Troubleshooting Sunn Alpha 112r

[01redcobra]

I have a Sunn Alpha 112R here with a problem similar to a post I saw titled "Loud Hum In Peavey Pacer Amp". I'm not too familiar with transistor amps and mostly just don't even look at them, but I am trying to do a favor for the owner so here we are. The amp has the loud hum when turned on and it's pushing the speaker cone out as Enzo described in that other thread. I am led to believe as per the diagnostics provided in that other thread that I am looking at bad output transistors. They are Motorola SJ954 which I cross referenced to NTE219's. What I need to know is, what other things should I be checking that could have caused these to go bad? Also, the amp was sold to the owner with no speaker and I'm confused as to why there are 3 leads out to the speaker ( a green, a black, and a white). Any help on these concerns would be truly appreciated. I have attached a schematic (hopefully).

[52 Bill]

The schematic that you posted shows a complementary pair of output transistors (NPN/PNP) so check the polarities of yours to see if this is the correct schematic. The SJ number sounds like a Peavey part, so they may have been replaced once before.

Check the power supply for plus and minus 45 volts dc. Again the schematic shows that each polarity is fused so check the fuses.

Leave the speaker disconnected until you get the amp circuit straightened out. Measure the voltage on the output, it may help in diagnosing your problem.

Test the output transistors with your meter to see if they are shorted or not. The problem could be something else.

[Mark Black]

I notice the schematic shows numbers "3055" and "2955" for the outputs. So 2N3055 and 2N2955 (very common old-school devices).

A lot of us around here just use the MJ15024 and MJ15025 parts for all-around replacements on this type of output.

[01redcobra]

So in my haste, I didn't notice that one of the transistors was the sj954 but the other is a 223056 which is the equivalent to an nte130. I measured 13vdc on the speaker lead. I came up with +48v on the 223056 but only 33v on the sj954. While checking the voltages, I accidentally shorted the probe tip on 2 leads on the tip29 and now It keeps blowing all the fuses. Better I should have stayed out of here.

[Enzo]

SJ numbers were Motorola house numbers not just for Peavey. Other companies had SJ numbers assigned.


CObra, stay away from the overpriced NTE stuff. We can come up with substitutions just as good as they can, and we won;t charge you a ton to do it. Depending upon the shape, 2N3055 or MJ3055 would be the likely transistors to use.

You want the easy way out? Other than taking the amp to a competent tech? Replace both outputs with MJ15003 and MJ15004 or MJ15024 and MJ15025, assuming those are TO3 shape tansistors. Then replace the TIP29 and TIP30 ones. Make sure to get TIP29C and TIP30C, not A or B. Replace the MPSA43 and also whatever Q102 is that I can;t read. Probably an MPSA93 would be good there. Not done yet. Check ALL those resistors, the couple 10 watt 0.33 ohm by the output transistors, also the 100 ohm between the two TIPs, and those MPSA each have a looks like 180 ohm, check those. And right in the middle is looks like Q109, with a resistor ans a trimmer control by it. Check the resistor and control and figure out what Q109 is and put a new one in.

Be careful, everyone here has had a probe slip at some point. Now that you are aware that it can happen, stay alert.

[01redcobra]

Thanks for all the info Enzo! Now if I can get the nuts off the potentiometer threads (rusted in place), I might be able to get somewhere with this amp. Thanks also to 52 Bill and Mark Black.

[Enzo]

A couple drops of penetrating oil or even WD40 on those threads and let it sit overnight.