Marshall 8080 - Lack of volume on boost channel.

[J M Fahey]

Well, I think your HV and tube circuit will work now, but we need first to repair your power amp, or further testing will be impossible.
With the series lamp permanently engaged until we tell you otherwise, turn your amp on and measure +B, -B and DC voltage on the "hot" speaker out.
This Valvestate power amp is somewhat more complicated than it should, so post or link the power amp schematic so we all refer to the same and agree on part numbers.
Don't worry, we are real close now.
Edit: now I see the schematic at the thread beginning.

[Enzo]

I agree that your high voltage transformer and power supply circuit will probably be fine when installed back into the amp.

One thing to keep in mind when working on stuff is that nothing is certain. Just because something once worked doesn't mean it is permanent. When fuses blow, all the procedures go back to zero. What I mean is that you cannot decide that the power amp must be OK because it sounded good before. As you are finding out, the reason your fuses are blowing is that the power stage now has shorted output transistors.


Here is a TIP. (Pun intended)

Those output transistors are types BDV65 and BDV64 originally. It will be cheaper and easier, at least for most folks, to replace them with TIP142 and TIP147 transistors instead.

[Trevor]

Today I took a few more measurements as J M Fahey suggested. I still have the light bulb attached in series with the live wire coming into the main transformer. The fuse is holding strong when powered. This gives me the opertunity to take readings around the board.

Input voltages to the bridge rectifier are:

+ 34 VAC
- 34 VAC

Output voltages at the bridge rectifier are:

+ 43.1 VDC
- 43.1 VDC

The hot speaker wire has a DC voltage of:

-2.25 VDC (Bear in mind both power transistors are not in the board)

I also placed an order for some spare parts today. I have on order:

* 3 x TIP142
* 3 x TIP147
* A new diode for the HT circuit (D3)
* New capacitors for the HT circuit
* More fuses

In your last messages you mentioned that you thought the power amp and HT pre-amp circuit would be ok once I put everything back together...I am still unsure of what caused the power transistors to fail and ultimately short circuit. Surely there must be a fundamental issue that is still requiring a fix.

Many thanks,

Trevor

[Enzo]

Not really. You usually never know what caused the outputs to fail. Once one does, it most times takes out one of the opposite polarity. But as to why the first one fails? WHo knows. Rarely is there a one to one correlation with anything. You could have plugged a speaker cord into it at some point in the past, and ther was a static electrical charge on the cable. That discharge into the output stage could have weakened a transistor allowing it to fail a month later. A brief intermittent short in the speaker cab or connections could have weakened something. if a cord pulls out of a speaker cab jack part way by accident, the jack bushing can wind up shorting across the amp output.

Those are just a few of endless possibilities.

[Trevor]

Those output transistors are types BDV65 and BDV64 originally. It will be cheaper and easier, at least for most folks, to replace them with TIP142 and TIP147 transistors instead.

Both of the transistors I removed have exactly the same markings. Which transistor on the board should be replaced with the TIP142 and which with the TIP147.

Also, the transistor insulators seem to have degraded with age. Will the new power transistors come with new insulators or should I buy those too? If so, where can I get them and what size do i need?

Many thanks,

Trevor Keast

[J M Fahey]

Hi Trevor.
Maybe the difference is quite small, but they are not the same.
Usually they do not come marked as BDV6x but with a nice script M (from Marshall) or a letter T (from) and a couple numbers, but don't worry: The Tip142 has its center pin connected to +B (+42V) and the 147 has its cented pin connected to -B (-42V).
Some call Pin: Leg or Tab, use whatever your local Culture uses.
We all use English here, but different countries use different words.
Buy the micas, TO218 or TO247 size, it's the same; some call them "Plastic TO3 size" but that's not very precise.
Also buy some heat sink/thermal transfer grease and just in case some plastic shoulder washers.
Some cases are full plastic on the top and don't need them, others are half plastic half metal and do need them.
Besides real micas, you can use silpads, which are pieces of rubber not unlike a piece of a kid's party balloon, in theory they are the same and easier to apply (they don't use grease) but I do not trust them.
Manufacturers love them because they can save 2 minutes salary, on an oriental guy who earns 60U$S a month
Don't believe me? : official minimum wages in GuangDong Province, China : qualified worker: 0.64U$S an hour; non qualified worker: 0.39 U$S an hour.
Oh well.

[Enzo]

Oh let's be fair.

If you crimp a mica washer, it can snap or flake off, while a silpad simply bends. Mica washers must be coated with silicone on both sides. That means messier work and grease dispensing equipment, and if you want to be real official, a chemical impact assessment. SIlpads are just all around more efficient to use.

