Leak Stereo70 - UK Vintage Radio Repair and Restoration Discussion Forum

PatMorgan · 14th Mar 2018, 11:31 pm

This is my first post.
Im hoping someone on here may have knowledge of the Leak S70
I've had two of these for some years now- one had no Right chanel so it was an easy repar to repalce the drivers and follower transistors- the Outputs seemed fine.
After running it for a few hours over a week of so, the sound sort of 'went awol' and now i have poor sound quality and volume - but on tape in- the sound is back to normal for monitor, but obviousy with no tone control.

Ive held back for now swapping boards from my other S70 as I dont want to start a load of trouble that I cant fix.
Yes, I set the IQ's when I repalced the tansistors and reversed the 200 ohm press,as the track was bad on the offending power output board.

Could I have blown the preamp boards? and what would blow on them?- it seems funny that the sound just 'went'.
I was probably feeding too high and input to the tuner 2 input, but id expected clipping if I was overdriving the BC148'S?

Id appreciate some comments or help.
Thanks.

Radio Wrangler · 15th Mar 2018, 5:29 am

Slow and careful work should get you there.

Just work your way through checking the bias voltages on all the devices in the chain, and stick an ESR meter on all the electrolytic capacitors.

If you have an oscilloscope you can use a probe to follow a signal down the chain and find where it vanishes.

You can probe-inject a little signal through a dc blocking capacitor and work your way backwards up the chain listening for the signal until you find where it disappears.

There are many approaches. Start with one which fits your available equipment. Avoid the urge to try quick fixes. Nice when they work, but can sow confusion and waste time when they don't. Logical fault-finding will allow you to trace any fault. Familiarity and experience will allow you to quickly fix the common faults, but leaves you back at square one for anything else, so the slow way is the certain route. Less profitable in a repair shop.

David

Hartley118 · 15th Mar 2018, 9:12 am

If you suspect a blown transistor, it can be helpful to use a digital multimeter on the 'diode setting to check that each transistor still contains two semiconductor junctions. If it does, it's probably OK.

With power off, look for a forward voltage drop of 0.5 - 0.7 V on the base-emitter and collector-base junctions. This is for silicon transistors: with germanium devices you'll see 0.2 to 0.3 V. Open circuits or shorts are bad news. Pin-out connections vary, so you may need to experiment a little.

Martin

nutteronthebus · 15th Mar 2018, 2:35 pm

Hi this is a link for the cct diagram https://www.radiomuseum.org/r/leak_stereo_70.html
I do have a lot of info on the Leak but too large to post ( diagram 10.5Mb ) PM me and I will burn it to disk and post it to you

Dave

dave cox · 15th Mar 2018, 4:31 pm

If it works OK on the tape input then the power amp is working. That is a good thing!

dc

PatMorgan · 15th Mar 2018, 8:11 pm

Thanks very much for all your replies. Some go above my head and are limited to the 2 meters I have and certainly no scope!
I thought I'd throw it out on the off chance someone had this sort of thing with an S70 before- I dont want to try the quick fix of swapping my good boards from my good amp.

To Dave- Thanks for the offer- I've the WD and service leaflet that Leak produced back in the day.
I've never had or used a signal injector but Ill keep my eye out for a scope on Ebay- I've been warned off digital ones and anything is going to over tax me anyway as the last time I used one was as an apprentice in my first year back in 1972.

I've a nagging suspicion this is something to do with the switches on the front panel but as David said in the opening reply- Slowly and Methodically. Seems odd that it works as they do- very flat when the Tape Monitor switch is depressed - which looking at the WD - bypasses the preamp.

Of the two you can maybe see in the thumbnail- the bottom one with the awful case is the one in question and it did sound better than the one I'm using (the upper)- it has a snitch more bass- which suits the music I listen to.

Whaam68 · 15th Mar 2018, 9:09 pm

From memory the tape input bypasses the preamp section? If so it might be worth recapping the preamp boards....and see if that cures it?
Mike

vidjoman · 15th Mar 2018, 9:35 pm

I presume you have cleaned the switches and the circuit board edge connections.

