LED (Lead) blinking off in MK IV

[Blueshaggis]

Hey guys, if my Lead light (by the little masters on front) is blinking
on and off a bit but stops afterwhile could it be a preamp tube (and
which one(s) or power tube or maybe just an LED problem? It's a
bit noisy and feedbacky in Lead mode and R2 (but got better again).
It stopped doing all that again and was sounding great tonight.....weird.
Any thoughts?

 

[Blueracer]

Hi Blueshaggis,

The Mark IV has many many many LED driven switches (not to be confused with the LEDs on your front panel) shown as LDRs in the amplifier's schematics that are actuated when switching between R1, R2 and Lead channels. The indicator LED on the front of your amplifier is most likely tied to the same circuitry that contols these LDRs. I say most likely because the schematics that I have found on the internet do not show the circuits that switch these functions for the Mark IV. If the front panel LED is flickering it may indicate a marginal voltage for actuating the LED. If this is the case then the LDRs also may have too low a voltage and switching may be incomplete for these devices. Hence the noise you are hearing in the R2 and Lead modes.

So much for background. The good news is that it most likely is not a tube. What may be occuring is there may be a component whose value has drifted or a cold solder of perhaps something entirely different. It is hard to tell without investigating further. Also note that the problem for the moment has gone away. Hopefully it will stay away but very often they tend to come back and get worse with time. Once the cause of these switching issues are found the fix is usually not difficult. It may mean a touched up solder joint or a component replacement or two. Hope this helpful.

Gary

 

[Blueshaggis]

Thanks Gary, let's hope it just stays away then. I don't relish the
thought of taking the chassis out, packing it and shipping off to Boogie
nor the expense involved. I was actually HOPING it might just be a tube
or two.

 

[rockhound76s]

I'm currently having the same problem with my Mark IV short head, only its with my RHY 1 channel LED light above the RHY 1 Master volume.

Yesterday, while rehearsing I noticed that the light was flickering. Today, the light is completely out, but I haven't had any loss in tone. I am assuming it is just a old bulb that needs replacing.

Is this a simple matter of opening up the chassis and replacing a bulb, or is it something I need to have checked out by a tech? (I understand the short heads chassis can be tedious to remove, so before I do anything I thought I'd ask you guys.)

Any help is appreciated.

 

[screengrid]

Hi, as a owner of a MK IV A too, which I bought new in 1991 in Germany, I remember , that I also had fore a very short time problems with a flickering LED. As far as I remember, it occured while switching from R1 to Lead. First the Lead LED was normal on for short time but then dark for some seconds. After I switched some times back and forward between the channels, the Lead LED was flickering for some seconds. After that, all was normal again and since this day it never occured again until today.
Like already mentioned by Gary, the MK IV works with a lot of LDR's to do all the switching functions. According to the schematic I have, I can count 22 LDR's beside two relays. Further, according to the drawn switching matrix, section 1 and 2, some IC's and transistors are included as following:

footswitch / switching matrix section 1:

1 x 74C922 which is a 16 key encoder ( MM74C922 which is a C-MOS chip)
1 x 74LS14 ( 6 x schmitt triggers/dual gate hex inverter ) (SN74LS14)
1x 74LS138 (1of8 decoder/demultiplexer) (SN74LS138)


MK IV / switching matrix section 2

1x 74LS14
1x 74LS138
4 x transistor ( Q 5-Q 8 )
2 x relays

operating voltage is +15V and +5V

no problem to get these IC's if defect, which are fairly cheap beside the encoder, which cost around 6 EUR in Germany.

hope that helps a little.

regards
Arno

 

[Blueracer]

Hi rockhound76s,

I believe the lights are LEDs and are normally soldered in rather than being a bulb and socket type of arrangement. I have not been inside a Mark IV so I don't know for sure but this is the normal way LEDs are configured. I am sometimes surprised how different manufacturers configure things. LEDs have a mean time between failure (MTBF) of something between 100,000 and 1,000,000 hours. To put this in perspective a year has 8760 hours. It is doubtful that the LED has failed (but always a possibility) rather I would suspect a cold solder joint, a loose wire or something similar particularly since your amplifier doesn't seem to have any other symptoms (I am assuming it switches channels normally). If you have any sort of electronics background you could go inside your amp and try to locate the problem. If not it is probably better to let a tech do it. Hope this is helpful.

Gary

 

[Nick_cor]

EVERYONE ALL YOU DO IS TOUCH THE LED , USE VERY LITTLE FORCE AND JUST PUSH THEM IN, IF THE LIGHT DOES NOT COME BACK FULL IMMIDEATLY THEN THEY'RE BURNT OUT!

 

[zool]

Dood.

Flick it with your finger and see if it stays on.

 

[Nick_cor]

Dood.

Flick it with your finger and see if it stays on.

Wow, do you really think that was nesserary? i mean, you just repeated what I said man, i hope no one gives you credit :roll: