Dynaco PAT-4 Rebuild

[podbaydoors]

Recently acquired a PAT-4A for $30.00 on CL.
Was functional as brought home, but was completely original.
Existing caps were all 1971 vintage, so assuming that was the build year.
Just finished stage one of the rebuild.
What a pain it is to flip up those boards without unsoldering all the leads from everywhere else.
Replaced every one of the old electrolytics as per stock schematic, with the exception of the PS filter caps.
Of which all sections were replaced with 1000uf 100v which I tied together with a pull tie, used hot melt for permanence and epoxied onto the floor of the unit.
Subsequent to the recap I ran a Sony CD player into the unit and sent the output into a rebuilt ST-150.
This was driving a pair of AR-2Ax.
For all the bad press the PAT-4 has gotten over the years, I was pleasantly surprised at how nice this puppy sounds. I also ran an AR-Xa with a Shure M-91E into the phono input. The quality was quite acceptable.
The phono input was a wee bit noisier than I'd like.
Next step with the rebuild will be to replace all the carbon comp resistors with new metal film.
After all that is complete, I'll be able to listen to what the original design intent actually sounds like.
At that point, I'll try the various mods, starting with the darlington swapout.
Why do all this to a lowly PAT-4?
When I was a kid, I wanted to assemble a system containing a PAT-4 and a ST-120
I rebuilt and tried out the 120 with a PAS-3, which was OK but the 150 is in a different league.
Here a couple of pics taken after re-assembling the unit prior to putting the cover back on.




Any comments or PAT-4 rebuild experiences that anyone else may have had would be most appreciated.

Cheers!,

Lou

 

[Lavane]

Nice! I have one but haven't gotten to it yet. I have a FM3 on the bench right now. Yours is much cleaner inside than mine is.

 

[podbaydoors]

Nice! I have one but haven't gotten to it yet. I have a FM3 on the bench right now. Yours is much cleaner inside than mine is.

It really was pretty clean inside.
There was no residue of cigarette smoke, which is somewhat unusual considering how many folks were heavy smokers back then.
Being a PAT-4A, it was factory assembled, so the soldering was top notch.
I'll also be replacing the twisted pairs with shielded cable ( Not sure whether it will significantly help but why not?)

 

[Lavane]

The tone controls were missing when I got mine. I found some NOS stereo pots for a SCA35 they just dont have the concentric shafts for seperate bass and treble per channel but I can live with that. Itsnice you found a factory wired unit. Mine isn't. Ill be replacing most of the wiring I think.

 

[eizner23]

Nice work. I have a recapped/ restored pat-4 that generates a constant waterfall like white noise even when volume is at 0. Noise is terribly loud in big tall room with wood floors. Is yours silent?

I plan to replace all resistors with 1% metal films and some Dales to try to mitigate this noise.

Other than that this preamp sounds phenomenal.

 

[ConradH]

I picked one up a few years ago at a hamfest for $15. They said, "oh, that's just a PAT-4", while they were selling the PAS and other tube stuff for far more. I did some mods and it sounds great. It does make some transients on turn-on, so a power amp with a delay is desirable. I just ignore it. I still need to change out all the RCA jacks, as they're just enough under size to be a problem with modern cables. Mods here- http://www.conradhoffman.com/PAT4.htm

 

[TPettenati]

I replaced the twisted pairs that go from the balance pot to the preamp boards with some small co-ax. Found this tip somewhere on UpdateMyDynaco. Seems to help the noise floor just a bit.

 

[podbaydoors]

Nice work. I have a recapped/ restored pat-4 that generates a constant waterfall like white noise even when volume is at 0. Noise is terribly loud in big tall room with wood floors. Is yours silent?

I plan to replace all resistors with 1% metal films and some Dales to try to mitigate this noise.

Other than that this preamp sounds phenomenal.

The line level inputs were pretty much dead quiet.
The phono input had barely discernible white noise when turned up higher than I'd be using anyway.
That said, the M-91ED has a relatively generous output voltage, low level cartridges would not do well with this as it sits.
I'm guessing that the replacement of the carbon comp resistors will help significantly,
but it may be the transistors and the nature of the circuit.
I'll post results when done with that stage.

 

[TomReceiver]

I have a whole thread on how I liked mine until one channel became lower than the other. I spray and worked the controls, e-mail Frank Alstein the Dynaco modder and he said it wasn't worth to be fixed by him. Others on the forum chimed in as well. I'm not one to change resistors or caps. In the mean time I got a NAD 1020a which blows the doors off the Dynaco even when working properly. Good luck w/ your machine...Oh, mine was $60 off C/L about 2 years ago

 

[podbaydoors]

I have a whole thread on how I liked mine until one channel became lower than the other. I spray and worked the controls, e-mail Frank Alstein the Dynaco modder and he said it wasn't worth to be fixed by him. Others on the forum chimed in as well. I'm not one to change resistors or caps. In the mean time I got a NAD 1020a which blows the doors off the Dynaco even when working properly. Good luck w/ your machine...Oh, mine was $60 off C/L about 2 years ago

The acquisition and rebuild of this preamp really began as a labor of nostalgia and " gee, I really wanted one of those back in the day " but after getting them I got to wanting to recreate approximately how it would have have sounded new. There are certainly more modern examples, but for me at least, the chase is a big part of the fun.

