Hi, all
I’m servicing a 74 130 HD that’s been abused. I’ve successfully completed a cap job and and other minor items, but the PT is violently vibrating. Tubes or not, the issue is there and it’s blowing fuses.
- known good tubes
- PT tests good
- OT?
I am new to these amps. I’ll check the ot to see if it’s shorted. Wouldn’t shock me - it’s being held on by two wood screws.
Have any of you seen this issue?
I noticed my power
I noticed my power transformer was buzzing/vibrating noticeably before I reflowed the solder connection from the PT to the rectifier board. After this, the PT seemed to calm down. Maybe check the connections?
I've read elsewhere that the bolts on the PT may be a little loose and this could cause vibration. Though it sounds like you've got more going on if you're blowing fuses.
I'm interested to hear what you find out about the OT. Best of luck!
Double Check
1. Check to see if one of the Caps was installed backwards...reversed polarity.
2. Check to see if there is a shorted power diode on the rectifier board.
3. Check for wayward solder blobs.
You're right, There is a short somewhere? Does it make a difference if it is in Standby Mode? Good Luck. -mgriffin
Didn’t see email
Didn’t see email notifications, so pardon the delay. Ot is fine! I did find two faulty diodes on the rectifier board. The cap jobs went without a hitch. I’m trying to debug two issues:
1. Uneven heater voltage
2. No sound at the output - not even a slight hun or hiss
Here are some measurements:
V2-5: el34 hi
1 .4mvdc
2 13.7mvac
3 726vdc
4 365.5vdc
5 -40vdc
6 -40vdc
7 6.66vac
8 84.7mvdc
V1: PI (12ax7)
1 293vdc
2 20.6vdc
3 46.4vdc
4/5 39.4mvac
6 207.7vdc
7 .1mvdc
8 1.82vdc
9 6.72vac
I don’t understand what’s up with the faulty heater string. I’ve measured it from lead to lead and get around 7vac, so it should be fine. However once in circuit with or without tubes, I get mv on one end and v on the other.
Thanks for the comments!
Bad op amp?
I can't help out with the uneven heater voltage problem. That one's above my pay grade.
As far as not getting sound at the output, I found when trying to install new LM307H op-amps (did this to see if I could improve the sound quality), one went "bad" as I was playing and completely shut off any sound at all getting through to the speakers. No hum or hiss of any kind.
Were you able to get sound out of it before you made any changes?
Thanks for giving it a shot.
Thanks for giving it a shot. I just figured out the main issue. I can’t believe I missed the obvious. The heater voltage is fine. The main problem with no sound was this:
The previous owner used WOOD SCREWS to secure the output transformer. One of the screws was shorting the impedance selector to the chassis!! After using proper hardware, the issue is gone lol.
Now I have a new issue: the reverb/trem channel’s volume is nearly silent. I’m working on the chassis outside of the head can obviously, but I’m wondering if the absence of the tank is causing this issue. The volume on this channel is a dual-ganged pot and the input signal seems to split off of it directly to feed the reverb circuit. Will see if I can test the LM307Hs.
Thanks!
Glad to hear the progress!
Glad to hear the progress!
I play a 212 HD 130 combo. I just played with the reverb tank cables disconnected from the chassis. I was able to play through the amp just fine, albeit without any reverb. So, you should be able to get signal through that channel without the tank connected.
I pulled up this older thread that might have some useful information: http://pacair.com/mmamps3/node/1003
One suggestion there is to look at IC-3 and IC-4, swapping those with IC-1 and IC-2 to see if you get a louder signal in channel 2.
Thank you! Updates soon.
Thank you! Updates soon.
Finally revived the new
Finally received the new opamps today and replaced IC3 & IC4. No change. Volume is just barely above the acoustic volume of the guitar! Stumped.
Saw in another thread that
Saw in another thread that you fixed the issue! Glad to hear it!
Thank you! Amazing what one
Thank you! Amazing what one can do with a schematic and a little understanding of signal flow lol.