impact / midrange definition. Its sonic signature appears to be weighted towards the mid-to-top end with delicate highs and incredible realism.

 

[solderjunkie]

I'm just looking to clean up the hum while adding some more oomph and body to the low-end and midrange with higher capacitance/voltage electrolytic replacements.

Have you replaced the power cord with a three-prong grounded type with the earth wire grounded directly to the chassis? I have two of these (A300's) that both hummed badly when I first acquired them and the most significant hum reduction to near silence was from simply doing this.

Agree that these are very nice sounding amplifiers. I use HSU Research HB1-MK2 bookshelf speakers with a Dayton Audio powered subwoofer which gives me plenty of volume and a very dynamic range. Even without the sub there is plenty of low-end. Rather than attempting to re-voice the amp have you thought about the speakers you are using?

 

[kward]

You don't want to twist the neg leads of C24 and C25 together--not directly anyway. The thing you must do is connect the anode of the diode and the neg lead of C24A directly together. Don't connect any other ground wires on that line between those two points because there is high ripple current flowing on that part of the ground circuit. To connect C24B to ground, connect another ground wire from the negative lead of C24A to the negative lead of C24B. At that point it is fine to connect that to the ground bus with another length of wire.

 

[onanysunday]

solderjunkie - Thanks for the power cord suggestion. I had forgotten about replacing it with a higher quality, thicker cord. It's good to hear you've had success reducing hum with a third-prong grounded to the chassis. I read some people said it had potential to cause even more humming, creating an additional earth/ground loop? Glad to hear this isn't the case. I was thinking of adding an earth connection between the amp and something else, just not through the power cord, but maybe those two scenarios are identical in practice? I do know that when I connected a thin wire between the phono ground post on an another amp and the mounting screw on its outlet cover, it did reduce hum. However, I'm no expert on humming or the difference between earthing and grouding.

kward - Thanks for the heads up on not connecting all the new, separate negative capacitor leads together for the given multi-caps: C24,C25. However, I'm a little confused. According to the schematic. It looks like, if anything, the cathode of diode would be connecting? to C24a negative and not the other way around. Also, which of the two diodes were you referring to. According to my underside photo there are two, top and bottom.

After re-reading some old posts and gathering new information, I am getting close to understanding it. From what I gather, I leave the outside mounting tabs on the old multi-caps intact while I clip off the inner, electrified tabs which are re-used and soldered to the new electrolytics. The mystery yellow wire is still confusing which also connects the old multi caps to various tubes. But I gather I just leave this line alone because I believe it is just for.. something else not relating to a recap? Some of the multis have more than one black wire attached to their leads, but maybe all black leads are separate grounds all leading to the same place and can be used or connected to one another interchangeably when connecting the negative leads of the new caps? I think the new capacitor solder connections will just be sort of floating wires underneath the chassis, whereas before they were all firmly connected to the multi cap. I have worked on a lot of solid-state equipment, not so much tube tech. It's hard for my mind to go from a flat PCB to a 3 dimensional puzzle of wiring with schematics that can seem incomplete or hard to understand.

Also, the C24 multi-cap posts aren't labeled underneath like C25 is, making it harder to determine which are the Square, Triangle and Crescent sections. I know which positive leg of the new caps go where, but can't identify which old posts they attach to. One was easy, C24a + to red/white. C23 was also easy as it was only one cap. The others, not so much. Especially C24c which appears to be shown on another circuit in a different part of the schematic? I can sort of read these things half the time somewhat, but not really.

 

[onanysunday]

solderjunkie- Before the A30k, I was listening to a newer Yaqin MC-13S hooked up with a pair of Rectilinear XII with upgraded crossovers, re-doped cloth surrounds on the mids and woofer surrounds replaced as well as new spiders. Original, inverted cloth surrounds were getting saggy -causing the voice coil to rub with heavy excursion. The speakers have wonderful presence and power and are very balanced sounding from top to bottom without the need for a sub. However, they can't be super efficient. I don't know the sensitivity of the Rectilinear or the RMS wattage of the MC-13 but it could be more than the h/k which could explain the hk's perceived softer output especially on the low end.

Btw, I love HSU products. I've had both their stf-2 and vtf2 mk4 subs. Most musical subs I have ever heard.

