H/K Citation 11 grounding issues

I am very close to finishing my H/K Citation 11, 12 & 15 restoration. Citation 15 needed only basic recapping of electrolytics and grounded mains lead. I recapped 12 with correct dia CDE computer grade electrolytics at 12 000UF/63V and used decent quality smaller electrolytics. Grounded mains lead was installed. I twisted live and neutral leads inside chassis for lower radiated hum. Also second ground lift resistor was added, as chassis is now grounded by mains cable. There is considerable DC offset on both outputs, but it seems that there is not too much trouble about this issue.

11 proved to be more troublesome. My unit seems to be similar to Sir Byrd´s Citation 11/HK 430 recap tread and H/K Technical manual starting serial no. 2060001.

There was considerable ground loop hum. It took some time to find that 10 ohm ground lift resistor was added between main filter cap and chassis ground. For some odd reason there were two black wires running from chassis end of ground lift resistor to output and high level input RCA:s, even inputs were already grounded to signal ground. Cutting these wires cured the ground loop hum.

As sound was edgy and switches and pots performed well, I considered output relay must be cause for edginess. I simply bypassed relay connection to output by wire link. This cured most of edginess and had no adverse effect for output muting.

Two under chassis 500 UF axial electrolytisc were grounded to right channel output ground, even one cap was used for smoothing left and other one right channel line stage supply. I added two way turret and second ground return wire to left channel output ground and performance improved clearly.

I noticed also that both L and R channel Line stage inputs are grounded directly L channel EQ board... this seems a bit odd... but it is difficult to trace where all ground wires go.

Everything except ground lift resistor seems to be original. Most likely there has also been faulty ground lift conversion bulletin available somewhere, as Sir Byrds 11 does have exactly same faulty conversion. Somehow I think that there is still some unsolved grounding problem.

As Citation 12 is performing quite well now... maybe slight edginess and not too good image depth, but pretty involving and dynamic. Citation 15 is very good by any standard. Therefore I suppose that it is reasonable to expect that 11 will be performing close to this standard, but my 11 is quite far from this now. Maybe David or somebody else do have any suggestions or idea how to proceed.

Best Regards

Kimmo

kimmo said:

I noticed also that both L and R channel Line stage inputs are grounded directly L channel EQ board... this seems a bit odd... but it is difficult to trace where all ground wires go.

Click to expand...

Line stage inputs were actually grounded to R channel EQ board output.

I removed L cannel line stage input grounding wire from R channel EQ board output ground and wired it to L channel EQ board output. This improved sound quality considerably.

I must keep digging deeper in this... it seems that the Citation 11 engineering is not 100% perfect at all.

Best Regards

Kimmo

Citation 11 grounding

I have now sorted some grounding issues of my Citation 11 since my previous tread.

L and R phono input and output grounds were originally separated, all other signal grounds were tied together at terminal board near input selector. I tied all left channel grounds to one point and all right channel grounds to other point. Both L and R channel high cut filters used also one channel for grounding 10n capacitors. I rewired circuit for using L and R channel grounds for L and R channel filters.

Now left and right channel signal grounds are tied together only at main filter capacitor. Ground lift resistor is also installed between main filter capacitor negative terminal and chassis. I also upgraded signal ground wiring running between input selector switch, pots, EQ board and output board to slightly lower resistance wire.

I also tried if shorting ground lift resistor would be OK. At least in my setup ground lift produced more balanced sound, even there were no hum problems with signal and chassis grounds tied together. This all made line amp sound considerably better. It seems that output is now more accurate and signal modulated noise is reduced. There is still 0,5 mV noise and 1 mV hum/ripple at min and max volume settings, and 1 mV when volume is at 25K ohm. 15 hz noise that was clearly visible after recapping, has been reduced such a way that it is invisible with my modest 25M scope.

When I measured how L and R channel EQ:s track to ea. other, I discovered that R channel 320 and 1K sliders do not have any effect. However the sliders were OK, but it was actually odd to find that one 50mH and two 300mH coils had been open circuited. When I measured good coil from L channel, I discovered that 50mH coil does have 40 ohm DCR and 300mH coil does have 170 ohm DCR. When I tried to find replacements from Miller or Hammond, ferrite cored RF coils do have roughly 4 times higher DCR:s and 300mH replacements are too bulky to be fitted in the can.

