Trouble with Teisco Checkmate Recap

[fishyguy192]

am trying to put new caps in a teisco checkmate that I just got. It had been messed with a little in the past so I figured I would get the power section back to spec. I replaced the 100mf/250v caps with 100mf/350v caps and the 20mf/500v caps with 22mf/500v. I turned the amp on to check voltages for the caps and I was getting 320v on the first cap and then the second was reading 560v which is obviously very high. I also am not getting any output. I have obviously messed something up but from the schematic it looks like everything is following where it should be. The caps polarity seems correct but I am not sure if it is actually good. I am wondering if anyone with a keen eye could look at a picture of what I have and compare it to the schematic. Also ignore the red written in caps from the schematic picture. One red wire from the transformer goes to the 4.7k (5k on the schematic) resistor and then to the 100mf filter cap which leads to the negative side of the next 100mf cap. The other red wire goes to the positive of one diode and the negative of another diode. Red goes to positive of one diode then through diode and negative end goes to the next 100mf cap.
Just confused why I’m getting really strange voltage and no output.

I made this post on the glowing tube forum thread but didn’t realize there was also a tech thread. I am going to try to do some more voltage readings straight from the transformer tomorrow to see if I have something messed up from there.

 

[Peegoo]

@fishyguy192

From what I can see in the pic the electrolytics' polarity is good, and the two diodes are pointed the correct way 'round per the schematic.

You didn't mention if the amp was working or not before you tore into it. Did you confirm correct voltages coming off the PT secondaries before you soldered everything in?

 

[tubejockey]

Those voltage-doubler type power supplies are not great at regulation. You may want to check the bias on the output stage, If the amp is not pulling enough current (biased too cold), it will have high voltages.

BTW, you may want to get some heat-shrink tubing over some of those exposed leads hanging out in the wind so you don't get a big bright blue flashing ozone generating surprise.

 

[ElPositivo]

Check all your grounds first with a continuity check.

 

[fishyguy192]

.

 

[fishyguy192]

Those voltage-doubler type power supplies are not great at regulation. You may want to check the bias on the output stage, If the amp is not pulling enough current (biased too cold), it will have high voltages.

BTW, you may want to get some heat-shrink tubing over some of those exposed leads hanging out in the wind so you don't get a big bright blue flashing ozone generating surprise.

How do I check the bias of the output stage? Is that controlled by that first resistor coming from the primary on the schematic? Also heat shrink sounds like a good idea. I will do that!

 

[fishyguy192]

@fishyguy192

From what I can see in the pic the electrolytics' polarity is good, and the two diodes are pointed the correct way 'round per the schematic.

You didn't mention if the amp was working or not before you tore into it. Did you confirm correct voltages coming off the PT secondaries before you soldered everything in?

The amp wasn’t really working before but I didn’t really try it because it hadn’t been used in years and I was told it wasn’t working so I didn’t want to plug it in in case it would damage it. I’m going to check voltage off the transformer today and post back here

 

[fishyguy192]

Check all your grounds first with a continuity check.

Edit:I just realized that heaters are supposed to have a DC reference so one side is hooked up to ground.

I’m checking my grounds and I am getting continuity from my heaters to ground. I shouldn’t be getting that right? Everything else so far is checking out with continuity.

 

[Wally]

I’m checking my grounds and I am getting continuity from my heaters to ground. I shouldn’t be getting that right? Everything else so far is checking out with continuity.

The heater filament shows a center tap at ground, so yes you would read some continuity to ground.
it would be good to see the entire schematic. It would be good to have a voltage chart.

How do I check the bias of the output stage? Is that controlled by that first resistor coming from the primary on the schematic? Also heat shrink sounds like a good idea. I will do that!

The schematic shows a fixed biased output. A voltage chart would include that voltage coming off of that bias adjustment pot and feeding the control grids of the power tubes.
I see that the schematic calls for 7189A power tubes. 7189As and 7189/6BQ5/EL84 tubes are not necessarily interchangeable. That depends on how the power tube sockets are wired as to whether or not one can sub them for each other. One can go to Tube Data Sheet Locator to see the difference in the internal connections on the 7189A compared to the other three EL84 types.

 

[fishyguy192]

The heater filament shows a center tap at ground, so yes you would read some continuity to ground.
it would be good to see the entire schematic. It would be good to have a voltage chart.

The schematic shows a fixed biased output. A voltage chart would include that voltage coming off of that bias adjustment pot and feeding the control grids of the power tubes.
I see that the schematic calls for 7189A power tubes. 7189As and 7189/6BQ5/EL84 tubes are not necessarily interchangeable. That depends on how the power tube sockets are wired as to whether or not one can sub them for each other. One can go to Tube Data Sheet Locator to see the difference in the internal connections on the 7189A compared to the other three EL84 types.

I haven’t had a chance to grab voltages yet but here is a schematic. I’m going to build a light bulb limiter tomorrow to try and have some more safety stuff and even power it up with a variac I just got.

 

[dan40]

I’m going to build a light bulb limiter tomorrow to try and have some more safety stuff and even power it up with a variac I just got.