 

[TomReceiver]

Podbay dude, that's fine if you know how to do the work on them. If you remember back in the day they had factory made units and also Dyna Kits for the home budding electronics engineer. With the hints that Mec gave me I will not throw mine out or sell it even if all is ok but since I do have the NAD Pat-4 will reside in my closet for the time being. Another example were the Gimpod boards for Polk SDA x-over models. This was an interactive site were it showed you where to put the caps,resistors and jumpers. Even that seemed so easy I wasn't going to do them and sent mine out. When Polk went to the Vanilla format that site was deleted.

 

[Jim Marantz]

Nice job so far. If you search through old Boston Audio Society (BAS) newsletters on the web, you can pick up a lot of Dynaco tricks as the engineers or service techs would occasionally write in with improvements on the current gear of the day. I've always found some good gems in those bulletins.

 

[milanomike]

I have a PAT 4 that I got cheap on the bay that had the UpdateMyDynaco power supply, thought was OK, but then added from the same place the line distortion reducer and the phono upgrade. Bam, this thing is better than real decent now, not as quiet as my Hafler Dh 110, but really quite acceptable in that regard. I really like it. Hiss very low, but they have a transformer that will reduce it even further. Thinking about it. I'm no expert troubleshooter, but can solder well and build a kit. Fun kit, real fine instructions.

 

[pdm4606]

Love mine. I did the "UpdateMyDynaco" distortion reducer and power supply. It sounds so fine and clean now.

 

[podbaydoors]

Nice job so far. If you search through old Boston Audio Society (BAS) newsletters on the web, you can pick up a lot of Dynaco tricks as the engineers or service techs would occasionally write in with improvements on the current gear of the day. I've always found some good gems in those bulletins.

Wow! hadn't ever heard of the group.
What a wonder how something meant for an esoteric few, written forty years ago, finds it's way into internet archives.
Very cool, thanks.

 

[podbaydoors]

I have a PAT 4 that I got cheap on the bay that had the UpdateMyDynaco power supply, thought was OK, but then added from the same place the line distortion reducer and the phono upgrade. Bam, this thing is better than real decent now, not as quiet as my Hafler Dh 110, but really quite acceptable in that regard. I really like it. Hiss very low, but they have a transformer that will reduce it even further. Thinking about it. I'm no expert troubleshooter, but can solder well and build a kit. Fun kit, real fine instructions.

I seem to be happy with the rebuild of the original power supply with a few additional MFD's, but the the line distortion reducer and the phono upgrade will be definitely in the lineup.

 

[merashallan]

I seem to be happy with the rebuild of the original power supply with a few additional MFD's, but the the line distortion reducer and the phono upgrade will be definitely in the lineup.

If you want to do the phono upgrade you will need the power supply upgrade (or another source of -17.5V). I also would recommend rewiring the amp for 5 high-level inputs as without the mod the phono preamp input is connected to the phono preamp output when switching between spare and phono causing a huge transient at the output.

 

[podbaydoors]

If you want to do the phono upgrade you will need the power supply upgrade (or another source of -17.5V). I also would recommend rewiring the amp for 5 high-level inputs as without the mod the phono preamp input is connected to the phono preamp output when switching between spare and phono causing a huge transient at the output.

Thanks for the heads up! Yes, I see that now, maybe the phono and PS upgrade can wait.
My ST-150 amp developed a mysterious very intermittent pop and that has now taken front burner.

 

[merashallan]

Thanks for the heads up! Yes, I see that now, maybe the phono and PS upgrade can wait.
My ST-150 amp developed a mysterious very intermittent pop and that has now taken front burner.

Well when you do find the time the phone and PS upgrade are nice. I really appreciate not having a pop when switching to/from phono anymore. I suspect I could have built an amp from scratch with all the effort I have put in to my PAT-4, but making something old work well again is more fun.

 

[eizner23]

I have a whole thread on how I liked mine until one channel became lower than the other. I spray and worked the controls, e-mail Frank Alstein the Dynaco modder and he said it wasn't worth to be fixed by him. Others on the forum chimed in as well. I'm not one to change resistors or caps. In the mean time I got a NAD 1020a which blows the doors off the Dynaco even when working properly. Good luck w/ your machine...Oh, mine was $60 off C/L about 2 years ago

I am experiencing this same issue...
I read somewhere the track resistance in the left and right volume pots are not the same due to age- often off by a few thousand. To fix this, its recommended to change these two pots. There was a reference to using Radio Shack parts.

I was getting noise through the amps even when the PAT-4 was turned off. Replacing the 2 rectifier diodes fixed that noise leak.

Still tracing the noise dragon, I replaced all the resistors on the PCBs with 1% metal film as recommended. Many resistors were over by a lot and out of spec. This reduced the noise by half. Good enough to put back into rotation.

I have the power supply mod with the 3 caps and 2 resistors mentioned here http://www.avahifi.com/images/avahifi/root/audio_basics/ab_pdf/ab1986.pdf starting on page 29.
Maybe a better power supply mod will reduce the noise in my unit even further?