Pioneer SA-750

Z

zemansky

New Member
Hi guys!
I'm new here and wanted to share a few pics of my first amp and ask for some advice.

First some specs:

Power output: 50 watts per channel into 8 ohms
Frequency response: 10Hz to 70kHz
Total harmonic distortion: 0.05%
Damping factor: 60
Input sensitivity: 2.5mV (MM), 150mV (line)
Signal to noise ratio: 74dB (MM), 102dB (line)
Output: 150mV (line)
Speaker load impedance: 6 to 16 ohms
Dimensions: 420 x 98 x 287mm
Weight: 5.5kg
Year: 1984





I moved this week and saw the amp on a used goods shop so I went for it.
It has taken a bit of a beating (first picture left down corner) but it seems to be fully functional.
Only issue I had was when connecting it to my notebook's headphones jack (with the notebook's level turned a bit down, I know..don't kill me). I don't know the level of that signal but the speakers kept going in and out of service (sometimes one and sometimes the other). After disconnecting and reconnecting the signal everything went back to normal... Don't know..
For now I'm using it with a Technics Tuner ST-C 01 and driving a couple of Technics SB-F2 (borrowed).
Next thing on my list will be my own speakers and a cd player. Any recommendations will be much appreciated.

I'm studing electronic engineering so I'm not afraid to take it apart and replace some components ,in fact, I enjoy it!. Also I can make a few measurements at my university's lab so let me know if you wantto know something in particular.

Any recommendations on some component to change will be welcome!
 
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I was having a bit of a problem with the channels. Every once in a while one would go down... After disconnectin and connecting the RCAs it seemed to go back to normal.
This was happening a lot with the AUX channel and a bit with the TUNER.
After checking and rechecking I noticed that pushing the channel selection buttons I would get some noise in the channel. So I now had something to go with...
I to the control board (AWG-092) apart and started measuring resistance on the swithes
2, 300, 600, 740, 4, etc... ohms!! On the same contacts after pushing and releasing the button several times.
My god this thing is dirty!!



I have to take the whole thing from the board and it has like 50 solder points... Guess I'll do it in the morning...

I'm thinking on using an ultrasonic cleaner with isopropyl alcohol, anybody can tell me if this is a good idea?

Thanks!
 
I would clean the all of the switches with a contact cleaner rather than disassemble at this point. Deoxit from Caig Labs is a good safe for plastics contact cleaner that is used by many here on AK.
 
sigh contact cleaner gives you clean corrosion, deoxit LOOSENS the corrosion so the contacts can be scrubbed clean by the friction of repeated (50x) actuation.
 
No intent to confuse, imho its just semantics. Caig Labs quote: "Featuring Environmentally Safe Contact Cleaners / Connector Enhancing Treatments: DeoxIT® D-Series, DeoxIT® Gold G-Series (formerly ProGold), DeoxIT® Shield S-Series (formerly PreservIT), DeoxIT® Fader F-Series (formerly CaiLube MCL) and the NEW, HAND-E-GLOVE®"
 
HI, New to this site just wanted to know where to order parts,
need: two big Capacitors the 8600uf and all four outputs also.
Any feedback appreciated!
Tim
 
Thanks guys for all the responses!
I'm trying to find deoxit in my country (Argentina) but so far no luck.
I cleaned them as good as I could and now they are working great. I'll monitor them from time to time until I get my hands on Deoxit.

If I don't see any burst capacitor do you think it's still necesary to change them? Any transistor you may know I show change just to improve it?

Thanks again!!
 
No problematic transistors I know of.

Check for any small to-92 transistors that have a thicker center lead, Pioneer did some mis-design around them. The center lead was supposed to conduct heat down to a large copper area on the pc board where the heat could be transferred to the air. Pioneer did NOT connect these to a large (4 square cm) continuous foil area to shed the heat, thus the circuit board may have shifted color due to the localized heat. My replacements use larger to-126 transistors with optional heat sinks.
edit: check q305-q308 inclusive for evidence of overheating

That's about all I can add right now.
 
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No weirdo transistor so far

q305-q308:
I gave them a look and they don't show any sign of overheating. Is there a more technical way of testing this? Should I measure the joints volts drop or something like that?

Thank you very much!
 
nope, either a thermal ($$) viewer, IR (laser?) thermometer or the "ouch" finger test.

if no signs of overheating, good - the board changes color, the leads blacken and the epoxy of the transistor can shift colors.
 
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