Phase Linear 400 Series 2 Repair

[jblmar]

Hi Ron,

The 300DC is a high feedback amplifier. Any bias would make it measure okay.

DC offset and bias current are two things you should always check on a repaired amplifier. You might measure under bias THD if you were to load the output with 4 ohm and about 1 Vrms output at 10 KHz or so. Higher powers and lower frequencies tend to measure much better.

The 'scope is only useful for looking at residuals.

-Chris

Edit: Do you still have the original parts? What was the reason for the replacement?

Hi Chris,
I changed the bias diode on the right channel after a thermal runaway condition developed. Replaced the output transistors with newer, at the time Motorola MJ 150xx. I forget the exact number but I believe they are the 15022/23. Sounds great.
While I was in there I did a complete recap and cleaned all contacts including the relay. That was back in 2002. Never looked back.



Ron

 

[anatech]

Hi Ron,

I changed the bias diode on the right channel after a thermal runaway condition developed.

That's interesting. Was it a varistor? Often the leads get broken at the body if they are handled excessively.

While I was in there I did a complete recap and cleaned all contacts including the relay.

I did the same thing to mine, and changed a few. The difference was very positive and this is my favorite amp still. I need a new relay though.

Hi Richard,

I ordered a new set of six MJ21196's and will try it out.

Sounds good. How about the mica insulators and new thermal compound??

-Chris

 

[rwagoner]

Hi Richard,

Sounds good. How about the mica insulators and new thermal compound??

-Chris

Of course! I took mental notes and reread every reply to make sure I was looking at the right things. I also plan to use the 100-watt bulb in series when I fire it back up. The only thing I have not done yet is check whatever I need to check in the power supply. I think it is probably fine, since only one channel had any bad transistors, but I should probably check it anyway. Are there upgrades I can do on it?

Thanks for everything!

Richard

 

[jblmar]

Hi Chris,
Yes, the varistor lead was broken at the base of the component. Not 100% sure if it broke durning handling or was defective. Either way, it was the source of the problem.

The relay should be common and fairly easy to get. If you need the Marantz part number, let me know. The part number should include the DC voltage and poles/throw.

Tremendous difference with a full recap and board contact cleaning with Deoxit.

Ron

 

[anatech]

Hi Ron,

The relay should be common and fairly easy to get.

Yes, I have one but I'm waiting for speaker plugs so I can install some new Cyrus Mono X amplifiers. Then I can take the Marantz out of service and replace that last part.

If you need the Marantz part number, let me know. The part number should include the DC voltage and poles/throw.

That's one manual I kept. The part number won't help though. It's going to be a double pole, double throw type. If the N.C. pins are not used, you simply cut them off.

board contact cleaning with Deoxit.

You shouldn't do that! De-oxit (as with any cleaner) will creep and get into all kinds of components. Good thing you replaced the caps because they are the first to go. So try not to wash PCB's on the component side - ever. There are even some transistors it can wick into and wreck them. That was hard to find. WD-40 seems to be the universal cleaning agent people tend to use.

My advice, blow the dust off and that's it!

Hi Richard,

I think it is probably fine, since only one channel had any bad transistors, but I should probably check it anyway.

Your power supply should be fine. Just check the seals on the main caps.

Are there upgrades I can do on it?

First, get it working again. Then think carefully about anything you want to try. I'll hint that bypassing can be good and increasing the size of the main capacitors can be bad.

-Chris

 

[jblmar]

Hi Chris,
I didn't clean the board, just the gold plated connectors.
I agree with you that cleaning the bord itself is a very bad idea. Besides possible compromise to components, you run the risk of removing component designation numbers. On the foil side, the trace insulation can also be compromised leaving the unit open to possible shorts or shock hazard.
I ONLY cleaned the contact to contact area. Results were great.

Ron

 

[anatech]

Hi Ron,
Thanks for clarifying.

On the foil side, the trace insulation can also be compromised leaving the unit open to possible shorts or shock hazard.

Normally a non-residue cleaner would be used here. Any edge contacts should be avoided as they may become contaminated. The solder mask, if used, is generally resistant to these cleaners.

Cleaning any edge contacts is a normal thing to do. It's become so second nature that I don't even realize I do it.

-Chris

 

[jblmar]

Hi Ron,
Thanks for clarifying.


Normally a non-residue cleaner would be used here. Any edge contacts should be avoided as they may become contaminated. The solder mask, if used, is generally resistant to these cleaners.

Cleaning any edge contacts is a normal thing to do. It's become so second nature that I don't even realize I do it.

-Chris

Same here. Standard routine.

Ron

 

[rwagoner]

Back in the Saddle

I got the transistors a couple days ago; today I rechecked some resistors and capacitors. Everything looked OK so I installed the new transistors -- MJ21196G (I feel so green), put everything together and ... IT WORKED!

I may later change some of the suggested capacitors and resistors; I would have done so already but I could not find the exact values needed locally and wanted to get it back together. As I said, noting seemed out of spec, including the 4.7K 5 watt resistors. Sounds great. I think I'll leave it quasi for now ... I'm still a bit too ignorant of the differences and benefits.

Thanks again for all the help. I couldn't have done it without you.

Richard

 

[anatech]

Hi Richard,
Great news! I'm glad it's running for you now.

-Chris

 

[Bigred]

Excellent! Love to hear success stories and see vintage gear keep on ticking