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Old 23rd August 2014, 12:46 PM   #21
dbear44 is offline dbear44  United States
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[QUOTE=ci11;4033864]Thanks, dbear44!


Once the bridges are done, next on the list would be to bump C102/C202 to 470F as in the Threshold 400A. These caps are noted as non-polar on the schematic so I still have to decide whether it's Mundorf ECap Raw or Muse ES. Unfortunately, I have yet to find any film cap that would fit in that space - the Mundorf at 25x38 barely

I am really anxious to replace those too as soon as I can get a parts order together. I will probably use Muse ES or Silmic II. I have no experience with the Mundorf E caps. Looks interesting. PCX has the 400uf on clearance now for 70% off so might be worth a try if the will fit.

For C110/210 I used Wima MKS 10uf@50v (Mouser part# 505-MKS210/50/20 ). These are great for replacing old electrolytics and tantalums as they will fit in many but not all situations. I buy them by the dozen.

for C112/212 there was a 1uf with a .47uf in paralell on the back side of the board. I replaced it with a 1.47uf Wima.
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Old 23rd August 2014, 02:05 PM   #22
ci11 is offline ci11  United States
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Originally Posted by dbear44 View Post
The IXYS bridges I got from PCX. I used VBE 26-12 NO7 which is rated at 32A and 1200V. This is probably way overkill since with the soft start circuit they are never stressed, but the price is not much different from the smaller ones so why not.
I can't say that it made much difference with the old cans. I think it is more like icing on the cake. If the cake is old and stale the best icing in the world isn't going to help much!

Which brings us back to those big cans. What condition they are in depends on a lot of factors. Was the amp used regularly or in storage for years? Was it kept in some hot attic or damp basement or unheated garage? My amp had not been used in years so maybe that is why I needed to change them. YMMV.

I do know that I tried bypassing them with .47uf Wima's (the only thing that I had 4 that matched) and was not happy with the results. The sound became bright and edgy on top while still being soft and congested in the rest of the range. I ended up taking them out. What was happening was when you put a very small value low ESR cap in paralell with an extremely large high ESR cap you are making a filter. The bypass only helps on the high frequencies leaving lower ones still straining to get current.

So what I am saying is if your cans are in OK shape you may be able to get away with bypasses if they are large enough to affect most of the frequency range. If the amp still sounds overly soft and sluggish with the bypasses then you should replace the cans.
Thanks for the confirmation on the IXYS bridges. I was looking at them at PCX as well although my current thinking is more toward building them up with PCBs, most likely the ones Per-Anders makes: Sjstrm Audio - RFB03 The high current ultra fast rectifier bridge. This provides more diode options as well as being a reversible upgrade, an important consideration for me.

As for the cans, I did measure them with my low ESR meter and they appeared to be within spec. The question would be whether a low ESR reading is a definitive indicator that the caps are functioning normally and any replacement would not yield significant gains. Since replacement options are limited and costly, it would be nice to know.

The PA-7 to me is a beautiful amplifier. It shares the same Nakamichi design language as their famous cassette decks, and stands out against most other amps at any price range. Everyone else look like electronics, but the PA-7 is art. Adding $200 worth of caps to a $750 amp is a major investment by any measure, but restoring art is very different than merely repairing an amp. So if the PSU can upgrade is as dramatic as you indicated, I'd do it in a heartbeat.

On bypassing, having added the 3.3F added subtle dynamic differences but the additional 10nF polystyrenes defined the soundstage in important ways, with convincing harmonics from drums, brass and woodwind taking on a palpable dimensionality that did not exist before. No harsh edginess at all.

With the soundstage in reasonably good shape, I am out to fix the bloated bass which some would say is "tube-like warmth". To me, it just sounds like it's trying hard to go lower but can't - like many under-powered tube amps. But the PA-7 has gobs of power, so perhaps changing the C102/C202 feedback cap from 10F to 470F will fix that. Or perhaps it's the PSU cans not being able to deliver the goods.

So the question is - did/do you hear the same bloated bass and did your PSU can upgrade eliminate the problem?
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Old 24th August 2014, 02:33 PM   #23
dbear44 is offline dbear44  United States
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With the new cans and 4.7uf bypasses the bass is still a little soft but much better than before. Not the ultimate in grip or slam but still very nice. I'd say that overall the sound is much like a fine high powered tube amp. I'd definitely try changing C102/202 first. I am anxious to do that myself. I am also going to order some Vishay MKP 1837 series for bypasses.

Also, it has been mentioned changing R155/255 from 1k to 220ohm and R157/257 from 5.5k to 2.2k. Can anyone elaborate on what affect this will have?
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Old 24th August 2014, 04:53 PM   #24
ci11 is offline ci11  United States
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Originally Posted by dbear44 View Post
With the new cans and 4.7uf bypasses the bass is still a little soft but much better than before. Not the ultimate in grip or slam but still very nice. I'd say that overall the sound is much like a fine high powered tube amp. I'd definitely try changing C102/202 first. I am anxious to do that myself. I am also going to order some Vishay MKP 1837 series for bypasses.

Also, it has been mentioned changing R155/255 from 1k to 220ohm and R157/257 from 5.5k to 2.2k. Can anyone elaborate on what affect this will have?
Thanks for your input on the cans. I am sensing that it is an improvement but short of shifting the signature of the amp from being quite deliberately "euphonic" to more of the "brutus"-like stance of Threshold amps. Definitely something to think about.

On the R155/255 and R157/257. The schematic shows that R155/255 act on the -60V rail and are ultimately controlled by VR102/202, the bias current setting pots. R157/257 are in the "protector" circuit which sense then mediate the +60 on signal spikes to prevent damage on a rail not designed to handled more.

