Randall RH200 Blown Transistors

[nick_watts90]

[QUOTE=J M Fahey;498858]

Ok so there is no cont. between any of the collector (middle) legs of the transistor and the heat sink and yes the lamp current limiter lights then goes out straight away and the amp runs fine.

I’m reading 73 VDC of each side of the power supply, and less than 0.017V DC at the speaker out (however with the positive probe on the tip and the negative prob on the sleeve it is -0.017V DC and +0.017 with the probes around the other way) its probably nothing as the reading is so low.

Also I get a reading of 0.00 across the ceramic wire wound resistors.

[J M Fahey]

Yes, those values look fine.
+/-70V rails are quite high, more than I expected but still within ratings, that amp is a BEAST, those rails mean 200W into 8 ohms and 300W into 4 ohms.

I get a reason of 0.00 across the ceramic wire wound resistors

Are you using the 200 milli Volt scale? "200mV" .
In any case, even if power transistors are somewhat underbiased I´d still leave them as is, first for thermal safety reasons and second because there is no adjustment provided anyway.

And being a tatooed pierced headbanger amp, I doubt any will complain about a little extra ... um .... distortion

Specially one that is heard only at .... um .... bedroom levels

I suggest now you turn amp ON, with bulb limiter, without speaker (long story), and only after it´s ON connect the speakers.
Do you hear anything weird? (guess not)

Then plug a guitar play a little, at low volume, think 1 or 2 W (TV set volume).
Does it appear to work fine?

Then the real test: plug amp straight into mains, connect speaker and play loud, at least for a couple minutes.
Does it work fine?

This is where you actually check whether the replacement transistors can take it ornot.
They should, but actual testing has the final word.

Good luck.

[nick_watts90]

Fully agree and add:

yes, it is that simple, you were shorting transistor case to heatsink so place mica+grease (best) or SilPads.
And before powering check that Collector (middle leg) is NOT in contact with heatsink or ground. No speaker attached.
If/whwn amp powers normally (lamp blinks and all but turns off) test power and speaker out rails: +V , -V , and Speaker out.
I expect +/- 40 to 50V DC and less than 100mV DC respectively.

Then measure voltage across wirewound ceramic emitter resistors R25/26/27/28/29/30 (one meter probe on each end) and scale 200mV DC.
Expect less than 50mV across each of them.
Closer to 5 or 10mV would be fine.

Still no speaker connected.

Once you confirm this, we proceed with some audio.

So these are the measurements I’m reading,
55VAC out of the transformer (X 2, with reference to GND)
73VDC on the positive rail
73VDC on the negative rail (These seem high)
18mVDC on the speaker output
0.00 across the resistors (R25,26 and R29,30... R27 and 28 aren’t in this amp)

It’s got me wondering what would jack up the rail voltage?

[nick_watts90]

Yes, those values look fine.
+/-70V rails are quite high, more than I expected but still within ratings, that amp is a BEAST, those rails mean 200W into 8 ohms and 300W into 4 ohms.

Are you using the 200 milli Volt scale? "200mV" .
In any case, even if power transistors are somewhat underbiased I´d still leave them as is, first for thermal safety reasons and second because there is no adjustment provided anyway.

And being a tatooed pierced headbanger amp, I doubt any will complain about a little extra ... um .... distortion

Specially one that is heard only at .... um .... bedroom levels

I suggest now you turn amp ON, with bulb limiter, without speaker (long story), and only after it´s ON connect the speakers.
Do you hear anything weird? (guess not)

Then plug a guitar play a little, at low volume, think 1 or 2 W (TV set volume).
Does it appear to work fine?

Then the real test: plug amp straight into mains, connect speaker and play loud, at least for a couple minutes.
Does it work fine?

This is where you actually check whether the replacement transistors can take it ornot.
They should, but actual testing has the final word.

Good luck.

Just saw this reply, thank you! I will proceed with the testing and probably wait for the loud test till it’s not... um... dark outside for the sake of my neighbours.

You’ve been extremely helpful and I hope this has its problems sorted and I can find a (tattooed, headbanger) home for it elsewhere so I can get something with tubes and more.... leather shoes and skinny jeans sounding