CA-F1 Preamp - Rough but Working
- Thread starter petesui
- Start date
I'd been on vacay, virtually vintage audio free for 10 days, and was outright jonesing for a Sansui hit. Couldn't be helped.
Got through the F-2993 tone and flat amp board today. Straightforward mostly. Trimmers, set bias, a handful of caps. New ground for me today replacing varistors, MV12 and MV103, with double and triple 1N148s daisy-chained in series with heat shrink. Kinda goofy looking. If there's a tidier configuration, it didn't present itself to me. I have six more ready to go for F-2992 EQ amp board up next. Things look to get gluey again there. Still haven't come across any out of spec components.
After finishing F-2993 I did a sound check. Although the sound is good and clean, manipulating the tone controls (which I've cleaned already) appear to cause the muting circuit to kick in and both channels drop to a faint audible signal for maybe 15-20 seconds before the signal comes back full strength. For now, I swapped only ZD601 with a new 1N4735A, and replaced the the relay driver, 2SD438 at TR602, with a new KSC2383. This seemed to stiffen up the relay response, but because both original components tested fine and there's lots of glue to clean up down stream on the EQ amp and the head amp, I elected not to chase the ghost down the rabbit hole just yet and finish up the cleaning and recapping on the remaining boards first. Note that the only transistors I've replaced thus far are the regulators on the power supply and TR602 here on F-2993. I'm especially keen to clean up the head amp as that's where the nastiest glue is concentrated under C01-04 and the neighboring transistors and resistor legs.
Moving on to F-2992 EQ amp the proceedings have been straightforward. I'm short four 4.7uF caps for C07-08 and C27-28. Wimas are en route from AFA. Otherwise, I've recapped, replaced the zeners and the varistors with my triple in series jobs, and installed the new trimmers. Waiting on the Wimas to set DC OV. This is also where 10 of the 12 black flags resided which I replaced at the outset of the project. Glue cleaned up as follows:
Moving on to F-2992 EQ amp the proceedings have been straightforward. I'm short four 4.7uF caps for C07-08 and C27-28. Wimas are en route from AFA. Otherwise, I've recapped, replaced the zeners and the varistors with my triple in series jobs, and installed the new trimmers. Waiting on the Wimas to set DC OV. This is also where 10 of the 12 black flags resided which I replaced at the outset of the project. Glue cleaned up as follows:
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Glad you asked as I meant to describe. I got the idea from Ben, proprietor of the Novalux Stereophonic YouTube channel (a fantastic tech and channel). I'm sure he's around AK somewhere but I don't know his handle. He uses a small heat gun tool to soften the glue. I don't like using sharp metal objects to scrape. You can see the scuffing and scraping I caused in this case. I've also found that plastic cell phone repair tools don't work well either, too soft (and they melt under the heat gun!).
In this case, I swabbed and soaked the glue thoroughly for several minutes with IPA. I cut the end of a bamboo skewer on the bias at an angle. Then, heating up the glue with the heat gun, I simultaneously used the bamboo skewer to effectively rub and erase the glue off the pcb. It worked quite well and was fairly efficient. Now, of course, I was concerned about heating up the pcb and surrounding components too much as everything definitely gets hot from the gun. For example, the chassis gets hot! It's all pretty conductive material, right? Thus, I think proceeding with caution is in order. Nonetheless, it's certainly quicker and more effective than anything else I've tried. The bamboo skewer is key, too. The fibers in the cut end make for excellent scrubbers and the angled edge a soft but firm scraper.
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It occurs to me that mine came with reducer nozzle to better direct heat at the target. I'll have to root around in my bins and find it.
Finished repopulating the F-2991 MC head amp. I also replaced all of TR01-12 which included 2SC1845, 2SA992, 2SD438 (with KSC2383), and 2SB560 (with KSA1013). At this time, I'm electing to leave the original trimmers in place. I'm not going to touch them. I do have replacements in stock, but the SM adjustment for setting minimum distortion in this circuit is beyond my present skillset and I don't have a distortion meter. Although I do have a couple of AT3100S MC cartridges for my XP-99s, MC phono playback is not high priority for me. The majority of my cart collection is MM. Thus, I'm going to leave well enough alone and not open a can of worms I can't fix. I suspect the MC stage will sound fine to my ears. We shall see.
