Restoring an old Crown IC-150 preamp - caps and IC's
Before I start - HELP!!! but PLEASE be gentle with me here
I recently inherited a Crown IC150 preamp and (expensively) wrecked the volume control pot while trying to correct a drop-out in it's rotation. Hopefully sorted, BUT, while it's in pieces, the component upgrade bug has bitten..
This sample appears to be a mid-period one - full manuals available here -
The mother board, what there is of it, is type 7923 and if any of you look at the circuit, most of the components are actually power supply and tone/loudness associated, the supply to the IC's, which do all the work, being simple? +/- 18V rails. Having heard HiFi Dave's sample, I was surprised his didn't take my fillings out - quite the opposite in fact..
Anyway, possible bodges/tweaks? The first thing is that this brand used the best available components back then in critical parts and these bits, caps, 1% resistors etc., really do seem to stand the test of time and thrashing.
The IC's originally supplied were LM301's, judged dreadful by today's standards (and even when the thing was new it seems). Whats in there is summat like 3532's. Works well if the preamp is run nearly flat out, but noise and distortion, together with a overly-crisp sound, develops at low volume levels. I have some 833N's I could drop straight in, the OPA604's seem popular and I'm tempted by the LME49710N, although this may be too "wide open" for the design.
The supply caps - four in main supply and one on phono board are tiny (by modern standards) 250uF 30V ones. I've earmarked some 1000uF long life/high ripple Panasonics which I'm hoping will be ok. The four 1uF tants on the main board have already gone, replaced by some Wima 1uF I had left over from the Quad 33. There are four bead tants of 4.7uF on the phono board, which I have no idea what to put in place (I've seen some bead-electrolytics the same size).
The main thing is, there are some odd value electrolytics there - 500Uf 3V and 25Uf 15V NP and a 5uF 30V in the power supply. Assuming I ought to replace them either now while the thing is in bits, or later, what do you suggest?
PLEASE chaps, before suggesting I either bin the internals and put a valve buffer in there, could you technically inclined glance over the schematics linked to and suggest modern values and types that should work, hopefully better.
Thanks in advance. I'm going to need some of the facilities on offer here up in the office system, so it would be nice to get the best out of this old timer if at all possible, with modern caps and IC's at least..
Hi Dave, having had a look you should be fine with a 470uf in place of the 500uf in the phono stage. As it's an electrolytic it won't have a close tolerance & as standard is probably +/- 20%. What i suggest is to fit a higher voltage cap at the same time as this will lower leakage & distortion, with modern components being smaller you should have no difficulty fitting in a 16 or 25V 470uf capacitor.
The 5uf is following what looks suspiciously like a tracking regulator to my eyes. As it's part of the PSU i'd suggest something with a nice low impedance. If you look about you may well find some Sanyo Oscons that'll drop straight in, failing that look for some Panasonic FC type. Again go for as high a voltage as you can find to fit in there
As for the 25uf NP there may well be some Nichicon caps that will do the trick, i think they are the MUSE range & these are non polarised, this one should do the trick
As an alternative purchase 4 x 47uf & mount two back to back & you'll end up with around about 23.5uf. As it's non polarised i must assume it'll be carrying audio & at a reasonable level, make sure the caps are good ones. Another alternative is to use 63V MKTs & parallel some up to give you 25uf, but only if you have the room
Hope that helps... By the way, good idea to increase the reservoir caps to 1000uf from 250uf
"We must believe in free will. We have no choice" Isaac Bashevis Singer
Thanks so much for this. I have a shopping list saved with RS, but want to get it right, as Crown, in fairness to them, used the best bits they could reasonably get back then and the longevity (retaining their specs) of their products over the decades speaks volumes. despite this item not being their best loved product (I believe John Curl stated it was the worst HiFi product ever........), I have hopes for positive outcome
Wonderful pre! I had one mid 70's feeding a 700 watt Phase Linear power amp.
So they do have hidden fans after all
Seriously, I understand that the severe limitations of the IC's used (by modern standards) meant that you really should have run the thing flat out down the pre to power interconnect (at pro 5V levels) and then adjusted overall gain at the power amp end, easy with many of the power houses available back then. As soon as the preamp volume control was backed off below 1 o'clock, noise and distortion stated to climb big-time. This is what replacing the IC's and caps is supposed to cure. I'm not expecting "Croft Magic" from it, but dave W's sample is more than good enough in stock form to make me keen to get it going again. The twin phono inputs will definitely be an advantage I think
Crown IC-150 upgrade
I second the suggestion from Mark re the Sanyo Oscons, although they are discontinued now you may still find some around. The Panasonic FCs also seem to be discontinued, but the FM series are comparable followed by the FR type.
The Nichicon Muse ES looks like the best series for bi-polars, and if you want to buy from a regular shop rather than eBay, look at http://www.hificollective.co.uk. The Nichicon FG series may also work for the other caps, but I haven't tried them so I don't know whether or not the claims for sonic superiority are justified.
As a source for parts also consider Farnell - they have a wider range than RS in my experience, and these days they have a minimum order of £20 + VAT and take credit card payments. The days of "We only deal with trade" seem to be long gone!
Hope this helps.
many thanks for suggestions folks
Farnell is a pain regarding domestic £20 orders, although I do have accounts with them, CPC and RS. I've also used HiFi Collective in the past and also recommend them highly for prompt service.
Quick question regarding volume pots. Once the main unit is complete I'll have a donor to play with eventually. There should be enough caps left over for it, so what about a basic volume control and hang the "loudness" part of the circuit, which is switched out most of the time, thereby not needing the extra taps? The stock law of the original (judging by the channel I didn't ruin) is 400K log. Would a 500K work in this basic circuit? It's easy to attach to the frame and hard wire down to the board. I could even try some fixed resistors to give a fixed gentle loudness contour possibly when that is selected????
500K ohm should be fine, make sure it's a decent one though. No point in putting better components in other places & not the volume control
"We must believe in free will. We have no choice" Isaac Bashevis Singer
you can get 470K Log pots or You could try a 1M or bigger if poss linear pot & attach a 400K precision resistor bridged across the input & ground on each channel.
Originally Posted by DSJR
Thanks again chaps All good fun innit
I have an RS Components shopping list saved and the caps I've saved so far are Panasonic FK mainly, as these seem to have the highest ripple? I'm assuming that these should be over-specified for the demands the circuit will put on them. there are some Os-Con 510uF too which will be ideal for where they'd need to go. Can't order these bits just yet, but can't wait to get the volume control in at least