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Messages - tonyharker

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Amplifier Discussion / Re: Polytone 102 Tremolo
« on: January 24, 2019, 02:37:54 PM »
The symbol is the Greek letter Lamda which indicates the resistor is light dependent. The resistor in a circle above R53 must be a light source (48v lamp?).  This is coupled optically to R16 which is a Light dependant resistor. 

Amplifier Discussion / Re: OK to run Marshall Lead 12 into 4 ohm load?
« on: November 26, 2018, 06:15:00 AM »
The output transistors are ok with the higher power, but the heatsinks are a little small.
All I can suggest is try it and see how hot it gets.  If you cannot hold your fingers on the heatsinks for a couple of seconds then they're too hot.

Amplifier Discussion / Re: SWR 550X Bass Amp Trouble
« on: September 21, 2018, 06:21:57 AM »
The first thing I would do is to check the output for DC. To me it sounds like one of the output transistors has failed.

You will have blown the power amplifier by connecting the external amplifier. It will be an IC or discrete transistor device.  A photo will show which.  If an IC it will probably easier to replace if you are competant to do that - can solder properly etc. A Transistor amp will be more complicated. If you are not sure it would be better to take it to a tech.

Amplifier Discussion / Re: Repairing a Roland JC-50
« on: June 25, 2018, 01:42:32 PM »
Power transistors, and driver transistors are in the top right of the schematic page 3, Q18, Q22, Q27 & Q28 for later serial numbers, 25xxxx or Q11, Q12, Q13, & Q14 on page 7, for earlier serial numbers 24xxxx. The box around Q13, Q14, Q27 & Q28 means they are on a heatsink or mounted on the chassis

Amplifier Discussion / Re: Sunn Coliseum 300 Issue
« on: June 09, 2018, 06:35:19 AM »
First thing clean both Send and Return jacks and try again.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: May 30, 2018, 01:35:32 PM »
Glad to hear it. Enjoy playing.

Tubes and Hybrids / Re: Tube shields on EL-84's
« on: May 08, 2018, 05:17:18 AM »
Don't use shields on power tubes like EL84s, you will only shorten their life and maybe overheat them.  Shields should only be used in preamp tubes where they are likely to pickup RF.  So hi gain hi impedance first stage amplifier tubes only. Tube retainers ie spring clips can be used on any stage.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 24, 2018, 10:35:34 AM »
Yes, although putting  7v  approx across the speakers doesn't seem right.  Try biasing at a lower voltage first say 3v and see what it sounds like. Do you know what power the speakers are?

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 24, 2018, 05:37:14 AM »
You need to adjust the 1M bias preset on the AD130 #5 which drives the speakers so that the collector is at half supply voltage. This is a class A stage driving the speakers directly.

Amplifier Discussion / Re: Hohner Pianet Amplifier Low Output
« on: April 15, 2018, 02:40:48 PM »
Sounds to me like a bias problem or the transistors could have failed after all this time. - they are germanium after all.
First check that there is a voltage supply out of the bottom power transistor (No 6) - this is a hum reducing capacitance multiplier stage and should output about 15v. If this is present then the other transistors need taking out and checking for gain and leakage.

The symptoms you describe appear to be one of the transistors not conducting and the increasing signal being eventually rectified by the base/emitter junction and switching it on.  This would sound loud but distorted.
After all this time it may be necessary to replace all the transistors if you can find them or equivalents, or replace the complete amplifier for a modern one.

The Newcomer's Forum / Re: Orange Crush 12 repair
« on: April 08, 2018, 09:09:44 AM »
You really need to find out why it is failing.  If you replace it odds are that it will fail again.. Op amps don't just fail for no reason!

Check T8 BF245 & D24 the slow start circuit.  Sounds like T8 is on all the time.

Amplifier Discussion / Re: Need Help Identifying a Diode.
« on: January 31, 2018, 01:57:57 PM »
Looks like a thermistor to me not a diode.  Diodes are usually cylindrical.  :)

Preamps and Effects / Re: Active pickupa
« on: January 07, 2018, 12:40:41 PM »
Is this an EMG 81? If so you need the connection board as shown here: .  Have you got it?

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