Dead V676

[timtom]

I got a non working v676, I recapped the elkos and tantalums except c107.
I started tracking the power lines, R713 was burnt, I replaced it but at ST701/1 I only got ~3V left. It should be ~24V, right?

I suspect the transistors to be broken. Is there a way to measure them? Can they also be broken if the measure correctly with a Multimeter Diode Test?

 

[rock soderstrom]

but at ST701/1 I only got ~3V left. It should be ~24V, right?

Yes, I would expect almost 23V there, 3 volts is definitely too low, something is pulling your supply voltage down. See attchment for voltages.

I suspect the transistors to be broken.

This can, but does not have to be the fault.

Is there a way to measure them?

Yes, you can also check the transistor in the circuit itself, for example, by temporary deactivating and bridging the entire darlington series regulator and seeing if your operating voltage is still too low.

If so, your fault is not in the series regulator (transistor further upstream?), if not, then these components are suspect.


How high is the voltage at point ST701/9?

Can they also be broken if the measure correctly with a Multimeter Diode Test?

In my experience, transistors that are measure correctly are not broken...

 

[timtom]

How high is the voltage at point ST701/9?

It's 24V.

Yes, you can also check the transistor in the circuit itself, for example, by temporary deactivating and bridging the entire darlington series regulator and seeing if your operating voltage is still too low.

I'll do that.
I also checked the card it outside the v676 and with 24V at ST701/9 there is still 3v at the ST701/1.

 

[Rob Flinn]

Pull the daughter cards & see if you get a better voltage at ST701/1. If you don't you have a blown BSY90. You really need to verify this first.

You can test most of the other cards out of circuit with a bench PSU, sig gen & scope.

I was repairing a V676 recently which was blowing the BSY90 when the gain switch was in it's lowest position (line mode). It was one tantalum that was short.

 

[timtom]

Pull the daughter cards & see if you get a better voltage at ST701/1. If you don't you have a blown BSY90. You really need to verify this first.

You can test most of the other cards out of circuit with a bench PSU, sig gen & scope.

I was repairing a V676 recently which was blowing the BSY90 when the gain switch was in it's lowest position (line mode). It was one tantalum that was short.

I pulled the other cards and the voltage was still low.
Did you replace the BSY90 with another transistor? It's hard to find a BSY90. Another posts mentions the BC517 as replacement.

 

[Rob Flinn]

Yes I used something else. I got through a few transistors before I got it working again so just used what I had in my parts collection. I can't remember exactly but one sub was a bc441 as I had some lying about. It's just used as a pass transistor for regulating the voltage so any npn with the same sort of current rating should work.

 

[timtom]

I replaced bc107 of the darlington as it measured incorrectly.



When I bridge the darlington, I get corrected 24v.

Definitely something with BSY90.

 

[rock soderstrom]

Let me check my old transistor collection, maybe you got luck and I will find some BSY90. Otherwise you can use substituts like Rob mentioned.

https://www.radiomuseum.org/tubes/tube_bsy90.html

 

[rock soderstrom]

Sorry, no BSY90 in stock, only BSW65, BSX45 and a lot of Germanium transistors for fuzz pedals

Sunday is the right day to sort through old inherited stock...

 

[timtom]

Sorry, no BSY90 in stock, only BSW65, BSX45 and a lot of Germanium transistors for fuzz pedals

Sunday is the right day to sort through old inherited stock...

Thanks for looking. I'll find a replacement.

In theory, how bad is it to bridge this darlington and use the amplifier?

 

[rock soderstrom]

In theory, how bad is it to bridge this darlington and use the amplifier?

I don't know, the old guys must have designed it with a target in mind. It probably ensures consistent results under varying conditions, perhaps also a slightly better signal-to-noise ratio? Try it out.

 

[timtom]

I replaced the darlington with a bc517.
If I measure now without ground I got 23,1V when I connect the ground the voltage slowly rises until ~13V.

The test circuit outside the card works fine.

 

[Rob Flinn]

Not really getting why you replaced the 2 transistors with one darlington. It's normally just the bsy90 that blows. Also when you test the circuit off the board it has no load.

 

[timtom]

I just had no replacement for the bsy90 here. Now everything is working with the bc517 and I get 23v at ST701/1.
I confused R713 with 4,7k instead of 4,7 ohm.

Edit:
The Fault was broken T704 (BC107b) and a burned R173.

BSY90 actually works back in t he circuit with a new BC107b and a correct Resistor.

 

[Rob Flinn]

The R713 on your pcb doesn't look like the original transistor ..... Difficult to say looking from above, can you post a picture from the side ?

 

[timtom]

The R713 on your pcb doesn't look like the original transistor ..... Difficult to say looking from above, can you post a picture from the side ?

This resistor was burned from the beginning and I added a 4,7k instead of a 4,7 ohm in there.

 

[Rob Flinn]

This resistor was burned from the beginning and I added a 4,7k instead of a 4,7 ohm in there.

You will probably lose some voltage when all the other boards are installed, with it at 4k7.

 

[timtom]

Voltages are fine now and the amplifier works but heavily distorts. When tracking the signal path it stays clean until St701/7 and after that is distorted.
It is the same board that had the broken parts.

Measering the voltages from the v676a service manual the are all fine except the ones at St701/7 is way to low.

 

[timtom]

This is how I added the bc517 darlington. Is this correct?

 

[rock soderstrom]

This is how I added the bc517 darlington. Is this correct?
View attachment 122379

Does it work?