Yamaha A-1 high voltages

[ZeRask]

Hello, AK’ers

I hope some Yamaha A-1 experts here can help me troubleshoot a weird voltage issue with my receiver.

I attempted a recap/servicing on my A-1, which went well until the Electrolytics board, which was supposed to be the last one. I studied everything related to the subject on AK and after each replacement prior to ElCap board I managed to dial spot on DC offset and Idling current per service manual.

I am novice in receiver recapping, and made some stupid mistakes, such as not using DBT and using a regular screwdriver for the adjustments (I did not know any better but I’ve learned my lesson). While I tried to be very careful, I must have shorted something, which resulted in a blown fuse, along with the destruction on the left channel components - fusible resistor FR101, driver transistors TR135, TR133 as well as both output transistors TR1 & TR2. The other channel is fine.

I replaced all damaged components with the original parts from another A-1 board and built a dim bulb tester. Upon the receiver power on the 150W bulb either goes bright then goes dim, or just goes dim on and stays this way (see the picture). The speaker relay won’t engage. With the receiver running through the DBT I checked the voltages on test points and on the ElCap board. The right channel looks OK and I can dial in DC offset and Idle current, however there’s around +/- 40V flowing through the components on the different boards from the right channel. Below are the measurements I’m getting.

Any advise will be very much appreciated.

 

[mbz]

Will take a look at voltages a bit later, just fyi, the speaker relay only engagues when you have no fault and you press the speaker button on front panel. You certainly have a fault... need to step out...

 

[ZeRask]

Will take a look at voltages a bit later, just fyi, the speaker relay only engagues when you have no fault and you press the speaker button on front panel. You certainly have a fault... need to step out...

Thanks, mbz. That was my understanding of how it works and that's what prompted me to start testing voltages. The relay is brand new btw.

 

[avionic]

Pull TR133 and 135 and check it, out of circuit, for shorts.

 

[ZeRask]

Pull TR133 and 135 and check it, out of circuit, for shorts.

Thanks, avionic. I already did test them in the component tester. If I read this correctly, there are no shorts.

 

[mbz]

TR133b at +43V appears to be the high point, can you measure which point is higher O+ or TR133b by clipping red probe to O+ wire and black probe to TR133b, 2V scale and carefully note +/- sign alternatively you can measure at R101.

Measure TR133 Vbe = 2.5V looks abnormal, tempted to replace.

 

[ZeRask]

TR133b at +43V appears to be the high point, can you measure which point is higher O+ or TR133b by clipping red probe to O+ wire and black probe to TR133b, 2V scale and carefully note +/- sign alternatively you can measure at R101.

Measure TR133 Vbe = 2.5V looks abnormal, tempted to replace.

Traveling this weekend and I will do the measurements when I'm home. Although I'd be surprised if TR133 is a culprit - I tested it before I installed it. I'll keep you posted.

 

[ZeRask]

TR133b at +43V appears to be the high point, can you measure which point is higher O+ or TR133b by clipping red probe to O+ wire and black probe to TR133b, 2V scale and carefully note +/- sign alternatively you can measure at R101.

Measure TR133 Vbe = 2.5V looks abnormal, tempted to replace.

Good day.
I'm back in town and will be measuring this evening.
Just want to make sure that I follow your advise correctly. For Vbe measurements, do I do it in situ or should the transistor be pulled?

 

[mbz]

For Vbe measurements, do I do it in situ or should the transistor be pulled?

Try in situ if you can do this safely, with unit powered up.

 

[Nashou66]

Its easiest to do with top and bottom off, standing the amp on end transformer side down to prevent top heavy tip over.

Measure from bottom.

Athanasios

 

[ZeRask]

TR133b at +43V appears to be the high point, can you measure which point is higher O+ or TR133b by clipping red probe to O+ wire and black probe to TR133b, 2V scale and carefully note +/- sign alternatively you can measure at R101.

Measure TR133 Vbe = 2.5V looks abnormal, tempted to replace.

