Sun(0))) Beta Bass, Need Help On Repair

[wirelessliquid]

Hi I'm from the UK near the Portsmouth area and I need help fixing an old sun beta bass, It's a solid state Class A - B amplifier and the fuses on the power board keep blowing. Circuit diagram and Circuit board layout:

https://i.imgur.com/rWF2J8X.png
https://i.imgur.com/JCLCOlG.png

Fuses F2 and F3 keep blowing when I power up the board, I've replaced and checked components Q7, Q8, Q9, Q10, Q15, Q16, Q17, Q18, Q19 and both of the big capacitors. I've also checked the Ohms across R41 with R42 turned all the way down which reads 140 Ohms and with R42 all the way up reads 3.68Kohms. I'm confused and can't see any reason these fuses keep blowing when ever I power up the amp and I'm really running out of ideas but would love to get this amp working. Some photos of the amps condition.

https://i.imgur.com/hJ8sSZA.jpg
https://i.imgur.com/b9Fd7Tp.jpg
https://i.imgur.com/zq3r3GF.jpg
https://i.imgur.com/peeypDt.jpg

Any help or ideas would be great

[Enzo]

For starts, did you check all the associated resistors? Open resistors can cause problems as easily as shorted transistors.

Q13 is the bias transistor, is it open?

[wirelessliquid]

Is Q13 next to R17 and below R42 on the board? If so I'll check this when I'm home from work

Which resistors would be the associates? just check all of them on the board?

[J M Fahey]

Build a lamp bulb limiter (search MEF) , use a 40 to 60W lamp (real 40/60W , not "equivalent" such as CFL or LED) and plug amp there.
Fuses will not blow, lamp will glow bright instead, but at least let you can take some voltage measurements.

Not sure how you tested transistors, I expect one or more shorted, but as Enzo says, they might also be good but with bias transistor open they get overbiased and pass tons of current.

Also test amp *without* any load, either speaker or resistor.

Try to find a better schematic, the one you posted is unreadable.

The amp seems to be in very good condition and is worth repairing.

[wirelessliquid]

I'm working on building one of those at the moment, Would the bulb only need to be 40 - 60 watts? the amp says it has a 325 watt draw.

Tested the transistors with the diode test function on a multimeter

I've tried for 3 months to find better schematics but the amp is from 1979 and these are the only ones floating around on the internet which I have had to pull out of the only service manual which is missing the parts list aswell. You can find it here: https://www.google.com/url?sa=t&rct=...qvyo3RS7cq6S8D

If you can help find any other documents that would greatly appreciated

[J M Fahey]

325W is the draw full power squarewave into minimum load.
At idle, no signal, no load, an SS amp at pulls 15/30W .
Lamp will blink and then turn off or glow dull orange or red.
If bright, amp is still unrepaired.

[Jazz P Bass]

Oh, and use the limiter for idle voltage measurements only.

Do Not attempt to run the amp with a signal & the output loaded.

[Enzo]

Resistors. BY associated, I mean resistors involved with the transistors. The power transistors have those low ohm high power resistors, those can open. Other resistors that connect directly to any transistor that fails.

[loudthud]

Here's a scan of the power amp schematic. I know it says Alpha Slave but it's the same as the Beta series power amp except for that little bit on the left where input jacks and level control are shown.

[wirelessliquid]

This is a great help thanks loudthud

[CORYK]

I have this same amp. I had an open Q16. I now have audio, but the amp gets extremely hot. Any ideas what would cause it to get hot? Also, R25 gets so hot it unsolders itself from the board!