In addition, I have concerns about the output transistor emitter resistors, R125-138/R225-238, which are located on the other side of the PCB, below the heat sink extension. These are film resistors rated at 2W, not cement resistors commonly found in other amps capable of delivering 300W into 4 Ohms. With prolonged exposure to heat in a thermally confined area, their stability may well be in question after all these years. Any bias or rail voltage/bias increases may exacerbate the problem unless they are also upgraded.

These being pretty dramatic steps, I would also like to know if anyone has actual experience in doing so.
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Old 24th August 2014, 09:25 PM   #25
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Quote:
Originally Posted by ci11 View Post
To me, it just sounds like it's trying hard to go lower but can't - like many under-powered tube amps. But the PA-7 has gobs of power, so perhaps changing the C102/C202 feedback cap from 10F to 470F will fix that. Or perhaps it's the PSU cans not being able to deliver the goods.

So the question is - did/do you hear the same bloated bass and did your PSU can upgrade eliminate the problem?
My friend and I upgraded three PA-7's and I think it's the feedback caps not the cans.

Those two caps were changed on both of my PA-7's by a friend of mine and it made a world of difference in the bass response. I probably need to open them up and check though because I think he used smaller value than 470 micro farad. (I can't make the values out from the pics I had sent you)

I was thinking of changing the big cans on both mine but my friend who is a technician said they are still fine. He put smaller bypass caps on um.

Last edited by Mercury3; 24th August 2014 at 09:30 PM.
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Old 24th August 2014, 10:14 PM   #26
ci11 is offline ci11  United States
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Originally Posted by Mercury3 View Post
Those two caps were changed on both of my PA-7's by a friend of mine and it made a world of difference in the bass response. I probably need to open them up and check though because I think he used smaller value than 470 micro farad. (I can't make the values out from the pics I had sent you)
The caps your friend added in one of your amps are 50F non-polarized. This is what I could make out from your pictures. 50F would certainly be an improvement over 10F. According to factory schematics, the PA-5 and PA-7 Mk II's both use a 220F 16V bipolar in the equivalent positions instead of the 10F 200V used in the original, and the caps are now C104L/C104R. The Threshold 400A is at 470F polarized. 470F would yield a corner frequency of 2 Hz, and this is why I am leaning that way.

Those who have gone down this path before, please chime in - thanks!!
Attached Images
File Type: jpg PA5 PA7 Mk II Feedback cap value.jpg (78.4 KB, 112 views)
File Type: png 400A Feedback Cap.png (31.5 KB, 108 views)
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Old 24th August 2014, 10:22 PM   #27
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Quote:
Originally Posted by ci11 View Post
The caps your friend added in one of your amps are 50F non-polarized. This is what I could make out from your pictures. 50F would certainly be an improvement over 10F. According to factory schematics, the PA-5 and PA-7 Mk II's both use a 220F 16V bipolar in the equivalent positions instead of the 10F 200V used in the original, and the caps are now C104L/C104R. The Threshold 400A is at 470F polarized. 470F would yield a corner frequency of 2 Hz, and this is why I am leaning that way.

Those who have gone down this path before, please chime in - thanks!!
Cool thanks! If mine are only 50 then I think I'll up mine too. When you find the caps that fit would you mind posting?

Thanks!
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Old 24th August 2014, 10:41 PM   #28
ci11 is offline ci11  United States
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Originally Posted by Mercury3 View Post
Cool thanks! If mine are only 50 then I think I'll up mine too. When you find the caps that fit would you mind posting?

Thanks!
There are at least a couple of options: the Mundorf ECap Bipolar Raw at 470F 63V MUNDORF E-CAP AC Series Bi-Polar Plain and Raw Electrolytic Capacitors from PCX; and the Nichicon Muse ES UES1V471MHM Nichicon | Mouser from Mouser. The Mundorf at 25x38 is pretty big and will be a very tight fit for my setup but the Muse ES definitely will fit at a puny 16x25.
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Old 25th August 2014, 04:11 AM   #29
dbear44 is offline dbear44  United States
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Originally Posted by ci11 View Post

In addition, I have concerns about the output transistor emitter resistors, R125-138/R225-238, which are located on the other side of the PCB, below the heat sink extension. These are film resistors rated at 2W, not cement resistors commonly found in other amps capable of delivering 300W into 4 Ohms. With prolonged exposure to heat in a thermally confined area, their stability may well be in question after all these years. Any bias or rail voltage/bias increases may exacerbate the problem unless they are also upgraded.

These being pretty dramatic steps, I would also like to know if anyone has actual experience in doing so.
Thanks ci11. I may try changing the emitter resistors down the road. It looks like you could get at them by removing the heatsink assembly without having to take appart the boards. Here are a couple of suggestions for parts: MOSX5C1R0J KOA Speer | Mouser and https://www.partsconnexion.com/resistors_ohmite_5.html. Any preference of metal oxide Vs NI wirewound? Other suggestions?
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Old 25th August 2014, 02:13 PM   #30
ci11 is offline ci11  United States
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Originally Posted by dbear44 View Post
Thanks ci11. I may try changing the emitter resistors down the road. It looks like you could get at them by removing the heatsink assembly without having to take appart the boards. Here are a couple of suggestions for parts: MOSX5C1R0J KOA Speer | Mouser and https://www.partsconnexion.com/resistors_ohmite_5.html. Any preference of metal oxide Vs NI wirewound? Other suggestions?
I was looking at these WHS5-1R0JT075 Welwyn Components / TT Electronics | Mouser because they are 5W, cement filled on a ceramic core, and are surge rated. They should be mounted 4mm off the PCB to dissipate power correctly as they currently are.
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