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Harman Kardon SR 900 refurb (and tweaks?)

CosmoK

CosmoK

Active Member
I am refurbishing an H/K SR 900. A little background on this unit: it was possibly the first successful solid-state receiver hitting the market in 1964 or 1965 and selling for around $450. Inside it is all point to point wiring, with two output transformers and all(?) germanium transistors.

Here's a great restoration video of a different unit by MGUESS on youtube:

After replacing all the electrolytic capacitors it sounds pretty good. But, as I plan to give this to a close friend for his daily listening, I'd like to make it sound even better if possible.

Here are two minor issues:
(A) There is 60 Hz hum through the speakers even at zero volume on both channels. I've verified the frequency with a Fourier transform on an oscilloscope.
(B) I suspect the germanium transistors are noisy. The receiver's sound is not particularly detailed.

Here are my questions:

(1) Should I replace the power supply diodes with modern fast recovery diodes? I don't know the actual part numbers because the schematic is blurry (see schematic 1) and the parts use H/K's numbering system, but the manual lists some 10 A - 100 V diodes, which I assume are in the rectifier, and also some 0.75 A - 100 V diodes that are in the power supply circuit. I'm pretty sure the 10 A diodes are screwed through the chassis and secured with a nut.

(2) Should I replace the small germanium transistor in the common-emitter amplifier just downstream of the tone controls? They're called Q15 and Q22 in the lower left of schematic 3. I was thinking to drop in a silcon NPN workhorse like a KSC1845 or KSC1815. I know the germanium transitors have different diode drops, gains, and leakage, but does it matter here, will the feedback take care of the differences?

(3) Are there any other non-drastic things one could do to tweak the amplifier section of this reciever and improve its sound?
 

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CosmoK said:
There is 60 Hz hum through the speakers even at zero volume on both channels.
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Coming through both speakers points to the power supply.
Suspected cause
C124 -- ? can't decipher the schematic
C125 -- 3000µf/30vdc
C128A -- 3000µf/30vdc
C128B -- 200µf/30vdc
C126A -- 400µf/35vdc
C126B -- 200µf/35vdc
C126C -- 1000µf/40vdc
C127 -- 100µf/35vdc
 
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Since I replaced all the big caps, should I try installing some modern diodes? looking for noisy resistors?

The other thing I've read about is shielding. The old caps were in cans, but the new (much smaller ones) are hanging off the chassis (like everything else in there). Is shielding possibly a problem?

I'm mostly looking for a list of priorities from folks who've been to a rodeo like this before. :-)
 
CosmoK said:
(3) Are there any other non-drastic things one could do to tweak the amplifier section of this reciever and improve its sound?
Click to expand...
Make sure the power supply is 100% clean DC power.
 
You can try the rectifier diodes. Was this 60 hz present before you started replacing parts ?
 
Thanks avionic, this is great advice.

When I get back at it, I will step through the power supply with a DMM and/or oscilloscope and see if I can spot any problems. Before I started, I had no sound on one channel and good but not great sound on the other. I don't recall the 60 Hz hum, but its quiet enough that I can't hear it when I'm working in my basement and the furnace kicks on. It's annoying in the living room though.

Is there a standard modern diode that you would use for the 10A-100V spec? 1N5401 is common, but only 3A-100V. Should I look for some 10A1's?

avionic said:
I'm assuming this is plume full of noisy carbon composite resistors -- all junk IMHO.
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I already found a bad resistor coming off the +25V power supply. I will look for more and keep an assortment on hand after I place my next digikey/mouser order.

avionic said:
Good luck finding replacement germanium transistors.
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Well, I was hoping to keep the drivers and outputs but possibly replace a transitor further upstream. My hope was that a silicon transistor would work in the common-emitter circuit designed for a germanium... or at least work after changing the a resistor or two... but maybe that's not feasible or not worth the work.

I have no desire to replace the germanium drivers or outputs. If they are the cause of the noise, I'll just live with it.

avionic said:
Like most P-2-P wiring .. Wire routing is KEY to reducing induce noise.
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This is my first P-2-P project, so that's good to know. Again, if the factory wiring is the problem, I'll just have to live with it.
 
avionic said:
Make sure the power supply is 100% clean DC power.
Click to expand...

I inventoried the voltages on the power supply (schematic 1 above) and found all output voltages had some amount of sawtooth AC on them.

40 V output was 45 V DC + 1 V AC at 60 Hz
+/- 23.5 V output were 23.5 V DC + 0.5 V AC at 120 Hz
30 V, 25 V, 22V, and 10 V supplies were within 1 V DC and 50-100 mV AC.

These all seem ugly, but I've never looked at power supply voltages in detail before (the oscilliscope is a new addition).

