Phase Linear Model 300 Series Two overheating on one side at low volumes

[jtatknox]

I have a new-to-me Phase Linear Model 300 Series Two that I purchased on eBay as testing/working. Cosmetically very clean, original caps. When I plug it in, it emits a humming sound from the amplifier and plays clean stereo audio but one of the two heat sinks heats up to 220F within a couple of minutes when playing quietly or with no load and shuts the amp down. Speakers are Seas A26 – not hard to drive.

My first inclination is to return but would prefer if it could be kept alive since it is otherwise clean. Thoughts on whether this would be an easy repair? I'm located in Brooklyn, NY.

Thanks, all, for your thoughts and expertise.

 

[laatsch55]

A known fault of a 300 is the bias transistor leads getting broke. The way they are mounted, any board movement works on the leads. Have you checked DC offset?? It's different than bias, I can't remember where to check bias on a 300 it's been years. Those are RARE output transistors in that puppy, be careful with them

 

[jtatknox]

I took the lid off to observe only and noticed how those notorious cards could move around...

DC offset is < 3 mV each side.

Sure would be great if it's just a bias transistor because it's playing music great for the short time it can stay running.

 

[laatsch55]

The 300 is a sweet sounding amp, had the best THD specs of the whole bunch really.

From the 300 service manual; Excessive overheating

1----Adjust bias--unplug all inputs and outputs, measure the DC voltage across R123, or R124 Should be in the .35 to .45 volts DC. Adjust bias trim pot (R116) to obtain the correct voltage.
2---Check and replace pre-drivers Q102, Q103....

Pretty simple....NOTE- that is not all that could be heating up the heatsink...

 

[jtatknox]

Thanks for this concrete info. I can at least check those DC voltages as a start.

Is audible hum from the amp but not the outputs consistent w/ a bias adjustment issue or does that sound like something more sinister?

 

[laatsch55]

Loose transformer mounting bolts??

 

[laatsch55]

When poking around that board, PLEASE power down and let the caps drain....a probe slip can and will wipe out those rare outputs.. The bias resistor is close to those, a 22 ohm....

 

[laatsch55]

From a service bulletin (undated) a parasitic oscillation can also affect heat and cause audible buzzes or whine. Addition of a slowdown cap is recommended after verifying oscillation with a scope...

 

[jtatknox]

I inspected the board for damage and the only thing I see is that the ceramic capacitor w/ one insulated leg is looking like it's starting to crumble and has a shorter insulated leg than the other channel. Transformer solidly bolted down.

The bias trim pot looks to be the white/yellow plastic screw gear right in the middle of the board. Can you help me identify the bias measurement resistors from this photograph? None of them are numbered on the board and I am worried I'll trace the circuit incorrectly.

 

[J!m]

I would replace those crumbly caps before bothering with any adjustments... (I would, anyway)

 

[laatsch55]

The red,red, black , gold resistor just above the 4 big brown emitter resistor is R124
R123 is on the backside of the board. Put your leads on each side of either one..

 

[jtatknox]

The bias voltage on the working side is dead on 0.35V. The bias voltage on the overheating side was 0.8V and increased to 1.1V when turning the bias pot w/ a chopstick. Changing the pot across the entire range could not lower the potential below 0.8V. Now I guess the questions I have are: is this misbiased enough to cause the rapid overheating? If so, WHY is it so much higher than expected and can I fix it without a scope or an engineering degree?

Thanks, everyone, for your help so far. Much closer to diagnosing the root cause.

 

[Gepetto]

The bias voltage on the working side is dead on 0.35V. The bias voltage on the overheating side was 0.8V and increased to 1.1V when turning the bias pot w/ a chopstick. Changing the pot across the entire range could not lower the potential below 0.8V. Now I guess the questions I have are: is this misbiased enough to cause the rapid overheating? If so, WHY is it so much higher than expected and can I fix it without a scope or an engineering degree?

Thanks, everyone, for your help so far. Much closer to diagnosing the root cause.

Yes and anybody's guess...

Is the photo you show above the failing channel?

Have you looked for bad solder joints on the backside of the PCB assembly?

To troubleshoot this, first recommend installing a short wire jumper from C-E on the bias transistor.

When looking at your photo, the pinout is B-C-E from left to right. You will want to jumper from the middle pin to the one that is farthest to the right.

DO NOT DO THIS WHEN POWERED UP. Allow the amp to fully discharge for 5-10 minutes before working on it.

 

[jtatknox]

Yes, that's a photo of the failing channel. I looked at the solder joints on the back side and they all appear equally crappy. I'd be afraid to try to reflow them all and burn out a component w/ my heavy hand.

I can try this jumper concept but I want to make sure I have the right transistor. You're talking about the one at the very top of the board? Still learning the topology here.

 

[jtatknox]

LOL

I bought two of these on eBay and one is a toaster oven and the other was just declared lost or stolen by FedEx. No scans in seven days.

 

[Gepetto]

If you are not comfortable working on electronics, you are wading into deep waters. Especially with what Lee mentioned, those obsolete output devices that PL used in this amp.

Recommend you consider sending it to one of the very skilled techs that haunt this forum every day...

 

[Gepetto]

At a bias of 0.8V your amp will be toast on that channel very quickly...it should be 0.35. You are probably buzzing because the amp is putting itself into current limit protection mode.

 

[jtatknox]

Sage advice from all here, thank you. Learning is part of the hobby for me, but I will also gladly take it to someone local if anyone can recommend a good technician in the NYC area.

 

[jtatknox]

Well, the "lost or stolen" one showed up today without warning. I brought it up on DBT and and measured DC offset less than 5 mV each side. Happily playing music now.

The broken one that started this thread I didn't originally bring up on a DBT because it was sold as tested/working. This afternoon I started it up on DBT and lo and behold the DBT remains dimly lit! It still plays music normally when DBT bypassed.

I'm still interested in working out the problems on the broken amp if no one knows a local PL technician.

My current questions for the kind experts on this board:

1. Is "tweeter hiss" normal on these amps? I am hearing tweeter hiss and a very faint midwoofer tone from both amps in both speakers when I put my head close to the speakers. The hiss does not increase w/ source volume. I assume this is just a feature of playing with high power amps that are run wide open all the time? If not normal, what factors can contribute to tweeter hiss? I do plan to replace filter caps at a minimum.

2. How does the DBT lighting up change approach to diagnosis?

3. If approach the same, I'd like to learn what the expected outcome would be jumping C to E on the bias transistor and where the transistor is located.

Thanks everyone for your valuable time and consideration.

 

[Gepetto]

Well, the "lost or stolen" one showed up today without warning. I brought it up on DBT and and measured DC offset less than 5 mV each side. Happily playing music now.

The broken one that started this thread I didn't originally bring up on a DBT because it was sold as tested/working. This afternoon I started it up on DBT and lo and behold the DBT remains dimly lit! It still plays music normally when DBT bypassed.

I'm still interested in working out the problems on the broken amp if no one knows a local PL technician.

My current questions for the kind experts on this board:

1. Is "tweeter hiss" normal on these amps? I am hearing tweeter hiss and a very faint midwoofer tone from both amps in both speakers when I put my head close to the speakers. The hiss does not increase w/ source volume. I assume this is just a feature of playing with high power amps that are run wide open all the time? If not normal, what factors can contribute to tweeter hiss? I do plan to replace filter caps at a minimum.

2. How does the DBT lighting up change approach to diagnosis?

3. If approach the same, I'd like to learn what the expected outcome would be jumping C to E on the bias transistor and where the transistor is located.

Thanks everyone for your valuable time and consideration.