DCR of 50mH coil can be sorted by reducing 470 ohm series resistor enough, but shorting resistor is not enough for 2 300mH coils. And their size is also too much. So... if anybody do have any idea where to find 10mm x 18mm sized inductors, or even close to this, at 300mH/170ohm DCR or have any advice how to rebuild the originals. They have most likely been produced for audio, as core material must be the reason for low DCR and small size. :screwy::screwy::screwy:

Best Regards

Kimmo

Citation 11 grounding

I have now sorted out one more grounding issue in my Citation 11 since my previous tread.

Output relay control voltage ground currents did flow from power supply PCB to power transformer CT via line amplifier PCB using 15 cm length of audio ground. I disconnected control voltage ground from right channel audio ground at line amplifier PCB. After this, I grounded power supply PCB control voltage ground pin to main reservoir negative terminal. I used too thin grounding wire and this created considerable noise in right cannel output, maybe 2-4 mV with plenty of upper harmonics. When I increased wire gauge and used slightly shorter wire link, right channel hum decreased to less than 1mV level.

It has been quite surprising that there has been no response to this grounding tread. These hardly noticeable tweaks have improved performance of old preamplifier much more than installing exotic caps, resistors, wire or semiconductors... cost of these mods have been next to nothing. The amplifier is not exactly original anymore, but in my opinion, Citation should have been built like this.

Best Regards

Kimmo

Citation 11 & ARC D76A

I suppose that my Citation 11 rebuild is now finished, or there is not too much to do if fairly original appereance and topology is to be maintained.

My first impression was that Citation 12 and 15 were the best products in my 11,12 & 15 set and 11 was not too impressive. Now when I have used them together and with other gear, it seems that 12 was the limiting part of this set up.

When I used 11 with restored 12 and restored Audio Research D76A, it was 100% clear that 12 was the reason for conquested and one dimensional HF, especially with complex and full blown material. LF performance of 12 was clearly less defined than with ARC D76A.

Citation 11 and ARC D76A combination produced open and three dimensional sound. LF was more accurate than with 12. Low level definition was not as good as more recent high quality gear do have, but not too bad at all.

There is still small amount of line related noise with 11. It might be due to the odd looking main filter capacitor earthling arrangement. I have tried to find better ideas without too much luck. However amount of this hum is not enough to be heard at listening position, but it is modulating audio signal anyway.

It seems that Citation 11 is little gem, when rebuilt properly and when grounding issues have been sorted out. It would be nice to find and rebuild ARC SP3 for comparison...

Best Regards

Kimmo

Pics or circuit diagrams would be much appreciated! Sometimes it's a little tough to follow the text explanation alone. Looking to get my citation 11 to run as smoothly as yours

Pics about Citation upgrades

I took some pics about my Citation 11. You can also see Citation 12, 15 and 17 combo behind Citation 11.

1st inside pic will show that I used mostly Nichicon audio and low impedance PSU caps for recap.

From 2nd inside pic lower left corner you can see how L & R channel input grounds are now separated on tag board . Heavier gauge ground wire is now used from tag board to pots and from pots to switches and later amplifier stages. Unit is very late production as mains transformer build date is 20/76. Note green/yellow main safety ground wire connected to chassis in lower right corner of photo.

In 3rd inside pic you can see how bk and rd wires bypass relay contacts when mute is not operating. Relay shorts output when mute is in use. Gray jumper wire is somehow related to separating audio and relay control grounds. Please note from following pics that PSU and line amp connectors do have separate control ground pins, but they were not used in this amp. Maybe they have been used in earlier models.

From inside pic 4 you can see that 10nF high cut caps are not connected to one channel anymore. Signal wiring is also twisted to ground wire of same channel from eQ switch on front panel to eQ board. L channel 470 uF PSU cap is not connected to R channel output ground anymore.

From last pic you can see that I replaced line board to output connector wiring with heavier gauge ground wire... but noticed no difference. I would not do it again. Output relay wiring is not too well seen, but in original tread it was explained.

Best Regards

Kimmo

PS there were also some other mods in original tread, hopefully pics will show now,how they were made

you're the man Kimmo, thanks