You can start it up on the limiter to check for shorts but be aware that taking voltage readings while plugged into the limiter will result in low voltage readings throughout the amp. When checking the voltages, set your variac for whatever AC voltage that will give you 6.3vac on the heater filaments.

 

[Wally]

I turned the amp on to check voltages for the caps and I was getting 320v on the first cap and then the second was reading 560v which is obviously very high

C26 and C28 are 100mfds/250vdc on the schematic. They are in series. The voltage off of the positive end of C26 is the B+ voltage that feeds the Center tap of the OT…and thereby the plates from each end of the primary winding. It also feeds the rest of the power rail as I am sure you understand
Is either one of these caps the cap you are calling the ‘first cap’ or ‘second cap‘? In effect, these two caps in series act as one 50mfd cap at 700vdc….since you are using 100mfds/350vdc caps…, and this series arrangement establishes the first stage of filtering. What is your B+ voltage?
When taking the voltage measurement at the plate of the power tubes, you should hear a slight pop in the speaker. If not, you have a problem right there. If you hear that pop, then as you move from the power amp toward the input stage along the signal path…not reverb or trem tubes….you should hear progressively louder pops. The input stage should yield the loudest pop. Anytime the pop is not louder than the previous test, you have found a problem area. Taking voltages can reveal more than just voltages.

 

[fishyguy192]

C26 and C28 are 100mfds/250vdc on the schematic. They are in series. The voltage off of the positive end of C26 is the B+ voltage that feeds the Center tap of the OT…and thereby the plates from each end of the primary winding. It also feeds the rest of the power rail as I am sure you understand
Is either one of these caps the cap you are calling the ‘first cap’ or ‘second cap‘? In effect, these two caps in series act as one 50mfd cap at 700vdc….since you are using 100mfds/350vdc caps…, and this series arrangement establishes the first stage of filtering. What is your B+ voltage?
When taking the voltage measurement at the plate of the power tubes, you should hear a slight pop in the speaker. If not, you have a problem right there. If you hear that pop, then as you move from the power amp toward the input stage along the signal path…not reverb or trem tubes….you should hear progressively louder pops. The input stage should yield the loudest pop. Anytime the pop is not louder than the previous test, you have found a problem area. Taking voltages can reveal more than just voltages.

Thanks for the great response! I thought the voltage rating on the caps was for the safe voltage operating range not that it would be 100mf at 350. So that means it’s probably not working in spec because of that right? The one I was calling 1st cap is C28. C28 I was getting around 320V and C26 I was over 500!
Goes without saying I’m still learning hahaha. Thank you for the response!

 

[Wally]

Goes without saying I’m still learning

As am I.

 

[Lowerleftcoast]

The one I was calling 1st cap is C28. C28 I was getting around 320V and C26 I was over 500!

Are these voltages without a load? I have read a few threads mentioning B+ ~ 420.

 

[fishyguy192]

Are these voltages without a load? I have read a few threads mentioning B+ ~ 420.

All tubes in

 

[fishyguy192]

Just did some voltage checks.
C28 255
C26 502
C24 426
C12 345
C10 205

7189#1 grid pin 9-408 Plate pin 7-500
7189#2 grid pin 9-408 Plate pin 7-500

12ax7 connected to R9 pin 1- 99
Pin 6- 116
12ax7 connected to R10 pin 1- 142
Pin 6- 100
12au7 pin 1- 25
Pin 6- 162
12ax7 only one side connected
pin 6 102

On 12ax7 closest to the input I wasn’t getting any popping when I was taking a reading but I did on the second 12ax7 I did get some popping. While taking readings my 7189 power tubes were hot plating really badly. Should I put the 100mf/250v caps in instead of the 100mf350v?

 

[Lowerleftcoast]

Should I put the 100mf/250v caps in instead of the 100mf350v?

I would use the 350v caps for C26 and C28. The 250v caps would be maxed out.

Does *hot plating* equal red plating?

The 7189s pin2 should have a negative DC voltage on them (bias voltage). I guessing it should be in the -35 to -42 volt neighborhood. The more negative this voltage the cooler the tubes will run. The schematic shows a bias pot. Use the pot to get a more negative voltage reading on pin2.

Use all high voltage safety procedures.

 

[fishyguy192]

I would use the 350v caps for C26 and C28. The 250v caps would be maxed out.

Does *hot plating* equal red plating?

The 7189s pin2 should have a negative DC voltage on them (bias voltage). I guessing it should be in the -35 to -42 volt neighborhood. The more negative this voltage the cooler the tubes will run. The schematic shows a bias pot. Use the pot to get a more negative voltage reading on pin2.

Use all high voltage safety procedures.

I haven’t noticed a bias pot on this amp but will certainly give it a look! Yeah they are red playing badly. Hopefully that is the fix with the bias. Still having some issues with no output at the moment but hopefully that will be fixed with the bias and I think possibly the first preamp tube section is giving me issues since I don’t get any pop when I check for voltage

 

[fishyguy192]

The schematic shows a bias pot.

Where are you seeing this in the schematic? I’m not seeing it anywhere on my amp.