Marshall SL5

[guitardad]

Have an SL5 that comes on but volume just dies (maybe 1/2w) within 15 sec. It still has both channels (clean and od) working, just really low volume (not even like the 1w mode). Every once in a while it comes in regardless of 1 or 5w settings. Kind of a PITA to work on with the way the pre/output board is vertical. I was hoping to make working and non working voltage just for reference, but this just goes out too quick. It only seems normal when its really cold. I tried using freeze spray to bring it back to no avail. Pulled and inspected all boards, tube swap etc. Still acting up.

Problem has been reported on a few other forums, but no fixes indicated (just told to return). Not an option.



Voltages look ok based on what I would assume normal (plates grids cathodes htrs). Must be signal loss. Do these have relay issues? I would assume not the prob since I can get both Clea and OD, just the volume is really low.

SL5.pdf

Thx

[g1]

In the C-2 area of the schematic, there is shown a reset line.
Is there digital circuitry that is not shown on this drawing?

[MarkusBass]

It has custom digital reverb (not shown on the schematic). But you can see Reverb send and return circuits.
The amp is quite simple. I would measure signal on tubes , e.g. on cathode of V3A. I had similar case in Aguilar DB751. I had to draw the schematic first (since it is not available) and having it was a simple task. On anode of one tube there was signal and after capacitor the amplitude was many times smaller (sometimes - exactly as you have). After replacing the capacitor the amp was fixed. I suspect something very similar in your case. You have to find a clever way to find it quickly. In my case it was (having the schematic) signal generator and oscilloscope (and maybe a little bit of luck).

Mark

[52 Bill]

Yes, divide and conquer. I'd start by injecting a signal into the power tube input grid and see if the sound is good there.Then move forward through the amp to find out where the signal dies out.

[guitardad]

Yes, divide and conquer. I'd start by injecting a signal into the power tube input grid and see if the sound is good there.Then move forward through the amp to find out where the signal dies out.

Signal initially is fine all the way to the op of EL34 when unit is cold (been off for 6 hrs or more). Once it comes on signal is strong then starts to die in less than minute. Looking to be a bad OT. I've seen the same problem with Egnater R20's. I know they had a rash of bad Chinese OT's and this one is MIC so probably the same junk. Voltages and all look fine too, so I assume internal short OT when under load.

[MarkusBass]

So you already know what is the problem . It's a pity because people here are trying to help you without knowing this. In this case I suggest replacing the OT - such a small transformer is only several dollars.

Mark

[guitardad]

So you already know what is the problem . It's a pity because people here are trying to help you without knowing this. In this case I suggest replacing the OT - such a small transformer is only several dollars.

Mark

IDK if it's the OT or not. I asked here since the issue is very odd (from my experiences), but this unit model seemed to have a few frequent complaints/symptoms on other sites. Thought I'd take a stab that others here may have seen the problem.

I didn't want to spend a lot of time on something that may be RMA'd , likely to happen now since the cust just wanted me to see if it was something simple like bad solder, connector etc.

[MarkusBass]

What exactly is "IDK", or "RMA'd" ? Something from this list: https://en.wikipedia.org/wiki/RMA ?
Seriously, the amp is so simple that I'm surprised that people reported problems with this amp and the problems were not solved. Send them to me

Mark

[Enzo]

Output fades away. You say some Egnater models fade away too? Fading usually means a tube heater loses power. The problem with the internet is that a few problems have been reported. It can't be few AND frequent. If we find six examples of a similar complaint, then how many thousands of them did not have the complaint?

If one half of a 12AX7 is not going dark with the fade, my first suspects are an unterminated grid, a loss of ground connection through a grid resistor somewhere.

Probe pin 7 of V3 with your meter. Does that happen to liven it up? Unground your meter and probe the same point. Now your probe wires act as an antenna to inject a hum signal. Does that come out diminished or strong? That should tell you if the problem is before or after that point.

You said the signal is OK at the plate of the EL34 when cold, so does that signal diminish as it warms? If so, I don't suspect the transformer. If the signal remained there but the output fades, then I might like the transformer.

[Enzo]

RMA is a return authorization - a warranty exchange. Which makes a mystery of return is not an option.

[DrGonz78]

"IDK" means... I don't know. Yeah but whose on first? Idunno

[J M Fahey]

That "fading" problem sometimes is a lost ground or bias reference (open resistor / cracked track or solder / poor-dirty-open connector) so amp starts but coupling capacitor gets charged and bias flies away, one way or the other, muting gain stage.

The scope is your friend, I'd clip it to out tube grid or , suitably attenuated, tube plate, inject some signal and watch it die away.

A shorted turns OT attenuates signal a lot but does not self recover as described, just stays dead (or in coma).

[guitardad]

Output fades away. You say some Egnater models fade away too? Fading usually means a tube heater loses power. The problem with the internet is that a few problems have been reported. It can't be few AND frequent. If we find six examples of a similar complaint, then how many thousands of them did not have the complaint?

If one half of a 12AX7 is not going dark with the fade, my first suspects are an unterminated grid, a loss of ground connection through a grid resistor somewhere.

Probe pin 7 of V3 with your meter. Does that happen to liven it up? Unground your meter and probe the same point. Now your probe wires act as an antenna to inject a hum signal. Does that come out diminished or strong? That should tell you if the problem is before or after that point.

You said the signal is OK at the plate of the EL34 when cold, so does that signal diminish as it warms? If so, I don't suspect the transformer. If the signal remained there but the output fades, then I might like the transformer.

We've had about 4-5 Egnater Rebels at the shop with bad OT. Symptoms not like this SL5. At first, even they were extending the warranties.

This SL5 the customer said "fades out after a few minutes." For us, when cold the unit worked, then after about 45 sec to a minute the volume just dropped to maybe a 1/2/ 1/4w regardless of power settings (1 or 5w mode). That symptom remains for hours. Checked connections, used coolant spray, nothing changed.

Problem is it runs for a minute then goes to very low volume. This has been reported several times on several other sites including Marshall forum and the suggestion posted was to return the unit (RMA). Being so, I thought I'd ask here to see if there was anything common. Apparently there isn't.

The Cust didn't want to RMA (return to dealer) the unit since it was NLA (no longer available), supposedly a rarity, and they didn't want to pay $80-100 to ship it out, and the dealer offered them only a suitable replacement since SL5's are no longer made. They did not like the choices offered, so here it is.

I subbed the transformer from a Class 5 and the unit is working. I too, was not suspecting the OT at first, thinking other causes, and hoping for something more obvious. I'll check with supplier tomorrow for actual stock.

Just the first transformer issue like this I have seen (works then goes out). More often they are open, shorted, and have no sound or low sound under load. On the other hand its rare I see an amp with a bad OT.

Thx