Cambridge audio a1 mk1 not powering on. - Page 20

RS232 · 08:30 PM

To tiefbassuebertr the amp is question is the original A1 also known as A1 mk1 and is a full discrete components audio amplifier (no integrated stuff anywhere).
There are 3 different versions of this amplifier.
1st one is the original one described above
2nd one has a E I transformer and a LM4765
3rd one has a toroid transformer and is based on the TDA1514.
None of them share the preamp board/design.
To Antspants check associated resistors and trimer as this last one can "dig" a hole in the track inside it and give strange readings.
PS: Don't forget that the BD139 is mounted with face to heatsink.
Pictures of 2 different version os the A1 will post pic tomorrow for the MK1

Antspants · 28th February 2021, 09:22 AM

Hi thanks for that will double check everything going to replace the bd139 just in case its OK on tester but failing in circuit probably replace timer as well. Looks like there are a few variations

RS232 · 11:15 AM

VBE multiplier circuit: Q611 (BD139), C611 (100nf), R620 (470r), R621 (1K5) and P601 (470r).
If everything reads rights reset idle current (compre with good channel) and test if noise/distortion is still there.
If it is still there disconnect the preamp board (had several issues with it) and test again.
Repaired 2 different A1 mk1 with sound issues caused by the preamp board.

Antspants · 28th February 2021, 05:19 PM

Hi there thanks for all the advice had a disaster today and shorted output transistor! Will report more fully tomorrow

Ian Finch · 28th February 2021, 09:43 PM

Welcome to the "oops" club. Using plain and slippery meter probes? If so, get yourself some hook probes or fit some cheap IC clips to probe leads before trying to manage both probes and adjust a loose meter (or some similarly awkward operation) at the same time. Fortunately, the output transistors are about the cheapest genuine spec. power transistors anywhere.

OldDIY · 1st March 2021, 12:02 AM

You are alive and well. This is the main thing.
Minimize operations under voltage. Be attentive and careful.

mjona · 2nd March 2021, 03:55 AM

Quote:

Originally Posted by Antspants

Hi there thanks for all the advice had a disaster today and shorted output transistor! Will report more fully tomorrow

There is an image in post 155 which shows an insulating washer to isolate screw securing the tab of the BD139 to the heat sink. Without this you are reliant of the paint for isolating the collector from other parts of the amplifier - possibly shorting to earth and similarly the base of the TIP3055.

Antspants · 2nd March 2021, 10:18 AM

Hi I must say a big thanks to everyone all the replies have been really helpful . I'm much more au fait with valve technology having repaired and modded mainly guitar and bass kit for over 40 years and had 2 years of tutorials from a ex senior Marshall designer, never really got into these ew fangled sid state stuff except for relatively simple pedals and amp stuff! Stereo stuff is way more complicated and without a circuit diagram I'm fighting a loosing battle. Brief recap as I might have one more go, right channel very distorted, full vbe multiplier supply of 1.1v on base of bd139, 30mv on output transistors left channel 1.1v supply with vtage drop to 5.9v on base of bd139 and around 10mv on emitters of output transistors .Assumed open base on left hand bd139 replaced it in correct orientation now had 0.88v instead of 1.1v and still full supply voltage of .88v on bd139 base checked both associated transistors which both have 32v on collector but only 0.4v on bases and 0.88v on emitters, right channel was still OK with 0.6v on bases and 1.1v on emitters. Double checking readings is when I shorted q313 both channel supply rail fuses blew q313 and both left hand channel output transistors
Blew. As I hadn't sorted vbe multiplier issues and couldn't find any other problems I'm not
convinced its worth pursuing any
Further without a circuit diagram although I don't like been defeated and I've only ly failed once to repair a bass amp with loads of smd in it, vile stuff especially when manufacturers refusing g to supply diagrams and parts anyway thanks again will let you know what I end up doing if not proceeding the amp or parts will free to a good home for postage cost

Antspants · 2nd March 2021, 10:21 AM

Here is a pic of the offending probes

RS232 · 2nd March 2021, 12:21 PM

If you give up on this one I would like to give it a go.
Russc offered me a board for beer money you could get in touch with him I think he still has the board.

