Transistor equivalents?

[Searing]

I am working on a Marshall amp and cannot find these two transistors or their equivalents:


T8818


T8730


Any thoughts?  Thanks.

[PRR]

> cannot find these

Neither can we (I bet).

How about the name/number of the amp or a link to the schematic? Marshall didn't make intergalactic missiles. The transistor is something earth-ordinary. The code number is a private-spec so Marshall could take advantage of odd lots, maybe 13V parts or low gain. Sometimes a glance at a plan suggests some common-spec part will do the job.

And why do you think you want to replace these? Transistors sure do die, but are not always the best first bet.

[Searing]

It’s a SS Marshall 3315.  To be honest, I am replacing parts haphazardly trying to cure the low volume, distorted output issue.  I have never had an amp give me such a headache!

[sluckey]

It’s a SS Marshall 3315.  To be honest, I am replacing parts haphazardly trying to cure the low volume, distorted output issue.  I have never had an amp give me such a headache!

That's a very hard way to fix a SS amp! You may even end up feeding it transistor after transistor only to watch the amp eat them as soon as you power on.

[Latole]

It’s a SS Marshall 3315.  To be honest, I am replacing parts haphazardly trying to cure the low volume, distorted output issue.  I have never had an amp give me such a headache!

Don't do that, you may do have more issue

[tubeswell]

I am working on a Marshall amp and cannot find these two transistors or their equivalents:

T8818

T8730

Any thoughts?  Thanks.

Are they NPN-PNP pairs? (Without looking at a schematic)


Whereabouts in the circuit are they? What are the voltages? And what are the load resistances?

[mresistor]

Tubeswell..  you mean Darlington Pairs.. ?   
here is a schematic  https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-150W-3315-5150-Schematic.pdf


op  can you look at schematic and tell us which transitors you are talking about... ie   TR1  TR2  etc.

[shooter]

now that's a blurmatic 


if my crossed eye's seen what I think they saw, the amp can be "broke in 1/2" real easy.
so jack in a good signal to the PA section
jack out the pre into a known good PA

[mresistor]

Sorry Shooter - the one in the library wasn't any better - it's what we got..

[mresistor]

https://www.ebay.com/itm/ECG152-8818-Transistor-ECG-PHILIPS/202107132339


(mods - looks like my post count doesn't increase)

[shooter]

it's  good enough to figure out which 1/2 is bad, maybe even which 1/4, then drafting might be an option

[tubeswell]

Tubeswell..  you mean Darlington Pairs.. ?   
here is a schematic  https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-150W-3315-5150-Schematic.pdf


op  can you look at schematic and tell us which transitors you are talking about... ie   TR1  TR2  etc.

No. A Darlington pair is different


I was talking about pairs in a dual rail supply

[mresistor]

there does appear to be a pair in the PS - TR3 and TR7   and then TR4 and TR5 and TR8 and TR10

[tubeswell]

there does appear to be a pair in the PS - TR3 and TR7   and then TR4 and TR5 and TR8 and TR10

Yes that's what I was talking about. However I don't see 'T8818' or 'T8730' in that schematic (but I could have missed it)

[sluckey]

there does appear to be a pair in the PS - TR3 and TR7   and then TR4 and TR5 and TR8 and TR10

None of those are Darlingtons.

[mresistor]

yes  Sluckey  I know I am refering to what Tubeswell was mentioning.


I was talking about pairs in a dual rail supply

[mresistor]

Tubeswell..   I don't see those transistor designations in the schematic either..   which probably means the transistors in the amp are substitutes for the originals.

[tubeswell]

Which goes back to knowing where they were in the circuit.


However, the transistors might not be the problem- not enough details

[glass54]

Hi Searing
Unfortunately some of us have experience in repairs of this era Marshalls (or was that error??  )
Can you clarify if this is the correct output Schematic, please.
Kind regards
Mirek

[DummyLoad]

pictures of the PCB you're working on & with a photo editor, circle the parts that you have replaced. high resolution pictures where we can clearly see the reference designations would be helpful. all those semiconductors are common parts and still available.


TR5 is labeled A06 which is actually a MPSA06 - likely mounted on the heatsink in proximity of TR8 & TR10.


--pete

[Searing]

These are the transistors near the input. 


Yes. Mirek, that is the output schematic of the amp.


The volume is low, very distorted, and when I turn the amp near full volume, it cuts out completely.


I have replaced all electrolytics, the two 680ohm resistors in the output section.  I have replaced all IC’s, and transistors other than the ones this thread is about. 

[tubeswell]

These are the transistors near the input. 

MC1430 and TL072 op amps. So whereabouts on your board are the T8818 and the T8730?

[PRR]

And are there any voltages in there?

[glass54]

Please forgive me Searing, BUT the only transistor in the Preamp that I can see is TR1 (BC184, muting control for Reverb) and the rest of the Preamp "transistors" are J series FETs and labelled probably correctly. I'm a little baffled like Tubeswell 
Can you please clarify if the fault is in the Preamp or Power Amp, you can inject into JS2 "PA In"
Regards
Mirek

[Searing]

Using a known good power amp, I have discovered that the preamp section is working.  So, I am getting weak, distorted output from the power section.  I have changed all electrolytics, the two 680 ohm 3 watt resistors (one was waaay out of spec), and I changed most transistors but two being a 212 and a SA06. 

[Searing]

So, I removed the two main output transistors and saw an excessive amount of solder running down the leads.  Used my Hako de soldering gun, and sucked it off.  I put the transistors back in, and the amp worked!  Must have been shorting out between the transistor leads and the chassis? 

[PRR]

> sucked it off

I love a happy ending.

[glass54]

Hi Searing,
(Just got back to the computer here in Aussie)
Thank you for the update  and I'm glad you had success.
I was going to run you through a step by step process to cure the Marshall but we don't need to. 
The Solid State Marshalls of that era had limited success and a lot was to do with lack of detail to assembly.
Also suffered a lot of Dry Joints and PCB fractures around Output devices.
Kind regards
Mirek

[Searing]

Just want to say thank you to all for the help!  I greatly appreciate it.