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latest project realistic model STA-240 receiver - Printable Version

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RE: latest project realistic model STA-240 receiver - morzh - 03-08-2026

Rich

Measure the voltage under load, with at least the amp turned on: this will draw the quiescent current (they put it through to get rid of crossover distortion) which for CLASS AB could very well be in 100-300mA (gets the output pretty warm).
This will load the power supply. Somehow I think it is unregulated type, just the bridge and the caps. These will show much higher volts without the load.
You need the one with the amps on.
Unless you already have them on. Then I agree, not too much margin, but good caps will last awhile.

RE: latest project realistic model STA-240 receiver - MrFixr55 - 03-08-2026

+1 on what Mike (Morzh) states. The originals were 50WV. The difference between 8,000uF and 8200uF is only about 2.5%. If there is no load on the power supply, the output voltage will be higher.

There is a load on the supply due to the biasing of the amp. If this is a class A amp, there would be a minor difference tween no signal and full tilt boogie. For class AB, the difference is significant.

Certainly with program, the voltage could drop.

I don't know if the load is affected by whether or not a speaker is connected. I assume with amp on, no program that if there is an electrolytic cap in series with the speaker (load) there would be no difference in current. If it is direct coupled, there may or may not be current drawn. I would have to assume no current difference with speaker or without speaker for direct coupling because if there was current the speaker voice coil would be energized and the cone would be sitting at a position different from resting position (a mechanical bias due to other than zero volts on voice coil. My brother borrowed a bass amp and speaker and the speaker blew due to a defect in the amp that caused a "DC bias voltage" to be placed on the speaker, which caused a positive excursion of the voice coil. Add to that the signal from the bass guitar, the voice coil moved outside the magnet and the spider tore.
I told the guy to take it in for service as it was new and I didn't want to tamper with it. Besides, with no advanced test equipment, no schematic and no time, I did not want to bother with it.

I just got finished reading several articles on rating of caps. I was always taught (from the 1940s vintage electronics books and NRI course I learned from) that one needed to choose a working voltage for an electrolytic near the actual voltage. For example for an AC-DC AA5 radio, a 150 WV cap was appropriate, a 350WV cap would not form correctly and the actual capacity would be less, possibly much less than design rated capacity of the cap. These articles, however, recommend to allow at least 10% margin to allow for spikes which could destroy the dielectric. They do talk further about derating, that the WV chosen should be about 120% of the "rail" voltage. So if you could find 63V caps, they would be better. However, the set (granted about 40 years old) was designed with 50WV caps.

My $0.02 worth

RE: latest project realistic model STA-240 receiver - radiorich - 03-08-2026

Hello MrFixr55,
Far as those two Electrolytics go they are Nichicon here is the link too mouser

mouser nichicon

I took my second measurement with the unit running and playing music, and it came out to about 46 volts. I’m not pushing this 70-watt-per-channel receiver very hard, though. Mike, yes, it’s an unregulated power supply with just a bridge rectifier and a power transformer.

Sincerely Richard

RE: latest project realistic model STA-240 receiver - morzh - 03-08-2026

MrFixr

>>I don't know if the load is affected by whether or not a speaker is connected.

No it won't. The speaker is connected to GND on the other end. So, if there is a cap, obviously no cuyrrebt flows.
And if it is the DC amp, with bi-polar supply, the output will be at 0V with no signal, and 0V to 0V makes no current.
So the only current to speak about will be the bias (quiescent) through current which, again, in my experience will be on the order of 200-300mA for 50W output with +/-35V and up power supplies.

RE: latest project realistic model STA-240 receiver - radiorich - 03-08-2026

Hello guys,
Here’s another issue to address before giving it back. In the photo below, you can see the large tuning pulley on the input of the variable tuning capacitor, which connects to a smaller drive gear that operates the tuning. The problem is that the small gear tends to come off the end of the shaft if you pull on it.



Sincerely Richard

RE: latest project realistic model STA-240 receiver - morzh - 03-09-2026

You mean, there is no set screw or...what holds it?

RE: latest project realistic model STA-240 receiver - MrFixr55 - 03-09-2026

Wonder if it was supposed to be a press fit. If so, Loc-Tite (or other appropriate adhesive) the gear to the shaft.

RE: latest project realistic model STA-240 receiver - MrFixr55 - 03-09-2026

Hi Morzh,

I figured empirically that there would be no current other than quiescent current due to bias. I was musing.

RE: latest project realistic model STA-240 receiver - morzh - 03-09-2026

To add to the musing, if it is a stereo (methinx it is) then there will be two times the quiescent current. So, yeah, it should drain that voltage a bit.

RE: latest project realistic model STA-240 receiver - radiorich - 03-10-2026

Hello mr Fixxr,
I was thinking it was original press fit !!
Sounds about right to amount .

Sincerely Richard

RE: latest project realistic model STA-240 receiver - Arran - 03-10-2026

Richard;
Do you mean the pinion gear on the end of the tuning shaft, the small one that the large pair, with the spring, meshes into? Is it plastic or metal? It may be a pain to remove though.
Regards
Arran

RE: latest project realistic model STA-240 receiver - radiorich - 03-10-2026

Hello Arran,
Yes, that exactly what I was talking about I just forgot the gears name this last few days have been tough because sore tooth .

Sincerely Richard

RE: latest project realistic model STA-240 receiver - MrFixr55 - 03-10-2026

Morzh, I'm half Muser, from my Dad's side.

RE: latest project realistic model STA-240 receiver - radiorich - 03-15-2026

Hello Morzh,
I measured the rail voltage under load today, and with strong bass, it drops to around 45 volts. While the set was playing, I also cleaned the wooden cover, removed the grille for a wash, and finished it off with a coat of Howard's Bee's Wax polish.

Sincerely Richard

RE: latest project realistic model STA-240 receiver - morzh - 03-15-2026

Rich


Here's the thing.
I do not think you will have a problem, at least not for quite a while; this thing might outlive you and me, but:

You are right, a good safety margin would be 20% or so. Now.
As we established, the rectifier up to that output of 45V is unregulated.
Which means, it is also proportional to what the AC voltage in your outlet is today.

Today the Mains AC typically is 122-127V. In 70s it would be more like 110V-115V.
Let's say you have 127V and it was 115V. Or 122V/110V.
This is 10%.

So if you do the math, the voltage back then would be 40.5V, not 45V. Or 42V vs 46V.
And yes, at the time Japanese, while making high quality items, did not designa forever devices and would not use good 20% margin like we like it today.

So, 63V rated caps (which today would be the same size as then 50V) would be more appropriate.
However, you do not exceed WV and so it is not a gross violation of anything.
63V is better.

Whatever caps you picked, if you choose low ESR / High Ripple Current rated caps, this will make them that much more reliable.