Quad 33 and FM3

[Keithstv]

Perhaps they got a good deal when buying the caps or 1uF were in short supply.

Keith

[Synchrodyne]

One of those 0.68 µF capacitors might be the switch filter for the MC1310, and if so, it is definitely different to the 0.25 µF value that appears to be normal, as shown in a couple of circuits:

       

Some commentary on this is provided by RCA in the data for its CA1310, equivalent to the MC1310:

   

I imagine that there might be an upper limit to how much capacitance could be used here without adversely affecting the MC1310 mono/stereo switch operation.

On the other hand, it looks as if Quad used a lower value input coupling capacitor than the 2 µF that was considered normal.

Here‘s another example (Sugden) of a deployed MC1310 with a 1 µF input coupling capacitor and a 0.22 µF switch capacitor:

   


Cheers,

Steve

[ppppenguin]

My gut feeling is to use the 1u polyesters that I have in stock, especially as they will fit very neatly. The alternative is some 0.68u axial polyesters I have which will fit if mounted vertically but not very elegant. Or since I'll be doing an RS order soon I could get these for a tidy result and leave the design as Quad intended: http://uk.rs-online.com/web/p/polyester-...s/1262269/

[Keithstv]

C100 & C104 are the input coupling caps and changing them to 1uF would probably not have any effect. The data sheet I have, dated 1974, recommends a 2uF input capacitor but a lower value is permissible if reduced separation at low frequencies is acceptable.

C105 is the filter capacitor for the stereo switch level detector. The application circuit in the data sheet has this as 0.25uF. It gives the time constant for this as C105 x 30k +/- 30%. I would guess that increasing it from 0.68uF to 1uF would marginally affect the speed of switching from mono to stereo and vice-versa.

In reality I doubt if you'd notice any changes.

Keith

[ppppenguin]

I too looked at the data sheet and can't see how fitting 1u in all those positions could make any significant difference.

[ppppenguin]

RS order arrived and I fitted all the caps. I ordered 0.68u polyesters to replace the tants. At just over £2 for a pack of 10 it wasn't worth arguing.

FWIW the 3 caps in the main PSU which are 400, 640 and 640u on the schematic and 1000, 1000 and 680u in real life were all replaced with 1000u. I think some have recommended replacing the 12V zener with a 15V part to give extra headroom but since I've not had problems I'll leave it alone.

Fast forward to yesterday evening with caps all replaced and a few minutes to spare before going out for the evening. Put the 33 back in its case, hook it up to the 405 etc and it has stopped working. Oh *****. All I get is some very faint sounds and quiet-ish buzz.

Now back on the bench and time for some faultfinding. There weren't any exploding electrolytics so I've probably got them the right way round but that's the first thing to check.

[ppppenguin]

Power supply was OK so I put it back in the system. This time I got intermittent sound with both voume and balace controls very crackly. Using the pushbuttons gave horrible pops and bangs. First thought was DC in the pots and switches. I'd replaced the 100u caps in series with the balance control but they shouldn't affect the volume control or switches.

I'll make a few DC measurements around the controls and see if I can reason out what's happened. For now it's a bit mysterious.

I should add the all the other electrolytics (on the plug-in boards) were replaced less htan 5 years ago.

[Nick]

If it's both channels that are bad, that sounds suspiciously like a PSU or connectivity issue...

You presumably dismantled part of the unit to do the work - is there some mechanical displacement anywhere that could have that effect - a wrongly re-installed connector maybe?

[ppppenguin]

The only things I disconnected were the L and N to the mains transformer. The only relevant feeds from the PSU are 0V and +12V. the 12V is spot on and it wouldn't work at all with if one of those was missing. I also unplugged the PCBs but I've reseated those to no good effect. When I next get it on the bench I'll put a meter on the pots and see if there's any DC. It just sounds and feels like DC on pots and switches even though this is logically impossible for all of them at once.

There might be some fractured wire I haven't found possibly part of 0V continuity.

[ppppenguin]

I disconnected the pair of 100u electrolytics that I replaced. Mde no difference. No DC that I can find on the volume control. Might be something horribly intermittemt - tapping gently in all sorts of places gives nasty noises. Hard to say. Anyway, it's on the bench now and I'll hook it up to a small powered speaker. At least I can then troubleshoot without worrying about the loud thumps and bangs that aren't good news with a Quad 405 and a pair of ESL63s.