Randall RG200

[lowell]

I cannot find C5198, A1941, C4793, or A1837 for this amp.

I'm wondering if someone can confirm that the 2SC5242 is suitable as replacement for C5198.
http://www.toshiba.com/taec/componen...c//66/7886.pdf
http://www.toshiba.com/taec/componen...c//66/7895.pdf

Also cannot find the complementary A1941... so found complementary to the 2SC5242 which is the 2SA1962.
I cannot find a datasheet for the 2SA1962 but assume if the 5242 works then this should too.

Will the 2SC5171 work for the C4793?
http://www.toshiba.com/taec/componen...c//66/7846.pdf
http://www.toshiba.com/taec/componen...c//66/7870.pdf

Please confirm or correct me if I'm wrong here. Cannot locate a schem for this amp.

[Phostenix]

I couldn't find any 2SC or 2SA xstrs that I was looking for the other day from Mouser, Allied, Digikey, or Newark. I bought something from Suburban Electronics a while back off ebay & got a mailer from them yesterday - it had all kinds of 2SC & 2SA parts. All 4 of the parts you listed show up on their website.

Suburban Electronics

Might be worth a call.

[Enzo]

I am always HIGHLY suspicious of those parts sellers that send out little paper flyers with cheap transistor prices.

I have had good luck with B&D enterprises - no fakes so far. B&D Enterprises - electronics, projection lamps for hitachi, sony, mitsubishi, panasonic, samsung
And MCM has been good to me.

MCM shows them all in stock, though one buyer was not happy with his 5198.

Subs are extremely flexible in SS power amps as long as you keep the minimums. If you can't find your parts, then use pairs like
2SC3280/2SA1301 or 2SC3281/2SA1302 or most anything

[Gtr_tech]

I looked for that pair at MCM and thay have a "review" section on the page for the component. They had fakes on that pair according to the review. MCM is off my list of reliable vendors for replacement semi's. When the bean counters take over, all bets are off. The counterfeit problem is getting out of hand.

[lowell]

thanks guys, just ordered from B&D.

[lowell]

so I replace all the outputs and reassembled only to get no sound. The pre out is good, headphone and speakers are not. I then connected my signal probe to the speaker out and the signal is great, it's there and loud. Then as soon as I connect a speaker to the 2 speaker clips... nothing. There is 12vdc on the speaker out so something is passing DC. Where to look? All .33 ballast resistors are good.

[Gtr_tech]

Check the driver section.....sounds like a leaky x-istor.

[lowell]

I just replaced the drivers too, sorry forgot to mention that. Also the +/- supply voltages are good too. Still DC on output.

[J M Fahey]

Hi Lowell.
1) As I understand it, you have 12VDC on the output pin, *with* a speaker connected (I may be mistaken).
What do you measure without load?.
2) How do you know you have signal there?
Are you using a scope?
Does the signal level vary with input signal?
*Maybe* your "loud signal" is what your signal probe "reads" (mistakenly) when actually it's some DC voltage.
3) With no signal present, measure VDC relative to ground, on Q1 and Q2 bases, on their emitters (collector of Q3), Q3 Vbe, and voltage across R7 (it *should* be around 9V)
A defective "slow turn on/thermal clamp" circuit might cause the differential input to go crazy. I've never trusted this particular kind.
4) If all cited test well, measure Vce and Vbe of: Q4 and Q5, which form a sort of current mirror; Q5 providing most of the amp's gain; Q7 which is its active load.
All these form the main amp, regarding gain, stability, DC offset, etc; the rest of the circuit basically providing only (big) current gain.
5) Sorry, but I don't understand very well Q6 and its surroundings.
It's the bias circuit but it doesn't yet look good to me. Oh well, I'll have to brew some more coffee.
Anyway, please measure the voltage from Q5's collector to Q7's collector and the voltage across C22, which *should* be somewhat less, around 2.6V, but who knows? this is a sick chap.
If you are using a series lamp with a 40 to 60W bulb, I'd unsolder one end of D3 and D4 just to put the protection circuit out of the picture.
6) Last but not least, *or* you might start here, measure Vce and Vbe at: Q8, Q12, Q16 and Q9, Q13 and Q17.
Post all values measured in some kind of table, or the post will become very confusing.
Good hunting.
PS: can you post the rest of the RG200 schematic ? just for the library.
PS2: I assume you have already measured and discarded obvious transistor shorts, we're digging somewhat deeper here.

[Enzo]

Just a quick look at Q6, I would think it is used here like a diode. Stick it on the heat sink for thermal tracking. SO Q6 R47 D2 form a string to keep the driver bases appropriately spaced. I would then further guess that the reason R47 is not specified wouold be that its value is selected to set the bias for the particular amp. Of course that could be wrong.

[lowell]

Now I remeasured and the DC on the output is 14vdc. I have no load connected. What I don't get is that I usually have hum when this is the case, however this thing is DEAD quiet when I connect a speaker...?

I know there is signal there because when I touch a signal probe (capacitor) to the speaker out I can hear the signal fine and it does change when the volume is adjusted.

I'll check those other parts.

[lowell]

R7 has 0v across it. Q1 has E3.5v, B12v, and C74v on it's 3 pins. Q2 has E12v, B12v, and C74v on it's 3 pins. That's off if you say it should be 9v. Any clue or should I get the other readings too?

FYI Q1, Q2, and Q3 test good w/ voltage drop measurement and R7 is 3.9k.

[J M Fahey]

Hi Lowell.
To begin with, your amp is muted.
0V across R7 tells me that.
Why is it muted, I don't know yet. There must be some fault condition, we'll get to that working methodically.
To un-mute it by force, unsolder the top (in the schematic, not necessarily on the board) pin of R44, where it joins Q22's collector, and wire it with a short piece of wire to ground, for example R3's or C3's ground pad.
Of course, not load attached and always through the series lamp.
Measure the DC output voltage, the new V B/C/E to ground at Q1 & Q2, and the Vbe and Vce at Q3 *referred to -B or -HV"
By the way, tell me what + and -HV are.
For clarity, please don't write

E3.5v, B12v, and C74v

, but E:3.5V ; B:12V and C:74V, or I'll intuitively read it as , for example, C7:4V.
Too late to teach new tricks to the old dog
Just to gain some time, unsolder D3's top pin and D4's bottom pin, and leave them "in the air" until the amp works.
Basically we are simplifying it into a basic, textbook example, later we'll reconnect bells and whistles.

[lowell]

JM what do you mean by seriers lamp? Are you referring to a lightbulb limiter? And if so aren't the voltages readings going to be way off?

[Jazz P Bass]

L B L

Yeah, I think he means light bulb limiter.
Anyway, I had a Yorkville that blew up bad.
Along with all the ouput transistors it took out the zener on the base (of your amp) Q3. Q3 was also fried.
Can you check this zener. What is the base voltage of Q3?
I can't seem to wrap my brain around Q1 Base at +12Vdc.
The bias path is R3-RT1-R41. These go to the power supply ZD3 & C18 & finally R46 to V-
These all are minus voltages.