Ok, so I measured the voltage between O+ and Tr133b, and it's at -0.00 at both 2V and 200mV range.
The TR133Vbe is 2.4V
What do you think that means?

 

[mbz]

I measured the voltage between O+ and Tr133b, and it's at -0.00 at both 2V and 200mV range.

Red probe on O+ and a -ve result suggests TR133B is slightly higher than O+, so it might be a case of leakage from the collector.
The zero result just indicates a very low current flow/microamps.

The TR133Vbe is 2.4V

TR133 2SC1913 Vbesat is 1.5V si it's a little high for that

The A-1 is a difficult troubleshoot since mostly direct coupled, I'd be leaning to replace TR133, also replaace TR135 as a set. Have you replaced TR1,2, this may influence choice of drivers, maybe MJE15032,3??? others better at transistor subs...

 

[ZeRask]

Red probe on O+ and a -ve result suggests TR133B is slightly higher than O+, so it might be a case of leakage from the collector.
The zero result just indicates a very low current flow/microamps.


TR133 2SC1913 Vbesat is 1.5V si it's a little high for that

The A-1 is a difficult troubleshoot since mostly direct coupled, I'd be leaning to replace TR133, also replaace TR135 as a set. Have you replaced TR1,2, this may influence choice of drivers, maybe MJE15032,3??? others better at transistor

The problem is that TR133 and TR135 have already been replaced. I pulled them from another A-1's elcap board. Tested them before installation too.
Same with TR 1 & 2. They were taken from another A-1 and tested prior to installation. All 4 transistors are the originals. I doubt it that replacing them will resolve anything, since this step has been accomplished. If I understand this correctly, unless a faulty transistor can measure within specs on the component tester they should be OK.

It's difficult for me to come up with theories, due to the lack of knowledge but my uneducated (and maybe stupid) theory is that there is something that is letting through those 41v in the left channel across 3 different boards. Can it be something upstream? Drive board maybe?
I also recall seeing a post where someone was measuring the near rail voltages on output transistors and that was caused by loose connectors within TO3 sockets.

 

[Nashou66]

Are the output tranny's ok?

Also check the emitter resistors

 

[ZeRask]

Are the output tranny's ok?

Also check the emitter resistors

Tested prior to installing both OT's and ER's snd they were OK. Ran the receiver through dim bulb tester only. If something happened to them after installing this must be caused by something else

 

[Nashou66]

Tested prior to installing both OT's and ER's snd they were OK. Ran the receiver through dim bulb tester only. If something happened to them after installing this must be caused by something else

I always use a 470 ohm 1-2 watt resistor across the Base and Emitter pads to replicate the outputs while trouble shooting in place of the outputs. This way if something goes wacky it won't damage good expensive outputs .

Id recheck them again just to be sure, they might still be ok , hopefully .

Athanasios

 

[ZeRask]

I always use a 470 ohm 1-2 watt resistor across the Base and Emitter pads to replicate the outputs while trouble shooting in place of the outputs. This way if something goes wacky it won't damage good expensive outputs .

Id recheck them again just to be sure, they might still be ok , hopefully .

Athanasios

Thanks Athanasios. I will recheck. But say if they're not OK, what could cause their destruction and the voltages I described in the 1st post?

 

[mbz]

The high point at TR133b may be caused by TR525 being off, the "directions" testing of O+ and TR133b was not a strong result of 0mV.

TR525 is controlled by TR511,3,5,7 and IC701(dual fet). If you wish to try something left field then reflow the solder connections on IC701.

 

[ZeRask]

The high point at TR133b may be caused by TR525 being off, the "directions" testing of O+ and TR133b was not a strong result of 0mV.

TR525 is controlled by TR511,3,5,7 and IC701(dual fet). If you wish to try something left field then reflow the solder connections on IC701.

Thanks a lot! Will try and report. This might take me a few days but I will follow up with the results.

 

[mbz]

Could you plz try connecting red meter probe to O+ pin and black probe to any black speaker connector (=GND), measure dc voltage while adjusting VR501 (slowly), wanting about +1.3Vdc. Does the O+ voltage change with VR501 adjustment?