HH Scott A-436 Restoration

jheu02

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I picked up what was listed as "only for parts, not worth a restoration" sale on that auction site, along with the matching T-526 tuner. But, I wanted a challenge, so purchased it anyway. Here's photos of how the unit arrived (there'll be a separate thread for the tuner). Though I can't really dig into it right now, as I promised the spousal unit I'd finish off the bar I'm building and get the basement and garage straightened out, I couldn't help but at least put it on the dim bulb and do some basic checks. I only had a 2A slow blow fuse (unit states 3.5A) and 3 amp fast blows (vs 3.5A) for the speaker protection circuits, but voltages checked out, Bias was almost spot on at 8mV, and DC Offset was 8mV Left and 20mV Right. Haven't hooked up speakers yet, but I may do that test later tonight just to get a "before" sound impression.

So, here's another "Ugly Betty" restoration on its way. I'll be using parts on hand for the most part, but I'll most likely buy some new rack handles since they appear to be standard drawer pulls. Suggestions welcome as to how to deal with the corrosion and rust (wire brush, Naval Jelly?, just use cold galvanizing paint?...), transistor subs if needed (I may replace the differential pairs [2SC1000BLs] with BC550Cs since they're also low noise and have a high beta group like the originals. I'll also probably swap out resistors in the signal path for high quality Dale RN60Cs. But, I want to listen to it first to get some impressions about the last gasp of breath for HH Scott gear.

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Inside bottom cover
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Power Supply
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Phono Preamp
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Main Amp
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Meter Board
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Tone Board
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Looks like it will take a while to clean up. At least there is a Service Manual for it on HiFI Engine. Have seen this amp before in another thread. Good luck restoring the amp.
 
Well, I got to do a quick listen...sort of. Exercised all the switches several times, got everything hooked up, switches set, no sound. Meters were moving though. Knowing there was a bit of corrosion, I sprayed the Speaker 1 terminals with deOxit and slid the speaker pins in and out several times to facilitate cleaning. This got me a slight power on bump, but still no sound. Then I pried (yes, it required a little leverage) the pre out, main in connectors to separate the pre and power amp sections and see where the problem lay. Set it up for the A-436 to drive my Kenwood Basic M1. We have sound! All the switches functioned properly, though the tone sliders are pretty stiff. There's bleed through even with the volume pot at minimum, not sure if that indicates cap leakage, overload signal from the CDP I was using, or just cost cutting in the design. Next I used the Basic C2 to drive the A-436 amp section. Once again, meters moved but no sound. Sprayed speaker 2 and tried that, no sound. Then tried the headphone jack. Though I had to hold things in place while using phone earbuds and a 1/4" adapter, I did get sound. So, pre and power amp sections function, but there's a problem with the signal getting to the speakers themselves.

So, I'll have to give the speaker selector switches a THOROUGH cleaning and also check and perhaps even replace the speaker terminals. But the good news is that everything appears functional for the most part and can only improve with cleaning and parts replacement.

Though not a detailed listen at this point, overall sound impression is positive. There's decent soundstage and imaging, and the tone is decent with flat controls.
 
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Hi, I have a mint one, but one of the vu meters are broken, someone konws where I can buy one? thanks Al
 

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Hi, I have a mint one, but one of the vu meters are broken, someone konws where I can buy one? thanks Al
Alferox, your Scott is beautiful. PM me if would be interested in seliing it. I've been looking for one.
 
Hello jheu02! Being the owner of an A436 since 1976 I was interested in seeing what others thought about it, so I googles it and found your pretty interesting thread. Did you ever get her past your inspection and up and running? Looks like a lot of work would have been needed to got it all cleaned up. Hopefully it panned out well for you in the end, (crossing fingers for you here).
On another note, I was hoping you might be able to give me some direction in finding a replacement part for mine that has been broken off for years. I'm not any kind of electronics tech, but I figured if I can provide the part, I can take it to someone that can fix it for me.
The part as you will see in the pic attached, is one of the front toggle switches - for "Tape 1/Tape 2". I basically only use it anymore as a preamp, but seeing her with the missing tooth is beginning to bug me.
c-A436-front-p.jpg
Please, if you, or any other member may be able to help, I'd appreciate the direction immensely!
 
No, other things got in the way. You might be able to find a switch on ebay. I tried to get a part number off the Service manual, but it only lists Scott internal part number, so you'll probably have to open the unit and get a part number off the actual switch, or find a similar vintage tape selector switch from a parts unit.
 
