UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Closed Thread
 
Thread Tools
Old 24th May 2021, 6:41 pm   #1
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Cyrus III amplifier - output stage failure.

Hello!

Has anyone any experience of repairing the output stages on one of these amps? The one I'm involved with has both o/p transistors s/c, and the driver transistor s/c for the -Ve half cycle also.

The parts above are readily available, but this amp is virtually all surface mount design with a sprinkling of tantalum capacitors as well! This does present certain challenges as regards servicing. I've checked all the discrete components in the driver/output section of the current amplifier of the output stage and they check OK. The Lh channel is OK, just the Rh that has failed.

There has not been any previous repair attempts, the PCB is original. There are some posts over on the DIYaudio forum with dire warnings of disturbing the heatsinks or replacing the insulating washers with the wrong types. What did spook me was reading that the early stages in the differential amp or voltage amplifier can suffer from intermittent transistor breakdown. This something that I've not come across before, has anyone else had any experience of similar?

I'm not asking for any specific component level testing procedure, but more any general advice on this model.

Thanks, SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 25th May 2021, 7:17 pm   #2
Jez1234
Pentode
 
Join Date: Mar 2021
Location: Morpeth, Northumberland, UK.
Posts: 111
Default Re: Cyrus III amplifier - output stage failure.

Same procedure as anything else really... There's nothing "magical" about this specific amp. Be sure you've found all the faults before powering up or you'll likely blow the replacement parts you just fitted. SMD makes things more fiddly etc but that's about it.
Jez1234 is offline  
Old 25th May 2021, 9:15 pm   #3
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

Everything pre driver stage appears OK. But before powering up fully, it will get a lamp limiter and a variac and mid-point monitoring during power on. I've often wondered about changing the emitter resistors in output stages from the .22 fitted to say 22ohm. This would obviously limit the current during testing, and if there was a fault in the voltage amp stage, I think the amp would still work, but produce very little power, but show up any voltage offset etc.

Waiting for the drivers at the moment, will post again when they're fitted.

SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 25th May 2021, 9:49 pm   #4
McMurdo
Dekatron
 
McMurdo's Avatar
 
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 4,645
Default Re: Cyrus III amplifier - output stage failure.

I wouldnt bother messing with the emitter resistors, particularly if they're used as part of the VI feedback limiting circuit. Just use a lamp limiter on the mains. If it powers up ok, check for dc offset on the output, run a sine wave through it and scope it, check for symmetrical limiting/clipping as applicable and then try it on a cheap speaker with some decent audio. If it seems stable and distortion free you can take the lamp out of circuit.

These are bit tightly packed aren't they! Rather you than me
__________________
Kevin
McMurdo is offline  
Old 25th May 2021, 10:22 pm   #5
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

Thanks Kevin. To be honest I'll simply limit the current with various wattage lamps and see. Only if I'm really stuck will I go further with the current limiting. Apart from the spares this amp had cost me nothing. So there's really not a lot to lose.

Regards, SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 27th Jun 2021, 8:36 pm   #6
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

Fitted the replacement output and driver transistors, and with current limiting, it powered on. Mid point at 8mV comparable to the other channel. With either a 60 or 40watt lamp as a limiter, there was too much mains drop to power the control micro's 5v regulator. So after much checking, it powered on with full mains with no drama's.

Next job is to re-mount the amp pcb into the chassis. The heatsink on these amps is the actuall top aluminium case. The output transistors are clamped to the amp's sides. There's a post on one of the the audio forums about the heasink surfaces being rough, and not polished, which in turn has lead to poor poor thermal transfer. This was the case on my amp. I've removed the torroidal transformer and polished the heatsink faces smooth.

It seemed a little low output on the Rh channel on CD compared to the left. But on the AV input, both channels had identical outputs, so there may be a second fault on the input select side of things.

At least it's running! SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 27th Jun 2021, 11:41 pm   #7
Refugee
Dekatron
 
Refugee's Avatar
 
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,387
Default Re: Cyrus III amplifier - output stage failure.

Before final test I would be tempted to blow the boards out with compressed air to get rid of any tin whiskers on the unleaded solder.
Refugee is online now  
Old 28th Jun 2021, 9:22 pm   #8
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

Refugee: Thanks for the advice. I wasn't aware that the tin-whisker effect was on unleaded solder. I've not got a air-line at home, so a stiff brush may have to suffice. SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 28th Jun 2021, 10:14 pm   #9
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 19,543
Default Re: Cyrus III amplifier - output stage failure.

The lead in leaded solder acts to make the tin content behave. In unleaded solder and tin plated steel, it gets a free rein to grow dendrites.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 29th Jun 2021, 2:03 am   #10
Refugee
Dekatron
 
Refugee's Avatar
 
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,387
Default Re: Cyrus III amplifier - output stage failure.

I would not rule out a tin whisker from causing the original fault.
Refugee is online now  
Old 3rd Jul 2021, 7:28 pm   #11
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

Thank you for all the replies, I've followed Refugee's sage advice.

The the main amp board is now back in the chassis, which is the the top cover and heatsink. Quite a fiddly job getting the output transistors and isolating washers all aligned.

I mentioned on my last post about a channel imbalance on certain inputs - this is still present. On the the tuner and Au inputs both outputs are identical. But on CD and VI the signal is much greater on the left than before. To make things clear, the Rh isn't low, it's the left that actually increases in level.

If you look at the circuit snipits on IC107A pin 1 from my test disc the level rose from 1v P.P to 3vP.P when I swopped from tuner to CD.

