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

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Old 23rd Jan 2022, 6:34 pm   #21
nigelreltub
Tetrode
 
Join Date: Nov 2021
Location: East Yorkshire, UK.
Posts: 74
Default Re: Armstrong 526 Rebuild/Restoration

No luck with the M8 Stereo card as yet.

I have finally got round to checking/looking at the bias measurements. The amplifier has had the 2n3055 silicone mod/upgrade and all new capacitors on both A15 boards and new main capacitors.

There is a difference of around 11mv between the channels (7.8mv chA 19mv chB). This after the amplifier has been on for 30 minutes. Power supply voltage at 48.5vdc.

Initial thoughts are the transistors? TR's 33/34/36/22 but all measure equally the same on both boards as per the measurements on the "Component Cross Ref sheet".

CZ10 readings at room temp (removed from boards to check)
chB
1278 - 13k8
1280 - 18k23
chA
1278 - 14k34
1280 - 14k22

Increase temp to approx 34 degrees, constant for all readings
chB
1278 - 10k
1280 - 12k5
chA
1278 - 10k
1280 - 10k8

Could it be the 1280 CZ10 at the higher 12k5 when warm?

Looking at the information sheet they seem to work as a in rush current device to protect lamps and valves, I have also read you could add a 10/15r resistor in series to bring the mv up on the low channel with a small difference, would these need possibly a 2k resistor to bring the others up nearer to the 12k?

Could it be something else.

Any advice, thoughts please on a solution or a replacement for the CZ10's if you think this is the problem?
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