My DYNA PAT5 preamp repair thread

[JRUBIN]

This thread will document my repairs to the PAT5 Pre-amplifier as a spinoff of my other AMP thread http://audiokarma.org/forums/index.php?threads/st70-7199-6gh8a-amplitude-vs-frequency.694384/

This is my solid state preamp for the ST70. Which naturally, doesn't work correctly.
My other is the PAS 3, one factory and one custom MOD

The PAT5 exhibited a moderately loud white noise from both channels on all inputs with volume all the way off. I first checked the power supply.


After removing the PS unit PC32, i removed and tested all electrolytic. Looks like this board has been a lot of heat.

The 220uf were reconditioned on the IM-11 and are down to 5uA at 100VDC. Nothing amazing but acceptible. The 2 100uf's were trashed, so I replaced them. The 1uf was tested good, but I replaced it anyway

The diodes and 1 resistor tested good, shown below are also the two broken 100uf16V caps

The Heathkit was used for testing and reconditioning pf the 220uf as well as the replacement caps

Board re-assembled after repairs

Another shot

Back in the Unit... I did some quick measurements, have a feeling something is pulling down the 42.5VDC rail more than it should so I will have to look at the PC 33 cards and test their components.

 

[JRUBIN]

Here we see the outputs of the Left and right channels. Both have noise but are not the same noise.... both channels are differently broken

Here we see one of the channels showing one connection to the output jack and one to pin 6 of the IC103 of the same channel. The output artifact appears to originate on the pin 6 of the IC103
removal of the +15v at pin 19 quiets the circuit, however, the 15v powers the IC103

 

[JRUBIN]

I installed some disconnect pins on the areas I was testing, sort of a one pin terminal. This allowed me to not have to keep soldering and unsoldering things
as well as tie a signal to ground directly or through a capacitor.

During testing I noticed one electrolytic was highly reactive to its surroundings. I pulled it out and got no reading on it the 4.7uf measured .1uf and had an open ESR value
Since this effect was apparent on both channels I prepared myself for replacing both

C22 - 4.7uf CAP shown above its location on the schematic

Temporary C22 replacement in circuit board, also the pin 14 showing the quick disconnect connector

LEFT and RIGHT before any modification with inputs shorted to ground

First cap replaced........ (LEFT)

Second cap replaced (RIGHT)

No doubt replacing the remaining small electrolytics would further benefit the overall performance... Thats the next step........

 

[JRUBIN]

This is as low as my scope goes. Today the power supply and the phono boards get all new electrolytic capacitors




POWER SUPPLY - Raised off of the HOT PCB

Both PHONO boards recapped

960Uv ripple on the outputs , but I havent done the main boards yet. Still leaky caps

 

[TPettenati]

Thanks for all the detailed info. I like your methodology, find and fix the faults first before any large scale cap replacement.
I have a PAT5 that is all original and in daily use. It's in my garage/shop, so any noise floor is probably overlooked. Not exactly critical listening.
No doubt it would benefit from some cap replacement, but I really need to do some baseline measurements first.

 

[JRUBIN]

Once the caps are done it will go onto the HP311A for a torture session just like every other AMP

 

[JRUBIN]

Almost finished, a few caps I ordered were wrong Axial vs. Radial.... So it will take a bit longer. Each main board need only 2 caps replaced, the others are done

 

[JRUBIN]

Just my luck, the transistor on the power supply died. Ive had to order another one.

 

[JRUBIN]

All electrolytic caps replaced.
The 2n3053 was replaced and restored the 42V rail

The verdict on the HP311A was .05% which is the best quality input wave I have, so there was no measurable increase in distortion. Not only that, it was measured using the phono input
which means a lot more components to get through. Closing the books on this project