AWESOME photos, thank you! Perfectly aligned, made is east to superimpose for clarity:
Your 2nd schematic was close, but not quite there. Gleaned from your photos, this works:
Delay & coil current. With C2 = 10uF, delay is shorter than original, @ 2+ seconds. Graph shows 10, 15 & 20uF if you tweak it.
This circuit does NOT implement the lower hold current.
Relay coil draw is higher @ 31mA, but without specs for that particular coil, I used the same resistance @ inductance as the IDEC coil. In reality it might be less, or it might be what that relay needs (that relay has a very odd footprint).
Coil current could be reduced by adding 50-200 ohms instead of the jumper they placed in the original circuit location.
Good news, total draw from both +18 & +16 combined is less than 1mA. They're only used to drive the transistor bases. The majority of power comes from the +24v: 31mA for coil, and another @4mA thru the drive transistors.
I don't know why they chose to fed both +18 and +16 separately, both halves can be run from either one alone, and it still works as expected in simulation. But sims don't always account for every nuance.
Another history piece to add to the Apt files archive! Thanks!
I reverted to the original muting circuit this afternoon. I had to use 1k ohm 1/2 watt for R90 because I messed up ordering the 820 ohm alternative. I used KSC1845 for Q17 and Q19, and 2N5087 for Q18. Everything else matches the schematic/PCB values. The relay is a RY2V-U-DC24 from Galco.
Unfortunately it only works about half of the time. The other times it never unmutes on start up, and I have to manually mute and unmute to get the relay to close.
I plan to put back the old op amps and see if it the muting circuit becomes reliable. I'm really glad I used the DIP sockets.
Unfortunately it only works about half of the time. The other times it never unmutes on start up, and I have to manually mute and unmute to get the relay to close.
I plan to put back the old op amps and see if it the muting circuit becomes reliable. I'm really glad I used the DIP sockets.
Interesting thread. Would you post the asc file for the simulation? I found a 2SC1345 model if there is interest in applying it here.
.MODEL 2SC1345 npn(Is=300.3E-18 Xti=3 Eg=1.11 Vaf=100 Bf=424.8 Ise=300.3E-18 Ne=1.329 Ikf=.5 Nk=.9239 Xtb=1.5 Var=100 Br=1 Isc=0 Nc=2 Ikr=0 Rc=0 Cjc=2.167p Mjc=.1631 Vjc=4.728 Fc=.5 Cje=8.215p Mje=.3932 Vje=.3905 Tr=10n Tf=368p Itf=1 Xtf=0 Vtf=10)I swapped out the OPA2134s for the original op amps and muting delay works now. The delay was only about 1-1.5 seconds on startup so I added a second 10uF cap in parallel with the existing one (under the PCB). Now the delay is about 3-4 seconds.
I’m going to call it good. The new owner can swap in newer op amps with lower power consumption if they want. I don’t think it’s worth swapping the transformer to make the OPA2134s work.
I’m going to call it good. The new owner can swap in newer op amps with lower power consumption if they want. I don’t think it’s worth swapping the transformer to make the OPA2134s work.
Here ya go. I had the 2SC1345 model, but I don't use it as it's not a viable part anymore.
Good to hear ya got it working! Shame about the opamps though.
The very short delay is odd. Makes me think something else may be amiss.
All, This thread has proven invaluable to the APT community. I for one have found your experimenting beyond my capability, but worthwhile because i am going to reinstall the original circuit back into my original 1980 unit. I also will put in a new IDEC relay, except i will install a relay socket onto the board to eliminate any future soldering on the board. I will be using the 2n4401/2n4403 transistors as i do not have the originals 5087 and ksc parts. Thanks to all.
Community, sorry to ask this question as this thread is a year old. Would a 2sc1815 NPN work for the two relay transistors in this circuit? I have 100 of these available and some 2n5550 and bc547's. Finally getting back into this unit. Thanks to anyone that can say yes or no to the NPN choices.
