Recap of SAE P102 Preamp

[clearchris]

I have an SAE P102 preamp from the mid 1980s. So it's around 35 years old and hasn't been performing well lately, I'm sure it's overdue for a re-cap. I have mostly worked on computer equipment and random commercial equipment, but seldom audio.

Are there any special things I should pay attention to when re-capping a preamp? I generally buy low ESR caps, does this matter for audio, and it is preferred? Do the caps need to be matched between the audio channels? Anything else I should worry about?

Thanks!

Edit: wow, this pre-amp became a classic, two up on ebay for parts at $200+ each. I'm certain I didn't spend that much for it 10-15 years ago.

[redwire]

Re: Recap of SAE P102 Preamp

The SAE P102 uses many CD4052 analog mux IC's, so it is sensitive to static electricity and they do get zapped if you mess around with the RCA jacks a lot. If you recap it, be careful not to nail it with ESD. A distorted channel can be caused by a damaged mux IC.

I think around 50 electrolytic capacitors inside. You don't need lowest ESR capacitors for audio, that is a more a requirement for switching power supplies.
I've used Panasonic FM series but they do not go below 22uF and there are mostly 4.7uF 50V for signal coupling and 1uF, in the SAE P102. Then I would use Nichicon HE. No need to match them exactly.
There are many audiophool capacitors out there and you can spend big $ if you like.
I would make a list of the caps in Excel spreadsheet, from the service manual.

[clearchris]

Re: Recap of SAE P102 Preamp

Hey, thanks, I hadn't considered that there would actually be a service manual available. Good tips on the ESD, I normally don't take any ESD precautions.

Thankfully, the preamp lives in my rack, I don't mess with the RCA jacks hardly at all, maybe once a year max. I also have it behind a cyclades controlled power system, when it loses power, it waits 60 seconds and powers up the rack one by one. For some reason, we lose power a fair amount where I am now.

I only really see two symptoms, sometimes it takes a while of being plugged in before it will turn on successfully (after a power outage). Lately, there's also some warbling when powered up. It will go away after a while, which is why I'm suspecting the caps need to be warmed/charged up before it operates properly.

Yeah, people do pay some money on "audiophool" (ha!) caps, but I'm not going to go that crazy. A good recap with nichicon/panasonic, or other good jap caps is fine with me.

I'm also considering replacing the mylar caps with film/foil caps, do you have a brand/series you prefer for this application? From the service manual there are around 10 or so low uF mylar caps. From my research, mylar caps don't degrade over time, but apparently cap technology has improved to the point where it might be worth replacing them while I'm in there. I don't plan on opening this preamp up for another 10 years or so, so now is the time...

Panasonic FMs are my go-to when they are available.

[redwire]

Re: Recap of SAE P102 Preamp

I don't know what you mean by 'warbling'?
The only thing odd is the MCU has an R-C oscillator that you adjust trimpot to 2MHz. It might be way off. Otherwise, I would start by making sure the supply rails are OK. I found the SAE P102 service manual/schematic from hifiengine.

To improve fidelity, I would change out the existing op-amps (adding good DIP sockets) NJR Radio NJM4560, NJM4558 for better faire. Night and day difference, you can try OPA2134 but these are BiFET so for the phono stage something else (BJT input) is best to keep the low noise.

Also would use film caps in place of the 4.7uF electrolytics in the signal path.
Use something with 5mm lead-spacing 4.7uF 63V like Kemet R82 R83CC4470AA30J or Wima MKS2-4.7/50/20A polyester. I don't use PET dielectric for audio. Polypropylene is usually too big to fit.
For the existing small value caps, lowest distortion is C0G ceramic, polystrene (Mouser Xicon but a bit big and awkward) sounds the best.

[clearchris]

Re: Recap of SAE P102 Preamp

By warbling, there's an oscillating sound, "wub wub wub wub wub wub wub" about once a second. It goes away after a while.

Thanks for the tip on the 2mhz oscillator, I assume I'd need an oscilloscope for that? It's entirely possible that it needs to be tuned.

Great info on the opamps, I'll do that also. I'm thinking two NJM4558DD (extra low distortion model) for the two in the phono section and OPA2134 for the rest. They are about 6x the price, but I plan on keeping this until I can't use it anymore. Great tip on the DIP sockets, there was a mention of baking the OPA2134 chips before soldering and while I have that capabliity, the DIP sockets sounds like a quicker and safer solution. I have popped my fair share of chips my not dry-baking first.

