Looking at a silent STA-2250

quaddriver

120 What?'s per channel
Subscriber
It has multiple minor fleas and the most confusing protection circuit ever (IMHO)

but I might need outputs. I took out SCR601 for protection (driven off a simple current sampler from the outputs) and the relay comes on after 4-5s seconds and stays on. using headphones I hear faint music one channel. no fuses ever went and it could be just the drivers went and outputs are ok (a tiny bit of DC offset on the speakers)

here are the originals:

2SD1110 120v, 7A, 80w, 14mhz
2SB849 - compliment

all in a TO3P case. Im only getting sparse data from semicon, no good datasheet.

using the parameters are a basis to mouser, Im kinda leaning at:

NJW0281G and NJW0302G: 250V, 15A, 150W 30Mhz (seems like overkill, but its meaty overkill?)

also, using tape out to test the tuner et al, I found on phono, lots-o-noise. Its using 2SC1222 differnetials and a 2SC750/2SC1222 amp stage for phone. are the 1222's on the kill list? easy sub, but still. or is it the input and output tants that are going haywire? (have normal subs for all)
 
What is the output Tx emitter current and offset voltage (measured before the relay contacts) running at with the protection operated? When you say 'complex', do you mean it monitors emitter current, offset voltage and heatsink temperature, or is there something else as well. Quite often there is a start-up delay circuit included and this can sometimes trip back in as they normally just use a timing cap through a resistor. The overheat detection varistors can also cause problems.

You said they were complicated. My Rotel has all of the above on each of it's 6 power amplifier circuits together with remote power triggers and input signal sensing.

If all voltages, currents and temperatures look O.K. it may be worthwhile bypassing the protecting relay to a cheap test speaker to find out if the protection circuit has actually tripped correctly or just developed a fault itself and is tripping incorrectly.
 
I didnt say complicated or complex, I said confusing. they use 5 transistors, 1 scr, 2 relays and a crapload of supporting characters. nice try but overkill.

anyways, dont have the ability to measure it now, Im into the unit, power supply, phono amp then off to tonal board.

I think whatever happened, happened, and it busted protection. all replaceable cheap parts
 
On the off chance that it does need outputs, I prefer Sanken.

http://www.semicon.sanken-ele.co.jp/sk_content/2sa1694_ds_en.pdf

IME, it's fairly unusual for outputs to fail in a manner other than shorted.
I agree, I think and hope its the drivers that went. we know protection tripped, but when I removed it thinking I have an A or B only situation, and powered up, no fuses went and the relay came in. so these outputs are not shorted. thats not to say they might not be ugly, but they aint shorted.
 
That's a receiver, right? I have had more than a few that had so many dirty switches/controls that I couldn't get any sound out of them at all initially.
The tripped protection circuit is interesting though.
 
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