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Cambridge audio a1 mk1 not powering on.
Cambridge audio a1 mk1 not powering on.
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Old 2nd March 2021, 12:30 PM   #201
Antspants is offline Antspants
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OK cheers will have a look, transformer is fine
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Old 2nd March 2021, 05:17 PM   #202
mjona is online now mjona  New Zealand
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Quote:
Originally Posted by Antspants View Post
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
Don't give up so easily, take a photo of the circuit board and post the image. It should not be too hard for members to work out the general structure and get a handle on problems.

A transistor with a base emitter voltage of 0.6 Volts should be OK. Don't use your probes to measure between these connections - this practice is too risky as you have experienced.

Instead put the black probe into the speaker black output terminal and use the red probe to touch the emitter first and the base second and subtract the difference from the higher reading noting whether plus or minus.

To be systematic write the transistor Q number and the readings for each. A spreadsheet would be a useful way to record the detail and allow alterations to be made with progress in tracking faults. Usually circuit diagrams have some voltage readings shown at various points.

To avoid short circuits you could use a length of insulation from a length of suitable gauge hook up wire or similar with the latter stripped out, and use that as a sleeve to insulate your red probe so only a small area around the tip is exposed.

This leaves you with a free hand to turn off the power immediately if you have to do that while taking measurements.
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Old 4th March 2021, 03:10 PM   #203
Antspants is offline Antspants
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Thanks for all the advice on balance I'll give it another go as Power supply is fine it survived! One issue is the transistor I shorted has so far proved difficult to identify I've attached photos hopefully someone can clarify the code, I've tried a few permutations. Also apologies on previous posts as looks like auto correct has kicked in hope you could all still decipher info
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File Type: jpg 20210303_172947.jpg (474.7 KB, 53 views)
File Type: jpg 20210303_172941.jpg (503.3 KB, 52 views)
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Old 4th March 2021, 05:10 PM   #204
Mooly is offline Mooly  United Kingdom
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Cambridge audio a1 mk1 not powering on.
Philips BC640
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Old 5th March 2021, 09:06 AM   #205
Antspants is offline Antspants
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Brill thanks
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Old 5th March 2021, 12:31 PM   #206
tiefbassuebertr is offline tiefbassuebertr  Germany
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Quote:
Originally Posted by Mooly View Post
Philips BC640
yes, go to page 3 (marking codes) under
BC640 pdf, BC640 description, BC640 datasheets, BC640 view ::: ALLDATASHEET :::
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Old 5th March 2021, 01:25 PM   #207
Mooly is offline Mooly  United Kingdom
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Cambridge audio a1 mk1 not powering on.
Thanks. The giveaway for me was the 'PH' bit. I used to see those markings a lot in various equipment years ago.
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Old 11th March 2021, 10:18 AM   #208
Antspants is offline Antspants
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Hi going to give it another go found a faulty transistor its possible it was fine before my accident but maybe it was the original fault will buy parts today will let you know how I get one
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