You weren't kidding about there being 50 caps. I just realized that the service manual uses "ditto marks", I haven't seen those in use since I was in grade school.

[clearchris]

Re: Recap of SAE P102 Preamp

Ok, I finally got the preamp on the bench. I counted 79 caps including one underneath.

I also noticed that one led in the line level display is out. Since disassembly of the display looks absolutely abhorrent, possibly the worst to disassemble I have ever seen, almost 20 leds are soldered in place and need to be desoldered from the back to remove the board, I actually tested the LEDs first with my lcr-t4 tester. All leds tested good. I traced the wires back and the resistor also tests ok, so it looks like the LB1409 level meter driver has a bad segment. These seem pretty hard to source, I don't see anyone that carries them except for a few vendors on ebay. Compatible chips seem to be nte1513 and SK9250 which also seem to be pretty rare. For now I'm going to get some dip16 sockets, desolder and replace both of them later so they are matched. Anyone know a more common chip?

Now I get my spreadsheet out and figure out a parts list. Will post it when done in case anyone wants it.

[clearchris]

Re: Recap of SAE P102 Preamp

All electolytic caps have been replaced and I put sockets in for all the opamps. My frequency counter is going to take a while to arrive, so I'm going going to be able to adjust the oscillator quite yet. I didn't replace the mylar caps as I have the v1 board and the v2 board (detailed in the service manual) seems to change quite a few of the small value caps. I'm going to have to pull and measure or see if I can read the values with a small inspection mirror.

So here's a question. There was some custom work done, and most of it I understand and agree with. However, there are some resistors added to both sides of an opamp V- input that lead to what I believe is ground (but I could be wrong here). I don't understand what they were trying to do, or why you would bother to alter one of the two opamps that feed the LCD driver. Anyone have any ideas?

[budm]

Re: Recap of SAE P102 Preamp

Show us the circuit/service manual.

[clearchris]

Re: Recap of SAE P102 Preamp

Sure! The files are too big to post here, so I'll link to my google drive.

Service Manual (v2 board, I have v1, haven't found service manual for v1 board, but it's pretty close)
https://drive.google.com/open?id=1m7...uMd8cXeOIdPWi2

Close up of underneath board. Resistor connected to pin two of NJM4560D (IC401), also another resistor connected to trace of pin six. Resistors meet in middle, soldered to third trace.
https://drive.google.com/open?id=1cx...-7-x5YvIakqBRM

Larger view. There's also some diodes connected for protection from clipping I believe. Also a 1uf cap, it's on VDD of a MC14051 multiplexer (ext pro IC205). Also, black flying leads connect VDD and VEE of input MC14051 multiplexer and output MC14051 multiplexer (IC305).
https://drive.google.com/open?id=1d2...8597g-Ofao564r

Top view: There is a pair of flying resistors and a jumper that appears to have been relocated. Pic shows new NJM4558 that I installed with socket. Opamps in the sound path have been replaced by me with OPA2134 opamps except for the phono line. All others replaced with NJM4558.
https://drive.google.com/open?id=1d3...f6r7tVe6Y3nUxX

[clearchris]

Re: Recap of SAE P102 Preamp

Some of this may have been factory. I found two misprints on the board, and I just noticed that the sheathing on the flying resistors resembles the sheathing on the ground cap attached to the chassis. So it's possible they made these changes after the boards had been produced. But the preamp has almost definitely been worked on before. The solder connections on the caps didn't look like the other joints and there was plenty of crusty flux. Took me some time to get it clean, surprisingly the board looks pretty darn good, especially considering that I had to flip the board over probably 200 times to replace all the caps, and someone else probably did something similar. Or maybe there's some tricks I don't know...

[budm]

Re: Recap of SAE P102 Preamp

" I don't understand what they were trying to do, or why you would bother to alter one of the two opamps that feed the LCD driver. Anyone have any ideas?"
Which OP Amp is that in the circuit?
BTW, I could not download the manual from your link but I found the SM on line., it is not that big file.

[clearchris]

Re: Recap of SAE P102 Preamp

It's IC401.

Cool, I didn't think I could attach it, I usually have to reduce the file size of pics quite a bit to post anything here.

[budm]

Re: Recap of SAE P102 Preamp

"I don't understand what they were trying to do, or why you would bother to alter one of the two opamps that feed the LCD driver. Anyone have any ideas?"

IC401 (Dual OP Amp) output is driving 4052 which drive the LED circuit, I do not see IC 401 for driving LCD.