Trevor · 16th Mar 2018, 9:00 am

I agree with wham 68 re the caps you can also switch the pre-amp boards around if the fault moves with the board you know where to start. Their is generally no danger in swapping pre boards

Trev

PatMorgan · 17th Mar 2018, 11:39 am

Quote:

Originally Posted by vidjoman

I presume you have cleaned the switches and the circuit board edge connections.

As much as you can- yes! I used switch cleaner on the pots and bank of switches. I plan to remove wires and see if the fault alters (one at a time as they are not all colour-coded- well, if they are my eyes can't pick up the difference!)

One thing I notice/recall is that when the volumume is completely off, there's still a fair bit of 'sound' coming through.

PatMorgan · 17th Mar 2018, 11:41 am

Quote:

Originally Posted by Trevor

I agree with wham 68 re the caps you can also switch the pre-amp boards around if the fault moves with the board you know where to start. Their is generally no danger in swapping pre boards

Its the same on both channels- so Ive tried that.

mark2collection · 17th Mar 2018, 2:00 pm

If it's anything like my Stereo 70, the inability to 'mute' the sound by turning the volume control right down required a new potentiometer.

Between ground & the centre (wiper) of the pot, mine read something like 25 ohms, in the end I gave in & replaced it with an Alps item, as using the amp late at night/early morning became a nuisance, as even at minimum volume, it was too loud!

Sound going AWOL, most likely the tape monitor switch itself. Check it with a multimeter, on ohms, amp off. Even after cleaning, mine gave troubles. With the amp off, meter attached & wiggling the switch showed it was faulty.

Over time, & using the amp more often, exercising the switches before use, it's been great.

Might be worth cleaning the PCB edge connectors & sockets while you're in there.

Mark

PatMorgan · 22nd Mar 2018, 6:38 pm

Its been a bit cold recently to go out and play fault-find, but today I bit the bullet and put my pre-amp boards into my known working S70 AMP and transfered the same fault over- then put my known board back in and the amp worked fine.
So, my dud amp has two preamp boards defective - that's where the sound has 'gone' I'll remove the transistors and test, as I doubt all of them have failed. One of the problems with the boards being plug-in is the difficulty to measure pin voltages- so it will be trial and error.

dave cox · 23rd Mar 2018, 12:27 pm

While you have the transistors out, check the value of all high value resistors (they have a habit of going very high) and any small electrolytics for leakage (especially) & capasitance.

dc

PatMorgan · 23rd Mar 2018, 2:43 pm

All- problem solved- I know have 2 working S70'S!

T4 was not switching on, so very little gain on T5- and I had 5v on the emitter, so the BC148's were repalced and sound has returned. same fault on both channels.

The good thing with these old amps is the wiring diagram which is relatively easy to follow, but you dont expect the same fault to 2 channels. It might have been overdriven by the earlier stages as the output from my minidisc is higher than the recommended input level- Ive attenuated it now via the switch on the back.

Trevor · 23rd Mar 2018, 4:38 pm

Bc148 lock-fit we have been discussing these on the forum known to have a higher failure mode than the normal type of transistors
Trev

PatMorgan · 23rd Mar 2018, 6:24 pm

I've replaced them with 337's- But there's still a few more on each preamp board to fail. Somewhere in my loft ive a Substitution book, but it will be at least 40 years old!

Trevor · 23rd Mar 2018, 7:24 pm

bc109 =bc149 =bc184
bc108 = bc148 =bc 182

Trev

PatMorgan · 23rd Mar 2018, 10:11 pm

Thanks Trevor.

So far no difference noted with the BC337's in place of T4, T5.

I must get a 200Ω trimmer as I can feel (heat) the IQ has wandered again on one channel- I'd already reveresed it as one leg is shorted to wiper. It's seen better days.

PatMorgan · 23rd Mar 2018, 11:07 pm

I find they are ok providing that the magic spot- the 30ma is hit, but oldcarbon tracks etc.

