Laney VH100R switch on delay issues

[Zouto]

Hi,
I got a Laney VH100R with serious switch-on-delay troubles. The relay switches ON after 40 seconds, the amp is working fine for half an hour but then suddenly the relay is tripping and switching OFF, you can hear it clicking. After turning off the power switch for some time it works again. Sometimes the amp is working for several hours without relay tripping.Unfortunately most parts of the switch-on-delay circuit are smd parts, which are hard to exchange and tend to be unreliable in hot tube amps.Modern Laneys use them extensively, so servicing is quite a drag.
I think it's sort of a thermal problem.Any ideas how to fix that amp?
For Schematics see:
http://music-electronics-forum.com/t16301/
Help much appreciated,
thanks
Zouto

[Zouto]

There is some unknown part in the schematic called
100?C Trip in Power TX.
It might be the culprit.
Any idea what it is, a fuse in the PT???
Thanks for help
Zouto

[Jazz P Bass]

Laney delay/on circuit

The 100?C is a temperature switch.
There are a number of ways the delay on circuit can be faulty.
First & foremost, relay 5 must have +15Vdc at the cathode of the flyback diode.
Monitor that point & see if it is failing.
If it does fail here, it may be the 100?C switch, it's connections or the +15Vdc rail may be at fault.
The second critical part is TS9. It must conduct a ground reference for the relay.
It appears that TS8 keeps TS9 off until C47 charges through R53.
If you remove TS8 the delay part will be disabled.

[Zouto]

Hi Jazz P Bass, thanks a lot for your great explanation.
I found out that the +15Vdc at the cathode of the flyback diode stays solid even when relay5 cuts B+ Voltage by fault. So the culprits must be TS8 or 9.
Since I do not have any SMD tools at all it might be too complicated to exchange them.I could not find any informations about PF3820 (T8).
Could it cause damage to the toroidal transformer if I would disable that unreliable delay circuit to switch the standby witout any delay as usual?
Zouto

[olddawg]

In the past with thermally intermitant smd transistors I have been able to bend little feet on the leads of an appropriate cross refference and solder it to the pads. They breath a little better that way. If amp vibration makes this unreliable, you can drill lead holes with a dremel tool and a small bit if it isn't a double layer board. Can you isolate the problem with a can of freeze, a blow dryer, and a scope?

[Zouto]

In the past with thermally intermitant smd transistors I have been able to bend little feet on the leads of an appropriate cross refference and solder it to the pads. They breath a little better that way. If amp vibration makes this unreliable, you can drill lead holes with a dremel tool and a small bit if it isn't a double layer board. Can you isolate the problem with a can of freeze, a blow dryer, and a scope?

Thanks, I'd really like to do it, but I can't find any substitutes for BC847B and PF3820. Otherwise it would be easy to exchange the those transistors.
One of them seems to be the culprit.

[Zouto]

The 100?C is a temperature switch.
There are a number of ways the delay on circuit can be faulty.
First & foremost, relay 5 must have +15Vdc at the cathode of the flyback diode.
Monitor that point & see if it is failing.
If it does fail here, it may be the 100?C switch, it's connections or the +15Vdc rail may be at fault.
The second critical part is TS9. It must conduct a ground reference for the relay.
It appears that TS8 keeps TS9 off until C47 charges through R53.
If you remove TS8 the delay part will be disabled.

Can I just remove TS8 to disable that crappy delay circuit? Is this delay sort of a surge protection for the toroidal PT?

[Zouto]

Any suggestions, ideas.......?????

[Enzo]

It's not really a crappy circuit, it just isn;t working in your amp.

First, that "100?C trip" thing is a thermal switch inside the power transformer, and since you found the +15 that comes through it to be steady at the relay, it must be OK.

Have you just resoldered those two transistors? That might be all they need.

Press down on each component in this circuit including the relay. Does that cause the chatter to start or to stop? If so, ther is a solder connection that failed.

It might not be a transistor at all. Instead of guessing, FIND OUT.

The way this works is not complex. When power is first applied, C27 charges up through 1meg R54. Once the voltage charges up enough, the voltage at the gate of TS8 turns it OFF. It's a JFET. WHile TR8 is ON, it holds the base of TS9 down so it cannot conduct, leaving the relay OFF. Once TS8 goes off, then 22k R55 is free to pull the base of TS9 up, turning it on and enabling the relay.

You found the voltage at the top of the relay was steady. When the relay starts to chatter, what is happening at the collector of TS9? DO not assume there is continuity from the collector if TS9 to the anode of the flyback diode. There SHOULD be but don;t assume it. If the relay goes out, and voltage at the collector of TS9 goes up to 15v, then the transistor is going off or is open, but we know there is continuity. But if the relay goes off and the collector drops to zero, then we know we have lost continuity either to the relay or through its coil.

Likewise, what happenes at the base of TS9, or if that is not convenient, the voltage across R55? When the relay drops out or chatters, is that voltage shifting up and down, likely from zero to +.5V? Indicating that TS9 is being turned of and on. Or does it go up to some voltage and stay there, indicating a bad TS9?

How about the voltage across C27 or R53? It ought to charge up to 7-8v and stay there. Does it? or does it jump around? If it is unstable, then we can;t expect the transistors to be either.

This is classic troubleshooting. The cap charges to turn off TS8 to turn on TS9 to energize the relay. By checking each step of this process we can determine exactly what is at fault.

[Zouto]

Enzo, thanks a lot. I'll check all the parts you mentioned.I've resoldered all parts of the relay/delay circuit thoroughly.The problem is, that the amp sometimes stays stable for long time, even hours, but then suddenly the relay turns off B+ for some time. So next thing will be to exchange all the smd parts with standard stuff. The whole pcb does not look reliable at all with lots of sloppy soldered smd parts and connections underneath, not easy to reach, to check and to take measurements on the powered amp.The coating of the pbc covers also some soldering points and has to be removed. Final check and quality control seem to be an issue on Laney amps.
Zouto