hi , need a little help with blown '70s H&H MA100 amp

[bobster]

hi guys , just looking for some pointers and help to try and get this amp going again....
im not a tech and just have some limited know how [ can solder and use a meter etc ] but ill list some symptoms which may help you to suggest some areas for me to look.

for a while , the amp was starting to build up a hum when on for about a half hour or so. the hum got grradually worse and louder the longer the amp was on.

so , i changed main filters 4700uf 60v but no difference.

i then turned her up loud to see what happened and she went nuclear with the hum and then i got no signal just hum even with volume off. amp lights up etc but just hums loudly.

i then tried the speaker cable for voltage and got about 40v there.

so , i got a new bridge rectifier bridge KBPC35-06 and popped it in but this didnt cure it and i still have the same issue

while it was still working , i took some readings on the transistors on the power board but not the two main power transistors as i could not reach them.

Q1  - 2SD381,         b -47v c -0v e - 0v
Q2  - 2SB536,         b - 1v  c -47v e - 48v
Q3  - BC204B,         c - 47v b - 47v  e-48v
Q4  - BC207B,         c - 1v   b -0v   e - 0v
Q5  - BC204B,         c - 0v   b -0v   e - 0v
Q6  - 2SD381,         b  -1v   c - 47v  e - 0v

any ideas gentlemen?
many thanks for any help
cheers - bob [ in UK ]

[J M Fahey]

Hi bobster, sorry about what happened.
Start by googling around, trying to get a schematic maybe in some HH lover's forum, or Laney, the present owners.
Maybe a Laney M(something)100 is close enough.
There might even be a copy here in SSGuitar, or Music Electronics Forum.
Also, with amp  unplugged from mains, and multimeter on Diode scale, check for shorts between each transistor's legs.
Good luck.

[pyromaniac_]

Well the voltage readings on the transistors looks really fucked up, so start looking for shortcuts there. Also if the base voltage of the transistor is less than about 0,7 V (the forward voltage drop) no current can pass through it. So maybe you should figure out why you read 0V on the base pins of Q4 and Q5. It could be a shortcut, but also something else. Start to try to get the schematic anyway.

[bobster]

yeah , those tranny voltages look skewed even with my limited know how..
i was going to replace all the smaller transistors and see if that helped . i'll get round to that soon as i get some time.
dont know if these symptoms would happen if the power trannys went south?
all parts on the boards look and feel secure and there are no visible burnouts anywhere
hmmm

do i need to unsolder the transistors to test for shorts on the diode scale?
thanks - bob

[teemuk]

The schematic (see attachment). There are at least two versions but they are not drastically different. The overall circuit is about the same but there are few differences in component values, and if I remember correctly some components were missing from the first version.

Edit: Apparently the semiconductor models do not match. Let's hope there isn't an entirely different version of the circuit... 

[bobster]

hey thanks for digging that up!
yeah the semiconductors numbers dont match ,, o dear.

its a shame this thing sounded so cool and clear with guitar. id like to try and fix it myself as a learning thing before i give in and hand it in to a shop........
cheers just now

bob

[J M Fahey]

IF it were a "reissue" actually made by a subcontractor in Asia, it might be loaded with Oriental semiconductors, but since it is not, probably they just show a hurried technician who used the "transistor-of-the-day" he had at that moment in his spare parts bin.
Check it carefully against the schematic, and restore it to original condition.
First of all print a copy of Teemu's schematic, or screen-capture it and save as a .gif, so you can edit it and place actual component values close to schematic ones, to check how close (or far) you are.
I'm sure your original board will be a good match, maybe not so the possibly replaced components.
Good luck.
PS: refer to schematic part numbers or we can't follow you.