No, other things got in the way. You might be able to find a switch on ebay. I tried to get a part number off the Service manual, but it only lists Scott internal part number, so you'll probably have to open the unit and get a part number off the actual switch, or find a similar vintage tape selector switch from a parts unit.
Thanks for the reply John. Sorry to hear the status of your project.
I'll be on the hunt with Google I guess, see what pops up.
Do you have a copy of the service manual I can get, by chance? I know from past experience with my local service shop, it sometimes takes them forever to locate vintage docs, as was the case with my Nakamichi LX-3. I would love to have the manual at hand when I get to take it in for a long needed tune up and cleaning, the controls and slides are sounding a bit crusty. ...if you don't mind.
Thanks!
 
Wordy…if you just want the question, skip to the last 3 paragraphs.

While waiting for the main filter caps to come in so I can finish up another H/K A-402, I decided to pull this back out and see if it was worth the effort. Had problems getting some of the knobs off due to corrosion, with one plastic insert remaining fixed on the shaft, and one of the toggle levers completely pulled out of the housing of the switch while trying to get its end cap off. But I was able to get the faceplate off and gain some access. Got the whole switch panel out for ultrasonic cleaning, but discovered many bad/lifted traces which I had to repair upon reassembly. The large pins used to attach signal wires, and the way they were inserted were too much for the traces, and had been fiddled with before at some point. Anywho, got it all back together, gave the rest of the pots and switches a good cleaning, checked on DBT…passed, set the bias and checked dc offset….low mV there surprisingly, so good to go.

hooked it up and was only able to get sound out of the right channel. Eventually ran rca out to another amp and confirmed the preamp was working, sound out of both channels. Did a bunch of signal tracing with the tone function of my DMM which confirmed that the signal is getting through the speaker selector switch and to the speaker terminals, so problem must be on the amp board correct?

Pulled the board and all the outputs…appear to test ok using 6 way test. Next compared a bunch of resistors…nothing awry. Started pulling and testing all the transistors and checking against the good R channel…all look reasonable. Next, the diodes, again the same. Reflowed the board and found one trace at the signal in post non existent. Bingo, or at least I thought. Got everything cleaned and re-greased the outputs, replaced the ‘lyrics and wired everything back according to my photos.

DBT….pulsating glow. Then discovered one ground wire from the main filter cap bus bar wasn't connected. Got that back on. DBT…bright glow, no dimming. Rut row Raggy.

Because there's no easy way to pull the outputs without removing the whole board and heat sink, I de-soldered the power rail wires from where they enter the board, wire nutted them, and back on the DBT….nice dimming. Checked voltages at the cap terminals vs chassis ground and saw basically what I would expect on the DBT. +38V, -38.4V, sub board 12.9V

Here's where it gets interesting…I've gotten used to checking and making sure the caps discharge before working on it using a bleeder resistor wire I clamp to the chassis and main filters. This time I just decided to check voltage on the caps with power off w/o using the bleeder. The positive rail discharged on its own rather quickly. The negative rail is staying fairly stable, very slow rate of discharge. Could this be a main filter issue?

second question, is it ok to reconnect the rails one channel at a time and see if the bulb dims to narrow down the search for faulty components?

Dave (@avionic), any thoughts on how to further troubleshoot? Visual inspection of before/after component side shows all small transistors in the correct orientation.
 
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More data…

I ran jumper wires for each channel separately for the pos and negative rails. R channel the bulb dimmed, left channel it stayed bright. I then desoldered the output leads for the Left channel and the bulb dimmed. The outputs all passed the 6 way diode test earlier, so I went back to the speaker terminals.

I can get a good signal tone from the Positive terminals of both speaker 1 and 2 R channel inserting the probe into the speaker spring clip and tracing all the way back to the amp board where it connects to the speaker fuse. L channel I only get a tone to the board thru speaker 1 spring clip, no tone from L channel of speaker 2 spring clip. Now for the negative terminals, There's a “bus bar” tying all the rear negative parts back to the ground bar at the main filters. The ONLY spring clip I get a tone is the speaker 1 R channel. All the rest are dead. I can get a tone touching the rear part of the spring clip, but not through the spring clip.

Could this be “seen” by the amp as a short? I DO NOT get any continuity tone between the positive and negative speaker terminals themselves.
 
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Success, of sorts! Just unscrewed and capped off the wires to the speaker terminals and jumped the power rails to the board…DBT dimming! Now to figure out what to do about speaker terminals. I may be able to take them apart and wire brush them clean, but I also took the posts out of a spare wall mount bracket and may use them intstead.
 
I was able to make one good set of speaker posts out of the parts and after hooking everything back up...we have sound! Now that things are working, bias sets fine, DC offset is 60ish mV L channel, 25ish mV R channel. So, I'll probably match up some BC 546s for the differential pair and then look at the other 2SC1000s, finish the phono board, etc. Wasn't sure if this was going to be worth the effort, but what the hey....it's a hobby.