14th Mar 2018, 11:31 pm	#1
PatMorgan Triode Join Date: Mar 2018 Location: Maidstone, Kent, UK. Posts: 24	Leak Stereo70 This is my first post. Im hoping someone on here may have knowledge of the Leak S70 I've had two of these for some years now- one had no Right chanel so it was an easy repar to repalce the drivers and follower transistors- the Outputs seemed fine. After running it for a few hours over a week of so, the sound sort of 'went awol' and now i have poor sound quality and volume - but on tape in- the sound is back to normal for monitor, but obviousy with no tone control. Ive held back for now swapping boards from my other S70 as I dont want to start a load of trouble that I cant fix. Yes, I set the IQ's when I repalced the tansistors and reversed the 200 ohm press,as the track was bad on the offending power output board. Could I have blown the preamp boards? and what would blow on them?- it seems funny that the sound just 'went'. I was probably feeding too high and input to the tuner 2 input, but id expected clipping if I was overdriving the BC148'S? Id appreciate some comments or help. Thanks. Attached Thumbnails

15th Mar 2018, 5:29 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: Leak Stereo70 Slow and careful work should get you there. Just work your way through checking the bias voltages on all the devices in the chain, and stick an ESR meter on all the electrolytic capacitors. If you have an oscilloscope you can use a probe to follow a signal down the chain and find where it vanishes. You can probe-inject a little signal through a dc blocking capacitor and work your way backwards up the chain listening for the signal until you find where it disappears. There are many approaches. Start with one which fits your available equipment. Avoid the urge to try quick fixes. Nice when they work, but can sow confusion and waste time when they don't. Logical fault-finding will allow you to trace any fault. Familiarity and experience will allow you to quickly fix the common faults, but leaves you back at square one for anything else, so the slow way is the certain route. Less profitable in a repair shop. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

15th Mar 2018, 9:12 am	#3
Hartley118 Nonode Join Date: Aug 2015 Location: Cambridge, Cambs. UK. Posts: 2,198	Re: Leak Stereo70 If you suspect a blown transistor, it can be helpful to use a digital multimeter on the 'diode setting to check that each transistor still contains two semiconductor junctions. If it does, it's probably OK. With power off, look for a forward voltage drop of 0.5 - 0.7 V on the base-emitter and collector-base junctions. This is for silicon transistors: with germanium devices you'll see 0.2 to 0.3 V. Open circuits or shorts are bad news. Pin-out connections vary, so you may need to experiment a little. Martin __________________ BVWS Member

15th Mar 2018, 2:35 pm	#4
nutteronthebus Hexode Join Date: Jun 2011 Location: Glossop, Derbyshire, UK. Posts: 487	Re: Leak Stereo70 Hi this is a link for the cct diagram https://www.radiomuseum.org/r/leak_stereo_70.html I do have a lot of info on the Leak but too large to post ( diagram 10.5Mb ) PM me and I will burn it to disk and post it to you Dave __________________ Any tool can be used as a hammer but a screwdriver makes the best chisel

15th Mar 2018, 4:31 pm	#5
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: Leak Stereo70 If it works OK on the tape input then the power amp is working. That is a good thing! dc

15th Mar 2018, 8:11 pm	#6
PatMorgan Triode Join Date: Mar 2018 Location: Maidstone, Kent, UK. Posts: 24	Re: Leak Stereo70 Thanks very much for all your replies. Some go above my head and are limited to the 2 meters I have and certainly no scope! I thought I'd throw it out on the off chance someone had this sort of thing with an S70 before- I dont want to try the quick fix of swapping my good boards from my good amp. To Dave- Thanks for the offer- I've the WD and service leaflet that Leak produced back in the day. I've never had or used a signal injector but Ill keep my eye out for a scope on Ebay- I've been warned off digital ones and anything is going to over tax me anyway as the last time I used one was as an apprentice in my first year back in 1972. I've a nagging suspicion this is something to do with the switches on the front panel but as David said in the opening reply- Slowly and Methodically. Seems odd that it works as they do- very flat when the Tape Monitor switch is depressed - which looking at the WD - bypasses the preamp. Of the two you can maybe see in the thumbnail- the bottom one with the awful case is the one in question and it did sound better than the one I'm using (the upper)- it has a snitch more bass- which suits the music I listen to.

15th Mar 2018, 9:09 pm	#7
Whaam68 Hexode Join Date: Feb 2007 Location: Luton, Bedfordshire, UK. Posts: 456	Re: Leak Stereo70 From memory the tape input bypasses the preamp section? If so it might be worth recapping the preamp boards....and see if that cures it? Mike

15th Mar 2018, 9:35 pm	#8
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Leak Stereo70 I presume you have cleaned the switches and the circuit board edge connections.

