replacing 2-diode rectifier with a bridge and other PSU upgrades for old reciever - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 3rd April 2006, 09:06 PM   #1
diyAudio Member
 
bikehorn's Avatar
 
Join Date: Aug 2004
Location: toronto
Default replacing 2-diode rectifier with a bridge and other PSU upgrades for old reciever

I have a late 70' Superscope R-1270 reciever. It's not worth a lot of money and it's not highly collectible either, but i like the way it sounds especially considering i paid $18 for it.

since it's about 30 years old i figured a recap was in order...i don't have the patience to replace every damn cap on all the boards, so i have limited my recap to the power supply and larger electrolytics throughout the amp.

I figured i would upgrade the power amp's PSU while i was at it, since i sort of don't have a choice anyway. the amp was built with a dual 10,000 uF 50V electro for the power amp half of the PSU and since i've never really seen any dual caps anywhere except for low-value high voltage caps for tube amps, i pretty much discounted the possibility of a direct replacement.

I want to increase capacitance by replacing the 2x 10,000 uF cap with a single 15,000 uF cap per rail and i also want to replace the stock rectifier, which is a basic full-wave with a proper snubberised bridge. the large caps will also have bleeder resistors and bypass caps.

More or less this will mean disconnecting the higher-powered transformer leads from the PSU board and P2Ping the poweramp's new PSU starting with the bridge. mainly i am concerned about how much higher the DC voltage will be coming off of a diode bridge vs. a full-wave. given the 50 volt filter caps i would be given to thinking the secondary voltage going into the poweramp is 35 or 40 volts. there are 4 Marantz power transistors - 2 A757(NPN) and 2 C897(PNP). the replacements are Philips ECG 280 and ECG88. anyway, even if the voltage were higher my feeling is they would be alright since the whole thing only puts out 35 watts per pair of transistors and each transistor's absolute maximum disappation is 100 watts each(or in the PNP's case, 200). the heatsink gets warm after a while but that's it.

i'm not specifically looking for more power output - i would welcome it but i am really replacing the stock rectifier for better sound/efficiency(might as well use the whole transformer, right?), and so that i can snub it to reduce switching noise.

one more thing, i became interested in capacitance multiplier supplies while reading about Class A amps and thought about applying them in some DIY projects later on. the idea of being able to extract that sort of filtration from less components appeals to me, although the fact that its use results in a voltage drop kind of would limit my interest in its use to a new DIY project - except for the fact that i'm going to have some extra voltage laying around if i replace the rectifier. i figure at least a chunk of the voltage gained from the new rectifier would be negated in a capacitance multiplier since there seems to be typically a ~5 volt loss with their implementation. i thought it would be neat to have a whole bunch of filtration, like 60,000 uF per side in my reciever but there is hardly space inside to allow that using regular capacitors. without adding a multiplier i will be limited to 15,000 uF per rail, which doesn't bother me since it's still a gain, i just thought i would add a capacitance multiplier because i could and it would give me a little practice building one for future projects. any thoughts?

so, briefly:

am i going to run into trouble by installing a bridge rectifier in place of the old full-wave? how much DC voltage increase can i expect?

is adding a capacitance multiplier(mainly for the "because i can" factor) a bad idea? a really bad idea?

what else should i know about capacitance multipliers?

thanks.
  Reply With Quote
Old 4th April 2006, 02:57 AM   #2
Stocker is offline Stocker  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Austin
Some amplifier topologies do not take kindly to increased rail voltages and may tend to explode a little. Do not be discouraged by this, in light of the cost of the amplifier.

You will learn more than you thought you needed to know by digging this information up yourself, using a search engine like Google and one type of keyword to search for is something like bridge rectifier voltage multiplication or some such.

Do not assume anything with consumer electronics. 50V caps may be on rails from 0-60V or anywhere inbetween. MEASURE FIRST and see what you have!

Whatever you have, have fun; that is 1/2 the point of DIY!

Oh yes; increasing capacitance will not have much effect other than to add a slight bit of bass punch, if you double or quadruple the capacity... unless the originals were under-spec'd, then the rails will be quieter. Play this one by ear, literally.
__________________
Jesus loves you.
  Reply With Quote
Old 4th April 2006, 05:14 AM   #3
rpapps is offline rpapps  Antarctica
diyAudio Member
 
Join Date: Jul 2004
Location: nowhere of interest
Default Replacing 2 diode rectifier with bridge

Hi bikehorn
There are three ways to do this and two of them will end in tears.

If you leave the transformers center tap grounded and ground the negative end of the bridge, the bridge will go bang very quickly as the two diodes in the negative leg will each have half of the secondary voltage directly across them on alternate half cycles.