I cannot monitor the input signal to IC107-A as its so small. All I can think is that there is a problem with IC101/103. Both of these are surface mount DIL 16pin, I want to be as sure as I can before removal

Any thoughts about this?

At first I thought the RH was low, its actually that the LH is HIGH only on certain inputs.

Regards SJM.
Attached Thumbnails
Click image for larger version

Name:	20210703_194400.jpg
Views:	88
Size:	58.3 KB
ID:	237022   Click image for larger version

Name:	20210703_194413.jpg
Views:	56
Size:	59.0 KB
ID:	237023  
__________________
It's never been right since we've had it...
samjmann is offline  
Old 3rd Jul 2021, 9:01 pm   #12
Ted Kendall
Dekatron
 
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,010
Default Re: Cyrus III amplifier - output stage failure.

Not enough NFB around the op-amp on the left?
Ted Kendall is offline  
Old 3rd Jul 2021, 10:26 pm   #13
Refugee
Dekatron
 
Refugee's Avatar
 
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,387
Default Re: Cyrus III amplifier - output stage failure.

Give the preamp a going over with compressed air to get rid of the tin whiskers.
There is most likely one shunting the NFB down.
Refugee is online now  
Old 4th Jul 2021, 9:28 am   #14
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

Yes, the NFB was in my mind as well. The thing is that the op- amp has the NFB applied across it the same way for ALL inputs. I've got too much gain on the left, but only on CD and VI input. Tuner and AU are both producing the same output on both channels.

This is why, reluctantly, I think I'll have to change the two input multiplex IC's. Both of which are SMD. If they'd have been through the hole components, I'd have tried the other channel by substitution. These must be very low input/output impedance devices, I can't see any signals on them with a scope!!

The feedback I'll check next, and compare to the working channel.

I'll give the pcb a good clean in case of any 'tin-growth'.

Thanks again to Refugee and Ted for the replies.

SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 4th Jul 2021, 3:28 pm   #15
jamesperrett
Octode
 
Join Date: May 2007
Location: Liss, Hampshire, UK.
Posts: 1,498
Default Re: Cyrus III amplifier - output stage failure.

Have you checked R103 and R107? These are part of the gain setting circuit for those inputs.
jamesperrett is offline  
Old 4th Jul 2021, 10:45 pm   #16
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

No, I haven't checked these, but it's easily done. Will report back tomorrow.

I've gone ahead and ordered a pair of 74HC7053D IC's. To be honest I'm not sure that they're defective or not.

I might use 'Chipquick' low melting point solder to remove the Ic's. I've not used it before. Anyone had any experience of this? There's so many different soldering methods for SMD, we've all got methods that we feel comfortable with and those that we don't!

SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 5th Jul 2021, 1:35 am   #17
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 19,543
Default Re: Cyrus III amplifier - output stage failure.

The upper multiplex chip in that diagram switches only the shunt part of the feedback scaling resistors, and so the Rdson of the actual switch elements appear in series with the shunt resistor. This will act to lower the opamp gain, not increase it. To increase the gain would need either a wrong gain-setting shunt resistor or else some leakage path across that resistor.... maybe a failed-shunt ESD protector?

Alternatively, Rdson could have gone high on all the channels other than the high gain ones, reducing their gains.

Having Rdson get in on the gain setting act is not good design practice. There are well known ways of avoiding this.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 5th Jul 2021, 7:25 am   #18
Ted Kendall
Dekatron
 
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,010
Default Re: Cyrus III amplifier - output stage failure.

Quote:
Originally Posted by samjmann View Post
I might use 'Chipquick' low melting point solder to remove the Ic's. I've not used it before. Anyone had any experience of this? There's so many different soldering methods for SMD, we've all got methods that we feel comfortable with and those that we don't!
I've tried bismuth solder and it does work, but not significantly better than liquid flux and/or Servisol wick. You need to clean and re-tin the pads to get rid of the bismuth as well. I tried this on the DIM of my Volvo V70, where dry joints are a known issue, and eighteen months later I had to re-do some of the joimts, perhaps because I didn't clean thoroughly enough.
Ted Kendall is offline  
Old 5th Jul 2021, 4:11 pm   #19
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 19,543
Default Re: Cyrus III amplifier - output stage failure.

You can score the leads of the IC several times, untol they sever and the chip falls loose. You then warm and pick-off each lead individually, then clean the pads with solder wick before you lightly re-tin ready for the new device.

This is the option for when you know you don't need the device and you want to make minimum risk to the board.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 5th Jul 2021, 7:55 pm   #20
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 464
Default Re: Cyrus III amplifier - output stage failure.

Thanks again to all of you who have posted.

Got a little time tonight following the breakdown of our CH boiler...

From what RW posted, as far as I can work out on the LHC R117 (30.1K) acts as feedback between Pin1 of the op-amp and Pin15 of IC101.

When I did a so called 'point-to-point' check between IC107 Pin1 and IC101 Pin15 I got a reading of 30Mohm...

On the good RH channel, between the same two places, the reading is 28.5k.
So you were all correct, it does appear to be low NFB.

The resistor in question is R117 looks badly soldered at on end. Spiky rather than smooth like all those next to it. It might just be a dry joint.


To RW: Sorry to ask a dumb question, but what is Rdson?

To Ted: I've watched quite a bit of You Tube videos on Bismuth solder, it does look easy to use, but not cheap!

Best regards, SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Closed Thread

Thread Tools



All times are GMT. The time now is 11:50 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2022, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.