I've noticed a handful of 2SC1400 on the F-2993 tone and flat amp board which are on the noisy prone to fail list. I'm going to swap those out along with a single 2SC1634 and upgrade the remaining 2SD438 and 2SB560 here and on F-2992 eq amp. Just need those 4.7uF Wimas for C07/08/27/28 on F-2992 to land (they're in town but not out for delivery yet), and we should be good to go with final adjustments followed by a soundcheck. Here's my cleaned and capped head amp:
I've noticed a handful of 2SC1400 on the F-2993 tone and flat amp board which are on the noisy prone to fail list. I'm going to swap those out along with a single 2SC1634 and upgrade the remaining 2SD438 and 2SB560 here and on F-2992 eq amp. Just need those 4.7uF Wimas for C07/08/27/28 on F-2992 to land (they're in town but not out for delivery yet), and we should be good to go with final adjustments followed by a soundcheck. Here's my cleaned and capped head amp:
Ooh, thanks for the tip. These sound familiar. R81-82 and R83-84 are the emitter resistors and the test point for the DC adjustment. Makes sense. I don't have any handy, but will add them to my cart.For an additional noticeable boost in SQ you could replace the 33Ω resistors in the output section of F-2992 with ½watt metal film.
A few semi-final thoughts. Once I dropped the 4.7uF Wimas into the EQ amp, the board adjusted nicely. On initial soundcheck the balance adjustment was taking out the left channel and there was a little intermittent crackle and signal in and out with pressure on the knob. Sure enough, the solder joints on the tone controls were very thin to little material left. I reflowed all the tone control joints and gave the balance knob a second flush. Problem solved.
Speaking of controls, I have a question for the group. This is my first Alps style sealed volume potentiometer. It does have a little port available. Having never worked on one before, I left it alone. I did not treat or clean. It works smoothly and is quiet, the spin is firm. I opted to leave it alone. Would you do the same or give it a spritz?
Again, I thought this was a relatively easy piece to work on. Granted, I didn't encounter any tricky circuit or troubleshooting issues. The amp was basically working and really only needed heavy maintenance. Everything is laid out in the open in a straightforward accessible manner which made for good circuit study. I simply worked clockwise from the power supply. It was rather cruisy, if I may say so, in the grand scheme of my experience and projects so far.
It sounds great on the bench (Klipsch KG 1.5s and an Integra ADM-2.1 amp), and will sound better in a main system. I haven't figured out how I will deploy it yet, but ideas are rattling around. And certainly, should a BA-F1 present itself . . . I shall follow @Hyperion's tip and replace the emitter resistors. I've also noticed several fusible resistors that I may as well swap while I'm at it.
Speaking of controls, I have a question for the group. This is my first Alps style sealed volume potentiometer. It does have a little port available. Having never worked on one before, I left it alone. I did not treat or clean. It works smoothly and is quiet, the spin is firm. I opted to leave it alone. Would you do the same or give it a spritz?
Again, I thought this was a relatively easy piece to work on. Granted, I didn't encounter any tricky circuit or troubleshooting issues. The amp was basically working and really only needed heavy maintenance. Everything is laid out in the open in a straightforward accessible manner which made for good circuit study. I simply worked clockwise from the power supply. It was rather cruisy, if I may say so, in the grand scheme of my experience and projects so far.
It sounds great on the bench (Klipsch KG 1.5s and an Integra ADM-2.1 amp), and will sound better in a main system. I haven't figured out how I will deploy it yet, but ideas are rattling around. And certainly, should a BA-F1 present itself . . . I shall follow @Hyperion's tip and replace the emitter resistors. I've also noticed several fusible resistors that I may as well swap while I'm at it.