OP Amp:

[redwire]

Re: Recap of SAE P102 Preamp

The underside 1k5 resistors (R433 ?) pin 2 to GND, pin 6 to GND. I think they forgot them on the top.
Zener+diode clamp (to black wires) look like D201/D204, and D206/D306. I think they forgot them on the top?
On top, the 82R resistors are likely R427, R428.

I would say the PCB layout had some changes, but the schematic is still correct.

[clearchris]

Re: Recap of SAE P102 Preamp

@budm: You are right, there isn't an LCD in the box, don't know why I kept saying LCD. I meant LED. Thanks for the link, looks like I have some reading to do!

@redwire: Thanks! Yeah, I originally thought it was custom work, but yesterday things started seeming more like they were factory installed. On a hunch, I just did some searching, and I found a picture of a p102 that sure looks like it has the same samhwa caps installed. So maybe this item hasn't been worked on. Interestingly, the interior front frame was bent a bit, I took it out and hit it a few times with a hammer and punch to straighten it up a bit. If it hasn't been worked on, pretty sloppy work by the factory, both on the frame and the capacitors.

On the plus side, I can expect some serious improvements if those were really 30+ year old capacitors.

As an aside, on the newer revision p102, they silkscreened the bottom traces on the top of the board. That's a really nice touch that I haven't seen before. Here's the pic:

https://audiokarma.org/forums/index....air-tip.90708/

[clearchris]

Re: Recap of SAE P102 Preamp

I turned it back on, and man, it sounds great, out of one channel. The other channel, I get a super quiet buzzing and no music. Bah. I'm not an audiophile, but man, it just sounds super clean and clear now. I didn't know what I was missing.

So, thinking out loud here.
I get only one channel out of the speakers, one channel out of the headphone jack. So we can eliminate anything else in my stack.
The LEDs display good signal on both channels.
Same results if I plug into "tuner" or "aux".
If I change the balance and hit "stereo reverse" the second channel plays louder in the speaker that was working speaker.
If I hook up headphones to the "tape out" I get audio in both channels.

So, I think I can eliminate the MC14052B (IC201-IC205) as being a problem. I can also eliminate IC206 as being a problem since it feeds the LED drivers, and both channels are working properly there. So, assuming it was something I broke (the opamps didn't go easy into the sockets, sounded a bit crunchy) that leaves IC301, IC304, IC306. I have a spare NJM4558DD to swap out if needed, but given that I don't want to remove and replace these if I don't have to, I'm going to examine the board and see if I can figure out which one needs replacing.

I tried to test with the receiver part of a telephone tone set (aka fox and hound), but apparently telephone lines operate around 50v and audio is more around 1v. So that's not going to work. Does anyone have a quick and dirty way to trace out where I'm losing audio, or do I finally have to go buy an oscilloscope? One guy on youtube had a mini-amp that he set up with a probe, and that seemed to work pretty well, but if I'm going to buy something, it might as well be a cheap and cheesy scope.

[redwire]

Re: Recap of SAE P102 Preamp

It depends on how complicated the problem is, a scope is ideal but a simple LM386 signal tracer also works fine and they are under $2 for a kit. Or you could disconnect (at R601 220k) and use one headphone amp channel as a signal tracer to poke around and listen.
You can also troubleshoot by using a multimeter on DCV comparing each channel's readings. That will usually show an IC that has failed or a voltage that is wrong. Don't slip with the probe and short stuff out. The schematic shows some voltages. Op-amps should have less than 100mV at their output.

I usually start in the middle, at the volume control or tone controls or go right to the muting JFETs Q512/Q513 at the output looking for trouble there.

IC402 seems to be able to switch the LED VU meters before (tape in) or after (line out) of tone/volume control section. What happens to the LEDs, I assume line out displays only one channel.

[budm]

Re: Recap of SAE P102 Preamp

You can use one of the input of PC powered speaker as the audio probe by connecting 10K resistor in series with 0.47uf film cap to Left or Right channel of the input.

<------ resistor -------- cap ------- PC powered speaker Left or Right input. Gnd to the chassis.

[clearchris]

Re: Recap of SAE P102 Preamp

Ha! I knew you guys would have a quick and dirty solution. Don't want to wait for the $2 kit from china, so I'll get a $6 kit from a US shipper. I assume you mean somethnig like this: https://www.ebay.com/itm/LM386-MINI-...0AAOSw1MZZjUrf

The LEDs display both L&R channels.

I'll take a look at those, that should be easy and seem plausible.

[clearchris]

Re: Recap of SAE P102 Preamp

Budm: Darn, so close, could have been testing tonight! Alas, I have zero film caps.