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Yamaha A-1 high voltages

Nashou66 said:
Hard to say. I always check in circuit first if it reads weird I then remove . Its really the only way to be sure .

Check all fusible resistors . Some times you have to check everything, but look closely at the work you already did for poor solder joints , cracked traces, parts installed backwards etc.

Athanasios
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Hello again.
So, I finally had a chance to resume after I removed both TR525 and 527. I checked everything else and did not notice any anomalies. All remaining components measured the same in both channels.
Then I decided to fit TTA004b/TTC004b as a temporary replacement pair just to test if the amp will start working again.
When I powered it on through the DBT, the tested bulb went fully bright and the amp power on light didn't.
I then removed both TR's and powered on again - result.
I'm completely at a loss now, as I've no idea what went wrong.
 
did you put the right type of tranny in each spot?

PNP is 2SA914/TTA004b

NPN is 2SC1953/TTC004b

Also check for solder bridges.

Athanasios
 
Nashou66 said:
did you put the right type of tranny in each spot?

PNP is 2SA914/TTA004b

NPN is 2SC1953/TTC004b

Also check for solder bridges.

Athanasios
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I did put them in the right way. I'll check for bridges again. That was my 1st instinct but I didn't find anything.
Let's say there's no bridges, with both trannies removed is it normal for DBT to light up so bright?
 
if it stays on bright you have a short somewhere .

Need to go over your work since it wasn't doing that before , correct?
 
Nashou66 said:
if it stays on bright you have a short somewhere .

Need to go over your work since it wasn't doing that before , correct?
Click to expand...
Exactly. Only after the old trannies were removed and the new ones installed. I must note that I didn't push the drive board into its spot but it's resting on the wooden board.
 

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ZeRask said:
with both trannies removed is it normal for DBT to light up so bright?
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Amps should be designed not to go into meltdown should 1-2 components fail, however...
Yamaha certainly make an effort in preventing cascading fault.

Power up without TR525,6 means the +/-O pins will be at +/-50V so you might have blown TR133 and TR135 and FR101.
Test TR133,5 and outputs TR1,2, emitter resistors and FR101.

Since DBT is bright unlikely resistors are open, short transistors more likely.
 
mbz said:
Amps should be designed not to go into meltdown should 1-2 components fail, however...
Yamaha certainly make an effort in preventing cascading fault.

Power up without TR525,6 means the +/-O pins will be at +/-50V so you might have blown TR133 and TR135 and FR101.
Test TR133,5 and outputs TR1,2, emitter resistors and FR101.

Since DBT is bright unlikely resistors are open, short transistors more likely.
Click to expand...
Hmm.. I've no idea how I'd short them unless through a solder bridge. I've been especially careful this time
I'll recheck the OT's and 133, 135... It better be not them.
 
What was the first power up, with TTx004B or with TR525,6 removed

TR525 was suspect faulty possibly because of a downstream short. Replacing TR525 does not clear the downstream short.
What parts previously failed, these may have failed again.
 
mbz said:
What was the first power up, with TTx004B or with TR525,6 removed

TR525 was suspect faulty possibly because of a downstream short. Replacing TR525 does not clear the downstream short.
What parts previously failed, these may have failed again.
Click to expand...
The first power up was on TT's and DBT went bright. Both were consequently removed and rechecked, both are still working. Just to confirm - OT's and TR133, 135 are all downstream, right?
 
Hello, friends.
Me again. After a rather long break I finally had a chance to go back to my A-1 project. Checked everything up and down for oddities. No obvious shorts were found.
Installed TTA004b/TTC004b as a temporary replacement for TR525 & TR527 just to get the amp running. Powered on through the dim bulb tester and, oh miracle, the speaker switch did engage for the first time in months. The bulb isn't particularly dim, but isn't full bright either. I'd say it's at 50% brightness now.
However (there must always be a however with A-1s), there's some weir noise coming from the drive board. It goes away when I disengage "speakers" switch. I think it comes from speaker relay. Please see the video in the link.

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Any ideas is this is normal, considering the amp was ran through dim bulb tester?
 

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With the front plate off, all kinds of noises, hums, etc. are possible due to lack of proper grounding. I’d say you have an issue if you still heard the noise with the front cover installed and the front plate mounting nuts tightened.
 
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Sparkplug said:
With the front plate off, all kinds of noises, hums, etc. are possible due to lack of proper grounding. I’d say you have an issue if you still heard the noise with the front cover installed and the front plate mounting nuts tightened.
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Thanks for your suggestion. I had no idea the front plate is required for this
 
Did you ever change out the relay?

That can cause distortion. But also check the emitter resistors.

PS: Also...what size watt bulb are you using?

Athanasios
 
ZeRask said:
https:/

Thanks for your suggestion. I had no idea the front plate is required for this
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This, may or may not be the root cause of the noise you’re hearing, but making measurements with the unit apart is a no-no. Every mounting nut, every mounting screw can cause hums, sound artifacts, or outright malfunction in the unit if not properly installed. Especially those on the front plate used to secure volume, balance, etc. pots to the frame and front plate.

Once you ascertain that there are no short circuits, go directly to the mains, and make any measurements/adjustments while on mains. Dim bulb testers have one purpose, to protect the amp against short circuits and huge currents that result. Beyond that they just get in the way.
 
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Sparkplug said:
This, may or may not be the root cause of the noise you’re hearing, but making measurements with the unit apart is a no-no. Every mounting nut, every mounting screw can cause hums, sound artifacts, or outright malfunction in the unit if not properly installed. Especially those on the front plate used to secure volume, balance, etc. pots to the frame and front plate.

Once you ascertain that there are no short circuits, go directly to the mains, and make any measurements/adjustments while on mains. Dim bulb testers have one purpose, to protect the amp against short circuits and huge currents that result. Beyond that they just get in the way.
Click to expand...
Thanks again for your advise. I will put it back together just leaving the top and bottom covers out for adjustments.
 
Hello everyone. It's been a long while since my last post, but I finally to get to my A-1.
I ended up replacing the drive board TR525 - 528 with TTA004b/TTC004b pairs; reflown both IC's; then followed the advise to put the unit back together (except for top and bottom covers).
Great news is that after a year my A-1 has finally powered up normally and managed to dial in all adjustments per SM!
I played it for a while and eventually ran the receiver through a pair on 4Ohm Braun L830 speakers. No issues, sounds better than ever!

Thank you all for your valuable advise.

I then decided to replace the speaker terminals and reused the original plastic mounting plate by drilling the wholes to accommodate the new terminals. After I put top & bottom covers back and prior to powering it up I decided to check the new speaker terminals for shorts. To my surprise I get the continuity between the positive terminals and the chassis. The almighty chatgpt is telling me that some older this is possible in some older common-ground or single-ended output designs, but I am not sure if A-1 is sucha design.
Could someone please let me know if this is normal of if I should look for the shorts before I plug it back in?
 
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