The elecrolytic capacitors are all new, the only other thing between the outlet and the 40 V supply is the transformer and bridge rectifier.

The signal coming out of the transformer (connecting to the rectifier) was a 50 V sine wave at 60 Hz, except the upper and lower 5% of the wave was clipped.

I should note, this was done with the speakers slecter turned off, AUX input disconnected, tone out, and minimum volume.

Any ideas? New rectifier diodes? Search for loose ground wires?
 
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After some internet searching and oscilloscope probing...

I found that the hum gets louder when I wiggle the 1000 uF cap (C126C) that connects to the two 330 Ohm resistors in the power supply. The AC ripple goes from ~100 mV to about 5 V. So, I'll reflow the solder there and look into replacing the resistors (which are CC junk :)).

I also found that 100 mV ripple (or less) is the rule of thumb for filter capacitors. So, 400 mV on the +/- 23.5 V supply to the ouput transistors is high, but not insane. I guess.

To partially answer my original question concerning replacing the germanium resistors Q15 and Q22 with silicon (these are in the end of the tone section, right before the amplifier, schematic 3). It appears I'd need to replace the bias resistor and check/hope that the higher gain didn't destabilize the circuit. I'll hold off on this for now.

Dumb question: the AC signal from the transformer to the rectifier has 50 V AC amplitude [as in v = 50*sin(ft)], this should give me 2*50/pi = 32 V DC (minus 2 diode drops), but the schematic shows +/-23.5 V DC, and that's what I measure. What am I missing?
 
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I replaced the bridge diodes with a 15 A / 1,000 V module (Vishay GBPC1510). Since each of the original diodes was bolted to the chassis, taking them out produced extra space to bolt the new rectifier down.

I also found a filter cap that needed solder on one end. But, in the end I just replaced all the diodes and resistors in the power supply (except the 10 V zener, because I didn't have one on hand). I used 1N5404 for the two diodes in the voltage doubler that feeds the +30 V supply. I also moved a lot of junctions off the original can capacitor tabs (this took some problem solving), because I realized the grounded ends of the can caps were probably tied together inside the can, even if the can wasn't grounded. Meaning the original can caps were still a part of the circuit. Pretty dumb, but at least I didn't blow anything up.

I did some bias calculations for Q15 and Q22. The circuit seems relatively insensitive to changes in Vbe and hfe, which I think would be the main differences between germanium and silicon devices. The insensitivity would seem to stem from the fact that (1) this transistor is operating in type A amplification and (2) the circuit was (well) designed to be insensitive to hfe. At any rate, I dropped in a KSC1845 (with hfe of about 400) and the channel played, but was quieter than the original germanium at any given volume. After that I re-installed the germanium and called it a day. I would guess that if the germanium transistors ever fail, most of these circuits are simple enough to re-designed for a silicon transistor. But, I'm obviously missing something because my attempt at this was not a slam dunk.
 
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Some other oddities to note: there are two non-polarized 25 uF electrolytic caps in the phono stage that are clearly marked with a polarity in the schematic. I replaced these with polarized caps, but up a voltage rating. I *hope* the non-polarized caps were just to provide some security against bad input signals, and that the new caps can handle a small reverse voltage. Replacing these caps improved the signal in the phono section, which sounded like crap when I got it.

I also noticed a little bit of sound on the left channel even at zero volume. Research suggests this could be a dirty volume pot (even though I cleaned it) or a bad volume pot, either way, I'll live with it for now.

I noticed 9 V instead of 13 V on the collector of Q17 (the other channel had 12 V). I couldn't really figure out why Q17 was drawing more current, because the base voltage was pretty much dead on. I chalked this up to an old transistor with some odd behavior. But any excuse would do, because I was tired of poking around in there.

Lastly, for those interested, the receiver takes three 1847 incandescent lamps, which can be purchased through digikey. The front plate is only held on by two nuts under the volume and speaker selection knobs. The glass an be taken off with the black backdrop by removing the two lock nuts on the black screw posts on the back side of the backdrop (one also secures a lamp). Then the class comes out by removing the two metal clips. I VERY VERY gently wiped the back of the glass to clean off some milky stains, but not take off the dial paint. Seriously, one hard scrub and all the paint would be gone.
 
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At this point, the lights, meters, AUX input, FM stereo, and phono (lo) inputs are all working well. Good times.

How does it sound? It's got a full sound with plenty of bass. It has no problem pushing around my KLH 5s (unlike my similarly power-rated H/K 330B although that model wins in other areas). It is not particularly smooth or detailed, but it has a nice presence. Given the history of the model and it's unique design for a SS receiver (germanium transistors, transformer coupled, point-to-point wiring), I like it just fine. My buddy's going to put it in his basement to listen to classic rock.
 
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