 
Redid the phono amp board and the Tone B board (graphic EQ). Forgot to take a post pic of the tone board, but replaced the generic caps with UKL, the 2SC1000s with KSC1845s and the two 1K resistors in the output with MF. Deep clean and then lube of the sliders. Both boards got ultrasonic cleaning. The phono amp got an extensive rebuild, putting UKL and MF resistors at the inputs and outputs, matched KSC 1845s for the 2SC1000s, matched C0G caps replacing the generic ceramics, and increasing the main filter from 100uF to 220uF. All of the caps were out of spec in at least one area. All high in capacitance, but ESR and Vloss not bad for all. Inexpensive tester...take it FWIW. Also did the caps on the meter board, which also has fuse lamps to light up the meters. Found a blown trace which I thought was the reason the fuse lamps weren't lighting, but after all the work, they still didn't light. So, need to track that down. Also decided to see if I could match up the replaced 2SC1000s to fix the DC Offset issue. None were really closely matched, so KSC 1845s again, all matched. They have a lower Ic at 50mA vs the 100mA of the C1000s, but thought I'd try for the diff pair. Offset went from what's listed in the previous post to 130-140mV each channel. Tested the C1000s and found them not well matched at all (but it worked for some reason), so I'll adapt a trimmer/resistor combo to see if I can zero things out with the KSC1845s tomorrow.

Phono amp 'lytics

47/16 - 94.49uF .65ohm, 2.7% loss (these are the caps at the output)
47/16 - 108uF, .65ohm, 2.9% loss

4.7/25 - 7703uF, 1.7ohm, 1.3% loss (these are the input caps)
4.7/25 - 7416uF, 1.6ohm, 1.4% loss

4.7/25 - 7442uF, 1.6ohm, 1.3% loss (local filter to gnd)
4.7/25 - 7425uF, 1.6ohm, 1.2% loss

100/16 - 175.7uF, .31ohm, 1.6% loss (main power filter for board)

100/6.3 - 219.4uF, .4ohm, 4.9% loss (local filter)
100/6.3 - 210.5uF, .47ohm, 3.9% loss

Tone B Board only had two 'lytics

4.7/25 - 7908uF, 2.3ohm, 1.6% loss
4.7/25 - 8586uF, 2.1ohm, 1.6% loss

Phono Amp Post.jpg Tone B Trace Ultrasonic.jpg Tone B.JPG
 
Still need to track down why the meter lights aren't working. There's no current flow, so that's the basic problem. But not sure whether the wires are hooked up correctly or not. Looks factory, but I do know it's been worked on before, so who knows. Just doesn't seem like it makes a continuous loop like pictures on the scat.

In other news, the phono board rebuild sounds nice. And I added trimmers for adjusting DC offset. Didn't have any top adjust 5K, so repurposed some 4.7K trimmers that I think came off a Yamaha CA-800. 3.57K Rs in parallel in case the pot opens up. All hooked up on breadboard and screwed into the chassis.

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Love these threads. Curious if you used any Naval Jelly on the rust. It is highly nasty crap so you have to be very careful with it. Maybe it would be best to apply it selectively with a small paintbrush.

Great to see the progress you have made on this unit. Bringing something back to life is exciting. Those boards look really clean now.
 
OK, so here's the question with supplied scat, board printout and marked up actual board photo. What's wrong with the wiring or scat that the lights don't work? Caps have been replaced and I used some different 4.7K trimmers, but as far as I can tell, the lamps should be running directly off the transformer on AC power. Good continuity on fuse 903 and the meter fuse lamps.

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Well, it was the fuse lamps themselves after all. Even though they checked good continuity, and with a dropping resistor from a 9V battery, the element glowed, they still wouldn't light in circuit. Put in two of the LED fuse bulbs I'd been waiting to use in a different project and they lit up fine. :crazy::idea::banana:
 
Here's where we stand. Basically just need to wire brush and spray galvanize the chassis rust, and get some tone generator (or burn a 1000Hz test disc to set the meters properly. Got the new filter caps in today. Perfect diameter for the clamps but look miniscule in comparison with the original Panasonics. Anybody got a good faceplate? Planning on giving this to my younger brother, He visited two weeks ago, right as I was coming down with COVID (unbeknownst to me) and ended up catching it from me. He's unvaxxed and had a pretty high fever for awhile, but fortunately seems better now. Passed it to his housemates also. So, I guess I owe him something, and he's recently rediscovered vinyl (hence the extra work on the phono section).

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