If you unground the center tap of the transformer, the rest of the amplifier will go bang very quickly as you have now doubled the DC voltage.

If you connect the bridge between the center tap and one end only of the transformer, leaving the other end unconnected, it will work and you will have the same DC voltage as before.
( actually maybe a bit less as the half of the secondary winding that you are using now has to supply double the average current that it previously had to)

Personally, I'd stick with the two diode approach.
Cheers
Rob
  Reply With Quote
Old 4th April 2006, 10:20 AM   #4
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
I think your 2 x 10000uF caps are fairly large as is and not very much to be gained by going further. IMO, larger caps often sound worse than smaller caps. Your DC rail will probably not get much higher with larger caps as yours are already large.

Just my opinion, I would consider replacing the electros with a better brand if possible, or even better, replace them with 3 x 3300uF for each 10000uF, and leave the rectifiers.
  Reply With Quote
Old 4th April 2006, 07:13 PM   #5
Stocker is offline Stocker  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Austin
That sounds like a winner to me.
__________________
Jesus loves you.
  Reply With Quote
Old 4th April 2006, 08:50 PM   #6
diyAudio Member
 
bikehorn's Avatar
 
Join Date: Aug 2004
Location: toronto
thanks guys. i've decided not to get too ambitious with this one and go for a bank of smaller paralleled caps. seems you can get them in multiples of 10 on eBay from JEA. i figure i might as well squeeze as many of them in there as is reasonably possible, so i will probably end up with 5x 3300uF per rail. i suspect i will be too lazy to wire up 10 bleeder resistors especially given the limited space.

one more thing, i looked a little more carefully and the rectifier seems to be a full-wave centre-tap, not just full-wave. my understanding based on what i've read means that the DC voltage between the FW and FWCT are the same, but the increased efficiency of the centre-tap(or a bridge for that matter) means a lower transformer VA rating is required to power the same amp, right? just wondering for personal reference.
  Reply With Quote
Old 4th April 2006, 11:20 PM   #7
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
For each bank of 5 parallelled capacitors you will only need one bleeder resistor.

I re-read your first post to try to establish your output stage topology, but am still unsure. Could it be that the winding with the centre tap feeds each end into a cap and the centre tap goes to ground? Is this then two supply rails (a + and _ )? If so, you may have half wave rectification. On a good note, this has the advantage of having the fundamental hum frequency at mains frequency 50/60Hz.
  Reply With Quote
Old 5th April 2006, 03:06 AM   #8
rpapps is offline rpapps  Antarctica
diyAudio Member
 
Join Date: Jul 2004
Location: nowhere of interest
Default Superscope R1270

Hi bikehorn
What type number are the output transistors?
If they're all the same then you've got a quasi-complementary output that requires a single supply.
If there are two different sets of numbers, then you've got a complementary output that requires a dual supply.
Once we have established that we can advise you better.
Cheers
Rob
  Reply With Quote
Old 5th April 2006, 05:06 AM   #9
diyAudio Member
 
bikehorn's Avatar
 
Join Date: Aug 2004
Location: toronto
i figured there would be some confusion. i'll try to clarify:

there's a single large EI transformer in the reciever. there's 2 secondaries and it's centre tapped. there's a PCB for the power supply just in front of it and more or less it is split down the middle as far as the side of the amp the components mounted on it serve, left side being poweramp and right side being preamp and tuner. the lower voltage secondaries are logically on the side closer to the preamp and the higher voltage is on the other side, along with a chassis-mounted dual 10,000 uF cap. the diode on the PCB for the poweramp side is a dual diode, with 3 terminals. positive, negative and common i guess. and since the centre-tap is grounded, i have to think it's a FWCT supply and not the plain old full wave i thought it was initially. the preamp supply is half-wave for sure - there is only one diode.

the transistors are Marantz A757 and C987, they are definitly complimentary pairs because i looked up the replacements. they are Philips ECG88 and ECG280, PNP and NPN respectively.
  Reply With Quote
Old 5th April 2006, 05:32 AM   #10
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
Quote:
Originally posted by bikehorn
there's 2 secondaries and it's centre tapped.
Sorry, I may have missed something, what is centre tapped; or perhaps you could draw a simple schematic between/inclusive of the secondaries to the caps.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
diode bridge - 10000uF cap - diode tube - small cap - HV+ engels Tubes / Valves 5 29th January 2008 10:16 PM
hybrid power supply: diode bridge into diode tube engels Tubes / Valves 8 24th September 2007 09:18 AM
Pcb for MUR860 (or some else TO220 diode), rectifier bridge peranders Swap Meet 14 17th September 2003 10:37 AM


New To Site? Need Help?

All times are GMT. The time now is 06:51 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2