16th Mar 2018, 9:00 am	#9
Trevor Octode Join Date: Aug 2006 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 1,082	Re: Leak Stereo70 I agree with wham 68 re the caps you can also switch the pre-amp boards around if the fault moves with the board you know where to start. Their is generally no danger in swapping pre boards Trev

17th Mar 2018, 2:00 pm	#12
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 471	Re: Leak Stereo70 If it's anything like my Stereo 70, the inability to 'mute' the sound by turning the volume control right down required a new potentiometer. Between ground & the centre (wiper) of the pot, mine read something like 25 ohms, in the end I gave in & replaced it with an Alps item, as using the amp late at night/early morning became a nuisance, as even at minimum volume, it was too loud! Sound going AWOL, most likely the tape monitor switch itself. Check it with a multimeter, on ohms, amp off. Even after cleaning, mine gave troubles. With the amp off, meter attached & wiggling the switch showed it was faulty. Over time, & using the amp more often, exercising the switches before use, it's been great. Might be worth cleaning the PCB edge connectors & sockets while you're in there. Mark

22nd Mar 2018, 6:38 pm	#13
PatMorgan Triode Join Date: Mar 2018 Location: Maidstone, Kent, UK. Posts: 24	Re: Leak Stereo70 Its been a bit cold recently to go out and play fault-find, but today I bit the bullet and put my pre-amp boards into my known working S70 AMP and transfered the same fault over- then put my known board back in and the amp worked fine. So, my dud amp has two preamp boards defective - that's where the sound has 'gone' I'll remove the transistors and test, as I doubt all of them have failed. One of the problems with the boards being plug-in is the difficulty to measure pin voltages- so it will be trial and error.

23rd Mar 2018, 12:27 pm	#14
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: Leak Stereo70 While you have the transistors out, check the value of all high value resistors (they have a habit of going very high) and any small electrolytics for leakage (especially) & capasitance. dc

23rd Mar 2018, 2:43 pm	#15
PatMorgan Triode Join Date: Mar 2018 Location: Maidstone, Kent, UK. Posts: 24	Re: Leak Stereo70 All- problem solved- I know have 2 working S70'S! T4 was not switching on, so very little gain on T5- and I had 5v on the emitter, so the BC148's were repalced and sound has returned. same fault on both channels. The good thing with these old amps is the wiring diagram which is relatively easy to follow, but you dont expect the same fault to 2 channels. It might have been overdriven by the earlier stages as the output from my minidisc is higher than the recommended input level- Ive attenuated it now via the switch on the back.

23rd Mar 2018, 4:38 pm	#16
Trevor Octode Join Date: Aug 2006 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 1,082	Re: Leak Stereo70 Bc148 lock-fit we have been discussing these on the forum known to have a higher failure mode than the normal type of transistors Trev

23rd Mar 2018, 6:24 pm	#17
PatMorgan Triode Join Date: Mar 2018 Location: Maidstone, Kent, UK. Posts: 24	Re: Leak Stereo70 I've replaced them with 337's- But there's still a few more on each preamp board to fail. Somewhere in my loft ive a Substitution book, but it will be at least 40 years old!

23rd Mar 2018, 7:24 pm	#18
Trevor Octode Join Date: Aug 2006 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 1,082	Re: Leak Stereo70 bc109 =bc149 =bc184 bc108 = bc148 =bc 182 Trev

23rd Mar 2018, 10:11 pm	#19
PatMorgan Triode Join Date: Mar 2018 Location: Maidstone, Kent, UK. Posts: 24	Re: Leak Stereo70 Thanks Trevor. So far no difference noted with the BC337's in place of T4, T5. I must get a 200Ω trimmer as I can feel (heat) the IQ has wandered again on one channel- I'd already reveresed it as one leg is shorted to wiper. It's seen better days.

23rd Mar 2018, 11:07 pm	#20
PatMorgan Triode Join Date: Mar 2018 Location: Maidstone, Kent, UK. Posts: 24	Re: Leak Stereo70 I find they are ok providing that the magic spot- the 30ma is hit, but oldcarbon tracks etc.