 

[kward]

onanysunday, here's what I mean with a picture:

I leave the outside mounting tabs on the old multi-caps intact while I clip off the inner, electrified tabs which are re-used and soldered to the new electrolytics.

I think the new capacitor solder connections will just be sort of floating wires underneath the chassis, whereas before they were all firmly connected to the multi cap.

You will want to install a new solder tab or terminal strip to achieve what is depicted above. Terminal strips look like this:



You will need to drill a small hole in the chassis and connect it with a 4-40 screw and nut. You can get these at Mouser, part number 534-817. I would not float these connections. Tie them down firmly to a terminal strip. Yes, clip off the positive terminals of the old can caps. You want to make sure the positive leads of these old caps are not connected to anything after you install the new caps.

The mystery yellow wire is still confusing which also connects the old multi caps to various tubes. But I gather I just leave this line alone because I believe it is just for.. something else not relating to a recap?

Where does that yellow wire connected to presently on the multi cap in the power supply? You want to refasten it to the same equivalent cap after you replace the caps, and leave it connected to whatever it is connected to on the other end that goes to the tubes. I can't quite tell from the picture previously posted where this wire goes, therefore I can't identify it on the schematic.

Also, the C24 multi-cap posts aren't labeled underneath like C25 is, making it harder to determine which are the Square, Triangle and Crescent sections.

You can locate them by proximity of other component connections. For example find R56 and R57. The caps that connect to either side of those resistors will be the caps in question. Take good close up pictures of the area before you start the work and refer to them as needed as you make the changeover to the new caps.

 

[kward]

Especially C24c which appears to be shown on another circuit in a different part of the schematic?

C24C is the cathode bypass cap for the output stage. In the original amp it was one of the sections in the multi-cap cans. It can be relocated to be nearer to the output stage tubes if you want. It just needs to conform to the specs shown on the schematic: 220 uF, 50V. (you won't be able to find a 200 uF cap most likely, but 220 uF is fine and is commonly available). By today's standards this will be a small cap about the size of your top most thumb joint. It is okay to use a larger voltage rating cap in this position, like 250V if they are easier to find.

 

[primosounds]

solderjunkie - Thanks for the power cord suggestion. I had forgotten about replacing it with a higher quality, thicker cord. It's good to hear you've had success reducing hum with a third-prong grounded to the chassis. I read some people said it had potential to cause even more humming, creating an additional earth/ground loop? Glad to hear this isn't the case.

A piece of equipment with a 2 prong power cord will not have hum issues if it is in proper working condition. Most vintage pieces have a ungrounded power cord and replacing the original power cord (unless it is defective) can be an issue for someone who wants the piece to be original down to the power cord. It is most likely that the hum on your HK is caused by the electrolytic caps. There are other areas in an amp that can cause hum, such as tubes, corroded contacts, heater cathode leakage, and are more likely than the power cord.

 

[solderjunkie]

A piece of equipment with a 2 prong power cord will not have hum issues if it is in proper working condition. Most vintage pieces have a ungrounded power cord and replacing the original power cord (unless it is defective) can be an issue for someone who wants the piece to be original down to the power cord. It is most likely that the hum on your HK is caused by the electrolytic caps. There are other areas in an amp that can cause hum, such as tubes, corroded contacts, heater cathode leakage, and are more likely than the power cord.

This amp is no longer even close to original so I guess that point is mute.

I too have had a lot of experience with vintage hi-fi and guitar amps that still had their original two-prong power cords and idled quietly. Yes, it could be the caps but it is also possible that it is simply the power cord. I've got two of these exact amps and save for their power cords, they are both 100% original, every cap, every resistor, down to their tubes. Significant hum before cords changed. Zero hum after cords changed.Coincidence? I think not.

 

[solderjunkie]

I picked up another completely origainal A300 with it's original two-prong power cord. The previous two examples also had their original cords and hummed badly until I replaced them with three-prong cords grounded to the chassis. This one too, had the hum issue that is so loud you can hear it over the music. I thought I'd do a quick vid of how the issue can be resolved by grounding the chassis to our electric service ground through the outlet box as a three-prong power cord would do.