Good luck

28th February 2021, 11:13 AM	#193
RS232 diyAudio Member Join Date: Jan 2006 Location: Swindon	VBE multiplier circuit: Q611 (BD139), C611 (100nf), R620 (470r), R621 (1K5) and P601 (470r). If everything reads rights reset idle current (compre with good channel) and test if noise/distortion is still there. If it is still there disconnect the preamp board (had several issues with it) and test again. Repaired 2 different A1 mk1 with sound issues caused by the preamp board. Last edited by RS232; 28th February 2021 at 11:15 AM.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Powering TDA7492P Audio Amp	Zerlen	Power Supplies	0	5th July 2017 08:39 PM
cambridge audio DAC 3	lister40	Digital Line Level	2	14th January 2013 04:53 AM
Cambridge Audio 640C No audio output PLEASE HELP!	Kajman	Digital Source	3	10th December 2007 09:45 AM
cambridge audio cd 4se	whats gan on	Digital Source	46	3rd April 2007 10:13 AM
Cambridge audio A3 i	homertooties	Solid State	3	20th November 2005 03:52 PM

New To Site?	Need Help?
Register to Participate Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

28th February 2021, 09:22 AM	#192
Antspants diyAudio Member Join Date: Jul 2020	Hi thanks for that will double check everything going to replace the bd139 just in case its OK on tester but failing in circuit probably replace timer as well. Looks like there are a few variations

28th February 2021, 05:19 PM	#194
Antspants diyAudio Member Join Date: Jul 2020	Hi there thanks for all the advice had a disaster today and shorted output transistor! Will report more fully tomorrow

28th February 2021, 09:43 PM	#195
Ian Finch diyAudio Member Join Date: Apr 2010 Location: Coffs Harbour, NSW	Welcome to the "oops" club. Using plain and slippery meter probes? If so, get yourself some hook probes or fit some cheap IC clips to probe leads before trying to manage both probes and adjust a loose meter (or some similarly awkward operation) at the same time. Fortunately, the output transistors are about the cheapest genuine spec. power transistors anywhere.

1st March 2021, 12:02 AM	#196
OldDIY diyAudio Member Join Date: Dec 2009	You are alive and well. This is the main thing. Minimize operations under voltage. Be attentive and careful.

2nd March 2021, 10:18 AM	#198
Antspants diyAudio Member Join Date: Jul 2020	Hi I must say a big thanks to everyone all the replies have been really helpful . I'm much more au fait with valve technology having repaired and modded mainly guitar and bass kit for over 40 years and had 2 years of tutorials from a ex senior Marshall designer, never really got into these ew fangled sid state stuff except for relatively simple pedals and amp stuff! Stereo stuff is way more complicated and without a circuit diagram I'm fighting a loosing battle. Brief recap as I might have one more go, right channel very distorted, full vbe multiplier supply of 1.1v on base of bd139, 30mv on output transistors left channel 1.1v supply with vtage drop to 5.9v on base of bd139 and around 10mv on emitters of output transistors .Assumed open base on left hand bd139 replaced it in correct orientation now had 0.88v instead of 1.1v and still full supply voltage of .88v on bd139 base checked both associated transistors which both have 32v on collector but only 0.4v on bases and 0.88v on emitters, right channel was still OK with 0.6v on bases and 1.1v on emitters. Double checking readings is when I shorted q313 both channel supply rail fuses blew q313 and both left hand channel output transistors Blew. As I hadn't sorted vbe multiplier issues and couldn't find any other problems I'm not convinced its worth pursuing any Further without a circuit diagram although I don't like been defeated and I've only ly failed once to repair a bass amp with loads of smd in it, vile stuff especially when manufacturers refusing g to supply diagrams and parts anyway thanks again will let you know what I end up doing if not proceeding the amp or parts will free to a good home for postage cost

2nd March 2021, 12:21 PM	#200
RS232 diyAudio Member Join Date: Jan 2006 Location: Swindon	If you give up on this one I would like to give it a go. Russc offered me a board for beer money you could get in touch with him I think he still has the board. Good luck