Magnatone 240SV power amp distorts when loaded

[Mr_bibbles]

Hey Folks,

https://www.magnatoneamps.com/schema...-schematic.jpg
https://www.magnatoneamps.com/schema...components.jpg

After a lot of elbow grease and tracing and repairs, I've finally got a Magnatone 240SV to output sound on both power amps. The previous owner had snapped 2 PC boards in half trying to take it out of the chassis, re-soldered the entire thing back together and done a lot of other monkey business, but now we are able to probe good audio all the way through until we get to the two speaker output leads with no load connected.

The signal passing through the leads on both power amps are the same volume and roughly the same quality, but when we hook up the "right" (when viewed on the schematic) power amp to a load, the signal instantly sounds like crossover distortion. Unplug the speaker, probe the terminal, and everything sounds good, so it is only acting up under load, meaning that we're looking at any components that interact with said load...

I do have bias voltage at the trimpot symmetrical to the other side at around .7. Trimpot measures at 50k total, 35K actual.
I do have 20v coming to r302 on both sides.
I have replaced all the electrolytics and measured each resistor on the "problem side".



MY guess is that when the amp sees the load, the outputs activate and one of the two is malfunctioning , or possibly the pre-drivers are not supplying enough voltage for the outputs to properly amplify? Can I switch the OT transistors to see if the problem follows the transistors?

I've learned a lot through these posts about SS outputs and am looking forward to improving my skillset, I have a lot of knowledge gaps in this area.

Thanks!

[The Dude]

If I'm reading your description correctly (it's a bit confusing to me), that symptom usually indicates that the thing can't supply enough current to drive a speaker and a problem in the output section. Compare voltages on the output transistors from good side to bad.

[g1]

You can check idle current (bias) by measuring voltage (mVDC) across each of R402 & R403 for both sides. If they are similar, bias is ok.

[Mr_bibbles]

Thank you fellas, I think you helped me zero right in on the issue.

"Bad" power amp side across the .39 (emitter?) resistors.

7.5 MVDC
0.0 MVDC

"Good" Power amp Side:

9.2 MVDC
17 MVDC

So now we know we're looking at a loss of bias across one half of the first power amp, specifically from the .39 Emitter ----> the output transistor.

Since reasonable bias voltage is across the other resistors, we know we're not looking at the supply but probably the transistor itself? The parts list specifies 2147 and 40312, but in this amp I see two transistors labeled"23-50415 6832" (the good side) and "GE16 7803" (the side with no bias voltage).

Perhaps someone tried to replace these already since the other side has "SK3027" (mirror position of the "good" one 23-50415 6832) and then a another "GE16 7803".

Should I start by swapping the good "ge16 7803" over to the malfunctioning side and see if that fixes our issue, and then sourcing a replacement? Or am I missing something fundamental here?

Thanks!

[g1]

Yes you can try that. But before that, make sure D401, R313, and R314 are ok. Also, are values of R401 (across D401) same for both sides?

[g1]

Another problem I hadn't noticed is that the outputs appear to be a mishmash of silicon and germanium.
The GE's are germanium, the SK is silicon, the 23-50415 I don't know. Does it measure closer to .3 or to .5 when checking forward bias?
Regardless, they should all be either Si or Ge, although the 'good' side seems functional.

[Mr_bibbles]

Yes you can try that. But before that, make sure D401, R313, and R314 are ok. Also, are values of R401 (across D401) same for both sides?

Thanks! R313 was about %30 under spec, replaced it for an improvement but still having issues with apparent crossover distortion.

Another problem I hadn't noticed is that the outputs appear to be a mishmash of silicon and germanium.
The GE's are germanium, the SK is silicon, the 23-50415 I don't know. Does it measure closer to .3 or to .5 when checking forward bias?
Regardless, they should all be either Si or Ge, although the 'good' side seems functional.

If I'm checking it correctly, the forward bias with a limiter is .65mv, across the legs of the Outputs right? Since everything else looks in order, should I go ahead and order a matched pair (or quad) of Outputs?

The side that works sounds strong but not exactly anything to write home about.

Thanks again everyone!

[g1]

Ideally I think you want to get away from germanium for reliability sake. I'm not sure what all is involved in conversion though, maybe just biasing, maybe more. Haven't done one myself.
You would need an NPN and a PNP per amp channel.

[Mr_bibbles]

Ideally I think you want to get away from germanium for reliability sake. I'm not sure what all is involved in conversion though, maybe just biasing, maybe more. Haven't done one myself.
You would need an NPN and a PNP per amp channel.

Thanks! Do you have any other advice for sourcing these, or do you have any suggestions on reliable packages that work with this amplifier?

Thanks again guys for everthing.

[g1]

Do you have any generic TO-3s that you use? We used to use a lot of MJ15015 & MJ15016, those might even be overkill here. Once you have them in you can find out if any adjustments need to be made to the circuit.

[Mr_bibbles]

Do you have any generic TO-3s that you use? We used to use a lot of MJ15015 & MJ15016, those might even be overkill here. Once you have them in you can find out if any adjustments need to be made to the circuit.

Awesome!

I'm under-educated when it comes to solid state gear, I would have guessed that most TO packages should be chosen to match the circuits rather than having a "general replacement". I suppose this is because most output sections are very similar, so you just need to make sure your handling is OK?? I think I have a few 15015 and 150016 around, if not I'll order a couple pairs.

Again I really appreciate everyone on this forum. I still remember like 6 years ago I tried to fix a lab series amp (my very first.. woof) and Dude, G1, and Enzo were all there giving me good advice. Hope you guys are getting something good for the holidays this year.

[The Dude]

If you have to order transistors, I'd get MJ15024 & MJ15025. They are basically the same, just rated higher voltage and will be more useful for other repairs. They are often cheaper than, or at least similar in price to the '15 & '16 because there are more manufactured.

[g1]

If you have to order transistors, I'd get MJ15024 & MJ15025. They are basically the same, just rated higher voltage and will be more useful for other repairs. They are often cheaper than, or at least similar in price to the '15 & '16 because there are more manufactured.

Good point. I was going to suggest them earlier but for some reason when I checked, they were showing much higher price. Not sure what happened there, now checking again they are not substantially more, so that is the way to go.

[52 Bill]

Back in the mid to late '60s, there were a lot of power amps that used a combination of PNP germanium and NPN silicon outputs. At the time PNP silicon transistors were difficult to make, so they were deemed too expensive especially with cheap germaniums in plentiful supply. Switching over to all silicon usually requires a bias resistor change to eliminate the resulting crossover distortion.

Are both of the original outputs in TO-3 cases?

[Mr_bibbles]

Back in the mid to late '60s, there were a lot of power amps that used a combination of PNP germanium and NPN silicon outputs. At the time PNP silicon transistors were difficult to make, so they were deemed too expensive especially with cheap germaniums in plentiful supply. Switching over to all silicon usually requires a bias resistor change to eliminate the resulting crossover distortion.

Are both of the original outputs in TO-3 cases?

Yes sir, and I've actually already replaced them with G1's suggestion! Bias returned but I was still having intermittentissues.... until I discovered a blown picofuse. Traced that to an internally mounted new fuse holder to replace it, which had anodized so badly that it wasn't conducting. Both power amps are working fine, which considering the condition of the innards, is a slight miracle.

Next up, the reverb is not functioning! Shouldn't be so hard now that I'm familiar. Thanks for everyone's help!