2SC1815 SHOULD work fine, but I don't have a spice model to run it in sim. It has the correct pinout.
I tested 2N5550 and BC547 in simulation (see post 23+, page 2), and both appear to work fine. However, they have DIFFERENT pinouts than the 2SC1345/KSC1845, so be careful with them.
Again, these are not critical parts, I suspect almost any general purpose NPN/PNP would work, but produce different delays (some longer, some shorter than stock).
2SC1345/KSC1845/2SC1815 are all ECB pinout
2N5550 is EBC
BC547 is CBE
I tested 2N5550 and BC547 in simulation (see post 23+, page 2), and both appear to work fine. However, they have DIFFERENT pinouts than the 2SC1345/KSC1845, so be careful with them.
Again, these are not critical parts, I suspect almost any general purpose NPN/PNP would work, but produce different delays (some longer, some shorter than stock).
2SC1345/KSC1845/2SC1815 are all ECB pinout
2N5550 is EBC
BC547 is CBE
Wow, the datasheet curves appear to indicate a much higher Vaf and a much lower Bf but hey that's just me. At least, at collector current levels typically seen in audio preamplifiers (Ic <= 25 mA).
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I hope this isn't a thread hijack as that would be a poor way to introduce myself. I just purchased this preamp locally. After using it in the system for awhile, decided to pop the covers off to figure out what was knocking around inside to discover the transformer loose. Not only loose, but no nuts inside at all and poorly executed elongated transformer mounting holes. The preamp appears to be completely functional excepting one headphone channel is out. I've ordered the necessary caps and will probably order the headphone RC4558P opamp as well. My question is whether I should replace the current transformer with the Hammond transformer as suggested by GKTAudio as a precautionary measure? Are any circuit mods required for the Hammond? Many thanks from a Seattle newbie to the forum.
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Benjamin, I lean towards stock. These are nice preamps, I own two, both stock. I would add new hardware to secure the transformer and look everywhere in the unit for that lost hardware. The weak spot is the relay and they are available at Graco. I noted this is the previous threads. I have owned mine since 1980. Besides extra relays, I bought a supply of 2n5484 Jfets and sc1845 to replace the the sc1345 from mouser. The 2n5484's are the ones used in my preamp. These preamps do not like static electricity so I have a discharge mat I touch before touching the preamp. From this thread other pampas draw more current. Please let this thread know if the 4558 fixes the problem. And lastly congratulations. I bought my last one for $200 in excellent condition.
I've seen several different factory-installed transformers and this doesn't look like any of them, though I agree - find the missing hardware!
There are at least two different relay types installed by Apt. I make a replacement adapter PCB with high quality Omron relay for those who want to upgrade the general purpose relay in these units that I sell in the Audiokarma forum.
There are at least two different relay types installed by Apt. I make a replacement adapter PCB with high quality Omron relay for those who want to upgrade the general purpose relay in these units that I sell in the Audiokarma forum.
GKT, I also like the Omron adapter, but I must admit ,I have very limited skills to make this change. I would say that is why I revert to stock and I am far from an audio electronics experts. I am 69 years old and do simple things where I can limit damage and mistakes 
. I had one stock relay last two years and one lasted 15 years in my original 1980 model. The second preamp had the original relay replaced in 2012 and it was a 1980 unit also. This forum is great for me and I am humble enough to say everyone at DIY knows way more than I do. It is a fun hobby.
. I had one stock relay last two years and one lasted 15 years in my original 1980 model. The second preamp had the original relay replaced in 2012 and it was a 1980 unit also. This forum is great for me and I am humble enough to say everyone at DIY knows way more than I do. It is a fun hobby.
It actually requires only desoldering and soldering skills. if you can safely replace transistors, you can easily install this adapter board - it's pin for pin compatible, just like soldering in a new original style relay. Your replacement will last plenty of time; mine are for those who need a replacement and want to use a better quality low-signal relay.