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I am repairing my son’s 8080 Valvestate which initially had no output on the Boost channel. I tracked this down to a failing IN4007 diode D3 which was found to be passing ac. Replaced this and all seemed OK, except that the Boost channel volume level is now much the same as Channel 1.
When in use the normal settings for the amp are both Channel 1 and Boost channel Gain pots set to 4 (assume 5 to be half way), and the Boost channel Volume pot is set to 7. Switching to the Boost Channel used to give a significant increase in volume. But not now.
I suspected that the valve output may be low so I put in a new one but there was no difference. I have taken a few measurements on the Boost channel, these are as follows;
The + and – 15 volts supplies are OK. Valve HT is 285v at the junction of R109 and R110 and 118 v at the bottom of R109 and the V1(a) anode.
The valve output at the junction of R71, R95, R36 & C34 is 118v dc.
With OD1 set and looking at the Boost channel path at the junction of the LEDs and R34 & R68 I see 3.2v pp. This is almost the maximum as increasing the input signal or gain only changes the waveform shape not the height.
Also at R68 and R69 junction (valve grid) I read 750mv pp and at the valve output at the junction of R71, R95, R36 & C34 I see a maximum of 28v pp.
I would really appreciate if someone could confirm that these voltages are as they should be.
I have been able to restore the Boost channel output to its correct level by increasing the gain of IC5B from 2 to 5 by putting 15k in parallel with R41 but this is not fixing the problem.
I have compared the voltage outputs at IC5 pin 5 and observed the following;
With the controls set as follows:
Channel 1 Gain =4, Boost channel Gain =4, Boost channel Volume =7. All tone settings at midway and Contour set for max signal. Signal generator frequency at 4.2Khz
Input signal level 400mv 800mv
Channel 1 Clean 4.6v 9.0v Channel 1 Crunch 6.3v 11.0v
Boost channel OD1 5.0v 4.7v
Boost channel OD2 5.1v 5.1v
Also with Boost channel at full volume I get a reading of OD1 = 10.0v and OD2 = 12.1, these are just a little more than the output from Channel 1
Along the way I have changed the filter capacitors C59 & C53 and changed the high voltage coupling capacitors C54 & C34.
So any ideas on what is causing the loss of output?
When in use the normal settings for the amp are both Channel 1 and Boost channel Gain pots set to 4 (assume 5 to be half way), and the Boost channel Volume pot is set to 7. Switching to the Boost Channel used to give a significant increase in volume. But not now.
I suspected that the valve output may be low so I put in a new one but there was no difference. I have taken a few measurements on the Boost channel, these are as follows;
The + and – 15 volts supplies are OK. Valve HT is 285v at the junction of R109 and R110 and 118 v at the bottom of R109 and the V1(a) anode.
The valve output at the junction of R71, R95, R36 & C34 is 118v dc.
With OD1 set and looking at the Boost channel path at the junction of the LEDs and R34 & R68 I see 3.2v pp. This is almost the maximum as increasing the input signal or gain only changes the waveform shape not the height.
Also at R68 and R69 junction (valve grid) I read 750mv pp and at the valve output at the junction of R71, R95, R36 & C34 I see a maximum of 28v pp.
I would really appreciate if someone could confirm that these voltages are as they should be.
I have been able to restore the Boost channel output to its correct level by increasing the gain of IC5B from 2 to 5 by putting 15k in parallel with R41 but this is not fixing the problem.
I have compared the voltage outputs at IC5 pin 5 and observed the following;
With the controls set as follows:
Channel 1 Gain =4, Boost channel Gain =4, Boost channel Volume =7. All tone settings at midway and Contour set for max signal. Signal generator frequency at 4.2Khz
Input signal level 400mv 800mv
Channel 1 Clean 4.6v 9.0v Channel 1 Crunch 6.3v 11.0v
Boost channel OD1 5.0v 4.7v
Boost channel OD2 5.1v 5.1v
Also with Boost channel at full volume I get a reading of OD1 = 10.0v and OD2 = 12.1, these are just a little more than the output from Channel 1
Along the way I have changed the filter capacitors C59 & C53 and changed the high voltage coupling capacitors C54 & C34.So any ideas on what is causing the loss of output?