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Yamaha A1 strange problem

Dr. Ear

Dr. Ear

Super Member
Two days ago I got an almost "like new looking" Yamaha A1-Amplifier, no scratches, perfect door hinges, no nicotine and almost no dust. The amplifier powered up and played for 1 hour or so, then the speaker relay suddenly cut off. No Natural Sound any more. Obviously a DC problem.
I got the A1 service manual and checked the adjustments. Power supply was 35V (perfect). Drive C Board: Left channel is OK but I am unable to adjust DC offset in the right channel. Once perfectly adjusted the value changes (± 50-100 mV!) when I press one of the illuminated buttons i.e. the Disc and/or Speaker button (the voltage on the 12V rail then drops to 11,5 V while 35V and 50V rails remain stable). You can even see this voltage drop because bulbs get a little bit dimmed.
There is also no chance to adjust the bias in the right channel. Value should be 7.4 mV but is 2,2 mV. When I try to adjust the bias the value jumps to 25 mV immediately (and vice versa back to 2,2 mV). The left channel is OK.
This afternoon I have recapped the power circuit pcb with no result.
Any ideas highly appreciated!
Attachment: schematics
 

Attachments

  • A1 schematics.jpg
    A1 schematics.jpg
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Dr. Ear said:
The left channel is OK.
This afternoon I have recapped the power circuit pcb with no result.
Any ideas highly appreciated!
Attachment: schematics
Click to expand...

The A-1 is well worth repairing. Can you post the circuit schematic from the manual? Compare the DC voltages between the working and unworking channel, especially around the input FET pair on the drive circuitry.
 
Thanks for the fast reply. Both channels are working BUT the right channel is difficult to adjust (DC-offset and bias). The problem must be located somewhere in the power supply, I think. The schematics of that section are attached.
 
Dr. Ear said:
Thanks for the fast reply. Both channels are working BUT the right channel is difficult to adjust (DC-offset and bias). The problem must be located somewhere in the power supply, I think. The schematics of that section are attached.
Click to expand...

Since the power supply is common to both channels it wouldn't seem that likely to me ... but you never know.
 
Check that bias pot out carefully. Is there a cold solder joint there ... or is the pot crapping out on you maybe? :scratch2: :D
 
Good idea, maybe I have two problems. I have just placed an order for Bourns multigang Trimpots.
 
Dr. Ear said:
Good idea, maybe I have two problems. I have just placed an order for Bourns multigang Trimpots.
Click to expand...

If the trim pots are the open faced carbon track type that were commonly used back then, you sure can't hurt things by replacing them. Multi-turn trims improve set ability and stability even more if you can find some to fit.
 
In the meanwhile I've found out that the DC problem must have its origin somewhere in the tone control circuit board: when I adjust the bias to ±0 mv with the disc button (bypasses tone control pcb) engaged and I either switch Disc off or activate tone controls by the separate button) voltage goes up by 70-100 mV.
The bias problem in the right channel is probably reuslt of a defective transistor, I have swapped the trimpots and nothing changed.
 
I would be happy if one of the pro's could give me a hint which transistor is most likely the cause for the unadjustable bias in the right channel.
(Schematics attached)
Thank you in advance.
 

Attachments

  • A1 Electrolytic.jpg
    A1 Electrolytic.jpg
    114.7 KB · Views: 75
Do you have any more pages from the service manual that you can post? I don't think the problem is on that board.
 
Dr. Ear said:
Two days ago I got an almost "like new looking" Yamaha A1-Amplifier, no scratches, perfect door hinges, no nicotine and almost no dust. The amplifier powered up and played for 1 hour or so, then the speaker relay suddenly cut off. No Natural Sound any more. Obviously a DC problem.
I got the A1 service manual and checked the adjustments. Power supply was 35V (perfect). Drive C Board: Left channel is OK but I am unable to adjust DC offset in the right channel. Once perfectly adjusted the value changes (± 50-100 mV!) when I press one of the illuminated buttons i.e. the Disc and/or Speaker button (the voltage on the 12V rail then drops to 11,5 V while 35V and 50V rails remain stable). You can even see this voltage drop because bulbs get a little bit dimmed.
There is also no chance to adjust the bias in the right channel. Value should be 7.4 mV but is 2,2 mV. When I try to adjust the bias the value jumps to 25 mV immediately (and vice versa back to 2,2 mV). The left channel is OK.
This afternoon I have recapped the power circuit pcb with no result.
Any ideas highly appreciated!
Attachment: schematics
Click to expand...

These are circuit board layout diagrams ..Not schematics.:no:
 
Does somebody have an explanation for this:
When tone controls are engaged, DC offset changes its value significantly when I adjust treble to minimum (bass has no effect). I have already replaced all caps on the tone control board, no change.
 
Thank you very much. Unfortunately the readability of this ancient scan is not very good (same with the file one can download at yamaha.co.jp). Next step is to change the bias transistors and the drivers. i have found out that the pre-owner did a modification, there are two small ceramic capacitors of unknown value between +O & -O (left channel, drive board) and earth. At the moment the amp is working, but I want it to be perfect. The problems are in the left channel and not in the right, as posted previously.
 
Dr. Ear said:
You are right, I've tried the power supply board from another A1 and the problem still occurs.
Click to expand...

You really need a schematic to trouble shoot these kinds of problems. You can try comparing the DC voltages at different circuit nodes like around IC701 and IC702 for clues.
 
The FETs are OK. This A-1 was not a good buy it seems. There are problems spread over the whole unit, e.g. power supply, drive board, tone control and elswhere. Must have been a lightning.
Maybe I will give up and take the virgin front panel, the cover plus some other parts and keep the rest as spare parts. Next I will use this material to restore my "old" A1 which is working fine but needs optical refinement to a large extent (the hinges are broken and there are major scratches and bends all over). Thank you very much for your help.
 
Well, I have an A-1 at Echowars's shop at the moment, and about a minute or two after power-up there's a loud pop in both channels and then the relay kicks-in and then returns and things are fine after that. In my case the DC offset is very stable and low (although I haven't been lucky enough to check it at the "pop"). Anyway, I'll report back once I hear from Echowars.

Once the amp makes it through the initial "pop" it just sounds wonderful, as always.
 
I would also prefer to take my A-1 to Echowars but I think his shop is a bit far from my home. :tears: Please let me hear what he found out!
 
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