On some of those units, MArshall was even shipping them with the TIP142/147 parts installed.


As far as I know, the difference between TO3P and TO218 is the tab. On the TO218, the tab is full width like a larger TO220. The TO3P has the tab corners exposed, but the body come up to the top so the mounting hole passes through it. TO247 are the larger rectangular ones with no tab showing other than tiny bits in the scallops on each side.

[Jazz P Bass]

Marshall 8080

Can you identify the markings on the transistors for us.
They cannot be the same.
If you look at the schematic the two output transistors are the ones in big circles.
The top transistor (hard to read-Q41 ?) has arrows that point out .
That one is an NPN type transistor.(Never Points iN). You would use the TIP 142 in this position.
The bottom transistor is a PNP type. (Points In Perpetually). Here is where you will place the TIP 147.
I would advise measuring the base voltage of each output transistor before you install the new transistors.
If they are both on hard, you are going to eat more transistors.

[Jazz P Bass]

Silicone Insulators

Here is a link for the insulators.
Link:Transistor Insulator To-3p Silicon Rubber, 10 Pieces/pack | Distributed By MCM | 21-1155 (211155)

[Trevor]

Can you identify the markings on the transistors for us.
They cannot be the same.

I understand from looking at the circuit diagram that they cannot be the same however these "Marshall" stamped power transistors do appear to be stamped the same so I just want to double check that I put the new ones in the right way around.

I would advise measuring the base voltage of each output transistor before you install the new transistors.
If they are both on hard, you are going to eat more transistors.

What does "if they are both on hard" mean? Please excuse my ignorance. What should the base voltage be for each one? Can I measure this with the transistors out of the board?

Many thanks!

Trevor

[J M Fahey]

Hi Trevor.
Luckily your transistors are *not* the same: they are clearly marked T64, which I guess must be BDV64; and T65, probably BDV65.
Anyway you will use TIPs.
Yes, you can measure on board, on their empty pads, without output transistors.
Your center pads must read +42V (or thereabouts), there you will fit the TIP 142; the other one will read -42, there you will fit the TIP 147.
Use silipads if you want, I agree they are less messy and more modern, yet I'd love not to be flogged with a seven tails whip because I trust micas+grease better.
Seen as in your picture, pins go base-collector-emitter left to right.
From base to base you must read no more than 2 volts; more precisely: from emitter to emitter no more than 15 to 30 mv .

[oc disorder]

TR 10 = t64 = BDV64 = TIP147
TR 8 = t65 = BDV65 = TIP142
A slightly clearer copy at Ampix
http://www.ampix.org/albums/marshall..._8100_8412.pdf

Without wishing to cause a ruckus well
"yet I'd love not to be flogged with a seven tails whip because I trust micas+grease better."
I remember a while ago reading someone doing a side by side comparison test with a
test bed and a digital thermometer.
One test used mica and thermal grease and the other silli pads.
My recollection was the mica and thermal grease was better at transfering heat.
Mind you silli pads are much eaiser to use and in Trevors application (unless he plays death metal
at 13 ie 2 above spinal tap ) they should be fine.
T'was wondering if there are any certified tests or am I splitting hairs (or should I say mica) !

[J M Fahey]

Hi ocdisorder, thanks, that one is readable !!!!
All others floating around are scans from the already unreadable Marshall book schematic.

Hi Trevor, thanks to that clean schematic I can speak with more precision: you should have around 2.35 V base to base (that you can measure without output transistors) and if all well, install them, remeasure base to base but more important: emitter to emitter.
Check first that the emitter resistors are not open.
It does not have a bias preset, but you can adjust it anyway.
You should not have more than 33mV across, which mean 50mA idle current.
Post what you read, and the first time use a lamp limiter or a Variac, if available.

[Trevor]

Could you explain how I can measure "base to base" and "emitter to emitter" voltages. Is it simply base pin to ground? And emitter pin to ground?

[J M Fahey]

Sorry Trevor, didn't make myself clear.
Base to base means multimeter on 20VDC scale (because we expect a value somewhat over 2 volts), red probe on the Tip142 base, black on Tip147 base.
You already know that base is the left of 3 pins/legs/whatever, as shown in your picture.
You should read around 2.35V .
E to E means red on 142 Emitter (right leg) , black on 147 Emitter, scale on 200mVDC, you expect 30 to 50 mV, Marshall does not specify it on the schematic (they should, it's important) but I give you reasonable values.Others may differ, of course.
You may have lower values, no big deal, but 100mV or more are to be corrected.
Good luck.
NOTE= be careful not to slip a test probe shorting something, hold them straight and firm.
No pets, kids, girlfriend, World Cup final match in the work area, please.